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Infrared Thermography Inspection of Fiber-Reinforced Polymer Composites Bonded to Concrete


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INFRARED THERMOGRAPHY INSPECTION OF FIBER-REINFORCED POLYMER COMPOSITES BONDED TO CONCRETE By JEFF ROBERT BROWN A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2005

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Copyright 2005 by Jeff Robert Brown

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To my wife, Heather, and daughter, Zo

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iv ACKNOWLEDGMENTS This work would not have been possible without the help and contributions of others. First, I would like to thank my co mmittee chair and advisor, Dr. Trey Hamilton, for his guidance and support thr oughout this effort. It was a truly an honor to work on this project and I will always appreciate the experience. Member s of my dissertation advisory committee are also acknowledged for th eir efforts: Dr. Andrew Boyd (Cochair), Dr. Gary Consolazio, Dr. Ron Cook, and Dr. E lliot Douglas. Additional thanks goes to Dr. Kurt Gurley for his assistance with my research and teaching here at UF. Chuck Broward provided esse ntial support during the labo ratory phase of this study. He has also been a wonderful friend. I can safely say that if it were not for Chuck and his passion for astronomy, I would never have seen the rings of Saturn or the transit of Venus. I would also like to thank Tony Murphy for providing excellent computer support. My masters thesis advisor, Dr. Sashi Kunnath, was responsible for sparking my interest in research and also been a wonde rful influence on my career. I cannot thank him enough for all of his help. Another co lleague from UCF, Dr. Mark Williams, was also a major influence and helped to get me es tablished here at the University of Florida. I would like to thank a number of my fr iends and colleagues for their help and support. Tony Michael and Markus Kutarba we re a tremendous help both in the lab and out in the field. We will always have some amazing stories to tell about two bridges in Jacksonville. Amber Paul assisted in the sp ecimen construction and data collection for

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v Phase II of this study. This work would neve r have been completed on-time without her help. Finally, I would like to thank Gustavo Alvarez for his help in the early stages of this project. My parents have also provided treme ndous support over the years, and their contributions are gratefully acknowledged. I have also been blessed with a family of my own, and none of this would have been possible without the support of my wi fe, Heather. Our daughter, Zo, also contributed in more ways than can be me ntioned. The only way I would do this again would be if we could do it together. Finally, this material is based upon work supported under a National Science Foundation Graduate Research Fellowship.

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vi TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iv LIST OF TABLES.............................................................................................................ix LIST OF FIGURES..........................................................................................................xii ABSTRACT.....................................................................................................................xix CHAPTER 1 INTRODUCTION........................................................................................................1 2 FIBER-REINFORCED POLYMER COMPOS ITES USED TO STRENGTHEN REINFORCED CONCRETE.......................................................................................9 Constituent Materials....................................................................................................9 Fibers......................................................................................................................9 Matrix Materials...................................................................................................11 Construction Methods and Applicati on Procedures for FRP Composites..................12 Composites Used in the Aerospace Industry........................................................12 Composites Used to Strengthen RC.....................................................................13 Locations of Defects in FRP Systems Bonded to Concrete.......................................14 Quality Control Standards..........................................................................................15 Research Significance.................................................................................................17 3 NONDESTRUCTIVE EVALUATION USING INFRARED THERMOGRAPHY...................................................................................................18 Infrared Thermography Fundamentals.......................................................................18 Detection of EM Radiation with an IR Camera..........................................................22 Thermal Imaging System Used in Current Study.......................................................24 Infrared Thermography Methods for NDE of Materials............................................25 Heating Methods...................................................................................................26 Image Acquisition.................................................................................................28 Data Analysis........................................................................................................29 Objectives of Current Research..................................................................................35

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vii 4 PHASE I EXPERIMENTAL WORK AND FIELD STUDY....................................36 Introduction.................................................................................................................36 Full-Scale AASHTO Girders......................................................................................37 Description of AASHTO Gi rders and FRP Systems............................................37 Infrared Inspection Procedures.............................................................................41 Initial IR Inspections.............................................................................................44 IR Inspections Performed During Load Testing...................................................49 IR Inspections of Known Debonded Areas After Failure....................................49 Summary of IR Inspection Results for Each FRP System...................................54 Field Inspection: Chaffee Road..................................................................................57 Summary of Findings for Phase I...............................................................................61 5 PHASE II: EXPERIMENTAL SETUP......................................................................65 Introduction.................................................................................................................65 Specimen Construction...............................................................................................66 FRP Composite Materials.....................................................................................67 Concrete Substrate................................................................................................68 Surface Preparation...............................................................................................70 Surface Saturation and Tack-Coat........................................................................70 Application of FRP Composite to Concrete.........................................................73 Construction Details for Each Series....................................................................74 Heating Methods and Thermal Imaging.....................................................................82 Flash Heating........................................................................................................83 Scan Heating.........................................................................................................86 Long-Pulse Heating..............................................................................................90 Sinusoidal Heating................................................................................................93 Comparison of Heating Configurations................................................................94 6 PHASE II: DATA COLLECTION AND ANALYSIS..............................................97 Introduction.................................................................................................................97 Pulse Thermography: Series A...................................................................................98 Specimen Heating and Data Collection................................................................98 Image Preprocessing.............................................................................................99 Defect Analysis...................................................................................................101 Proposed Method for Charac terizing Detectability............................................116 Experimental Results: Fl ash Heating (Series A)................................................118 Experimental Results: Scan Heating (Series A).................................................135 Experimental Results: LongPulse Heating (Series A)......................................143 Comparison of Heating Methods........................................................................145 General Detectability..........................................................................................146 Defect Characterization......................................................................................152 Summary of Pulse Thermography Results.........................................................166 Step Thermography Analysis...................................................................................167 Analysis Procedures............................................................................................167

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viii Summary of Step Thermography Results: Series A Specimens.........................188 Frequency Domain An alysis: Series A.....................................................................190 Sinusoidal Heating (Lock-In IRT)......................................................................190 Pulse Phase Thermography.................................................................................199 Comparison of Heating Methods and Analysis Techniques.....................................207 General Detectability..........................................................................................207 Defect Characterization......................................................................................210 Series B, C, D, and E Specimens..............................................................................212 Data Collection...................................................................................................214 Series B...............................................................................................................214 Series C...............................................................................................................218 Series D...............................................................................................................218 Series E...............................................................................................................219 7 SUMMARY AND RECOMMENDATIONS FOR FUTURE RESEARCH...........221 Summary...................................................................................................................221 Phase I.......................................................................................................................222 Laboratory Study................................................................................................222 Field Study..........................................................................................................223 Phase II.....................................................................................................................223 Heating Methods.................................................................................................223 Data Analysis Methods.......................................................................................224 FRP System Properties and IRT Results............................................................225 Recommendations for Deployment of IRT..............................................................225 Guidelines for Qualitative IRT Inspections........................................................225 Quantitative Analysis..........................................................................................226 Future Research........................................................................................................227 APPENDIX A TIME DOMAIN RESULTS: SERIES A.................................................................229 B SINUSOIDAL HEATING RE SULTS: SERIES A..................................................250 C SERIES B, C, D, AND E RESULTS.......................................................................255 D COMPOSITE PROPERTIES FO R SMALL-SCALE SPECIMENS.......................265 LIST OF REFERENCES.................................................................................................268 BIOGRAPHICAL SKETCH...........................................................................................271

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ix LIST OF TABLES Table page 2-1 Dry carbon-fiber properties used in aerospace industry........................................10 2-2 Properties of dry fibers for commerci ally available fiber-reinforced polymer systems used to strengthen reinforced concrete......................................11 2-3 Properties of epoxies used in commerc ially available fiber-reinforced polymer systems for strengthe ning reinforced concrete........................................12 4-1 Fiber-reinforced polymer system properties for full-scale AASHTO girders....................................................................................................................38 4-2 Summary of scanning speed and uniformity of heating........................................47 5-1 Overview of Specimen Matrix...............................................................................67 5-2 Material properties fo r fibers, epoxy, and lamina..................................................68 5-3 Concrete mix proportions used for Series A to E..................................................70 5-4 Series A details......................................................................................................76 5-5 Series B details.......................................................................................................79 5-6 Series C details.......................................................................................................80 5-7 Series D details......................................................................................................81 5-8 Series E details.......................................................................................................82 5-9 Surface temperature increase resu lts for different heating methods......................96 6-1 Summary of data collected for pulse analysis study..............................................99 6-2 Parameters computed for de fect area at each time step.......................................104 6-3 Parameters extracted from Tdef vs. time plot for each defect.............................109 6-4 Detectability classification based on COV of computed radii..........................118

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x 6-5 Flash heating results for Specimen A-1...............................................................120 6-6 Gradient area method results fo r Specimen A-1: flash heating...........................122 6-7 Summary of Results for Sp ecimen A-2: flash heating.........................................126 6-8 Summary of results for Sp ecimen A-3: flash heating..........................................128 6-9 General detectability results for flash heating.....................................................131 6-10 Signal to boundary noise ratio (SBR) results for flash heating...........................132 6-11 Ratio of parameters for air and epoxy-filled defects...........................................135 6-12 General detectability results for scan heating......................................................139 6-13 Signal to boundary noise ratio (SBR) results for scan heating............................139 6-14 Ratio of parameters for air and epoxy-filled defects (scan heating)....................142 6-15 General detectability resu lts for long-pulse heating............................................144 6-16 Defect data and predicted dept h for defects shown in Figure 6-48.....................162 6-17 Predicted and actual properties of defects in Figure 6-48....................................165 6-18 Typical thermal properties for materials of interest.............................................169 6-19 Summary statistics for de fect-free areas (Series A).............................................175 6-20 Parameters of interest for ch aracterizing defects from step thermography data...............................................................................................190 6-21 Frequencies investigated during sinusoidal heating experiments........................191 6-22 Recommended pulse dura tions and detection limits for sinusoidal heating (carbon-FRP systems)..........................................................................................198 6-23 Normalized temperature response @ t = 60 sec for properly saturated specimens.............................................................................................................216 A-1 Flash heating results for Series A........................................................................233 A-2 Scan heating results for Series A.........................................................................237 A-3 Long-pulse (30 sec) heat ing results for Series A.................................................241 A-4 Long-pulse (45 sec) heat ing results for Series A.................................................245

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xi A-5 Long-pulse (60 sec) heat ing results for Series A.................................................249 B-1 Sinusoidal heating results: vs. frequency plot parameters............................254 C-1 Series B (low saturation) summar y statistics for defect-free areas......................259 C-2 Series C (surface prep) summary statistics for defect-free areas.........................261 C-3 Series D (fiber saturatio n methods) summary statistics for defect-free areas.............................................................................................262 D-1 Series A composite properties.............................................................................266 D-2 Series B composite properties..............................................................................266 D-3 Series C composite properties..............................................................................266 D-4 Series D composite properties.............................................................................267 D-5 Series E composite properties..............................................................................267

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xii LIST OF FIGURES Figure page 1-1 Strengthening reinfor ced concrete beams................................................................2 1-2 Prestressed AASHTO girder dama ged by over height vehicle................................3 1-3 Application of FRP composite to strengthen existing structure..............................4 1-4 Reinforced concrete column wrapped with FRP.....................................................5 1-5 Vehicle impact damage to FRP composite s that occurred af ter installation...........5 1-6 Damage to FRP composite due to corrosion of internal reinforcing steel...............6 1-7 Surface temperature response due to external radiant heating.................................7 1-8 Infrared thermography inspecti on of FRP composite system..................................8 2-1 Location of potential unbonded, debonde d, and delaminated areas in FRP systems...................................................................................................................15 3-1 Incident radiation ( i) is reflected, transmitted or absorbed.................................20 3-2 Electromagnetic emission curves fo r objects at different temperatures................21 3-3 Atmospheric emission in the MWIR and LWIR spectral bands............................23 3-4 General schematic of a focal plane array (FPA) and associated optics.................24 3-5 Application of IR thermography to FRP composite bonded to concrete...............25 3-6 Surface heating and defect de tection for pulse thermography...............................31 3-7 Defect detection with lock-in thermography.........................................................34 4-1 Full-scale AASHTO type II gird er and load test setup..........................................37 4-2 Cross-section views of FRP systems.....................................................................39 4-3 Data collection for full-scale AASHTO girders....................................................43

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xiii 4-4 Subsurface defect found on Girder 3.....................................................................44 4-5 Subsurface defects found on Girder 3....................................................................45 4-6 Non-uniform surface heating of Girder 4..............................................................46 4-7 Thermal images collected for full-scale AASHTO girders...................................47 4-8 Background temperature increase vs position along length of girder...................48 4-9 Failure modes for full-scale AASHTO girders......................................................50 4-10 Defect signal strength ( Tdefect) vs. time for known debonded area......................51 4-11 Debonded area after failure for Girder 6................................................................53 4-12 Series of thermal images for air and epoxy filled defects......................................54 4-13 Polyurethane matrix shown after de bonding from concrete (Girder 4).................56 4-14 Vehicle impact damage sustai ned after FRP strengthening...................................58 4-15 Visual and thermal images of vehicle impact damage...........................................59 4-16 Infrared thermography insp ection of undamaged girder.......................................60 4-17 Damaged girder before new FRP system was applied...........................................61 5-1 TYFO SCH-41 carbon-fibers.................................................................................69 5-2 TYFO SEH-51 glass-fibers....................................................................................69 5-3 Surface preparation before FRP placement...........................................................70 5-4 Application of epoxy satu rant and tack-coat.........................................................71 5-5 Fiber saturation......................................................................................................73 5-6 Completed specimen..............................................................................................74 5-7 Defect configuration for Series A specimens........................................................76 5-8 Defect configuration for Series B specimens.........................................................80 5-9 Lap-splice configuration for Series E....................................................................82 5-10 Heat source and camera configuratio n for pulse heating experiments..................84 5-11 Typical thermal image collected during pulse heating experiment.......................84

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xiv 5-12 Surface temperature profile due to pulse heating..................................................87 5-13 Heat source used in s can heating experiments.......................................................88 5-14 Thermal images collected during scan heating experiment...................................88 5-15 Thermal image collected during scan heating experiment....................................89 5-16 Surface temperature profile for scan heating.........................................................89 5-17 Heat source and camera confi guration for long-pulse heating..............................91 5-18 Thermal image collected at t = 1 sec during long-pulse heating...........................91 5-19 Laboratory setup for long-pul se heating experiments...........................................92 5-20 Surface temperature profile for long-pulse heating (30 sec pulse)........................92 5-21 Surface temperature profile for long-pulse heating (60 sec pulse)........................93 5-22 Diagram for sinusoidal heating control and data acquisition.................................96 6-1 Application of 3x3 averaging f ilter applied to each pixel in thermal image.......................................................................................................102 6-2 Area identification for defect analysis.................................................................104 6-3 Constructing Tdef vs. time plots from area parameters......................................105 6-4 Thermal images and Tdef vs. time plot for Defect IB........................................106 6-5 Non-uniform heating and weak signals for defects.............................................107 6-6 Identification of important parameters for weak signals.....................................108 6-7 Signal for undetected defect.................................................................................109 6-8 Defect area computations using boundary trace method.....................................111 6-9 Surface temperature profile and gradient used to approximate the boundary of detected defects...............................................................................112 6-10 Reduced accuracy in area com putations due to a weak signal............................114 6-11 Coefficient of variation for ellip se radii (computed with NP = 250)...................115 6-12 Reduced accuracy in area computa tions due to non-uniform heating.................115 6-13 Reduced accuracy in area computations due to low image resolution................116

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xv 6-14 Detectability classification based on Tdef vs. time plot.....................................118 6-15 Flash heating results for Specimen A-1...............................................................120 6-16 Gradient images for defects.................................................................................122 6-17 Specimen A-1: Important parameters for defects................................................123 6-18 Thermal images for Specimen A-2: flash heating...............................................124 6-19 Flash heating results for Specime n A-2 : temperature vs. time data...................126 6-20 Unintentional defects betw een layers in Specimen A-2......................................126 6-21 Specimen A-2: important parameters for defects................................................127 6-22 Thermal images and Tdef vs. time plots for Specimen A-3...............................128 6-23 Specimen A-3: important parameters for defects................................................129 6-24 Thermal images and Tdef vs. time plots for Specimen A-4...............................130 6-25 Normalized Tmax for flash heating.....................................................................133 6-26 Time to maximum signal for flash heating..........................................................134 6-27 Signal half-life for flash heating..........................................................................134 6-28 Standard deviation of Defect IB perimeter..........................................................136 6-29 Data for Defects A25 and E25 (6.4 mm diameter)..............................................137 6-30 Thermal images for Defect IB (Specimen A-2)...................................................138 6-31 Normalized Tmax for scan heating.....................................................................140 6-32 Time to maximum signal for scan heating...........................................................141 6-33 Signal half-life for scan heating...........................................................................142 6-34 Normalized Tmax for long-pulse heating.........................................................144 6-35 Time to maximum signal for long-pulse heating.................................................145 6-36 Signal half-life fo r long-pulse heating.................................................................145 6-37 Legend for Figure 6-38........................................................................................146 6-38 Summary of general detect ability for flash, scan, and long-pulse heating................................................................................................147

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xvi 6-39 Comparison of Tmax for different heating methods...........................................149 6-40 Comparison of normalized Tmax for different heating methods........................151 6-41 Coefficient of variation (COV) of computed radii for different heating methods...................................................................................................153 6-42 Maximum signal vs. radii COV for all detected defects in Series A...................154 6-43 Time to maximum signal fo r different pulse durations.......................................155 6-44 Defect circumference (C) x depth (d) vs. tmax for flash experiments...................156 6-45 Signal half-life for different pulse durations........................................................158 6-46 Plot of defect circumference C x depth D vs. t1/2 for all heating methods...........159 6-47 Plot of defect circumference C x depth D vs. t1/2 for all heating methods...........160 6-48 Thermal image from long-pulse experiment (Series B and C specimens)..........161 6-49 Defect signal vs. time plot for defects shown in Figure 6-48..............................161 6-50 Characterization of Defect A3.............................................................................163 6-51 Characterization of Defect A4.............................................................................164 6-52 Temperature increase for select areas..................................................................166 6-53 Surface temperature increase due to uniform heat flux.......................................170 6-54 Normalizing T for two points on Specimen A-1...............................................173 6-55 T image for Series A specimens........................................................................173 6-56 Normalized T image for Series A specimens....................................................174 6-57 Defect-free areas for Series A specimens............................................................176 6-58 Mean value of Tnorm for defect-free areas on Series A......................................176 6-59 One-dimensional model of FRP systems.............................................................177 6-60 Computation of Tdef from normalized temperature data...................................179 6-61 Determining point at whic h defect is detected in Tdef plots..............................180 6-62 Surface plots of Def ect A50 (Specimen A-2)......................................................181 6-63 Two-dimensional correlation coe fficient, R, for Defect A50..............................182

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xvii 6-64 Defect signal vs. t1/2 for Specimen A-1................................................................183 6-65 Two-dimensional correlation coefficient vs. t1/2 for Specimen A-1....................184 6-66 Defect signal vs. t1/2 for Specimen A-2................................................................185 6-67 Two-dimensional correlation coefficient vs. t1/2 for Specimen A-2....................186 6-68 Normalized T images for Defect IB (Specimen A-2).......................................186 6-69 Defect signal vs. t1/2 for Specimen A-3................................................................187 6-70 Two-dimensional correlation coefficient vs. t1/2 for Specimen A-3....................188 6-71 Normalized temperature imag e for Specimen A-4 (t = 60 sec)...........................188 6-72 Data analysis for sinusoidal heating (pulse duration = 500 sec)..........................193 6-73 Sinusoidal heating result s for Series A specimens (Pulse Duration = 8.33 sec)..................................................................................194 6-74 Sinusoidal heating result s for Series A specimens (Pulse Duration = 25 sec).....................................................................................195 6-75 Sinusoidal heating result s for Series A specimens (Pulse Duration = 125 sec)...................................................................................196 6-76 Sinusoidal heating result s for Series A specimens (Pulse Duration = 500 sec)...................................................................................197 6-77 Application of PPT method to Specimen A-3.....................................................204 6-78 Comparison of time domain and frequency domain (PPT) results for Specimen A-3.....................................................................................205 6-79 Comparison of time domain and frequency domain (PPT) results for Specimen A-4.....................................................................................205 6-80 Defect signal (phase) vs. freque ncy plots for air-filled defects...........................206 6-81 Frequency domain re sults for Specimen A-3.......................................................207 6-82 Comparison of heating methods for Specimen A-4.............................................209 6-83 Comparison of data analysis techniques for Specimen A-3................................212 7-1 Field inspection (scan hea ting method) of FRP system.......................................226 A-1 Flash heating results: Th ermal images for Series A............................................230

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xviii A-2 Flash heating results: Tdef vs. time plots for Series A.......................................231 A-3 Scan heating results: Thermal images for Series A.............................................234 A-4 Scan heating results: Tdef vs. time plots for Series A........................................235 A-5 Long-pulse (30 sec) heating resu lts: Thermal images for Series A.....................238 A-6 Long-pulse (30 s ec) heating results: Tdef vs. time plots for Series A................239 A-7 Long-pulse (45 sec) heating resu lts: Thermal images for Series A.....................242 A-8 Long-pulse (45 s ec) heating results: Tdef vs. time plots for Series A................243 A-9 Long-pulse (60 sec) heating resu lts: Thermal images for Series A.....................246 A-10 Long-pulse (60 s ec) heating results: Tdef vs. time plots for Series A................247 B-1 Sinusoidal heating results: Phase images for Series A.......................................251 B-2 Sinusoidal heating results: vs. frequency plots for Series A........................252 C-1 Low matrix saturation (Series B carbon-fibers)...................................................256 C-2 Medium matrix saturati on (Series B carbon-fibers)............................................256 C-3 High matrix saturation (Series B carbon-fibers)..................................................257 C-4 Low matrix saturation (Series B glass-fibers).....................................................257 C-5 Medium matrix saturation (Series B glass-fibers)...............................................258 C-6 High matrix saturation (Series B glass-fibers).....................................................258 C-7 No surface preparation (Series C carbon-fibers).................................................260 C-8 Light blast surface preparat ion (Series C carbon-fibers).....................................260 C-9 Heavy blast surface preparat ion (Series C carbon-fibers)...................................261 C-10 Different fiber saturation me thods (Series D carbon-fibers)...............................262 C-11 Specimen E-1 (1-layer/3-layer/2-layer)...............................................................263 C-12 Specimen E-2 (2-layer/3-layer/2-layer)...............................................................263 C-13 Specimen E-3 (3-layer/4-layer/2-layer)...............................................................264

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xix Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy INFRARED THERMOGRAPHY INSPECTION OF FIBER-REINFORCED POLYMER COMPOSITES BONDED TO CONCRETE By Jeff Robert Brown August 2005 Chair: H.R. Hamilton III Cochair: Andrew J. Boyd Major Department: Civil and Coastal Engineering The use of fiber-reinforced polymer (FRP) composites to strengthen existing civil infrastructure is expanding rapidly. Many FRP systems used to strengthen reinforced concrete are applied using a w et layup method in which dry fibers are saturated on-site and then applied to the surface. Air voi ds entrapped between the FRP system and concrete as a result of improper insta llation reduce the integrity of the repair. The objective of this study was to investigate the use of infrared thermography (IRT) for evaluating bond in FR P composites applied to reinfo rced concrete. Phase I of this study examined FRP strengthening system s that were applied to full-scale bridge girders. IRT inspections were perfor med on four AASHTO t ype II girders with simulated impact damage that were loaded to failure. Phase I also contained a field inspection of an in-service bridge that was strengthened with FRP composites. The results of the field studies indicated that as the overall thickness of the FRP system

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xx increased the detectability of defects was diminished. In addition the installation procedures influenced IRT results. The use of excessive epoxy tack-coat was shown to reduce detectability and increase the required observation time. A second experimental study (Phase II) was conducted in which 34 small-scale specimens (15 cm x 30 cm) containing fabricated defects were inspected in a laboratory environment. These specimens were c onstructed using diffe rent FRP composite thicknesses (1mm to 4 mm) and matrix saturati on levels. Four heating methods were investigated (flash, scan, long-pulse, and si nusoidal), and quantitative analyses were performed on the thermal data usi ng currently available techniques. Data were used to establish detection limits for air and epoxy-filled voids in carbon FRP composites. It was shown that IRT is capable of detecting 19 mm diameter and larger defects in carbon FRP composites up to 4 mm thick. Quantitative data analysis techniques were also used to estimate the de pth and material composition of defects up to 2 mm below the surface. These data anal ysis techniques were also effective for enhancing detection of defects up to 4 mm below the surface.

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1 CHAPTER 1 INTRODUCTION Fiber reinforced polymer (FRP) composite s are currently used to repair and strengthen existing reinforced concrete struct ures. Several types of FRP repair systems are commonly encountered: surface bonded, near surface mounted (NSM), and FRP bars. Surface mounted systems consist of dry fibers that are saturated on-site with a matrix material and applied directly to the surface before the composite cures. NSM systems involve pre-cured FRP laminates that are bonded to the structure using an intermediate bonding agent. One advantage of pre-cured lami nates is that they are manufactured in a factory setting. The resulting laminates are ty pically of high quality and possess uniform material properties. FRP bars are also pre-cured composites that are intended to serve as additional reinforcement in reinforced conc rete (RC) structures. These systems are installed by cutting a groove in the surf ace of the concrete just large enough to accommodate the FRP bar. An intermediate bonding agent, typically a thickened epoxy, is then used to grout the bar in place. The advantages of using FRP composites for strengthening and repair include the high strength to weight ratio of FRP material s, ease of installa tion, and resistance to corrosion. Prior to the availability of compos ite materials, traditional repair and retrofit techniques involved attaching steel plates to members or outright replacement. This type of repair is often difficult to implement a nd requires the mobilization of heavy equipment simply to handle the repair materials.

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2 The American Concrete Institute (ACI) committee 440 has produced a document that engineers can use to specify FRP sy stem requirements based on design objectives (ACI 440-R02 2002). FRP composites are used primarily to: Increase flexural capacity of a member Increase shear capacity Provide additional confinement to increase concrete ductility For flexural strengthening, th e fibers are oriented along th e length of the beam axis to serve as added tensile reinforcement (Figure 1-1A). The FRP composite is typically applied to the tension face. To increase th e shear capacity of a member, unidirectional FRP composite is applied to the web with the fi bers oriented transverse to the beam axis (Figure 1-1B). Bi-directional FRP composites are also used with the goal of providing additional reinforcement in ar eas of high diagonal tension. A B Figure 1-1. Strengthening reinfo rced concrete beams. A) For flexure. B) For shear. A common thread with these two approaches is that they rely entirely on bond to transfer stresses between the c oncrete beam and fibers. Thes e applications are considered bond-critical. The adhesive and concrete substrate must be sound and of sufficient strength to transfer stress to the fibers. The bond is critical because there are no redundant load paths for stress to follow should the bond fail. Flexural Strengthening Shear Strengthening Externally Bonded FRP Composite

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3 A practical example of these t ypes of repairs is shown in Figure 1-2. In this case, an AASHTO girder was hit by an over height vehicle and was subsequently repaired with FRP composites. In addition to the heavy concrete spalli ng, a number of the girders prestressing strands were cut. The FRP com posite was applied to restore both flexural and shear capacity. Figure 1-3(a) demonstrates th e relative ease with which FRP composites can be applied to existing struct ures. FRP composites are easily conformed to the member and the orienta tion of fibers can be adjust ed depending on the particular strengthening application. Figure 1-2. Prestressed AASHTO girder dama ged by over height vehicle. Girder was repaired with FRP composites. The third approach, improving concrete c onfinement, is popular in seismically active regions and is generally refe rred to as column-wrapping. Figure 1-4 shows the use of column-wrapping at the base of a column or pier where a plas tic hinge is expected to form during extreme ground shaking. The FRP composite wrap confines the concrete, which improves the ductility and apparent strength as well as improving lap splice performance. Bond is not considered critical in this application since the fibers are continuously wound around the member. Column wrapping is commonly referred to as contact-critical.

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4 A B Figure 1-3. Application of FR P composite to strengthen exis ting structure. A) Workers applying carbon-fiber composite. B) completed project. If the composite is not installed prop erly and air-bubbles are present at the FRP/concrete interface, the system may not perform as desired. Figure 1-4 shows the severity of installation defects that can occu r in FRP composite systems. Another issue that can affect the overall performance of an FRP system is durability. Numerous researchers have cited durability of FRP composite systems as a major challenge confronting the industry (CER F 2001, Kharbari et al 2003, Nanni 2003). A number of factors can contribute to the degradation of an FRP compos ite system during its service life: Environmental exposure (moi sture, temperature cycles) Overloading resulting in partial debonding Vehicle impact Corrosion of internal reinforcing steel A number of the FRP repairs initiated by the Florida Department of Transportation (FDOT) to mitigate vehicle impact damage have also been struck and damaged by over height vehicles (Lammert 2003). Figure 1-5 provides two examples of damage to FRP systems resulting from vehicle impact.

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5 Figure 1-4. Reinforced concre te column wrapped with FRP. The large air bubbles are a result of installation defects. A B Figure 1-5. Vehicle impact damage to FRP co mposites that occurred after installation. A) Chaffee Rd./I-10 overpass near Jacksonville, Florida. B) 45th St./Florida Turnpike overpass in West Palm Beach, Florida. The expansive nature of corrosion byproduc ts result in cracking of the concrete substrate. If an FRP system is applie d to a member experiencing active corrosion, subsequent cracking of the conc rete substrate can lead to debonding or rupture of the FRP composite. This scenario was observed on an FRP repair that was applied to a US

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6 Highway 100 bridge spanning the intracoas tal waterway in Melbourne, Florida (Figure 16). This damage led to the removal a nd replacement of the entire FRP system Figure 1-6. Damage to FRP composite due to corrosion of internal reinforcing steel After an extensive survey on defects in FRP composites, Kaiser and Kharbari (2001a) concluded that the performance and ex pected lifetime of FR P repairs are largely dependent on the quality of installation and th e presence of defects. This work also highlights a need for long-term monitoring of performance and durability. Kaiser and Kharbari (2001b) also provide a description of NDE techniques that can be used to evaluate FRP composites. It is apparent that FRP co mposites are increasingly used to repair and strengthen reinforced concrete structures Because this is a relativel y new construction technique, methods of evaluating the inst allation quality and long-term efficacy are needed. ACI 440 cites acoustic sounding, ultrasonics, lase r shearography, and infrared thermography (IRT) as methods that can be used to evaluate these critical aspects of FRP composites. The objective of this research is to develop IRT technique s to evaluate bond quality in FRP composites applied to concrete. IR T is a non-contact remote sensing technique that can be used to measure the surface temperature of an object. The fundamental approach is shown in Figure 1-7. If the surface of a homogeneous material is heated Cracking and debonding due to internal corrosion of reinforcing steel

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7 using an external heat source, the increase in temperature on the surface will be uniform. If, however, the thermal front traveling from the surface into the material encounters an air-void or other discontinuity (defect), the relative rate of surface temperature increase above the defect will change. Depending on th e size and material characteristics of the subsurface defect, it may be possible to dete ct this change in temperature using an infrared camera. A sample application of IRT is shown in Figure 1-8. In this example, the surface of a carbon-fiber/epoxy FRP system was heated using a 500 W halogen lamp. The thermal image provided in Figure 1-8(a) indicates that a portion of the FRP is not bonded to the concrete substrate. The light co lors in the thermal image indicate higher temperatures. Portions of the FRP compos ite that are well bonded to the concrete substrate appear darker in the thermal image. Figure 1-7. Surface temperature response due to external radiant heating for homogeneous materials and materials with subsurface imperfections. Tambient Surface Temperature tp homogeneous material material w/ defect time1/2 Surface heating with external heat source

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8 This research investigated the use of IRT for detecting defects in FRP systems bonded to concrete. Specifically, a major goa l was to use IRT data to provide the following information about detected defects: Size Depth below the surface Material composition Other items that are addressed include: Detection limits Heating methods Data analysis procedures Previous research in this field has focu sed on the inspection of FRP composites that are commonly used in the aerospace industry. A number of data analysis techniques have been developed that can assist in using IRT results to character ize defects. None of these methods have been calibrated for use on FRP systems bonded to concrete. A B Figure 1-8. Infrared therm ography inspection of FRP comp osite system. A) Thermal image. B) Visual image. Debonded area indicated by yellow/white areas (higher temperatures)

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9 CHAPTER 2 FIBER-REINFORCED POLYMER COMPOSITES USED TO STRENGTHEN REINFORCED CONCRETE Constituent Materials The term composite is used to describe any material that is created by combining two or more materials on a macroscopic leve l. In the current study, the term composite will refer to a combination of a polymer matrix and fibers. The primary function of the fiber material is to carry load. The matrix serves as a binder that holds the composite together and transfers stress between fibers. Fibers The following fiber materials are commonly used in FRP composites (Gibson 1994): Glass Carbon Aramid Boron The type of fiber chosen for a specifi c application depends on the specific requirements for strength, toughness, stiffness, and service temperatur e. Glass (E-glass) and carbon-fibers are widely used for strengthening reinforced concrete. It is interesting to note th e wide array of material pr operties that are found amongst different types of carbon-fibers. The stre ngth, stiffness and thermal conductivity of carbon-fiber materials are highl y dependent on the manufactur ing process and on the base material from which the fibers are extrude d. Carbon-fibers can be divided into two general categories: PAN based fibers, wh ich are extruded from a polyacrlyonitrile

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10 precursor, and pitch based fibers, which are extruded from a petroleum based pitch precursor. PAN based fibers tend to have a lower modulus of elas ticity (207 to 310 GPa) and a higher ultimate tensile strength (3.8 to 5.2 GPa) than pitch based fibers (Table 2-1). Pitch based fibers are noted fo r their relatively high stiffness and can have a modulus of elasticity of ranging 379 to 965 GPa. A wide range of thermal conductivity values is associated wi th different carbonfiber types. Callister (1997) provides a th ermal conductivity for low modulus PAN based fibers of 8.5 W/m-k. The ASM Materials Handbook (Vol. 21, 2005) provides a thermal conductivity of 20 W/m-K for standard modulus PAN based carbon-fibers and values as high as 1100 W/m-K for ultra-high modulus pi tch based carbon-fibers. All quantities cited for thermal conductivity represent values in the longitudinal dire ction of the fibers. The glass-fibers used in st ructural engineering applica tions are typically E-glass, with a thermal conductivity of 1.3 W/m-K which is lower than all forms of carbon-fibers. Table 2-1. Dry carbon-fiber propertie s used in aerospace industry Manufacturer Designation Organic precursor Tensile strength (GPa) Modulus of elasticity (GPa) Hexcela AS4 PAN 4.27 228 Hexcel IM7 PAN 5.17 276 Cytecb Thornel-P55s Pitch 1.90 379 Cytec Thornel-P120s Pitch 2.41 827 Cytec K-1100 Pitch 3.10 965 a Hexcel (2005). http://www.hexcel.com/Products. 6400 West 5400 South, Salt Lake City, UT b Cytec Industries (2005) http://www.cytec.com/business/EngineeredMaterials/ CFInternet/cfthornelpitch.shtm. Cytec Industries Inc., 5 Garret Mountain Plaza, West Paterson, NJ Mechanical properties of dry carbon and glas s-fibers commonly used in structural engineering applicatio ns are provided in Table 2-2. Only one of the FRP system manufacturers listed in the table (VSL) explic itly identifies their carbon-fibers as PAN based. The modulus of elasticity of all carbonfibers provided in Table 2-2 is relatively

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11 consistent between the different FRP system manufacturers. These modulus values suggest that a PAN precursor is common am ongst the different carbon-fiber systems. Table 2-2. Properties of dry fibers for comm ercially available fiber-reinforced polymer systems used to strengthen reinforced concrete Manufacturer Designation Fiber type Tensile strength (GPa) Modulus of elasticity (GPa) Area density (g/m2) Percent elongation @break Fyfe Co.a SCH-41 Carbon 3.79 230 644 1.7 Fyfe Co. SCH-41S Carbon 3.79 230 644 1.7 Edge Compositesb VelaCarb 335 Carbon 4.48 234 335 1.9 Edge Composites VelaCarb 600 Carbon 4.48 234 600 1.9 VSLc V-Wrap C100 Carbon 3.79 228 300 1.5 VSL V-Wrap C150 Carbon 3.79 228 440 1.0 VSL V-Wrap C200 Carbon 3.79 228 600 1.5 Fyfe Co. SEH-51A E-Glass 3.24 72 915 4.5 Edge Composites Vela-Glass E-Glass 2.28 72 875 4.0 VSL V-Wrap EG50 E-Glass 2.28 72 900 4.0 a Fyfe Co. LLC (2005). http://www.fyfeco.com/products/compositesystems.html. Nancy Ridge Technology Center, 6310 Nancy Ridge Drive, Suite 103,San Diego, CA b Edge Structural Composites (2005). http://www.edgest.com/edgedatain tro.html. 145 Park Place Point, Richmond, CA c VSL (2005). http://www.vsl.net/s trengthening_products/vsl_frp_composites.html. 7455 New Ridge Road, Suite T, Hanover, MD. Matrix Materials A variety of matrix materials (resins) are commonly used in structural engineering applications: epoxy, polyester, vinylester, and polyurethane. Epoxies are commonly used in wet layup systems due to th e relatively long pot-life (usu ally on the order of several hours depending on the temperature). Typi cal mechanical properties of epoxies commonly used in structural engineer ing applications are provided in Table 2-3. Polyester resins and vinylester resins are used in spray-up app lications where chopped glass-fibers and matrix material are spraye d onto the surface. These materials tend to

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12 cure more rapidly than epoxies. Polyur ethane resin can be found in certain preimpregnated (prepreg) systems. An interesting feature of this matrix material is that water can be used to activate the curing process. Table 2-3. Properties of epoxies used in commercially available fiber-reinforced polymer systems for strengthening reinforced concrete Manufacturer Designation Tensile strength (MPa) Modulus of elasticity (GPa) Density (g/cm3) Glass transition temperature (C) Fyfe Co.a Tyfo S 72.4 3.2 1.1 82 Edge Compositesb Veloxx LR 44.8 2.1 NA 63 VSLc V-Wrap C100 55.2 3.4 NA NA a Fyfe Co. LLC (2005) b Edge Structural Composites (2005) c VSL (2005) The thermal conductivity associ ated with the different ma trix materials ranges from 0.15 W/m-K to 0.2 W/m-K (Callister 1994). Construction Methods and Application Procedures for FRP Composites Composites Used in the Aerospace Industry Major developments in the field of FRP composites occurred in the 1960s around the growth of the aerospace industry (Gibson 1994). A wide variety of fiber and matrix materials were developed along with a numbe r of advanced manufacturing procedures. Today, most composites used in the aerospac e industry consist of carbon-fibers and an epoxy matrix. Composite parts are typically constructed by placing layers of carbonfibers that have been pre-impregnated with the matrix material onto a mold with the desired fiber orientation. Parts are then placed in a vacuum bag and cured in an autoclave under high pressures at an elev ated temperature. The resu lting parts have a high fiber volume fraction (typically from 0.5 to 0.8) and a low void content (0.001 to 0.01 by volume).

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13 Composites Used to Strengthen RC FRP composites applied to RC are typical ly installed using a wet layup method. This procedure involves saturating dry fibers on-site and then appl ying the wet composite directly to the surface be ing strengthened. The composite is then allowed to cure in-situ. The resulting composites typically have low fi ber volume fractions (high matrix content) and a higher percentage of air voids than aerospace composites. The concrete substrate must be properly cleaned and contain no sharp protrusions before the saturated composite is applied to the surface. Th e level of surface preparation that is performed can vary significantly between different applications. In some cases the surface will be sandblasted while in other ca ses the surface might be ground smooth with a grinding wheel. Large imperfections in the concrete substr ate must be repaired by backfilling the damaged area with a cementitious material. Smaller imperfections, such as bug holes and formwork joints, can be repaired by filling the void with putty or thickened epoxy. It is important that any remain ing sharp edges are removed before the FRP composite is applied. The next step in surface preparation i nvolves saturating the surface with matrix material (epoxy). Concrete is a naturally po rous material that can absorb epoxy. If the saturated fabric was applied directly to dr y concrete, there would be a tendency for the concrete to pull matrix out of the fibers. This can result in air void s at the FRP/concrete interface. If the composite is being applied to an overhead surface, an additional layer of thickened epoxy tack-coat can be used to ensu re that the saturated fabric does not fall down before the matrix material cures.

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14 After the saturated fibers have been applied to the surfac e, a squeegee or roller is used to remove air bubbles and any excess ma trix material from the FRP composite. If the specific application calls for more than one layer of composite, the layers are applied one at a time. Once the composite has cure d, a final top-coat of epoxy is applied to provide an additional layer of protection for the composite. Another common application involves bonding precured FRP laminates to the structure. The FRP laminates are manufactured in a controlled environment using similar procedures to those described for the aeros pace composites. These precured laminates are then bonded to the stru cture using a thickened epoxy paste. Even though the composite material is not likely to contain defe cts such as air voids, there is a possibility that imperfections will exist along the thic kened epoxy bond line if the material is not applied properly. The wet layup method provides the mo st flexibility for RC strengthening applications. Different thicknesses can be obtained over critical areas and the fiber orientation can be easily adjusted depending on th e strengthening requirem ents. It is also possible to span long distances across b eams by splicing shorter pieces together. Locations of Defects in FRP Systems Bonded to Concrete Defects in FRP systems can result from improper installation or long-term degradation due to environmental factors. De fects in FRP systems can be classified in three ways: unbonded areas, debonded areas, and delaminated ar eas (adopted from Levar and Hamilton (2003)). The term unbonded refers to areas of the FRP system that were not properly bonded when the system first cured. The most common causes of unbonded areas are improper surface prepar ation of the concrete and attempting to apply material across sharp angles or re-ent rant corners. Debonded area s are locations in which bond

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15 that previously existed between the concre te and FRP has been destroyed. Debonded areas can occur at several locations in the composite/concrete interface region (Figure 21) and are usually a result of excessive load ing or impact. If the debonded area occurs due to excessive loading, it is common for the failure plane to occur a few millimeters below the adhesive concrete interface. It is also possible for the entire layer of concrete cover to separate from the beam at the leve l of the reinforcing st eel (Sebastian 2001). Delaminations are a lack of bond between diffe rent layers in a multi-layer FRP system. Delaminations can be a result of imprope r installation or ex cessive loading. Figure 2-1. Location of potential unbonde d, debonded, and delaminated areas in FRP systems Quality Control Standards Installation defects ar e likely to occur in FRP systems bonded to concrete. ACI document 440.2R-02 provides acceptance criteria for the allowable debonded area in wet layup FRP systems. These guidelines are intende d to be applied to the installation of new FRP systems and may be summarized as follows: FRP Layers Adhesive Layer Reinforcing Steel Concrete Cover Delamination FRP/Adhesive Interface Adhesive/Concrete Interface Delamination 1-5 mm from concrete surface Delamination of Concrete Cover

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16 Small delaminations less than 12.9 cm2 each are permissible as long as the delaminated area is less than 5% of the total laminate area and there are no more than 10 such delaminations per 0.93 m2. Large delaminations, greater than 161 cm2,can affect the performance of the installed FRP and should be repaired by sele ctively cutting away the affected sheet and applying an overlapping shee t patch of equivalent plies Delaminations less than 12.9 cm2 may be repaired by re sin injection or ply replacement, depending on the size and number of delaminations and their locations. The ACI document also identifies three NDE techniques that can be used to evaluate bond: acoustic s ounding (hammer sounding or coin -tap), ultrasonics, and thermography. No additional information is given regarding the deployment of a particular technique or the in terpretation of results. In addition, the document does not provide any references or cite specif ic data to justify these guidelines. NCHRP Report 514 (Mirmiran et al. 2004) also provides guidelines for the allowable debonded area in wet layup FRP sy stems. These requirements are more stringent than those prescribed by ACI. According to the NCHRP report, small debonded areas less than 6.4 mm in diameter are acceptable so long as ther e are less than five such defects in a 0.93 m2 area. Debonded areas with di ameters between 6.4 mm and 32 mm should be repaired by injecting the void with epoxy. Debonded areas with diameters between 32mm and 152 mm should be repair ed by cutting out the defective area and replacing the removed material with a new FR P composite patch that extends a distance of one inch beyond the borders of the original defect. Larger defects (great er than 152 mm in diameter) are to be repa ired in a similar manner except that the replacement patch should extend a distance of 152 mm beyond the defect area. The NCHRP report also recommends acoustic sounding as the primary NDE technique. The report states th at if an air-pocket is suspecte d, an acoustic tap test will be

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17 carried out with a hard object to identify delaminated areas by sound with at least one strike per 929 cm2. This report also acknowledges infrared thermography, microwave detection, and ultrasonics as additional NDE testing that may be performed. Guidelines provided by the International Conference of Building Officials (ICBO) in document AC125 (ICC Evaluation Services 2003) are similar to the standards laid out by ACI. This document recognizes that small diameter defects (1.6 to 3.2 mm in diameter) are naturally occurring and do not re quire any attention. Defects smaller than 12.9 cm2 are acceptable so long as there are fewer than 10 per 0.93 m2. Specific requirements for repair procedures are not provided, but the document does describe backfilling with epoxy and re placement of small areas as acceptable. The AC125 document also recommends a visual inspection of the cured FRP system in combination with acoustic sounding using a ball peen hammer to identify debonded areas. Research Significance The overall effect that def ects have on the short and l ong-term performance of FRP systems bonded to concrete is not well unders tood. ACI, NCHRP, and ICBO have all recognized that defects are an important issu e that must be addressed. The most common NDE method that is currently used to in spect FRP systems is acoustic sounding (coin tapping). This method is subjective and may not accurately identify or characterize defects. The focus of the current research effort was to develop a NDE technique for evaluating FRP composites bonded to concrete using infrared thermography (IRT). This technique can be used to eval uate bond in FRP systems imme diately after installation and throughout the service life of the repair.

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18 CHAPTER 3 NONDESTRUCTIVE EVALUATION USING INFRARED THERMOGRAPHY This chapter contains background informa tion about infrared thermography (IRT) and a review of previous research. The firs t section deals with the fundamentals of IRT and describes some of the basic technology used in thermal imaging systems. The following section addresses IRT as a nonde structive evaluation (NDE) technique. Infrared Thermography Fundamentals All objects at a temperature greater than 0 K emit electromagnetic (EM) radiation. Furthermore, this radiation is emitted across a range of wavelengths. Max Planck formally quantified the EM emissions of a blackbody (perfect emitter) in 1900 with the following relationship describing intensity of the emitted radiation as a function of wavelength and temperature of the object (Maldague 2001): 1 ) / exp( 2 ) (5 KT hc hc T I (3-1) I = spectral radiance (W m2 sr-1 m) = wavelength of emitted radiation ( m) T = temperature of the object (K) h = Plancks constant (6.63 x 10-34 J s) K = Boltzmann constant (1.38 x 10-23 J/K) c = speed of light in a vacuum (m/s) The wavelength at which the peak inte nsity occurs is given by the Wien displacement law: T c3 max (3-2)

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19 max = wavelength of peak intensity ( m) T = Temperature of the object (K) c3 = a radiation constant (2898 m K) This formula is obtained by taking the de rivative of Planks law (Equation 3-1) with respect to wavelength, and setting the re sult equal to zero. Another useful formula when considering EM emissions is the Stefan-Boltzmann Law. This law states that the total amount of radiation per unit ar ea, M, emitted by an object can be described by: 4T M (3-3) M = total radiant power emitted by object (W/m2) = emissivity of objects surface T = Temperature of the object (K) = Stefan-Boltzmann constant (5.67 x 10-8 W/m2-K4) The Stefan-Boltzmann law is simply the integration of Plancks law over all wavelengths. This relationship also cont ains a factor to account for the surface characteristic of the object: emissivity. Emissivity can be summarized by the following relationship: ) ( ) ( ) ( T I T I Tb o (3-4) = emissivity of the objects surface Io = Intensity of the radiation emitted by the surface Ib = Intensity of radiation emitted by a black body (perfect emitter) Another useful relationship describes wh at happens to the total radiation flux incident on an object. The total incident flux is the sum of the reflected, transmitted, and absorbed radiation. The behavior is illustrated in Figure 3-1 and is defined as: a t r i (3-5) i = Total incident flux r = Total flux reflected by the surface

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20 t = Total flux transmitted a = Total flux absorbed Equation 3-5 is often expressed as a fraction of the total incident flux and can be rewritten as follows: 1 (3-6) = % of flux reflected by the surface (reflectivity) = % of total flux transmitted (transmissivity) = % of total flux absorbed (absorptivity) Figure 3-1. Incident radiation ( i) is reflected, transmitted or absorbed (Maldague 2001) For simplicity, the discussion will now be limited to opaque objects that do not transmit incident radiation. If this is the case, Equation 3-6 can be simplified as: 1 (3-7) = % of flux reflected by the surface (reflectivity) = % of total flux absorbed (absorptivity) Finally, the relationship between absorptiv ity and emissivity can be expressed by Kirchoffs law which states that the two quantities are equal. Figure 3-2 A shows the electromagnetic (EM) radiation emission curves for several common objects at different temperatures. Th e curves illustrate that the intensity (brightness) of the EM emissions increase with the objects temp erature and that the wavelength containing the peak intensity increases as temperature decreases. i r a t

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21 Figure 3-2. Electromagnetic emission curves for objects at different temperatures Equation 3-1 to Equation 3-6 serve as th e foundation for both visible light imaging (photography) and thermal imaging (IR th ermography). The importance of these relationships is best demonstrated with several examples. First, consider the largest emitter of EM radiation that humans on earth are likely to experience: the sun. The surface temperature of the sun is approxima tely 5800 K, which, when substituted into Equation 3-1 results in relatively high inte nsities across the entire EM spectrum. The wavelength at which the peak intensity occurs, 0.5 m, can be determined using the Wien Displacement law. This turns out to be very convenient for humans since 0.5 m (500 nanometers) happens to fall very close to the center of the visible spectrum. When visible light from the sun strikes an object a portion of that light is absorbed and a portion is reflected (assuming the object is opa que). Visual imagi ng devices, including the human eye, are designed to capture this reflected energy and measure the intensity of the radiation that occurs w ithin the visible spectrum. Next, consider an object with a temper ature of 300 K (27C or 80F). The EM radiation emitted by an object at this temperature is limited to longer wavelengths outside 2500C 5500C 37C 500C vi s ib le Near IR Far IR UV Intensity Wavelen g th

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22 of the visible spectrum. Using the Wien disp lacement law, the peak intensity is found to occur at 9.7 m (9700 nanometers). This value falls wi thin the infrared (IR) region of the EM spectrum. Thermal imaging devices are de signed to capture and record the radiation emitted by an object in the IR region of the EM spectrum. Additional information about thermal imaging systems is provided in the following section. Detection of EM Radiation with an IR Camera The IR region is commonly divided in to five categories based on wavelength: Near IR (NIR) (0.7 1.4 m) Short wavelength IR (SWIR) (1.4 3 m) Mid wavelength IR (MWIR) (3 8 m) Long wavelength IR (LWIR) (8 15 m) Far IR (FIR) (15 1000 m) IR cameras measure surface temperature using EM radiation emitted by an object. The two primary regions of interest of the EM spectrum for IR cameras are referred to as mid-wavelength IR (MWIR) and long-wavele ngth IR (LWIR). MWIR cameras are sensitive to wavelengths betw een 3 and 5 m (this range ca n vary slightly depending on the particular detector and optics used) while LWIR cameras are primarily sensitive to wavelengths between 8 and 13 m. Figure 3-3 shows why sensors are banded in this manner. The figure plots the EM emissions of the earths atmosphere, which shows very high levels between 5 and 8 m. Based on Kirchoffs law, this translates into very high absorption. IR radiation emitted by the subject is effectively blocked by the atmosphere. MWIR cameras are typically more sensitive than LWIR cameras. However, both MWIR and LWIR cameras can accurately meas ure surface temperatures within the range of interest for IR inspections of comp osites bonded to concrete. A fundamental difference between the two types of cameras is that MWIR detectors often require some

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23 type of cryogenic cooling to avoid signal noise due to the EM emissions from the detector and surrounding electroni cs. This adds to the overall complexity of the thermal imaging system and requires an additional leve l of maintenance as compared to uncooled detectors. Figure 3-3. Atmospheric emission in the MWIR and LWIR spectral bands Many IR cameras made today operate in the LWIR region and use microbolometer focal plane array (FPA) technology. A bolometer is a type of thermal detector made of a material whose electrical conductivity varies with temperature change due to incident radiation. A microbolometer FPA is simply an array of extremely small bolometers (50 m x 50 m) onto which an image is projected (similar to a CCD digital camera). Typical FPA detectors might include a 320x240 array of microbolometers. The electrical signal that is developed by each bolomete r is converted to a single pixel containing temperature data by applyi ng an appropriate calibration factor. The electrical signal must be co rrected to ensure that the te mperature determined from the incident EM radiation matches the actual su rface temperature of the object. Factors that must be corrected for include: Emissivity of the objects surface Background temperature of any objects that mi ght reflect off the surface of interest Distance to the object Atmospheric temperature Relative humidity 3 m MWIR 12 m 8 m 5 m LWIR Emission Wavelen g th High

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24 The optical lenses for LWIR cameras are typically made of ge rmanium because of its high index of refraction (around 4.0) for wavelengths between 2 and 12 m and its high opacity to wavelengths outside of the 2 to 20 m band. This allows the lens to serve as a filter for the visible and UV radiati on that would otherwise be incident on the detector (resulting in noise). The remaini ng wavelengths outside of the 8-12 m band are removed using in-line spectral filters. This is important since EM radiation emissions by the atmosphere in the 5-8 m band would result in additional background noise. A general schematic of an FPA camera and associated optics is provided in Figure 3-4. Figure 3-4. General schematic of a focal plane array (FPA) and associated optics Thermal Imaging System Used in Current Study A FLIR ThermaCAM PM 695 infrared camer a was used in this study. This thermal imaging system operates in the 8 12 m (LWIR) wavelength band of the electromagnetic spectrum. An important feat ure of this camera is the ability to save thermal images digitally. Each pixel in the thermal image (320x240) is stored as a temperature value. This allows for easy post-processing of collected images using IR Radiation emitted by subject Germanium Lens Microbolometer Focal Plane Array Spectral Filter UV and Visible IR Radiation emitted by atmosphere Resulting Thermal Ima g e

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25 proprietary software. The maximum image save rate for this thermal imaging system is 5 frames per sec (5 Hz). Infrared Thermography Methods for NDE of Materials The fundamental concept behind using IRT as an NDE techni que is to apply heat to the surface of an object and gene rate a thermal front that trav els into the material. The increase in surface temperature should be uni form if the material is homogeneous. If the material contains defects below the surface, such as air voids, hot-spots will develop since the flow of heat from the surface to th e substrate is interrupted. These hot-spots can be detected with an IR camera. The thermal image provided in Figure 3-5A demonstrates this concept for an FRP system applied to RC. The hot-spots in the image result from very small air voids at th e FRP concrete interface. The visual image provided in Figure 3-5B was taken after a saw-cu t was made through the composite to examine the cross-section. The thickne ss of the FRP layer in this system was approximately 1 mm. The sizes of the air voids detected in this FRP system were extremely small (less than 6.4 mm across for the smallest dimension and less than 0.25 mm thick). A B Figure 3-5. Application of IR thermography to FRP composite bonded to concrete A Thermal image of FRP surface showing defects and B cross-section view showing air voids Air Void at FRPConcrete Interface Hot-Spots due to air voids 1mm thick FRP composite

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26 This technique has been applied by numer ous researchers to a wide variety of materials. There are currently ASTM standards available that describe procedures for detecting pavement delaminations in bri dge-decks due to corrosion (ASTM 2003) and for identifying wet insulation in roofing syst ems (ASTM 1997). A recent search of the ASTM standards database indicated that a new document is currently being drafted to address the NDE of polymer matrix composite s used in aerospace applications. Work item summary WK8211 cites IRT as an emer ging NDT [method] that [has] yet to be validated Considerable work does exist in the literatu re that investigates the use of IRT for identifying subsurface defects in materials. Much of the work on composites has focused on aerospace applications, though several res earchers have addressed the issue of FRP composites applied to concrete. This work will be discussed in greater detail in the following sections. There are three fundamental i ssues that must be addressed when using IRT as an NDE tool: Heating methods Image acquisition Data analysis Heating Methods A wide variety of heating methods can be used when inspecting FRP composites. Short duration heat pulses can be applied using a photography flash. Longer duration heat pulses can be generated with halogen or IR heating lamps. These heat sources transfer energy to the surface being inspect ed by radiation. The resulting surface

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27 temperature increase is dependent on th e intensity of the heat source and the configuration of the heat source with respect to the surface. Photography flashes offer a great deal of fl exibility with regards to controlling the intensity of the heat pulse. Different f-stop se ttings can be used to regulate the amount of energy released during each flash. A wide variety of models are also available with different maximum output capabilities. Flash sy stems are typically rated in terms of the amount of energy that can be stored in the sy stems capacitors. This quantity is measured in Joules or Watt-seconds. Models with relatively low output for IRT applications are typically rated from 500 to 1000 W-s. Models with relatively high output can be rated as high as 6400 W-s. As a general rule, higher inte nsity will translate into better IRT results since the difference in temperature betw een defects and defect free regions is proportional to the intensity of the applied he at. The cost of photography flash systems, however, is proportional to thei r maximum output capabilities. The overall cost of these systems can range from $1000 to $10000 depe nding on the maximum output and time required to recharge the cap acitors after each flash. Infrared heating lamps and halogen lamp s are also efficient means for heating samples during IRT inspections. The output of these lamps is measured in Watts and can range from 250 to 1000 W. A wide variety of lamp configura tions are available. Most IR heating lamps are designed to project a na rrow beam of energy. Halogen lamps are usually designed to illuminate large areas a nd tend to disperse the energy over a wider field. The specific requirements for surface temp erature increase depend on the thermal properties of the material under consideration and the depth below the surface that

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28 defects occur. Heating methods that work well for one material may not be appropriate for another. A major focus of the current st udy is to determine the required heat source intensity and configuration for inspecti ng FRP composites bonded to concrete. Image Acquisition IR cameras capture and record data in two basic formats: Intensity images Radiometric images Intensity images provide information a bout the relative temperature difference between objects or areas within the IR camera s field of view. Before intensity images are collected, the user is required to specify th e level and span of temperatures that will be encountered. The total span is typically di vided into 255 bins. In a standard grayscale image, the highest temperatures will appear as white and the coolest temperatures will appear as black. Depending on the sophistic ation of the thermal imaging system, the intensity image may or may not contain a te mperature scale in physical units. Another important thing to note about intensity images is that any intensity va lues that are greater than the specified level and span will be assi gned a value of 255, and any values less than the preset level and span will be assigned 0. If the span and level are not set properly, the image may appear underdeveloped (excessively dark) or overdeveloped (washed out). Radiometric images involve storing a te mperature value for each pixel in the thermal image, eliminating any requirements for presetting the span and level. This format facilitates post processing since therma l images can be viewed with any desired grayscale or color scale limits. It is also possible to access a specific temperature value in an image by specifying the coordinates of the point in terms of the row and column

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29 number. If a series of images are saved at a specified time interval, the temperature vs. time history for a single point (or series of points) can be extracted and analyzed. The image save rate is also a distinguis hing feature of thermal imaging systems. The most sophisticated research grade cameras can save thermal images at rates up to 120 frames per second. Specific image save ra te requirements depend on the nature of the material being inspected. If the thermal diffus ivity of the material is high and defects are located very close to the surf ace, a higher image save rate is required. Conversely, a lower image save rate is sufficient if the ther mal diffusivity is low and defects occur deep beneath the surface. Data Analysis There are two primary types of IRT analysis techniques: qualitative and quantitative. Qualitative inspections involve collecting thermal images and searching for any signs of non-uniformity in the resulting images. The thermal image provided in Figure 3-5 represents a qualitati ve analysis in that the thermal image indicates something of interest is occurring in the compos ite. Without additional information or an accompanying destructive test to reveal the sour ce of the hot-spots, very little can be said about the true nature of the defects. Levar and Hamilton (2003) conducte d a study involving qualitative IRT inspections of FRP composites bonded to RC. In this study, small-scale RC beams were strengthened in flexure and shear using CFRPs and loaded to fa ilure in laboratory testing. IRT inspections were performed after the FRP systems were installed and areas that appeared unbonded in the thermal images were recorded directly on the specimen. IRT inspections were also performed at various stages of loading a nd patterns of debonding were monitored. Important observations fr om these experiments were as follows: the

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30 total debonded area increased as the load was increased up to failure; and certain debonded areas appeared to have di fferent thermal signal strengths. The objective of quantitative IRT analysis is to use time dependent temperature data to assess defect charact eristics. The properties of interest in the current study include: Defect size (in physical units) Depth of the defect below the surface Material composition of the defect During a quantitative IRT experiment, thermal images are collected at a predetermined interval while th e surface of the object is bei ng heated and then while the surface cools. Specific points of interest can then be identified in these images and the temperature variation can be monitored as a function of time. Careful analysis of the results can help to establish where the defect is located in the FRP system. No standard test methods currently exist for performing quantitative IRT inspections. The following sections will highlig ht the basic principles behind some of the existing methods. Specific details regardi ng the implementation of each method for the current study will be presented in a later chapter. Pulse IRT Pulse IRT involves the applica tion of a short burst of hi gh intensity heat onto the surface of an object. The most common heat source is a photography flash apparatus. After the heat is applied to the surf ace, cooling will proceed as shown in Figure 3-6. If the thermal front encounters a defect as it trav els into the object, th e area above the defect will not cool as quickly. An important pa rameter to note is the time required for the perturbation to begin (tp). This value is proportional to the defect depth (zd). Another

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31 important parameter is the observed thermal contrast ( Tdef). This value is proportional to the size, depth, and thermal properties of the defect. Pulse IRT is commonly used in the NDE of materials with high thermal conductivities containing defects near the surf ace. A good example of this application is the NDE of aerospace structures made from FRP composites and/or metals (Kulowitch et al. 1995). The required heating and observation time is short, which results in the ability to inspect large areas very quickly. This, however, requires very high image acquisition rates that can translate into higher equipment costs. Anot her disadvantage is that the small amount of heat deposited on the su rface may not reveal deeper defects. Figure 3-6. Surface heating and defect de tection for pulse thermography (Maldague 2001) Step heating Step heating IRT involves a longer duration and lower energy heat input than pulse IRT. The temperature response on the surf ace of the specimen is monitored during heating and also after the heat source is removed. Materials with lower thermal diffusivities and deeper defects can often be evaluated with step heating IRT. Some advantages of this method include a low imag e acquisition rate and lo w cost heat sources (IR or halogen lamps). A disadvantage of this method is that it is sometimes difficult to tpulse Tambient Surface Temperature tp perturbation in surface cooling curve due to defect Tdef time

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32 apply heat uniformly to the surface being in spected. An interesting study was reported by Maierhofer et al (2003) in which a large block of concre te (1.5 m x 1.5 m x 0.5 m) containing fabricated defects up to 3 below the surface was inspecte d using step heating IRT. The surface was heated using an arra y of three 2400 Watt IR radiators for up to 60 minutes. A computer controlled arm was required to move the radiators across the surface in a manner that resulted in uniform heating. Starnes et al. (2003) used step heating IR T to identify and characterize defects in FRP systems bonded to concrete. Experimental results were presented for a small-scale specimen containing fabricated defects. Th e FRP system was comprised of a single 1.3 mm thick pre-cured carbon/epoxy lamina bonded to a concrete substrate with an epoxy adhesive. A total of 8 simulated defects were created by placing different materials between the lamina and the concrete. The fi rst step in the experiment was to simply detect the subsurface defects. This wa s accomplished by passing a 250 Watt IR lamp across the surface at a rate of 15 cm/sec. The lamp was held at a distance of 5 cm from the surface. This technique easily revealed all of the implanted defects. Once the location of each defect was established, a quantitative step heating experiment was performed to characterize the defect. A singl e lamp was aimed toward the defect at a distance of 33 cm and heat was applied to the surface for 10 sec. This configuration resulted in a defect signal strengt h of 2.7C for an air void. Starnes et al. (2003) also used the fin ite element method to simulate the heat transfer process involved in step heating. It was difficult to establish an appropriate thermal conductivity (k) value fo r the carbon/epoxy lamina used in the experiment. This was due to the large range of fiber volume fractions that are co mmonly encountered in

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33 FRP composites. For pre-cured laminates, fi ber volume fractions ar e typically greater than 70%. Wet layup composites typically have much lower values (<50%). The matter is further complicated by the wide range of published k values for plain carbon-fibers. Textbook values range from 8 to 500 W/m-K de pending on the modulus of the fiber and the type of precursor used (Pitch vs. PAN) (Callister 1997). A thermal conductivity of 2.9 W/m-k (perpendicular to the main fiber dire ction) was ultimately used in the finite element model. The finite element results we re compared to experimental results from one fabricated defect and good agreement was observed. Lock-in IRT Both pulse and step heating IRT rely on the ability of the IR camera to detect temperature differences on the surface above de fect and defect-free re gions. In lock-in IRT, thermal images are recorded while a m odulated heat source is used to heat the surface. Rather than monitor the temperature da ta at each pixel in a thermograph, lock-in thermography focuses on the phase shift of each pixel (see Figure 3-7). This technique results in cleaner thermal images and is also capable of detecting subsurface defects at greater depths. The major advantage of lock-in IRT is that phase images are not as sensitive to non-uniform surface heating. Different values of surface emissivities and reflection of the heat source also have limited effect on phase images (Maldague 2001). An interesting study was presented by Carlomagno et al. (2002) which compared pulsed and lock-in IRT in the NDE of historic frescoes An important finding was that lower net surface temperature increases are required for lock-in IRT than pulse or step-heating. This has implications for the current study since it is important to avoid heating the

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34 surface beyond the glass tr ansition temperature, Tg, of the FRP matrix (Kharbari et al 2003). Pulse phase IRT The experimental setup and data acquisi tion procedure used for pulse phase IRT (PPT) is similar to the pulse thermography procedure described above. After the series of thermal images is collected, a discrete F ourier transform operation is performed on each pixel of the images in the time domain. This operation results in a series of images in the frequency domain with each pixel consisting of an imaginary number. Phase images are obtained for each frequency by computing the inverse tangent of the imaginary part divided by the real part. Figure 3-7. Defect detecti on with lock-in thermography The advantage of this method is that th e resulting phase imag es are relatively independent of non-uniform heating. There is also a strong relationship between the frequency at which a defect first appears in th e phase images and the depth of the defect. Defects that are closer to the surface app ear in higher frequency phase images while deeper defects only appear at lower frequencies. A major disadvantage of this method is -0.5 0 0.5 1 1.5 2 2.5 0 5 10 15 20 2 5 -0.5 0 0.5 1 1.5 2 2.5 0 5 10 15 20 2 5 -0.5 0 0.5 1 1.5 2 2.5 0 5 10 15 20 2 5Modulated Heat Source Idealized temp response above defect free region Idealized temp response above subsurface defect defec t #1 #2 1/fmod Phase Image Note: Max defect for #1 occurs at lower f mod t h a n # 2

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35 that the amplitude of the def ect signal strength is significantly less than for long-pulse heating. Objectives of Current Research The overall objective of the current res earch was to develop IRT methods that could be used to detect defects in FRP syst ems bonded to concrete. Specifically, a major goal was to use IRT data to provide the follo wing information about detected defects: Size Depth below the surface Material composition Other items that are addressed include: Detection limits Heating methods Data analysis procedures Previous research in this field has focu sed on the inspection of FRP composites that are commonly used in the aerospace industry. A number of data anal ysis techniques have been developed that can assist in using IRT results to character ize defects. None of these methods have been calibrated for use on FRP systems bonded to concrete. The remainder of this dissertation is divi ded into two main sections: Phase I and Phase II. Results from Phase I of the current study are presented in Chapter 4. During Phase I, IRT was used to inspect FRP syst ems that were applied to full-scale AASHTO girders. Phase I contains a laboratory st udy and a field study. Findings from Phase I were used to develop a second laboratory st udy that was conducted in Phase II. Details of the Phase II experimental work are provided in Chapter 5 and Chapter 6.

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36 CHAPTER 4 PHASE I EXPERIMENTAL WORK AND FIELD STUDY Introduction This chapter describes experimental work that was performed in conjunction with a Florida Department of Transportation (FDOT ) project investigati ng the performance of FRP strengthening systems. The FDOT curren tly uses FRP composites to repair impactdamaged bridge girders. The objective of the FDOT study was to develop a quality products list (QPL) for FRP sy stems that are suitable fo r repairing impact damage suffered by bridges. The FDOT study involved full-scale load testing of six AASHTO Type-II bridge girders at the FDOTs structural research faci lity in Tallahassee, Florida. Impact damage was simulated at the midspan of each gird er by removing a section of concrete and cutting four prestressing strands Four different FRP system manufacturers then had the opportunity to design and insta ll an FRP system to restore the capacity of the damaged girder. These repairs were then validated by load testing each girder to failure. This project represented an excellent opport unity to investigate the use of IRT for evaluating the installation and performance of FRP systems. Each FRP system was inspected prior to the load test and then again at various stages of loading. The first part of this chapter results from this research. It should be noted, howev er, that the objective is to describe the IRT results and not the results from the load testing. This information is available in Lammert (2003).

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37 Full-Scale AASHTO Girders Description of AASHTO Girders and FRP Systems A typical AASHTO type II bridge girder that was used in this series of experiments is shown in Figure 4-1. The total depth of each girder was 122 cm (48 in.), which includes a 30.5 cm (12 in.) cast-in-place slab The distance between supports for each load test was 12.2 m (40 ft.). The critical dimensions with regards to the infrared inspections were the width of the girders tension face, 45.7 cm (18 in.), and the clearance between the girder and th e laboratory floor, 50.8 cm (20 in.). Figure 4-1. Full-scale AASHTO type II girder and load test setup Before the installation of each FRP strengthening system, vehicle impact damage was simulated by removing a section of concrete and cutting four pr estressing strands at midspan. This area was then patched with c oncrete to restore the original cross-section of the girder. Four different FRP strengthening systems were evaluated in this study (applied to Girder 3, 4, 5, and 6). The properties of each system are shown in Table 1. Each FRP system was independently designed by the sy stem manufacturer to restore the flexural capacity provided by the cut strands. The FRP system manufacturers also installed each

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38 system. During installation, each girder wa s raised to a height of 122 cm above the laboratory floor. This provided a challenge for the FRP installe rs by limiting access to the girders tension face. Table 4-1. Fiber-reinforced polymer syst em properties for full-scale AASHTO girders FRP system Girder Fiber Matrix Layers Thickness (mm)a Width of laminate (cm) Anchorage 3 Carbon Epoxy 4 4 / 7 40.6 None 4 Carbon Polyurethane 4 3.1 / 6 30.5 2-ply carbon 5 E-Glass Polyester Resin 1 12.7 / 3.5 9.8 45.7 12.7 mm 6 Carbon Epoxy 3 1.75 / 4.34 45.7 2-ply carbon a Data Sheet thickness / As-Built thickness Girder 3 The FRP system applied to Girder 3 cons isted of four layers of unidirectional carbon-fiber fabric (with aramid cross-fiber) and an epoxy ma trix. Each layer extended over the entire middle 6.1 m of the girder. A tack-coat (epoxy th ickened with silica fume) was first applied to the concrete su rface followed by the first layer of saturated carbon-fiber fabric. During installation, th ere was a tendency for the saturated carbon sheets to fall from the tension face. This prompted the inst allers to apply an additional coat of thickened epoxy between each layer of fabric. The final step was the application of an epoxy gel coat to the surface of the syst em. The thick layer of gel coat combined with the overhead application resulted in drip s forming before the matrix cured. These thickened areas affected the infrared inspec tions. There were also areas where the gel coat was thin, but no exposed unsaturated fi bers were observed. Acoustic sounding (coin tap) indicated that the syst em was well bonded to the concre te substrate and there were no visible abnormalities that would indicate debonded areas. The material data sheet (MDS) for this FRP system indicated a 1mm ply thickness resulting in a total laminate thickness of 4 mm. In order to verify this thickness, a small

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39 area of the strengthening system and concrete substrate (2 cm x 7 cm x 1.5 cm thick) was removed from the girder after load testing. The total thickness of the laminate varied between 6 mm and 7 mm (62.5% thicker than the MDS thickness). A 2 mm layer of thickened epoxy was observed between the se cond and third layers of carbon-fiber (Figure 4-2). A B Figure 4-2. Cross-section views of FRP systems. A) Girder 3. B) Girder 4 (note: principle fiber direction is out of page for Girder 4) Girder 4 The FRP system applied to Girder 4 consis ted of multiple layers of unidirectional carbon-fiber fabric that was pr e-impregnated with a water-activated polyurethane matrix. Four layers of carbon-fiber were applied to the middle 4.9 m of the girder; three layers Layer 1 Layer 4 Layer 3 Layer 2 Principle Fiber Direction: Layer 1 Layer 4 Layer 3 Layer 2 Concrete Thickened Epoxy Tack-Coat Principle Fiber Direction:

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40 extended over the middle 7.3 m; 2 layers extended over the middle 9.75 m; and a single layer was applied over the enti re length of 12.2 m. A pol yurethane tack-coat was first applied to the concrete followed by the two long est layers of the pre-impregnated fabric. These layers were then sprayed with water to in itiate the curing process. Finally, the two remaining layers were applied and sprayed w ith water. Anchorage for the FRP system was provided by two FRP stirrups (each was 2 plies oriented at 0 and 90 degrees) located at 12 feet on either side of midspan. A coin tap inspection of the installed sy stem did not indicate any debonded areas. The MDS thickness for this system was 0.78 mm per layer, which resulted in a total thickness at midspan of 3.1 mm. The measur ed thickness of the 4-ply laminate varied between 5 and 7 mm (Figure 4-2 B). Girder 5 The FRP system applied to Girder 5 wa s a sprayed-on mixture of chopped E-glassfibers and polyester resin. This process requires highly specialized equipment and is commonly employed in the fabrication of boa t hulls. The application method worked extremely well on vertical surface s (sides of the beam); howev er, it was difficult to apply material to the bottom of the girder. After a thin layer of glass and resin were applied with the spray gun, the material was pressed with a roller to condense the laminate. If too much glass and resin were sprayed onto the bo ttom, large sections tended to fall down. Sometimes this material would separate entir ely and fall to the floor, and other times it would simply cure as small draped areas. This resulted in a large number of visible surface and subsurface def ects in the laminate. The laminate was extended over the middle 6.1 m of the girder and stirrups were also sprayed onto the sides of the girder wh ere the laminate was terminated. The final

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41 measured thickness of the FRP system on the girders tension face varied between 3.5 mm and 9.8 mm. Additional material was also sprayed on the sides of the girders bulb to an average thickness of 12.7 mm. Girder 6 The FRP system applied to Girder 6 cons isted of three layers of unidirectional carbon-fiber fabric and an epoxy matrix. All layers extended over the middle 6.1 m of the girder. The data sheet indicated 0.58 mm ply thickness resulting in a total laminate thickness of 1.75 mm. Two additional plies of unidirectional fabric were used to anchor the FRP system at the termination points. Th is resulted in a total laminate thickness of 2.9 mm on the tension face at the termination points. The as-built thickness of this FRP system was not verified. Little or no excess matrix material was present on the surface of the installed system. A coin tap inspection indicated that the system was well bonded to the concrete substrate and there were no visible abnormalities. Infrared Inspection Procedures Thermal images were collected using the thermal imaging equipment described in Chapter 3. Heat sources used in this st udy included 125 Watt IR heating lamps and a 500 Watt halogen lamp. Limited access to the tens ion face of each girder along with the need for efficiency in evaluating the relatively large area prompted the development of two novel scanning procedures. In both procedur es, the heat source and IR camera were mounted to a rolling cart. The heat source wa s positioned on the leading edge of the cart and placed a distance of 7.6 cm from the FR P surface. The camera was positioned to view the FRP surface just behi nd the area being heated. As the cart was pushed along the floor, the IR camera recorded a series of images as the surface cooled.

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42 The cart configuration for the first procedure is shown in Figure 4-3 A. This resulted in a camera field of view (FOV) of only 22.9 cm x 17.1 cm. Consequently, two passes were required to inspect the entire 45.7 cm width of the girders tension face. This image was also slightly distorted since the an gle of incidence for the camera was not 90. The cart configuration for th e second procedure (shown in Figure 4-3 B) utilized firstsurface mirrors located near the ground to increase the cameras FOV to 56.7 cm x 42.5 cm. The image save rate for all inspections wa s set to one frame per two seconds (0.5 Hz). The fastest image save rate to the onboard PCMCIA storage ca rd is approximately 1 Hz. This rate, however, produces an unmanageable amount of data (each thermal image is 158 Kb). An even faster rate of up to seven frames per second is possible, but this requires a direct link to a laptop com puter. For the scanning speed used in these inspections, the rate of 0.5 Hz was found to be adequate. A typical series of thermal images containing a subsurface defect is shown in Figure 4-3 C. This particular series was recorded using the cart configuration shown in Figure 4-3 A. To characterize defects detected during each inspection, the defect signal strength, Tdef, was calculated as follows: background def defT T T (4-1) Tdef = temperature above the defect Tbackground = temperature of ad jacent defect free area The magnitude of Tdef was determined by identifying an appropriately sized area above the brightest portion of the defect a nd using the average temperature measured within that area. The standard deviation of temperature values within each area was

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43 typically less than 0.5C. A similar tec hnique was used to determine the corresponding Tbackground. A B C Figure 4-3. Data collection for full-scal e AASHTO girders. A) Scanning cart configuration for Girder 3. B) Girder s 4 to 6. C) Typical thermal images To make a valid comparison between def ect signal strengths, the amount of heat applied to the surface should be consistent during each inspection. Heating consistency for each scan was evaluated by monitoring Tbackground along the leading edge (edge closest to the heat source, as shown in Figure 4-4) of each thermal image in a series: ambient background backgroundT T T (4-2) Tambient = ambient temperature of th e girder prior to heating t = 0 s t = 12 s t = 8 s t = 4 s

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44 This quantity was also monitored along th e trailing edge (farthest away from the heat source) of each image in a series in orde r to evaluate the aver age cooling rate on the surface of the FRP. Figure 4-4. Subsurface defect found on Girder 3 Initial IR Inspections An initial infrared inspection was performe d on each girder prior to load testing. The objective was to identify any defects form ed during the installa tion. Girder 3 was inspected using the cart configuration shown in Figure 4-3 A. Girders 4-6 were inspected using the configur ation shown in Figure 4-3 B. The inspection of Girder 3 revealed 11 minor subsurface defects (< 12.9 cm2) and three moderate subsurface defects (> 12.9 cm2 but less than 161 cm2). Thermal images for two of these defects are shown in Figure 4-5. These images were recorded approximately six seconds after the area was heated. The computed signal strength for defect 1 and defect 2 was 7.5C and 15.1C, respectively. The difference in signal streng ths could be a result of several factors: defect depth, amount of heat applied to the surface, and the size of the defect. Stronger signal strengths are expected for defects that are closer to the surface (signal strength is Tambient = 19 C Tbackground = 36.8 C Tdefect = 53.1 C Tdefect = 16.3 C Size = 23 cm2 (3.6 in.2) leading edge trailing edge

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45 inversely proportional to defect depth). Applyi ng more heat to the su rface will also result in higher signal strengths. Finally, a larger su rface area will result in higher defect signal strengths since the heat applied above the defect must travel farther before it is absorbed by the concrete. Figure 4-5. Subsurface defects found on Girder 3 The initial infrared inspection performed on Girder 4 did not reveal any defects similar to those observed in Girder 3. Th ere were, however, two interesting observations made regarding the polyurethane matrix material and the uniformity of heating perpendicular to the girders length. Some areas of the FRP surface were covered with excess polyurethane matrix. This excess matrix material had the appearance of a thin layer of foam. The color of this layer was also much lighter than adjacent areas, which appeared black. An example of this occurrence is shown in Figure 4-6. The resulting Tbackground for the light colored area was 5.4C while the Tbackground for the dark color was 7.8C. Another source of non-uniform heating was streaking due to the narrow beam width of the IR heat lamps. The resulting Tbackground for the area directly in-line with the heat lamp was 9.7C while the Tbackground in the area between lamps was 7.7C. Defect 1 Tdefect = 7.5 C Tbackground = 12.7 C Size = 5.8 cm2 (0.9 in2) Defect 2 Tdefect = 15.1 C Tbackground = 14.3 C Size = 34.2 cm2 (5.3 in2)

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46 Figure 4-6. Non-uniform surface heating of Girder 4 A visual inspection of Girder 5 reveal ed numerous defects on the surface of the FRP system. These were a result chopped fibers falling down before the system fully cured. There were also a large number of defects visible just below the surface of the FRP, which were the result of improper satu ration of the chopped glass-fibers. These large imperfections near the surface interfer ed with the IRT inspection. The thermal images were crowded with these imperfections and it was difficult to distinguish defects that occurred near the surf ace and defects that occurred be low the surface. A typical thermal image is shown in Figure 4-7 A. All of the defects that were visible in the thermal image were also visible to the naked eye. Only one subsurface defect was detected dur ing the initial scan of Girder 6 (shown in Figure 4-7 B. The recorded defe ct signal strength was 7.4C and Tbackground was 8.5C. This defect occurred on the edge of the laminate and was not considered to be significant. A summary of the scanning speed for each initi al inspection is presented in Table 2. Tbackground was computed along the leading edge (c losest to heat s ource) and trailing edge (farthest from heat source) of the series of thermal images that were collected as the cart was pushed along the beam. The average speed was computed by dividing the total Tbackground = 7.7 C Tbackground = 9.7 C Tbackground = 5.4 C Tbackground = 7.8 C

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47 distance scanned by the total tim e required. These scanning rates are much slower than those reported by Starnes et al (2003). Th eir basic procedure for identifying subsurface defects involved passing a singl e 250 Watt IR heat lamp held a distance of 5 cm from the FRP surface at a speed of approximately 15 cm/s. This approach was adequate to detect defects beneath a 1.3 mm thick pre-cured CFRP laminate. A B Figure 4-7. Thermal images collected for fullscale AASHTO girders. A) Girder 5. B) Girder 6 Table 4-2. Summary of scanning speed and uniformity of heating Leading Edgea Trailing Edge Girder Scan Config. Avg. Speed (cm/s) Tbackground (C) Std. Dev. (C) Tbackground (C) Std. Dev. (C) Avg. Cooling (C/s) 3 Fig. 3 A 1.2 13 1.4 9.5 1.1 0.25 4 Fig. 3 B 2.77 11.3 2.4 6.0 1.8 0.35 5 Fig. 3 B 1.25 7.2 1.5 4.26 0.98 0.09 6 Fig. 3 B 2.1 10.4 1.9 1.67 0.88 0.44 a Leading Edge of image is closest to heat source For the current series of inspections, th e average cooling rate (ACR) on the surface of the FRP was computed as follows: Speed FOV t where t trailing T leading T ACRSD background background : ) ( ) ( (4-3) Size = 3.1 in.2 Tdefect = 7.4 C Tbackground = 8.5 C

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48 FOVSD is the cameras field of view in the direction of scanning and speed is the average speed. This calculation assumes th at the surface temperatur e cooling profile at every point is linear, which is not the case. The results are reported in this format for ease of comparison between FRP systems. Controlling the speed of the cart during each scan was difficult. Figure 4-8 shows the resulting Tbackground along the leading and trailing e dge of each thermal image vs. position for Girder 3 and Girder 6. The leading edge curve represents Tbackground measured just after an area enters the therma l image. The trailing edge curve represents Tbackground measured just before the same area leaves the image. The average time between these two curves can be calculated as t` in equation 4. The significant fluctuation observed in each curv e demonstrates the sensitivity of Tbackground to cart speed. In areas where the cart was push ed slowly, there was an increase in Tbackground while areas in which the cart was moved more quickly experienced a decrease in Tbackground. The standard deviation of Tbackground along the leading edge for Girder 3, 4, 5, and 6 was 1.4, 2.4, 1.5, and 1.9C, respectively. A B Figure 4-8. Background temperature increase vs. position along length of girder. A) Girder 6. B) Girder 3 0 2 4 6 8 10 12 14 16 0200400600 Position (cm) Tbackground (C) 0 2 4 6 8 10 12 14 16 18 0200400600 Position (cm) Tbackground (C)Leading Edge Leading Edge Trailing Edge Trailing Edge

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49 IR Inspections Performed During Load Testing Additional IR inspections were performed dur ing the load test of each girder. For Girder 3, the load was removed during each in spection. Girders 4-6 were inspected while the specimen was under loading. Table 3 contai ns a summary of the load levels at which each IR inspection was performed. The purpose of these inspections was to monitor the subsurface defects detected in the initial scan as well as to detect any new debonded areas resulting from the applied load. None of the additional inspect ions that were performed prior to failure revealed new defects or subsurface defect growth due to loading. IR Inspections of Known Debonded Areas After Failure The failure mode for Girder 3 was delaminati on of the concrete cover at the level of the girders pre-stressing tendons (see Figure 4-9). There were no visual or audible indications of FRP debond duri ng the loading. A 90 cm x 45 cm piece of the delaminated cover was recovered after load testing to perform a thorough IR inspection. This section did not contain any defects that were identi fied in the initial inspection, however, debonded areas were formed as a result of the delaminated cover concrete and FRP striking the floor afte r the girder failed. A series of IR inspections were performe d on this section using the same amount of heat that was applied during in itial inspections prior to load testing. These inspections did not reveal any debonded areas even though coin-tap testing did indicate that large areas of the FRP had separated from the c oncrete. A very important observation was made regarding a large section of FRP on th e edge of the sample that was no longer attached to any concrete (eff ectively an overhang). This area was expected to appear

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50 extremely bright after being heat ed with the halogen lamp to a Tbackground of 15C. However, no thermal signal was detected. Figure 4-9. Failure modes for full-scale AASHTO gi rders. A) Girder 3. B) Girder 4. C) Girder 5. D) Girder 6. Figure 4-10 shows results from an experiment in which a Tbackground of 33.3C was generated above the bonded area. Immediately following the re moval of the heat source, the temperature increase above the de bonded (non-bonded / overhanging) area was 28.6C. This resulted in a thermal signal of 4.61C at t = 0 seconds. This initial negative temperature difference was likely due to improper lamp positioning that resulted in non-uniform heating of the surface. Afte r 282 seconds of cooling, the thermal signal achieved its maximum value of 2.12C. Measurements were terminated after 594 seconds with a thermal signal of 1.69C. If the sample had been heated uniforml y, the maximum signal would likely have approached 6.7C (2.12C (-4.61C)). It is important to note that this would occur after A C D B

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51 4.7 minutes of cooling. These results sta nd in sharp contrast to the measurements obtained during the initial IR inspections (defect signal strengths between 10C and 15C that were visible after less than 2 seconds of cooling). Figure 4-10. Defect signal strength ( Tdefect) vs. time for known debonded area The failure mode for Girder 4 was de bonding of the FRP laminate. Debonding began in the middle of the specimen and pr ogressed outward towards both ends of the girder. At the north end of the girder, debonding caused the anchorage FRP to rupture and then continued to end of the laminate. On the south end of the girder, the FRP system ruptured in tension before the debonding reached the anchorage point. After the specimen failed, the majority of the FRP system was no longer bonded to the girder. There was, however, a short sec tion on the south end that remained partially bonded. The line of demarcation between th e bonded and debonded area of this section was easily recognized with a coin-tap inspecti on. An IR inspecti on of this debonded area was made using the same procedure outlined above (cart speed of approximately 3 cm/sec). This inspection did not reveal the debonded area. Another inspection was performed in which the lamp was passed over the debond line for 120 sec and the area was observed while cooling for 3 minutes. Again, the debonded area was not detected. -6 -5 -4 -3 -2 -1 0 1 2 3 0100200300400500600 Time (sec.)Tdefect (C) t = 282 s Tdefect = 2.12 C Tbackground = 33 C Bonded to Concrete Not Bonded to Concrete

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52 The failure mode for Girder 5 was tensil e rupture of the FRP laminate at the girders midspan. There were no audible i ndicators during loading that the FRP system ever debonded from the surface of the concrete. The area around the rupture point of the FRP system was thor oughly inspected after the girder failed. A large debonded area (approx. 15.2 x 45.7 cm) was identified adjacent to the rupture point on the bottom of the girder. This area co uld not be identified with the scanning procedure that was used during the initial inspection of this girder. The failure mode for Girder 6 was debonding of the FRP system. This debonding began at midspan and progressed outward towa rds the anchorage points (very similar to Girder 4). Audible indicator s of the debonding were also pr esent; however no IR scans were performed between the time they were fi rst heard and failure of the specimen. At the ultimate load, a portion of the FRP slipped at the north anchorag e point resulting in failure. Results from an IR inspection performed on the tension face of the girder at the north anchorage point are shown in Figure 4-11. A thin strip (approx. 15 cm wide) in the center of the beam remained bonded to the conc rete at the anchorage point. The adjacent debonded/delaminated areas are clearly distinguish able in the thermal image. The defect signal strength for the delaminated area varied from 5C to 9C. A small FRP test patch (21.6 cm x 45.7 cm ) was constructed on the side of Girder 6 near the support. This test patch consisted of a single layer of carbon-fiber fabric. The area chosen for the test patch contai ned numerous bug-holes and other surface imperfections. A single bug-hole near the center of the area wa s identified and filled with thickened epoxy paste prior to placement of the carbon-fiber. The remaining bug-holes

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53 were left unfilled. The test patch area was h eated for 18 seconds with an array of four 125 Watt IR heating bulbs. The resulting Tbackground above the defect fr ee area was 5C. The bug-holes were visible immediately af ter the heat source was removed. The reference hole (unfilled), shown in Figure 4-12, had a defect signal strength of 9.5C. The signal strength above the epoxy filled hol e was only 5.25C immediately after the heat was removed. After 8 seconds of cooling, the defect signal strengths above the filled and unfilled holes were equal at 5.0C. As th e area continued to cool, the signal strength above the unfilled hole decayed rapidly, a nd after 20 seconds only the epoxy filled hole continued to possess a si gnificant thermal signal. A B Figure 4-11. Debonded area after fa ilure for Girder 6. A) IR image. B) Visual image This finding has several implications. F illing the hole with epoxy will ensure that the FRP is bonded to the concrete; however there is still a difference in thermal conductivity between the epoxy filler and concre te substrate. This results in the appearance of a debonded area in thermal images. Careful scrutiny of the thermal signal vs. time can differentiate the epoxy filled void from an air filled defect; however this requires a series of thermal images to be reco rded with the camera in a static position. Tdefect = 5 C 9 C Tbackground = 15 C Bonded Area Bonded Area

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54 Also, it might be difficult to differentiate between these two thermal signals if a particular image does not contain both types for reference. A B C (d) Figure 4-12. Series of thermal images for air a nd epoxy filled defects. A) t = 0 sec. B) t = 8 sec. C) t = 20 sec. D) Tdefect vs. time plot. Summary of IR Inspection Results for Each FRP System The FRP system applied to Girder 3 cons isted of four layers of unidirectional carbon-fiber fabric with an epoxy matrix. Initial IR inspectio ns performed after installation revealed three subsurface defects having an area greater than 12.9 cm2. Defect signal strengths for these defects were greater than 10C and resulted from a Tbackground of approximately 13C. These defects were visible immediately after the heat source was removed. Additional IR inspec tions performed on a section of the FRP t=20 s Epoxy-filled Unfilled 0 2 4 6 8 10 12 0102030 Time (sec.)Tdefect (C) Unfilled Epoxyfilled t=8 s Epoxy-filled Unfilled Epoxy-filled Unfilled t=0 s Tbackground = 5 C 20.3 cm

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55 system with known debonded areas produced different results. A Tbackground of 33C resulted in Tdefect measurements of only 2.1C after 282 sec of cooling. The defects found during the initial inspection were very close to the surface signifying delaminations rather than debonded areas. The more important fi nding is that the initi al scanning technique would not have detected debonded areas since the amount of heat ap plied to th e surface was relatively low and the camera was not positio ned to record images when the defects maximum signal strength was reached. The FRP system applied to Girder 4 cons isted of four layers (gradually tapering down to a single layer) of unidirectional ca rbon-fiber fabric pre-impregnated with a polyurethane matrix. No subsurface def ects were detected during IR inspections performed after the installation of the FRP sy stem. An IR inspection was also performed on a known debonded area. This debonded ar ea was located on a portion of the FRP system that was partially attached to the gird er after failure. Results indicated that this particular FRP strengthening system is not well -suited to inspection with IRT. A closer inspection of this system after failure revealed a thin layer of polyurethane matrix between the FRP and concrete that resembled insulating foam (as shown in Figure 4-13). If this particular type of matrix material is effectively insulating the carbon-fibers from the concrete, subsurface defects will not resu lt in hot spots on the su rface after heating. Additional experiments under c ontrolled laboratory conditions are needed to determine the limits of detection. The FRP system applied to Girder 5 was a chopped glass / polyester resin mixture that was sprayed on the surface. Numerous surface and subsurface defects (also close to the surface) were clearly visible with the naked eye. IR inspec tions of this system clearly

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56 revealed these defects. The thickness of th e system, however, and possibly the insulating characteristic of the glass-fibers made the detection of debonded areas difficult. Figure 4-13. Polyurethane matrix shown af ter debonding from concrete (Girder 4) The FRP system applied to Girder 6 cons isted of three layers of unidirectional carbon-fiber fabric and an epoxy matrix. In itial IR inspections revealed only one subsurface defect. Additional IR inspectio ns performed on a known delaminated area indicated that IR thermography was capable of detecting defects beneath at least two layers of the FRP system. It should be noted that the installation procedure for this girder was very different from Girder 3 even t hough the system specifications are similar. Excess matrix material that was present in th e laminate on Girder 3 that was not observed on Girder 6. This reduction in matrix vol ume increased the effectiveness of the IR inspections. An IR inspecti on performed on a small test patch (single layer of carbonfiber) containing numerous unfilled bug-holes de monstrated that IR thermography can be very effective at detecting defects under a si ngle layer of FRP. This inspection also showed that epoxy-filled holes still possess a defect signal st rength; however the rate of decay of this signal is much slower than a simple air void.

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57 Field Inspection: Chaffee Road The Chaffee Road/Interstate 10 overpass (located in Jacksonville, Florida) suffered severe vehicle impact damage in July of 2001 (see Figure 4-14). The impact dislodged large sections of concrete and ruptured a number of prestressing strands. The most severe damage occurred on the fascia girder th at was hit first (east si de of bridge). The exterior girder on the west side of the br idge also experienced similar damage. The interior girders were not significantly affected Rather than replace these girders, the FDOT decided to repair the damaged concre te and then apply an externally bonded FRP strengthening system. This system was comp rised of multiple layers of 0-90 carbon fiber fabric and an epoxy matrix that fully encapsulated the middle 9 m of both exterior girders. The exact configuration of the FRP system was not available at the time of this study. Samples removed from the girder, however, contained two layers of the bidirectional fabric. There were no signs that excess epoxy was applied during the installation of the system. Chaffee Road has the unfortunate distinc tion of being the lowest overpass on I-10 westbound out of Jacksonville. As a result, a number of minor vehicle impact events occurred between the time the FRP system was installed and July of 2002. In June of 2003, another serious vehicle impact occurred (shown in Figure 4-14). Clearly the FRP system was in need of repair and the stra tegy adopted by the FDOT was to completely remove the existing FRP and restore the cross-s ection of the girder with concrete. After this was completed, a new FRP system was applied to strengthen the girder. Before the original system was removed, the author inspected the system using IRT. The primary goal of this inspection was to assess the affect of the vehicle impact damage on the FRP system (beyond what was clearly destroyed). This was also an

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58 excellent opportunity to a pply the IR inspection techni ques developed during the fullscale AASHTO girder tests in a field situation. A B Figure 4-14. Vehicle impact da mage sustained after FRP strengthening. A) July 2002. B) June 2003. Areas of the FRP system that were dama ged by the vehicle impact were heated using four 125 Watt IR heat lamps. The insp ection procedure required two people: one to operate the camera and one to heat the surface. The camera operator and the surface heater were lifted up to the gi rder in a mobile scissor lift positioned directly below the area being inspected. The surface was heated by passing the lamps over the surface at a distance of approximately 10 cm. The rate of motion of the heat lamps varied between scans, but the average Tbackground generated by the heat lamps was 10C. As the specimen was heated, the camera operator view ed the surface thr ough the IR camera and directed the heat lamp operator. While there was some evidence of debondi ng, thermal images indicated that significant damage was limited to the immedi ate area surrounding the point of impact (Figure 4-15). The debonded areas visible in the thermal images were verified with a coin tap inspection. This coin tap inspecti on also verified that areas which appeared bonded in the thermal images in fact were. July 2002 June 2003

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59 A B C D Figure 4-15. Visual and thermal images of vehicle impact damage. A) Damage to side of girder. B) Thermal image. C) Damage to tension face. D) Thermal image While work was being done to apply the new FRP system to the east girder, the IR inspection team was able to evaluate the FRP system that was originally applied to the west girder. Access to the girder was achieve d with a 2 m x 4 m scissor lift. The scanner cart configuration shown in Figure 4-3 B was deployed on the sc issor lift in an attempt to duplicate the experiments performed on the full-scale girders in the laboratory. Unfortunately, this met with little success. Unevenness of the scissor lift platform meant that the height of the lamps were in consta nt need of adjustment as the cart was pushed Area shown in thermal ima g e Tdefect = 10.1 C Tbackground = 11.0 C Area shown in thermal ima g e

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60 along the girder. Also the cart was not prope rly configured to account for the increased distance between the platform and the girder that was mandated by the platforms railing. As an alternative to the scanner cart the camera was placed on a tripod and the camera operator applied heat to the surface as the thermal images were recorded. This was effective at revealing subsurface defect s in the FRP system; however this method required a significant amount of time for setup between shots. A typical thermal image collected during this inspection is shown in Figure 4-16. A number of small defects were detected throughout the inspec ted area. One area that was particularly prone to debonding was the re-entrant corner wher e the bulb intersects the shear face. Figure 4-16. Infrared thermogra phy inspection of undamaged girder The IR inspection technique worked very well in this field inspection. A number of subsurface defects were identifie d in the original FRP system as well as a portion of the system that suffered additional vehicle imp act damage. Overall, the IR inspection indicated that much of the FRP strengthe ning system was still bonded after the major impact damage. This was verified as the workers attempted to remove the existing FRP system with pneumatic jackhammers and encount ered tremendous difficulty. Most of the system was so well bonded that it was left in place and the new system was installed over it (shown in Figure 4-17). An alternative repair pro cedure that might be considered is to 25 cm Tdefect = 9.8 C Tbackground = 4.4C

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61 remove the debonded laminate around the damaged areas and patch the damaged concrete. Once the patch is cu red, apply new FRP composite over the repaired area with an appropriate overlap onto the existing FRP system. It is not known, however, how this repair technique would affect the strengt hened flexural capacity of the girder. Figure 4-17. Damaged girder befo re new FRP system was applied Summary of Findings for Phase I Results from the full-scale AASHTO gi rder experiments and the field study conducted on the bridge at In terstate 10 and Chaffee Road provide insight into how IRT can and cannot be used to inspect FRP strengthening systems applied to civil infrastructure. The most important observation is that an IRT inspection procedure that is effective for one FRP system may not be applicable to another. In the case of the AASHTO girder expe riments, four independent FRP system manufacturers were given id entical strengthening requireme nts for a damaged girder. The FRP systems that were installed on each gi rder varied significantly: different fiber types, different matrix materials, differe nt thicknesses, and different installation procedures. In general, thicker FRP syst ems that contained more matrix material required longer heating times and l onger observation times during cooling. Existing FRP system that could not be removed

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62 The IRT inspection performed on the bridge at Interstate 10 and Chaffee Road also raised some important issues. The IRT insp ection procedure used for the original FRP system applied in 2001 did detect defects. These data, however, were not collected in a uniform manner and the heating process vari ed widely from one portion of the FRP system to the next. The general procedure i nvolved heating an area with four IR heating lamps and watching the surface cool with the IR camera. If no defects were observed in the thermal images, the process was repeated by heating the area for a longer duration. This approach gave the insp ectors a general idea of the required heating times for this specific FRP system. However, there was no rational basis for the heating time or the observation time with the IR camera. The radiometric data were reported for se lect defects by identifying the relative surface temperature increase for the defect and the adjacent defect free areas. These data represent an improvement over ba sic intensity images. At the very least it is possible to state what the temperature difference is for th e defect as opposed to stating the defect is hotter than the surroundings. Without a ra tional model or an extensive database of results to compare with these numbers, the radiometric temperatur e data do not provide additional information about the defect. It is not possible to dete rmine the depth below the surface or the material composition of the defect. Another finding of the Chafee Road study supports a conclusion made from the AASHTO girder experiments: the installation methods and resulting FRP thickness varies considerably and is not always known. Th e replacement system applied to the Chafee Road bridge was applied on top of significant portions of the original system. A large amount of thickened epoxy paste was also us ed to fill surface impe rfections. There is

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63 also a question about the final thickness of the replacement FRP system. The design for this system specified two layers of car bon/epoxy in the longitudinal direction and one layer of carbon/epoxy in the tran sverse direction for shear. Ho wever, installation of this system required a large number of lap splic es and overlaps between successive layers. This installation procedure resulted in a composite thickness as high as five layers in certain locations. These five layers were in addition to any porti ons of the original system that were not removed. Based on the findings of the Phase I expe rimental work, it was determined that additional laboratory work was n eeded. This work is described in the following chapters as Phase II experimental work. The objective of this work was to further investigate the effects of FRP system properties on IRT result s. The properties that were investigated included thickness of the FRP system, fibe r type, and matrix saturation levels. Phase II experimental work also investigat ed different methods for applying heat to the surface of the FRP system. The objective of this work was to develop a standardized heating procedure that can be us ed to inspect FRP composites in the field. Phase I results highlighted the fact that FRP systems applied to concrete cover a large surface area. The heating methods that were investigated in Ph ase II were designed such that they could be practically implemented in the field. Another significant finding from the Phas e I experimental work was that thicker FRP systems require substantially longer hea ting times to reveal defects. From a practical standpoint, the heat flux applied to the surface of the FRP is not uniform. Longer heating times translate into large thermal gradients across the area of the composite. If the overall temperature differen ce that develops for a defect is small, the

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64 defect signal may become lost in the therma l gradient that devel ops from non-uniform heating. The data analysis techniques describe d in Chapter 3 were developed by other researchers to address the issue of non-uniform heating. These data analysis techniques were also developed to assist in using IRT data to charac terize defects. The Phase II experimental work in the current study i nvestigated some of these data analysis techniques and focused on calib rating the different methods for use with FRP composites bonded to concrete.

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65 CHAPTER 5 PHASE II: EXPERIMENTAL SETUP Introduction The overall objective of the current research is to develop IRT methods that can be used to detect defects in FR P systems bonded to concrete. Sp ecifically, a major goal is to use IRT data to provide the following information about detected defects: Size Depth below the surface Material composition The results presented in Chapter 4 indi cate that a standardized approach for collecting IRT data is need ed. Although general heating methods and image capture techniques are available, the application of IRT to composites bonded to concrete has yet to be fully developed. Consequently, the fo llowing issues must be addressed before IRT can be used in the field environment: Heat intensity and duration Timing and duration of image capture Image capture rate Subject size in field of view of IR camera The results presented in Chapter 4 also indicate that FRP system properties can vary significantly from one application to the next. A primary objective of the Phase II research was to investigate how the follo wing FRP system properties can affect IRT results: Thickness of FRP composite Matrix saturation levels Fiber types

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66 Thickened epoxy tack-coat Lap splices Small-scale specimens containing fabric ated defects were constructed in a laboratory environment. Four experimental procedures were designed and implemented for heating the surface of the composite under consideration: Flash heating with a photographers flash Scan heating with tw o 500 W halogen lamps Long-Pulse heating with four 500 W halogen lamps Sinusoidal heating with four 500 W halogen lamps This chapter contains a description of the specimen matrix as well as the details of each heating method. Information regarding the noise characteristics of the IR camera used in the current study is also presente d. Results and analysis are provided in Chapter 6. Specimen Construction A total of 34 small-scale specimens were constructed and grouped into the five categories shown in Table 5-1. Series A, B, and C contain fabricated defects of known size and location. Series A seeks to invest igate the relationship between the smallest detectable defect and the thickness of the applied FRP composite system. Series B examines how different levels of fiber saturati on and fiber type affect IRT results. Series C includes specimens that were prepared us ing different degrees of surface preparation (sandblasting) and tack-coat. Series D and E were used to determine wh ether or not IRT is capable of providing any additional information about how an FRP composite system was constructed. Series D consists of three specimens that were c onstructed using different saturation methods for the fibers. Series E was used to investig ate the effect of lap splices on IRT results.

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67 Specific details regarding how each specimen was constructed are provided in a later section. Table 5-1. Overview of specimen matrix Series ID Number of specimens Variable investigated A 4 Defect size and detection limits B 18 Saturation levels and fiber type C 6 Surface prep and tack coat D 3 Saturation methods E 3 Lap splices A number of the steps and procedures that were used to construct each specimen are common to all of the specimens in each series. These general steps and procedures will be described first. Specific details for each series will be discussed after the general information has been provided. FRP Composite Materials All of the fiber and matrix materials used in this study were provided by Fyfe Co., LLC. These materials were chosen to repres ent common matrix and fiber types used in wet layup FRP strengthening systems. Pre-cure d laminates were not addressed in this study. Two types of fiber materi als were used in this study: Carbon (TYFO SCH-41) Glass (TYFO SEH-51A) TYFO SCH-41 carbon fiber is a unidirectional, stitched carbon-fabric. The dry fibers are shipped in a large roll that is 61 cm wide and 91 m long. The surface of the fabric that is bonded to the concrete is cove red with a very thin veil of multi-directional glass-fibers. These glass-fibers are held in place by the fabrics cross-stitching. The purpose of this veil is to keep the fabric inta ct during installation. These fibers also help to reinforce the matrix materi al at the FRP/concre te bond line and aid in shear transfer from the concrete into the composite. Figure 5-1 shows both the top and bottom surface

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68 of the dry carbon fibers. TYFO SEH-51A gla ss fiber is a uni-dire ctional, woven glass fabric (shown in Figure 5-2). The dry fibers are shi pped in a large roll that is 137 cm wide and 46 m long. Important values are summarized in Table 5-2. Table 5-2. Material properties for fibers, epoxy, and lamina Dry Fiber Properties Epoxy Properties Lamina Properties Fiber type Area density (g/m2) Tensile strength (MPa) Tg (C) Tensile strength (MPa) Thickness (mm) Tensile modulus (MPa) Tensile strength (MPa) Carbon 645 3790 82 72.4 1 82.0 834 Glass 915 3240 82 72.4 1.3 26.1 575 TYFO S epoxy was used as the matrix mate rial for all of the FRP composites used in this study. This epoxy is a two-part sy stem (component A and component B) that is shipped in two 19 L buckets. These buckets are typically pre-proportioned such that all of component B can be added directly in to the bucket containing component A. Component B is non-viscous (similar to water) and can be moved from one container to another very easily while component A is ve ry viscous. For this study, components A and B were proportioned by weight according to the manufacturers guidelines (A:B = 100:34.5). Mixing was performed in a 1 L plastic mixing cup using a drill-powered mixing blade for a minimum of 3 minutes. Ca re was taken not to mix the epoxy too fast to prevent the formation of air-bubbles. Typical mixes were approximately 500 mL in total volume. Concrete Substrate The first step in specimen construction involved casting five 61 cm x 61 cm x 5 cm concrete slabs. Concrete mix proportions were taken directly from PCAs Design and Control of Concrete Mixtures (1994). This was a non air-en trained mix with a target slump of 7.6 to 10 cm. Mix proportions are provided in Table 5-3. Steel plates were used as the bottom surface of the concrete fo rmwork and a thin layer of form-release oil

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69 was applied to the surface of th e steel before the concrete was placed. Prior to finishing, the concrete was consolidated in the formwork using a standard concrete vibrator. This was probably not the most effective means for consolidation considering the shallow depth of the slab (5 cm). A B Figure 5-1. TYFO SCH-41 carbon-f ibers (scale shown in inches ). A) Top surface. B) Surface bonded to concrete. Figure 5-2. TYFO SEH-51 glass-fi bers (scale shown in inches) The concrete was allowed to cure in the forms for two days. No additional curing was provided after the forms were removed. Th e next step was to cu t each of the large slabs into 30.5 cm x 15 cm x 5 cm concrete blocks. These blocks served as the base material for all 34 specimens that were constr ucted. The final thing to note about each

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70 block was that the FRP composite was applied to the surface of the block that had been in direct contact with the steel formwork Table 5-3. Concrete mix proportions used for Series A to E specimens Concrete Mix Proportions Water/Cement Ratio 0.45 Water (kg/m3) 217 Cement (kg/m3) 481 Fine Aggregate (kg/m3) 700 Coarse Aggregate (kg/m3) 902 Max Aggregate Size (mm) 13 Surface Preparation Each specimen received a light sandbla sting prior to plac ement of the FRP composite. Figure 5-3A provides a visual reference for the level of sandblasting that was achieved. The objective of Series C specimens was to investigate the affect that level of surface preparation has on IR thermography result s. Two samples of this series (C-5 and C-6) received additional sandblas ting up to the level shown in Figure 5-3B. This will be referred to as Heavy Blast in the section describing Series C. A B Figure 5-3. Surface preparation before FRP placem ent. A) Light blast. B) Heavy blast. Surface Saturation and Tack-Coat After sandblasting, a 10 cm wide velour pain t roller was used to apply a thin layer of epoxy saturant to the concre te surface. The amount of epoxy that was applied to each specimen was determined by weighing the pain t tray containing the epoxy before and

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71 after the surface was saturated. This epoxy (TYFO S) is the same epoxy that would later be used to saturate the fibers during composite construction. The epoxy was allowed to sit on the surf ace for approximately one hour before a layer of thickened epoxy tack-coat was appl ied (TYFO TC). The same procedure that was used to apply the saturant epoxy was also used for the tack-coat (see Figure 5-4). Figure 5-4. Application of epoxy saturant and tack-coat There are two basic options available for saturating fibers in a wet layup application: (1) machine saturation and (2) ha nd saturation. Machine saturation involves a large piece of equipment that consists of an epoxy bath and two heavy rollers. The dry fabric is first passed through the epoxy bath a nd then pressed between the rollers in order to fully impregnate the fabric. This pro cess also removes any excess epoxy from the composite. The gap distance between the two rollers can be controlled such that the resulting composite contains the desired am ount of matrix material. The installation guidelines provided by the FR P system manufacturer (Fyfe Co. LLC 2001) include specific fiber to matrix proportions if a machine saturator is used: 1.0 lb. of fibers to 1.0 lb. of matrix for carbon-fiber systems and 1.0 lb of fibers to 0.8 lb of matrix for glass-

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72 fiber systems (allowable tolerance is +/10 %) It is common to use a machine saturator for large jobs in which hand saturation of the fibers using a roller w ould be impractical. The primary advantage of machine saturation is that the resulting composite is relatively uniform and contains the correct proportion of fibers and matrix. For smaller jobs, it is very common to sa turate the fibers using a hand-roller method. To give the reader some perspectiv e on what might be considered a large or small job, it should be noted that the FRP system installed on the Chaffee Rd. bridge in 2003 was saturated by hand. Another project that the author was invol ved in required the application of 790 m of 15 cm wide by 4.6 m long carbon fiber strips. Again, all of the fibers were saturated by hand. Unfortunately, the hand-roller method is a very subjective procedure that may result in over/under/nonuniform saturation of the composite. The following steps are provided in the manufacturers (Fyfe Co. LLC 2001) specification for hand saturation of fibers: Make a saturation bath frame out of a sheet of plywood and two-by-fours (or similar materials creating the same effect ) using the two-by-fours for the sides of the bath and the plywood for the floor. Line the bath with plastic sheeting to create a non-permeable membrane for the epoxy. Pre-cut lengths and widths of fa bric necessary for application. Place dry fabric sheets in the bath and add epoxy. Work epoxy into the fabrics using gloved hands, a trowel, paint roller, or similar. After the fabric has been completely sa turated (both sides), remove excess epoxy by squeegying it out with the trowel or by blotting the excess re sin with the next dry fabric sheet to be saturated. (NOTE: Properly saturated fabric is completely saturated with no visible dry fi bers and minimal excess epoxy) The hand-roller method was used to saturate all of the fibers in the current study. Each pre-cut 15 cm x 30.5 cm piece of dry fibe rs was laid on a piece of visqueen with the

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73 surface to be bonded to the concrete facing up. TYFO S epoxy was then poured evenly over the surface. The amount of epoxy that was applied varied depending on the specimen being constructed (details for each specimen are provided below), but the 1:1 fiber to matrix ratio for carbon and 1.0:0.8 ratio for glass were chosen as the standard for a properly saturated composite. Th e same 10 cm velour roller (Figure 5-5B) that was used to saturate the surface and apply the tack coat was used to distribute the epoxy evenly throughout the fibers (Figure 5-5A). The total amount of epoxy that was used to saturate each layer was measured by wei ghing the dry fibers and then weighing the saturated composite. A B Figure 5-5. Fiber saturation. A) The hand-roll er method. B) 10 cm velour roller (scale shown in inches) Application of FRP Composite to Concrete After each layer of FRP composite was saturated on the visqueen, both the saturated fibers and the visqueen were pl aced on the surface of the specimen (visqueen side up). The visqueen was then peeled off leaving the saturated fibers attached to the specimen. Next, the piece of visqueen was weighed to account for any residual epoxy that did not become part of the final composite The 10 cm velour roller was then used to smooth the composite onto the specimen and remove air bubbles. For multi-layer

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74 systems, the process was repeated for each layer. After all of the laye rs were applied, the composite was allowed to cure for 24 hours. A final coat of TYFO S epoxy was then applied as a top-coat in accordance with th e manufacturers installation guidelines. The final step in the specimen pr eparation phase involved trimming the sharp edges where the composite extended beyond the edge of the conc rete substrate. A finished specimen is provided in Figure 5-6. Figure 5-6. Completed specimen Construction Details for Each Series Series A The objective of this subset of specimens is to investigate how the following parameters affect IRT results: Composite Thickness Defect Size Defect Material Composition A total of four specimens containing fabri cated defects were constructed for this series. Specimens A-1, A-2, A-3, and A-4 were constructed using one, two, three, and four layers of TYFO SCH-41 carbon fiber comp osite, respectively. The target fiber to matrix saturation level for each layer of com posite was 1:1. This level was achieved by

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75 carefully adding epoxy and weighing the compos ite before and after the roller was used to saturate the fibers. The resulting wei ght volume fraction for layer of the composite was 0.50. Appendix C contains specific details regarding all quantities of saturant, tackcoat, composite matrix, fibers, and topcoat that was used for each specimen. The fabricated defect configuration was th e same for all four specimens. Defects were created by drilling a series of holes (3 @ 6.4 mm, 3 @ 12. 7 mm, and 2 @ 19 mm) to a depth of 6.4 mm into the concrete substrate on the surface receiving the FRP composite. Several of the holes were backf illed with epoxy or insulating foam and the remaining holes were left empty. A detail ed layout of the def ects is provided in Figure 57. An interface bubble was also implanted by inserting a small nylon machine screw (#8) into the surface of the concrete. The machine screw was cut such that it protruded 3 mm above the surface of the concrete befo re the FRP composite was applied. The exact size of the interface bubble was difficult to control while the composite was being rolled onto the surface of the concrete. It was also difficult to apply successive layers of composite material over the interface bubble and ensure that no air voids developed between layers. The final dimensions for each interface bubble were obtained by measuring the size on the surface of the cured composite. Two measurements were made: one parallel (d||) and one perpendicular (d) to the principle fiber direction. Results are summarized in Table 5-4.

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76 Table 5-4. Series A details Interface bubble Specimen ID Fiber type Surface prep Weight vol. of fibers d|| (mm) d (mm) A-1 Carbon LB 0.50 51 18 A-2 Carbon LB 0.50 57 29 A-3 Carbon LB 0.50 57 25 A-4 Carbon LB 0.50 51 32 A B Figure 5-7. Defect configuration for Series A specimens. A) Plan view. B) Profile. Series B The objective of this subset of specimens was to investigate how the following parameters affect IRT results: Epoxy saturation levels Composite thickness Fiber type (carbon vs. glass) Inter-lamina defects vs. interface defects Interface Bubble #8-Nylon Machine Screw FRP Composite 6.4 mm deep hole (size varies) 3 mm protrusion Drawing Not to Sca l e Ai r -Filled E p ox y -Filled Foa m -Filled Interface Bubble 19 mm 12.7 mm 6.4 mm IB A75 A50 A25 E75 E50 E25 30 c m 15 c m Principle Fiber DirectionSpecimen Layers A-1 1 A-2 2 A-3 3 A-4 4

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77 A total of 18 specimens containing fabric ated defects were constructed for this series. These 18 specimens are subdivided into three groups based on the amount of epoxy that was used to saturate the fibers: low saturation (6 specimens designated L), medium saturation (6 specimens designate d M), and high satura tion (6 specimens designated H). Each of the six-specim en sub-groups contains four carbon-fiber systems (designated C) and two glass-fi ber systems (designated G). The final distinction to be made between each specimen of a sub-group is the number layers. The four carbon-fiber specimens each have 1, 2, 3, or 4 layers, and the two glass-fiber specimens have either 2 or 4 layers. To summarize, the specimen designated B-MC-3 is a three-layer carbon-fiber composite with medi um epoxy saturation. B-LG-4 is a fourlayer glass-fiber composite with low epoxy saturation. The six specimens constructed using the low saturation level each had a fiber weight fraction (wf) of approximately 0.67. For a carbon-fiber system, the 0.67 value represents one-half of the recommended amount of epoxy needed to saturate the fibers. For the glass-fiber system, one-half of th e recommended epoxy would result in a wf of 0.71. Attempts to saturate the glass fabric at this saturation leve l were unsuccessful, and it was clear that additional epoxy would be need ed in order to crea te a composite that appeared close to saturated. The final value of wf used for the glass-fiber composite was 0.67. The medium saturation level wa s achieved using a target wf of 0.5. This represents a properly saturated composite for the carbon-f iber systems and a slightly oversaturated composite for the glass-fiber systems.

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78 The high saturation level specimens we re constructed with a target wf of 0.4. This wf is obtained when the amount of epoxy used to saturate the composite exceeds the recommended level by 50%. Several trial sp ecimens were constructed in which this excess epoxy was applied direc tly to the dry fibers during the saturation process. The resulting composites were too saturated and it became diffic ult to keep all of the epoxy on the visqueen as the fibers were being wetted-out. A more effective strategy was to first saturate the fibers to the medium satura tion level and then apply the composite to the specimen. Next, additional thickened epoxy tack-coat (TYFO-TC) was applied to each layer using a roller. The amount of tack-c oat applied was measured by weighing the pan and roller before and after each application. This quantity was assumed to be matrix material that was incorporated into the composite and was used in the wf calculations. All specimens in Series B c ontained at least one fabricat ed defect. This defect always occurred at the FRP/concrete interf ace and was created using the same procedure described for the interface bubble in Series A. After each system had cured, an estimate of the defect size was made by measuring the dimensions of the bubble with a ruler. For the carbon-fiber systems, each of the bubbles as sumed an elliptical shape. The dimension of the ellipse in the direction of the fibers, d||, was typically larger than the dimension perpendicular to the fibers, d. For the glass-fiber system s, the shape of the interface bubble tended to be more round than elliptical. For FRP systems containing more than 1 layer, an additional inter-lamina bubble was created by placing a #8 nylon nut beneath the top layer of composite. This nut resulted in a defect similar in size and shape to the interface bubble. Measured

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79 dimensions for all of the fabricated defects are provided in Table 5-5. A schematic drawing of the defect configurati on for this series is provided in Figure 5-8. Table 5-5. Series B details Inter-lamina bubble Interface bubble Specimen ID Fiber type Saturation level wf d|| (mm) d (mm) d|| (mm) d (mm) B-LG-2 Glass Low 0.67 57 51 B-LG-4 Glass Low 0.68 57 44 B-LC-1 Carbon Low 0.67 57 19 B-LC-2 Carbon Low 0.68 38 19 51 25 B-LC-3 Carbon Low 0.67 51 29 57 19 B-LC-4 Carbon Low 0.64 51 25 51 22 B-MG-2 Glass Med 0.54 51 38 51 44 B-MG-4 Glass Med 0.53 51 38 51 51 B-MC-1 Carbon Med 0.52 76 25 B-MC-2 Carbon Med 0.48 44 22 57 38 B-MC-3 Carbon Med 0.50 51 22 57 35 B-MC-4 Carbon Med 0.50 44 25 57 32 B-HG-2 Glass High 0.44 57 57 B-HG-4 Glass High 0.43 57 57 B-HC-1 Carbon High 0.43 57 29 B-HC-2 Carbon High 0.37 76 25 83 38 B-HC-3 Carbon High 0.42 44 19 64 25 B-HC-4 Carbon High 0.41 38 19 51 25 Series C Series C contained a total of six specimen s. The objective of this series was to investigate the effects of concrete surface preparation and the us e of thickened epoxy tack-coat. Three different levels of surf ace preparation were used (two specimens for each method): none, light sandblasting, and heav y sandblasting. A visual reference for the light and heavy sand blasting was provided in Figure 5-3. For each of the surface preparation methods, one of the specimens r eceived a layer of thickened epoxy tack-coat and the other did not. The specimen matr ix for this series is summarized in Table 5-6. Each of these specimens was constr ucted using one layer of carbon-fiber composite. One fabricated defected was im planted in each specimen using the interface

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80 bubble procedure that was described above. In addition to inve stigating how surface preparation affects the IRT results for each de fect, these specimens were also used to examine how surface preparation affect s IRT results for non-defect areas. A B Figure 5-8. Defect configurat ion for Series B specimens. A) Plan view. B) Profile Table 5-6. Series C details Int. Bubble Specimen ID Surface prep Tackcoat wf d|| (mm) d (mm) C-1 None Yes 0.44 57 38 C-2 None No 0.42 64 32 C-3 LB Yes 0.45 57 32 C-4 LB No 0.44 51 25 C-5 HB Yes 0.46 64 32 C-6 HB No 0.43 57 32 FRP Composite (# of layers varies) Interface Bubble (always located at FRP/concrete interface) #8-Nylon Machine Screw 3 mm protrusion Drawing Not to Sca l e Inter-lamina Bubble (always located under top layer of FRP) #8Ny lon Nut Interface Bubble A1 30 c m 15 c m Principle Fiber Direction Inter-lamina Bubble A2 d || d

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81 Series D The objective of this series was to in vestigate how different fiber saturation techniques affect IRT results. Three different techniques were applie d to three different specimens: D-1: Dry fibers were placed directly onto specimen (aft er tack-coat was applied). A 10 cm paint roller was dipped in epoxy and then rolled over composite until it appeared saturated. D-2: Dry fibers were placed on a sheet of visqueen and epoxy was pressed into the surface using a roller. Heavy pressure was applied such that all excess epoxy was removed. After the fibers were saturated, the composite was placed on the tack coat and the visqueen was peeled off. D-3: Similar to D-2 except that light pr essure was applied with the roller and not all of the excess epoxy was removed. Th is resulted in a co mposite with more epoxy than D-2. All specimens received a light sandblast and a layer of thickened epoxy tack-coat. Only one layer of carbon-fibe r was applied to each specime n, and no fabricated defects were included. The specimen matrix for this series is summarized in Table 5-7 Table 5-7. Series D details Specimen ID Surface prep Saturation method wf D-1 LB Surface 0.50 D-2 LB Heavy Roller 0.51 D-3 LB Light Roller 0.40 Series E The final series contained three specimens. These specimens were constructed to represent different configurations. Lap-sp lices are commonly enc ountered in the field when multiple strips of FRP are required to achieve the desired length for the composite system. The specimen labeled E-2/3/1 consists of 2 layers of carbonfiber over the left 4

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82 in., 3 layers of carbon-fiber over the middle 10 cm, and 1 la yer of carbon-fiber over the right 7.6 cm Specimen E-2/3/2 consists of 2 layers, 3 layers and 2 layers over the left, center and right sections, respectively. Finally E-2/4/3 consists of 2 layers. 4 layers, and 3 layers. Table 5-8 provides a summary of the fiber wf for each of the specimens. A detailed schematic showing the lap-splice conf iguration for each specimen is provided in Figure 5-9 Table 5-8. Series E details Specimen ID Surface prep Fiber type wf E-2/3/1 LB Carbon 0.56 E-2/3/2 LB Carbon 0.51 E-2/4/3 LB Carbon 0.46 A B C Figure 5-9. Lap-splice configura tion for Series E. A) E-2/3/1. B) E-2/3/2. C) E-2/4/3 Heating Methods and Thermal Imaging This section describes the f our heating methods used in this study: flash heating, scan heating, long-pulse heating, and sinusoidal heating. Flas h heating, scan heating, and long-pulse heating each involved a different ge ometric configuration for the heat source, specimens, and camera. The sinusoidal heati ng used the same configuration as the longpulse heating. Concrete 10 cm Concrete Concrete Principal fiber direction in plane of page Principal fiber direction out of page 10 cm 10 cm 10 cm 10 cm 10 cm 10 cm 10 cm 10 cm

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83 Flash Heating Flash heating experiments were conduc ted using two 3.3 kJ Godard photography flash systems. Each flash system consisted of a power pack (used to store the charge) and a lamp head (used to distribute the lig ht energy). One of the power packs was established as the master unit and its lamp wa s fired with a manual trigger. The second power pack was established as a slave unit a nd was set to fire simultaneously with the master via a light sensor. The total puls e duration was 50 ms. A 17.8 cm diameter reflector shield was used on each lamp head. The heat source and camera configuration us ed in the pulse heating experiments is provided in Figure 5-10. A number of different c onfigurations were investigated in which the flash heads were moved further away from the specimen. This results in a larger area being heated by the flash (which is desirable in terms of efficiency). The tradeoff experienced for the increase in surf ace area is that the intensity of the heat deposited on the surface is reduced The configuration shown in Figure 5-10 was ultimately chosen because it provided enough inte nsity to develop a signal for defects in multi-layer systems. A typical thermal image collected for Specimen A-1 using the pulse heating configuration is provided in Figure 5-11. This image was saved one second after the flash was triggered (t = 1 sec). All future references regarding the time an image was saved will be made with respect to the end of heating. For example, an image saved at t = 0 sec refers to the thermal image collected immediately after the flash was fired. For these experiments, the image save rate was se t to 5 frames per second (maximum rate for this thermal imaging system). Since ther e was no common trigger for the flash and IR camera, the actual time may differ by as much as 0.2 sec.

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84 A B Figure 5-10. Heat source and camera configuration for pulse heating experiments. A) Plan view. B) Profile view. Figure 5-11. Typical thermal image colle cted during pulse heating experiment (Specimen A-1 @ t = 1 sec) 81 cm 3.3 kJ, 50 ms Photographic Flash 20 c m Specimen IR Camera 3.3 kJ, 50 ms Photographic Flash 40 c m 30 c m L1L1 L2 L2 12 in. 15 c m 94 cm 81 cm 1 m IR Camera Flash Head

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85 To characterize this heating configura tion, the surface temperature profile was measured along two lines (L1 and L2 in Figure 5-11) at time values of 0, 1, 2, 3, 4, and 5 sec. The horizontal temperature profile, L1, is shown in Figure 5-12A. The values provided in this figure represent the surface te mperature increase that was experienced by the specimen. The profile generated at t = 0 sec has a maximum value of 24.3C and a mean value of 20.0C. The profile generate d at t = 1 sec has significantly lower values for the max (8.1C) and mean (7.1C). The subs equent profiles generated at t = 2, 3, 4, and 5 sec follow the same general trend that wa s established by the prof ile at t = 1 sec. A likely explanation for the large differences between the profiles at t = 0 and t = 1 is that the surface of the specimen can reflect a large amount of the heat energy from the flash. It is difficult to say exactly how much of the IR energy coming off of the surface at t = 0 is a result of the specimens surface temperature or the re flected energy developed by the flash. For the purpose of comparison with other heating methods, only the surface temperature profile developed at t = 1 sec will be considered. Figure 5-12B provides the normalized surface temperature profile for L1 at t = 1 sec. These values were obtai ned by dividing the temperature profile obtained at t = 1 sec and dividing by the maximum increase ( Tmax). In a perfect setup, the maximum value (1) should occur in the middle of the specimen (d = 15.2 cm) a nd then experience little or no taper moving off in both directions towards the edges. For this case, the peak value occurs at a distance of 23 cm from the le ft of the specimen. A minimum value of approximately 0.5 was observed on the left hand side of the specimen and a minimum value of 0.75 was observed on the right had side This unevenness can be attributed to a slight misalignment of the flash lamps.

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86 Similar plots for the vertical profile, L2, are provided in Figure 5-12C and D. The peak and mean values obtained at t = 1 s ec were 7.8C and 6.9C, respectively. The normalized temperature profile plot is relativel y balanced with a peak value occurring at approximately 7 cm from the top of the specimen. The minimum values of normalized temperature increase obtained at the top a nd bottom were 0.8 and 0.75, respectively. Scan Heating One of the major limitations of the pulse heating setup is that only a small area can be heated and inspected at one time. The ge neral concept behind scan heating is that the heat source is moved across the surface of the composite being inspected and the IR camera is positioned to record the surface temp erature as the specimen cools. It is important to note that the camera position re mains fixed throughout the duration of the experiment. The heat source developed for this study is shown in Figure 5-13. Two 500 W halogen work lights were used as the energy source and were arranged as shown in the figure. It should be noted that the safety glass was removed. A thin heat shield was constructed using adhesive-backed sheet metal to help focus the energy from the lamps. The dimensions of the shield opening were 35.6 cm x 20.3 cm and the plane of the opening was offset a distance of 18.4 cm from the lamp bulb. This metal shield also helped to control reflections that were generated by the lamps.

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87 0 2 4 6 8 10 12 0 5 10 15 20 25 p T (C)Distance (in.) t = 0 s t = 1 s t = 2 s t = 3 s t = 4 s t = 5 s 0 2 4 6 8 10 12 0 0.2 0.4 0.6 0.8 1 @ T/TmaxDistance (in.) A B 0 1 2 3 4 5 6 0 5 10 15 20 25 p T (C)Distance (in.) t = 0 s t = 1 s t = 2 s t = 3 s t = 4 s t = 5 s 0 1 2 3 4 5 6 0 0.2 0.4 0.6 0.8 1 @ T/TmaxDistance (in.) C D Figure 5-12. Surface temperature profile due to pulse heating. A) Horizontal profile for L1. B) L1 Normalized @ t = 1 sec. C) Vertical profile for L2. D) L2 Normalized @ t = 1 sec. During each experiment, the heat shield ope ning was held at approximately 7.6 cm from the surface of the specimens. The heat source was moved from left to right at a constant rate of approximately 2.2 cm/sec. The total amount of time that any one point on a specimen was exposed to the heat source was 12 sec.

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88 Figure 5-13. Heat source used in scan h eating experiments (scale shown in inches) A series of thermal images that were colle cted during a scan heating experiment is provided in Figure 5-14. The distance from the camera to the specimens was held fixed at 152 cm. This configuration allowed fo r the simultaneous inspection of up to 4 specimens (1858 cm2). 30 40 50 60 70 80 30 40 50 60 70 80 A B 30 40 50 60 70 80 30 40 50 60 70 80 C D Figure 5-14. Thermal images collected during sc an heating experiment. A) t = 0 sec. B) t = 5 sec. C) t = 10 sec. D) t = 15 sec The surface temperature increase and unifo rmity analysis was conducted using the vertical line shown in Figure 5-15. This line passes through approximately the same

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89 location on Specimen A-1 as line L2 shown in Figure 5-11. This image was collected one second after the edge of th e heat shield passed over the line. The surface temperature increase profiles recorded at t = 0, 1, 2, 3, 4, and 5 sec are provided in Figure 5-16A. The maximum value recorded along this line at t = 1 sec was 25.0C (mean value = 23.5C). The normalized temperature profile pl ot for t = 1 sec is provided in Figure 5-16B. The maximum value occurs near the bottom of the specimen which is located near the center of the heating apparatus. Near the top of the specimen, a normalized temperature increase of 0.85 was recorded. L1 L1 Figure 5-15. Thermal image collected during scan heating experiment for Series A (t = 1 sec for line L1) 0 1 2 3 4 5 6 10 15 20 25 30 35 T (C)Distance (in.) t = 0 s t = 1 s t = 2 s t = 3 s t = 4 s t = 5 s 0 1 2 3 4 5 6 0 0.2 0.4 0.6 0.8 1 T/TmaxDistance (in.) C B Figure 5-16. Surface temperature profile for scan heating. A) Vertical Profile for L1. B) Normalized @ t = 1 sec

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90 Long-Pulse Heating The long-pulse heating configuration us ed in this study is provided in Figure 5-17. During each experiment, a total of four 500 W halogen lamps were used to provide a square heat pulse for a specifi ed duration (30, 45, and 60 sec pulse durations were used in the preliminary investig ations). The 500 W halogen lamps were similar to those used in the heat source for scan heating except that th e safety glass was left in place. During each experiment, a maximum of six specimens (2787 cm2) could be heated and observed simultaneously. A thermal image collected for Series A at t = 1 sec following a 30 sec pulse is provided in Figure 5-18. The horizontal and verti cal lines (L1 and L2) are located on Specimen A-1. The surface temperature incr ease profile for each of these lines is provided in Figure 5-20. Similar data for a pulse duration of 60 sec are provided in Figure 5-21. This analysis yields similar results fo r the 30 and 60 sec pulse durations. The horizontal line, L1, experiences a significant decrease in surface te mperature rise moving from right to left across the specimen. The minimum value of normalized surface temperature rise was 0.45 at the left edge (furthest away from the heat source). The vertical line, L2, experiences l ittle or no taper from the top to the bottom of the specimen. It should be noted, however, that this is stri ctly a function of the specimens position. The specimen positioned directly below A-1 does exhibit a temperature gradient along the vertical axis.

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91 A B Figure 5-17. Heat source and camera configuration for long-pu lse heating experiments. A) Plan view. B) Profile. L1L L2 L2 Figure 5-18. Thermal image collected at t = 1 sec during long-pulse heating experiment for Series A (30 second pulse) 60 in. 2 500 W Halogen Lamps 7.25 in. 6 Specimens in FOV IR Camera 2 500 W Halogen Lamps 30 in. 24 in. 2 @ 500 W 18 in. 6 in. 37 in. 40 in. 3 in.2 @ 500 W 46 in. 6 in. 60 in. IR Camera

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92 A B Figure 5-19. Laboratory setup fo r long-pulse heating experiments. A) Halogen lamps and IR camera. B) Typical thermal image containing 6 specimens 0 2 4 6 8 10 12 2 4 6 8 10 T (C)Distance (in.) t = 0 s t = 1 s t = 2 s t = 3 s t = 4 s t = 5 s 0 2 4 6 8 10 12 0 0.2 0.4 0.6 0.8 1 T/TmaxDistance (in.) A B 0 1 2 3 4 5 6 3 4 5 6 7 8 9 T (C)Distance (in.) t = 0 s t = 1 s t = 2 s t = 3 s t = 4 s t = 5 s 0 1 2 3 4 5 6 0 0.2 0.4 0.6 0.8 1 T/TmaxDistance (in.) C B Figure 5-20. Surface temperature profile for long-pulse heating (30 sec pulse). A) Horizontal profile for L1. B) L1 No rmalized @ t = 1 sec. C) Vertical profile for L2. D) L2 Normalized @ t = 1 sec.

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93 0 2 4 6 8 10 12 2 4 6 8 10 12 T (C)Distance (in.) t = 0 s t = 1 s t = 2 s t = 3 s t = 4 s t = 5 s 0 2 4 6 8 10 12 0 0.2 0.4 0.6 0.8 1 T/TmaxDistance (in.) A B 0 1 2 3 4 5 6 4 5 6 7 8 9 10 T (C)Distance (in.) t = 0 s t = 1 s t = 2 s t = 3 s t = 4 s t = 5 s 0 1 2 3 4 5 6 0 0.2 0.4 0.6 0.8 1 T/TmaxDistance (in.) C D Figure 5-21. Surface temperature profile for long-pulse heating (60 sec pulse). A) Horizontal profile for L1. B) L1 No rmalized @ t = 1 sec. C) Vertical profile for L2. D) L2 Normalized @ t = 1 sec. Sinusoidal Heating The sinusoidal heating experiments used the same heating and camera configuration that was describe d for the long-pulse heating. The only difference was that the shape of the heat pulse was sinusoidal in na ture rather than square. The test setup for sinusoidal heating included an analog dimmer th at was used to control the intensity of the four 500 W halogen lamps. The analog dimmer regulates the energy output of each lamp based on an input voltage (0 10 V). This voltage is supplied and manipulated by a laptop computer and an analog I/O data acquisition card. A Labview program was

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94 created in which the user specifies the fr equency and peak intensity of the desired sinusoidal pulse. This Labview program was also used to control the image save rate on the IR camera. Since a broad range of pulse frequenc ies were investigated, it was important to have the ability to control how many images were saved during a given experiment. For the highest pulse frequency that was investigat ed, 0.2 Hz (total pulse duration = 5 sec)), an image save rate of 2 frames per second will yield a total of 10 images. The same image save rate applied the lowest frequency investigated, 0.002 Hz (total pulse duration = 5 sec), would yield over 1000 images. A bloc k diagram representing the final test setup for sinusoidal heating is provided in Figure 5-22. Comparison of Heating Configurations A summary of the surface temperature prof ile results that were obtained for each heating method are provided in Table 5-9. These results are limited to the horizontal and vertical lines passing through the center of Specimen A-1. It should be noted that different specimens would produce different re sults based on the characteristics of the composite and the location of the specimen with respect to the heat source. The values for Tmax and Tmean were computed for the profile obt ained 1 sec after the heat source was removed. norm was computed by taking the sta ndard deviation of the normalized temperature increase profile at t = 1 sec. The scan heating method provided the la rgest mean surface temperature increase ( Tmean=23.5C). This increase was also relativel y uniform with respect to the vertical axis ( norm = 4.4% perpendicular to the direction of movement by the heat source). The major downside to this heating method is that surface being inspected is not heated at the same time. The rate at which the heat source is moved will also affect the magnitude of

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95 the temperature increase as well as the tota l duration of the heat pulse. A detailed investigation into these effects was not conducted. From an efficiency standpoint, the scan h eating method is also the most effective. The total area that was inspected in the original co nfiguration (1858 cm2) was limited by two factors: the width of th e heat source (35.6 cm) and the distance from the camera to the target (152 cm). By movi ng the camera farther from the target, the total length of the 35.6 cm wide swath under cons ideration can be increased. Each of the other methods was limited by the total area the heat source was capable of illuminating. Moving the camera farther from the target does have implications for the image resolution. This parameter will be discussed further in Chapte r 6 in the section on defect area analysis. Another important observation stems from a comparison of long-pulse heating results for the 30 sec and 60 sec pulse durat ions. Even though the pulse duration was doubled, the mean surface temperature rise only increased from 8.1C to 9.6C. Another observation is that the degree of non-uniformity was the same for both pulse durations. These results indicate that non-uniform heating is a variable that must be considered. For the long-pulse confi guration, the maximu m and minimum surface temperature increase across a single speci men can vary by as much as 55%. The implication of this finding is that defects loca ted at different positions with respect to the heat source are subjected to a different in tensity heat flux. A common strategy for limiting the effects of non-uniform heating is to reduce the size of the area under consideration. The heat sour ces described above could be reconfigured in such a way that a 15 cm x 15 cm area could be heated un iformly. A major goal of this research,

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96 however, is to investigate techniques that wi ll make IRT inspections practical on a large scale. Table 5-9. Surface Temperature Increase Results for Different Heating Methods Horizontal profile (t=1sec) Vertical profile (t=1sec) Heating method Total area heated (cm2) Tmax Tmean norm Tmax Tmean norm Pulse 464 8.1 7.1 12.3% 7.8 6.9 6.4% Scan 1858 NA NA NA 25.0 23.5 4.4% Long-pulse (30 sec) 2787 8.1 6.1 18.9% 7.0 6.8 2.6% Long-pulse (60 sec) 2787 9.6 7.2 18.8% 8.4 8.0 2.9% Figure 5-22. Diagram for sinusoidal h eating control and data acquisition 4 500 W Halo g en Lam p s IR Camera Laptop #1 Laptop #2 Analog Dimme r NI 6036E PCMCIA DAQ Card Trigger to save image Images Stored on Laptop #2 0 10 V analog signal Scaled power to lamps System Input: Peak Intensity Imax (0 10) Pulse Frequency fmod (Hz) # of Images Desired N Intensity time 1/fmo d Imax Indicates point where image is saved System Output

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97 CHAPTER 6 PHASE II: DATA COLLECTION AND ANALYSIS Introduction This chapter presents the data collect ion and processing techniques applied to Specimens A through E described in the previous chapter. The first step was to apply each of the following heating methods to the specimens in Series A: Flash heating Scan heating Long-pulse heating Sinusoidal heating Four analysis techniques for processing th ermal data were applied to the data collected for Series A: Pulse analysis (time domain) Step analysis (time domain) Lock-in analysis (frequency domain) Pulse phase analysis (frequency domain) These analysis techniques are all currently available and have been utilized in some form in various IRT applications. None, as yet, have been adapted for use on FRP composites bonded to concrete. Consequently, the analysis techniques were adapted and calibrated using the data collected from the Series A specimens. The adjustments included modification of heating duration and data capture duration as well as filtering algorithms and defect characterization. The objective was to calibrate the techniques for varying composite thicknesses, defect sizes, and defect material compositions. Following the extensive calibration and adaptation, the techniques were compared to determine the best overall approach to use with the remainder of the specimens.

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98 This chapter is divided into five main sections. The first two sections include results and discussion for the pulse and step analysis techniques. The third section examines the lock-in and pulse phase analysis techniques. The next section compares the different heating methods and analysis techniques that were applied to Series A. The last section covers results from Series B through E. Pulse Thermography: Series A During the pulse thermography experiments, a uniform heat flux was applied to the surface being inspected for a finite pulse duration, tp. In the current study, five pulse durations were investigated: 15 ms, 12 sec, 30 sec, 45 sec, and 60 sec. Specimen Heating and Data Collection Three of the experimental setups described in Chapter 4 were used: flash, scan, and long-pulse. Flash heating The four specimens in Series A were h eated with the flash system and thermal images were recorded at a rate of 5 frames /sec. Only one specimen was heated during each experiment. The first thermal image th at was considered in the analysis was collected immediately after th e flash was fired (t = 0 sec). Thermal images collected before the flash was fired were ignored. Thermal images were recorded for 240 seconds while the surface of the specimen cooled. The image save rate of 5 frames per second generated an extremely large volume of data (1200 images = 720 MB). It was dete rmined later that an image save rate of 1 frame per second was adequate. Once the st art image was identifie d in a series of thermal images, the remaining set of images were decimated such that only 1 in 5 frames remained. This resulted in a tota l of 241 images for each specimen (Fs = 1 sec).

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99 Scan heating In the scan heating experiment, all four specimens were heated using the modified halogen lamps. The specimens were arranged such that all four c ould be observed at the same time. Thermal images were collected at a rate of 1 frame/sec while the heat source was being moved across the specimens. Images were saved for an additional 240 seconds while the specimens cooled. Long-pulse heating Data collection for these experiments was si milar to scan heating in that all four specimens were heated at the same time. Three different pulse durations were investigated: 30 sec, 45 sec, and 60 sec. Thermal images were saved at a rate of 1 frame per second, and images were saved for 240 seconds while the specimens cooled. Summary of data collection for pulse analysis Five sets of data were collected. Each se t contained a series of thermal images for each of the specimens in Series A. These five data sets are summarized in Table 6-1. Table 6-1. Summary of data co llected for pulse analysis study Data set ID Pulse duration (sec) Experimental setup Image save rate (1/sec) Data collection begins Number of images P-0 0.015 Flash 1 end of heating 241 P-12 12 Scan 1 end of heating 241 P-30 30 Long-Pulse 1 end of heating 241 P-45 45 Long-Pulse 1 end of heating 241 P-60 60 Long-Pulse 1 end of heating 241 Image Preprocessing The thermal images collected in each expe riment were originally stored in a proprietary format. This file format is uni que to the thermal imaging system and a standalone software package (Thermacam Researcher 2001) is required to view the thermal images. This software also allows the user to perform analysis functions on a series of

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100 thermal images. These basic features includ e the ability to identify a single pixel of interest in a thermal image and extract th e temperature vs. time response. A similar analysis can be performed by se lecting an area of interest (squares or rectangles) and extracting user-specified parameters (maximum value, minimum value, average value, etc.) as a function of time. Unfortunately, this software package proved inadequate for the current study. The amount of time requi red to extract the necessary data to characterize a defect was prohibitive. This software package was also not capable of performing a frequency domain analysis. These shortcomings led to the developm ent of a new computer program using Matlab to analyze the data. The signal and image processing toolboxes that accompany Matlab make this platform well suited for a wide variety of matrix, data analysis and image processing related tasks. Matlab was also a convenient choice since the Thermacam Researcher 2001 software supports saving images in a standard Matlab format (*.mat). This new software program will be referred to as IMG_PROC in the text below. Data pre-processing cons isted of three steps: Convert images to Matlab format Generate series of thermal images for each specimen Spatial filtering to remove random noise The images were converted to Matlab format using the Thermacam Researcher 2001 software. Once converted, the new image files could be opened and viewed in the IMG_PROC environment. The next step wa s to isolate each specimen in a series of thermal images by cropping away any extraneous pixels outside of the specimen boundary. Depending on the heating method being used, a series of thermal images may

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101 contain data for more than one specimen. If this occurred, the cropping procedure was performed multiple times on the same series of images to generate a unique data set for each specimen. This operation provided two major advantages. First, the size of each image in a series was significantly reduced. This reduction in size l eads to a reduction in the time required to process data for each specimen. Second, cropping the image ensures the entire range of the colormap is applied to the specimen of interest. The next step in the pre-processing phas e was to apply spatial filtering to each image to remove random noise. All of the th ermal images that were collected in this study contain a random noise component. This random noise component was investigated and described in Chapter 5. An effective way to reduce the influence of this noise is to apply a 3x3 averaging filter to each pixel in a thermal image. The concept is illustrated in Figure 6-1A. The output value for each pixel is computed by averaging the original value of the pixel and the eight pixels that ar e immediately adjacent to the pixel of interest. Figure 6-1 B and C demonstrate the effects of spatia l filtering on a 20x20 square pixel area. Figure 6-1 B provides a surface plot of this area before the filter was applied. Figure 6-1 C shows the same area after the filter was applied. It is possible to increase the size of th e averaging filter and obtain a smoother image. The danger in this approach, however is that features of the image will be destroyed. A 3x3 filter was chosen since it is th e smallest filter that can be applied on a two-dimensional basis. Defect Analysis The next step in the analysis was to extract quantitative information for the defects contained in the specimens. There are two objectives for this pha se of the analysis:

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102 Generate a Tdef vs. time plot for each defect Estimate the area of the defect A B C Figure 6-1. Application of 3x3 averaging filter applied to eac h pixel in thermal image. A) 3x3 Averaging Filter. B) Surface pl ot of 20x20 pixel area. C) Filtered Image (3x3). Generating Tdef vs. time plots Tdef is defined as the difference in temper ature between the defect area and the surrounding defect-free area. The purpose of the Tdef vs. time plot is to monitor how this quantity changes as a function of time. There are two common options that were investigated for computing Tdef in a single thermal image: Select a single pixel above the defect a nd a single pixel adj acent to the defect (subtract the two values) Identify and average the temperature values in a small area above the defect and a small area adjacent to the def ect (subtract the two values) 1 2 1 3 3 2 1 3 2 2 3x3 Averaging Filter Input Output

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103 The first method immediately raises a very important issue: how are the points selected for the defect and de fect free areas? The defect itself might occupy as many as 500 pixels in a typical thermal image. The location of the maximum value is not necessarily fixed and a point that is identif ied in one thermal image may not remain the maximum as time progresses. Choosing a defect-free location also introduces subjectivity. It was shown in Chapter 4 th at the temperature profile along the surface varies considerably due to non-uniform hea ting. Furthermore, FRP composites are not homogeneous materials. Fiber patterns and matrix variation cause a certain amount of texture that will appear in the therma l images. The second method will reduce the influence of the variability. The areas to be averaged, however, st ill involve subjective choice. In the current study, a new method for computing Tdef vs. time is proposed. The first step in the procedure is to identify an area around a defect and draw a rectangle on the thermal image. The width of the line defi ning the rectangle is one pixel. The only requirement for the location of this rectangl e is that the sides are located a sufficient distance away from the defect. This concept is illustrated in Figure 6-2. Figure 6-2A shows an area drawn around Defect A75 on Specimen A-1. Figure 6-2C provides a surface plot of this area. The surface plot shows that the boun dary of the rectangle is not influenced by the presence of the defect. Figure 6-2B shows a smaller rectangle for the same defect. The surface plot for this area (Figure 6-2D) indicates that the temperature profile of the defect does in fluence the temperature profile of the rectangle boundary. Based on this distinction, the area shown in Figure 6-2A is considered properly defined while the area shown in Figure 6-2B is considered poorly defined.

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104 A1 24 25 26 27 A1 24 25 26 27 A B C D Figure 6-2. Area identification for defect analysis. A) Prope rly defined defect area. B) Poorly defined defect area. C) Surface plot of properly defined area. D) Surface plot of poorly defined area Once this area was defined for each defect, the next step was to compute the parameters shown in Table 6-2 at each time step in the series of thermal images. Table 6-2. Parameters computed for defect area at each time step Parameter Description Tmax Maximum temperature bounded by the area Tper_avg Average temperature along the perimeter of the area Tper_max Maximum temperature on the perimeter per Standard deviation of the temperature values on the perimeter Tdef is computed at each time step using the following equation: avg per defT T T_ max (6-1) Figure 6-3A provides a plot of Tmax and Tper_avg vs. time for the defect highlighted in Figure 6-2A. The corresponding Tdef vs. time plot is provided in Figure 6-3B. Once this Tdef vs. time plot has been generated, th e next step is to extract additional

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105 parameters from this plot that will be used to characterize the def ect. Three parameters are shown in Figure 6-3B. Tmax is defined as the maximum value of Tdef. tmax is corresponding time at which the maximum value occurs. t1/2 is the half-life of the Tdef signal and is defined as the time re quired for the signal to decay from Tmax to Tmax / 2. A B Figure 6-3. Constructing Tdef vs. time plots from area parameters. A) Tmax and Tper_avg vs. time. B) Tdef vs. time. The example provided above describes th e characterization procedure for a welldefined defect. A number of defects analyzed in this study produced markedly different Tdef vs. time plots. Figure 6-4A provides a Tdef vs. time plot for the interface bubble defect (IB) on Specimen A-3. This plot indicates a Tdef value of approximately 3.25C at t = 0 sec. This signal slowly decays to a local minimum value of 2.5C at t = 12 sec. At this time, the signal begins an upward trend until the absolute maximum value ( Tmax) is reached at t = 40 sec. An examination of the thermal images taken at t = 0, t = 12, and t = 40 sec help to explain the Tdef vs. time plot (Figure 6-4B, C, and D respectively). The source of the signal between t = 0 and t = 12 sec is not the defect of interest that is bounded by the area IB. This false signal is a result of minor imperfections in the FRP system and non-uniform heating of the specimen. At t = 12 sec, there is a perceptible 100 101 102 0 0.5 1 1.5 2 2.5 3 Tdef (C)Time (sec) Tmax tmax Tmax 2 t1/2 100 101 102 23 24 25 26 27 28 29 30 T (C)Time (sec)Tmax Tper_avg Tdef

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106 shift in the location of the maximum value towards the center of Defect IB. At t = 40 sec, the dominant source of the Tdef signal is the defect of interest and the Tmax value has been achieved. To characterize this Tdef vs. time plot, it is conv enient to introduce a new parameter: tb. tb is defined as the time at which the Tdef signal reaches a local minimum and then begins an upward climb towards Tmax. In this situation, Tmax need not be greater in magnitude than the largest value of the false signal. Tmax is simply defined as the next local maximum after tb. IB 34 35 36 37 38 A B IB 31 32 33 IB 28 29 30 31 C D Figure 6-4. Thermal images and Tdef vs. time plot for Defect IB (Specimen A-3). A) Tdef vs. time. B) Thermal image @ t = 0 sec. C) Thermal image @ t = tb (t = 12 sec). D) Thermal Image @ t = tmax (t = 40 sec) Another distinct Tdef vs. time plot is provided in Figure 6-5. This plot was generated for Defect E75 in Specimen A-3. Th e plot begins with a false signal of 1.6C 100 101 102 0.5 1 1.5 2 2.5 3 3.5 Tdef (C)Time (sec) False Signal tb Tmax tmax

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107 at t = 0 sec. This false signal is due to nonuniform heating. The signal decreases with a near linear slope (plotted on a log sc ale) up to t = 15 sec. Unlike the Tdef vs. time plot shown in Figure 6-4A, there is no distinct local mi nimum to indicate precisely when the defect begins to dominate the signal. The thermal image shown in Figure 6-4C clearly indicates that the defect is detected in the thermal image. However, it there is no well defined value for Tmax, tmax, or t1/2. An interesting observation was made dur ing further investigation into the parameters that were measured for the E75 area. Recall that the maximum temperature on the perimeter of the area boundary (Tper_max) was also recorded at each time step. Figure 6-5B provides a plot of Tper_max minus Tper_avg (labeled B in the figure). The data series labeled A is the original Tdef vs. time plot. The interesting thing happens when series B is subtracted from A (result is la beled C). This curve provides a very clear indication of when the defect be gins to dominate the signal. A B Figure 6-5. Non-uniform heating and weak signals for defects. A) Plot of Tdef vs. time. B) Plot of Tdef vs. time with perimeter difference removed. Figure 6-6 illustrates how this new curve can be used to identify tb, tmax, and Tmax. tb is simply the point at which the curve assumes a positive slope. tmax is chosen to be the 100 101 102 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Tdef (C)Time (sec) Tmax=?? t b =?? False Signal 100 101 102 0 0.5 1 1.5 2 T (C)Time (sec) A = Tdef B = Tper_max Tper_avg C = A B

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108 point at which curve C is maximum, and Tmax is the corresponding Tdef value measured at t = tmax (from the original Tdef vs. time plot). Figure 6-6 also illustrates another very important quantity: Tthresh. Tthresh is defined as the threshold value of T for curve C. A value of 0.2C was chosen for the current study. If the maximum value obtained by curve C is less than 0.2C, the defect is considered to be undetected. Figure 6-7 provides an example of a Tdef vs. time plot for an unde tected defect. This curve was generated for Defect A75 on Speci men A-3. Since the value of curve C never rises above the threshold value of 0.2C, the defect is consider ed undetected in the Tdef vs. time plot. Figure 6-6. Identification of impor tant parameters for weak signals Another parameter that was measured fo r each defect was the signal to boundary noise ratio (SBR). This quantit y is defined as the ratio of Tdef to the standard deviation of the temperature along the boundary of the area used to define the defect. A high SBR is indicative of a well-defined defect that is easy to detect in thermal images. Low SBR values are encountered when the computed Tdef is small or if the boundary of the area surrounding the defect lies in a region of the specimen that experienced non-uniform heating. The value of SBR that was extr acted for each defect was measured at tmax. 100 101 102 0 0.5 1 1.5 2 T (C)Time (sec) tb Tmax tmax Tthresh = 0.2C

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109 Figure 6-7. Signal for undetected defect The only remaining parameter that was extracted for each area was Tper. Tper is defined as the average temper ature rise experienced by the defect area boundary due to heating. This value wa s obtained by subtracting Tavg_per @ t = 240 sec from Tavg_per @ t = 0. Table 6-2 provides a summary of all the pa rameters that were extracted from each Tdef vs. time plot. Table 6-3. Paramete rs extracted from Tdef vs. time plot for each defect Parameter Description Tmax Maximum defect signal strength Tper Average temperature rise experienced by the perimeter of the defining area due to heating (Tper_avg @ t = 0 Tper_avg @ t = 240) tb Time at which Tdef becomes dominated by the defect of interest tmax Time at which Tmax occurs t1/2 Time required for Tmax to decay by a factor of two Area computations To determine the size of a defect from a th ermal image, it is possible to draw a line around the boundary of the defect and count th e number of pixels inside the boundary. This method will be referred to as the boundary trace method. The number of pixels can be converted to an area by applying a length factor that is obtained by identifying two points in a thermal image where the true se paration distance is know n (typically the two 100 101 102 0 0.5 1 1.5 2 T (C)Time (sec)A = Tdef B = Tper_max Tper_avg C = A B Tthresh = 0.2C

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110 bottom corners of a specimen). Matlabs i mprofile command was then used to draw a line between these two points and the number of pixels this line passes through was counted. The known distance divided by the numbe r of pixels can then be used as the length ratio for each pixel (measured in mm/pixel). Figure 6-8 illustrates the bounda ry trace method applied to Defect A75 and Defect IB on Specimen A-1. The first step in the anal ysis procedure is to identify the image that was collected at tmax (time of maximum defect signal strength). The color scale of the image is then adjusted such that the enti re scale is distributed across the range of temperature values encountered in the box used to define the defect. Next, Matlabs roipoly command is invoked and the user tra ces out the boundary of the defect in the thermal image. The results of this analys is for Defect A75 (19 mm diameter air-filled defect) are provided in Figure 6-8A. The total number of pixels bounded by the trace was 377. After applying the length factor for this image (1.1 mm/pixel or 1.2 mm2/pixel), the area of the defect was estimated to be 4.4 cm2. The true area for this defect was 2.8 cm2. The same procedure applied to Defect IB on Specimen A-1 resulted in an estimated area of 10.6 cm2. The true area for this defect was 1.2 cm2. Further experimentation with other de fects of known size indicated that the boundary trace method consistently overestimates the size of the defect. It is conceivable that this bias error could be quantified and then considered in future computations. The fact remains, however, that selecting the boundary of the defect will always require some degree of human judgment. Maldague (2001) outlines a procedure for approximating the size of a defect by computing the magnitude of the maximum temperature gradient at each pixel in a thermal image. The underlying principle for this procedure is that the

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111 location of maximum slope of the temperature fi eld corresponds to the edge of the defect below the surface. This procedure will be refe rred to as the gradient area method in the text below. A B Figure 6-8. Defect area computations us ing boundary trace method. A) Defect A75 (Specimen A-1). B) Defect IB (Specimen A-1) A Matlab routine was developed to automate this procedure. The first step is to identify the thermal image collected at tmax. The next step is to compute a gradient image of the box area used to define the defect. Th e magnitude of the gradient for each pixel in this area is computed using Matlabs built-i n gradient operator. The gradient image generated for Defect A75 (Specimen A-1) is provided in Figure 6-9A. Figure 6-9D provides a surface plot of the gradient image which illustrates the lo cation of the defect boundary. The next step is to locate the center of th e defect by identifyi ng the pixel with the smallest gradient near the center of the gradie nt image. Once the center of the defect has been identified, a line is cons tructed between the center and the upper left corner of the area containing the defect. The location of the maximum value along this line is determined and stored as one point on th e defects boundary. Next, a new line is constructed from the center po int to the pixel on the borde r just below the upper left 10 20 30 40 50 60 70 5 10 15 20 25 30 35 40 True Area = 7.1 cm2 # of Pixels = 882 Estimated Area = 10.6 cm2 5 10 15 20 25 30 35 40 5 10 15 20 25 30 35 40 True Area = 2.8 cm2 # of Pixels = 377 Estimated Area = 4.4 cm2

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112 pixel. The location of the maximum gradient is computed again and stored as the second point along the boundary of the defect. The proce ss is repeated for a series of lines drawn from the center point to each pixel along the boundary of the area. The final defect boundary computed generated for Defect A75 is provided in Figure 6-9A. Once this boundary has been determined, the number of pixels contained within the boundary is summed. The number of pixels can then be c onverted into physical units by scaling with an appropriate factor. A B C D Figure 6-9. Surface temperature profile and gradient used to approximate the boundary of detected defects. A) Gradient intensity w/ defect boundary (A75). B) Gradient intensity w/ defect boundary (IB). C) Surface temperature profile (A75). D) Surface gradient profile (A75). 10 20 30 40 50 60 70 5 10 15 20 25 30 35 True Area = 1.10 in.2 # of Pixels = 551 Estimated Area = 1.00 in.2 5 10 15 20 25 30 35 40 45 5 10 15 20 25 30 35 True Area = 0.44 in.2 # of Pixels = 224 Estimated Area = 0.41 in.2

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113 For Defect A75 and Defect IB on Specimen A1, this procedure results in a better estimate for the defect area than the bounda ry trace method. The estimated area for Defect A75 was 2.6 cm2 and the estimated area for Defect IB was 6.4 cm2 (see Figure 69B). Unfortunately, the performance of the gradient area method is not always guaranteed. Results from the current study indicate several factors that can reduce accuracy in area computations us ing the gradient area method: Weak signal for the defect (low SBR value) High temperature gradient due to non-uniform heating Insufficient pixel resolution for small defects An example of a defect which genera ted a weak signal is provided in Figure 6-10. This thermal image was collected for Defect A75 in Specimen A-3. The corresponding gradient image (shown in Figure 6-10B) does not provide a well-defined defect boundary. Oddly enough, the estimated area for this defect was 3.3 cm2 which is within 14% of the actual value (2.8 cm2). Based on observation of the gradient image, however, this estimate does not appear to be meaningful. The coefficient of variation (COV) of the radius values generated by the gradient area method is a useful quantity for assessing the quality of the def ect boundary. COV is defined as the standard deviation of a seri es of numbers divided by the mean value. When the gradient area method was applied usin g this center point, a total of 152 radius values were obtained. The appare nt center point of the defect in Figure 6-10B is labeled o. For the case shown in Figure 6-10B, the COV for these values was 0.54. The COV for the defect shown in Figure 6-9A, a well-defined defect, was 0.10. In an ideal case, the COV for a circular defect should be zero. For an elliptical defect, however, there will always be varia tion in the computed radius values. The

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114 magnitude of the COV will be dependant on the ratio of the primary axis dimensions of the ellipse. Figure 6-11 provides a graph of COV valu es plotted against the ratio of the radii of an ellipse. These COV values repr esent an ideal case for a perfectly defined boundary. This plot can be us ed to assess the quality of a defect boundary generated for an elliptical defect. Consider th e defect boundary that is shown in Figure 6-9 (Defect IB in Specimen A-1). The approximate ratio of th e principal axis dimensions for this defect is 0.35. This corresponds to an inherent COV of 0.35 on the graph in Figure 6-11. The computed COV for this defect was 0.33. This example illustrates that the absolute magnitude of COV should not be used to a ssess the quality of a computed defect boundary. Instead of the absolute magnitude the difference between the computed COV and the inherent COV for the defect under consideration should be used. A75 23.8 24 24.2 O 5 10 15 20 25 30 35 5 10 15 20 25 30 35 A B Figure 6-10. Reduced accuracy in area comput ations due to a weak signal. A) Thermal image for weak signal. B) Gradient intensity. The second source of reduced accuracy in area computations, non-uniform heating, is illustrated in Figure 6-12. The thermal image highlights the temperature gradient that develops across the box which was drawn around the defect. This thermal gradient is also apparent in the gradient image provided in Figure 6-12B. As a result, the defect boundary that was generated using the gradient area method is skewed to one side. The COV computed for the radius values was 0.25.

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115 Figure 6-11. Coefficient of variation fo r ellipse radii (computed with NP = 250) E75 30 30.5 31 31.5 O 5 10 15 20 2 4 6 8 10 12 14 16 18 20 A B Figure 6-12. Reduced accuracy in area com putations due to non-uni form heating. A) Thermal image for non-uniform heating. B) Gradient intensity. The final source of reduced accuracy encountered in the current study is illustrated in Figure 6-13. The thermal image shown in Figure 6-13A highlights Defect A75 on Specimen A-1. This thermal image was colle cted using the long-pul se heating setup in which the camera was a distance of 152 cm fr om the surface of the specimen. For this camera distance, the size of one pixel in the thermal image was determined to be 2 mm. Under these conditions, the entire diameter of Defect A75 (6.4 mm) is represented by just over three pixels. When the grad ient image is generated at tmax for this defect, there are not enough pixels available for a well defined center point to develop. The resulting defect boundary is then based on a center point that often li es on the boundary edge. This 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 COVR1/R2 R1 R2

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116 leads to a very high COV for the computed ra dii. A COV of 0.98 was computed for the defect boundary shown in Figure 6-13B. A25 29 29.5 30 O 2 4 6 8 10 12 14 16 18 2 4 6 8 10 12 14 16 A B Figure 6-13. Reduced accuracy in area comput ations due to low image resolution. A) Thermal image for small defect. B) Gradient intensity. Proposed Method for Characterizing Detectability This section outlines a proposed method fo r characterizing defect detectability. The first distinction that will be ma de is based on the shape of the Tdef vs. time plot. There were four basic shapes encountered during the IRT inspections of the Series A specimens. Each defect in these specimens will be classified Level I, Level II, Level III, or Level IV based on the shape of its Tdef vs. time plot. Leve l I defects assume a positive slope for t > 0 and achieve a single maximum value at t = tmax. Level II curves begin with a negative slope and reach a local minimum at t = tb. After reaching the local minimum, the curve assumes a positive slope for t > tb until a distinct local maximum is reached at t = tmax. Level III curves begin with a negative slope and never assume a positive slope. There is, however, a distinct tb that is recognizable when the difference in temperature on the perimeter of the defect s defining area is subtracted from the Tdef vs. time plot. This can also be recognized as an inflection point in the Tdef vs. time plot. The final classification, Level IV, is intended to describe defects that were not detected. A graphical depiction of each cl assification is provided in Figure 6-14.

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117 A second distinction is made based on the C OV of the computed radii of the defect using the gradient area method. A new quantity COV is introduced to describe the difference between the computed COV for the defect and the inherent COV for the shape of the defect. For circular defects, the computed COV and COV are equal since the inherent COV for a circle is zero. The inhere nt COV for an ellipti cal defect is obtained from the graph provided in Figure 6-11. The first category, A, is intended for welldefined defects whose COV values are less than 0.21. The second category, B, describes defects that are moderately defined by the gradient image and have COV values ranging between 0.21 and 0.4. The fina l category, C, is for poorly defined defects whose COV values are between 0.41 and 1.0. These categories are summarized in Table 6-4. This classification system provides 12 unique categories to describe general defect detectability. Detectability is influenced by a number of factors: defect size, defect composition, defect depth below the surface, material properties of the composite, and the heating method employed during the inspecti on. This classification system will make it possible to discuss detectabil ity from a broader perspective. For example, suppose that Defect E75 on Specimen A-3 was classified as Level III-C using the pulse heating method. During the step-heating method with a 60 sec pulse duration, however, the same defect was classified as a Level II-B. This would represent an improvement on the detectability scale. By repeating this process for all of the defects implanted in the Series A specimens (28 defects inspected using 5 heating methods = 140 obs ervations), it will be possible to assess detectab ility for each heating method.

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118 A B C D Figure 6-14. Detectabilit y classification based on Tdef vs. time plot Table 6-4. Detectabilit y classification based on COV of computed radii COV Classification 0 0.20 A 0.21 0.40 B 0.41 1.0 C Experimental Results: Fl ash Heating (Series A) The following defects were considered in this analysis: Air-filled Interface Bubble (IB) 19 mm diameter (A75) 12.7 mm diameter (A50) 6.4 mm diameter(A25) Epoxy-filled 19 mm diameter(E75) 12.7 mm diameter(E50) 6.4 mm diameter(E25) Tdef time Tdef time Level I Level II Tdef time Tdef time Level IV Level III

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119 A Tdef vs. time plot was constructed for each of the defects and the important parameters identified above were extracted from each curve. The following sections contain specific results for each specimen. Specimen A-1 Figure 6-15A provides a thermal image coll ected at t = 12 sec for Specimen A-1. The four air-filled defects are identified in this figure with boxes and the color scale for the thermal image is set to span the entire range of temperatures in the image. Figure 615B identifies the epoxy-filled defects. The colo r scale in this image has been set to span only the range of temperatures encountere d in the box drawn around Defect E75 (19 mm diameter epoxy-filled). All four of the air-f illed defects were detected (Lev el I or Level II according to the classification system described in the previous section). The largest Tdef was obtained for the interface bubble (IB). The Tmax value for IB was 3.3C and was recorded at t = 12 sec. The half-life for De fect IB was 38 sec. Defect A75 had a slightly lower Tmax (2.8C) than IB. This value was also reached in a shorter amount of time after the heat was removed (tmax = 10 sec). The half-life of Defect A75 (28 sec) was also less than IB. Results for defects A50 and A25 are summarized in Table 6-5. The three epoxy-filled defects were also detected. The Tmax value recorded for Defect E75 was 1.2C. The corresponding tmax and t1/2 for this defect were 14 sec and 47 sec, respectively. Defects E50 and E25 displa yed a similar trend to that observed for the air-filled defects: as the ar ea of the defect decreased, Tmax, tmax and t1/2 also decreased.

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120 IB A75 A50 A2512 25 26 27 E75 E50 E2512 24 24.5 25 A Air-Filled Defects B Epoxy-Filled Defects 100 101 102 0 0.5 1 1.5 2 2.5 3 3.5 Tdef (C)Time (sec) IB A75 A50 A25 100 101 102 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Tdef (C)Time (sec) E75 E50 E25 C D Figure 6-15. Flash heating results for Specime n A-1. A) Thermal image scaled for airfilled defects (t = 12 sec). B) Thermal image scaled for epoxy-filled defects (t = 12 sec). C) Tdef vs. time plot for air-filled defects. D) Tdef vs. time plot for epoxy-filled defects. Table 6-5. Flash heatin g results for Specimen A-1 Defect Defect Mat. Tper (C) Tmax (C) tb (sec) tmax (sec) t1/2 (sec) SBR IB Air 7.1 3.3 0 12 38 40.9 A75 Air 7.0 2.8 1 10 28 33.5 A50 Air 7.0 2.5 0 8 18 23.3 A25 Air 5.8 0.8 2 6 12 3.5 E75 Epoxy 6.7 1.2 1 12 47 6.6 E50 Epoxy 6.4 0.7 3 14 25 6.2 E25 Epoxy 5.3 0.4 6 5 20 1.9 Area computations were performed for each defect following the procedure that was previously outlined and results are summarized in Table 6-6. The column labeled Actual diameter provides the true average diam eter of the defect. For Defect IB, this was based on two length measurements: one in the direction of the fibers and one

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121 perpendicular to the fibers. The column la beled Actual Size provi des the true area of the defect based on the diameter measurements. The column labeled Image Diameter provides the average diameter computed for the defect usi ng the gradient area method. For the camera configuration used in the pulse heating experiments, the length ratio for each pixel was 1.1 mm/pixel. For the circular shaped air-filled defects (A75, A50, and A25), the average diameter computed using the gradient area method was within the distance represented by 1 pixel of the true diameter. The COV value computed for each of these defects varied considerably. The gradient images that were generated for def ects A75, A25, E75 and E75 are provided in Figure 6-16. These four defects all fall with in a different general detectability category based on the COV for the computed radii. The COV computed for Defect A75 was 0.10 (Category A) and the COV for Defect A25 wa s 0.28 (Category B). This difference in COV would indicate that the boundary generated for A75 was more consistent than the boundary determined for A25. This is not necessarily supported by visual inspection of the gradient images. The higher COV for De fect A25 can be attributed to a slight misalignment of the defects center point. De fect E75 had a COV of 0.20 (technically a Category A). The largest source of variation in the defects boundary occurs on the right side of the gradient image. This is likely due to a combination of weak signal and other imperfections in the composite above the defect. The gradient image generated for Defect E25 is provided in Figure 6-16D. This defect is poorly defined and the computed radius has a high COV of 0.79 (Category C). These results demonstrate that COV can pr ovide important insight into how well a defects boundary has been defined by the gr adient area method. It must be noted,

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122 however, that a visual inspection of the generated defect boundary is still required when interpreting the results. O O A A75: COV = 0.10 B A25: COV = 0.28 O O CE75: COV = 0.20 DE25: COV = 0.79 Figure 6-16. Gradient images for defects. A) A75 with COV = 0.10. B) A25 with COV = 0.28. B) E75 with COV = 0.20. D) E25 with COV = 0.79. Table 6-6. Gradient ar ea method results for Specimen A-1: flash heating Defect Actual diameter (mm) Actual size (cm2) Image diameter (mm) COV Image area (pixels) Image area (cm2) IB 34.0 7.1 29.2 0.33 542 6.3 A75 19.1 2.8 18.3 0.1 214 2.5 A50 12.7 1.3 12.2 0.11 91 1.0 A25 6.4 0.3 6.9 0.28 29 0.3 E75 19.1 2.8 21.3 0.2 312 3.6 E50 12.7 1.3 12.4 0.08 88 1.0 E25 6.4 0.3 8.6 0.79 54 0.6 Figure 6-17 provides bar charts for th e important parameters listed in Table 6-5. Results for the largest defect (IB) are presente d on the left side of the lower axis and

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123 results for the smallest defect ar e shown on the right. Values for Tmax increase with respect to defect diameter. Th e air-filled defects also produced consistently larger values of Tmax than epoxy-filled defects of the same size. Figure 6-17B provides the normalized Tmax vs. defect diameter. This value was computed by dividing the maximum defect signal strength by the temperature increase experienced by the perimeter of the area used to define the defect ( Tper in Table 6-5). Time to max and half-life of the signal also displa yed an upward trend with increasing defect diameter. For these qua ntities, however, the epoxy-filled defects generated consistently larger values than air-filled defects of the same diameter. 0 0.5 1 1.5 2 2.5 3 3.5 IBA/E-75A/E-50A/E-25Defect Tmax Air Epoxy 0.00 0.10 0.20 0.30 0.40 0.50 IBA/E-75A/E-50A/E-25Defect Tmax / Tper Air Epoxy A B 0 5 10 15 IBA/E-75A/E-50A/E-25Defecttmax Air Epoxy 0 10 20 30 40 50 IBA/E-75A/E-50A/E-25Defectt1/2 Air Epoxy C D Figure 6-17. Specimen A-1: Important paramete rs for defects with different diameters. A) Tmax. B) Normalized Tmax. C) tmax. D) t1/2.

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124 Specimen A-2 Specimen A-2 was constructed using two layers of carbon-fiber composite. The thermal image shown in Figure 6-18B was recorded at t = 16 sec. At this point in time, the boundaries of defects IB, A75, and A50 are defined in the thermal image. The exact time that Defect E75 becomes visi ble in the series of thermal images is not apparent. Figure 6-18C shows the thermal image at t = 16 sec with the color scale modified to encompass the temperature valu es encountered in the area defining E75. Some traces of the defect patte rn are visible inside of area E75, but similar patterns can also be detected out side of the area. Figure 6-18D shows the thermal image taken at t = 38 sec. The boundaries of E75 and E50 are better defined at this time at it is possible to say that the defects have been detected. A1 A2 25 26 27 28 29 A1 A2 A3 24 24.5 25 25.5 26 26.5 A B A1 A2 A3 E1 24.4 24.5 24.6 24.7 24.8 A1 A2 A3 E1 E2 23.5 24 24.5 25 A E t = 38 sec Figure 6-18. Thermal images for Specimen A-2: flash heating. A) t = 2 sec. B) t = 16 sec. C) t = 16 sec. D) t = 38 sec. The Tdef vs. time plots for the seven def ects of interest are provided in Figure 619. Data for the air-filled defects (shown in Figure 6-19A) provide additional insight into

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125 the observations made while inspecting the th ermal images. The curve for Defect IB assumes a positive slope at t = 0 sec and reaches a maximum value of 2.4C at t = 7 sec. The curve generated for Defect A75 also begins with a positive slope at t = 0. A local maximum of 1.1C occurs at t = 4 sec followe d by a local minimum at t = 9 sec. From there, the curve begins climbing again until a final local maximum of 1.1C is reached at t = 20 sec. The curve for Defect A50 displays a noticea bly different trend. The initial value for Tdef at t = 0 sec is 0.9C. The initial slope for this curve is negative and a local minimum of 0.5C is reached at t = 10 sec. At t = 23 sec, another local maximum is reached (0.7C). The most likely explanation for the differences in these three curves is illustrated in Figure 6-20. During specimen construc tion, it is possible that several unintentional defects were cr eated between the two layers of FRP composite. These unintentional defects, being cl oser to the surface than the interface defects, appear at earlier times in the thermal images. The ma ximum value for Defect IB and the initial local maximum experienced by A75 both occur at approximately the same time as the interface defects detected in Speci men A-1 (single-layer carbon). Defects E75 and E50 were also detected. A Tdef vs. time plot is provided in Figure 6-19B. The Tmax recorded for E75 and E50 was 0.5C (t = 40 sec) and 0.3C (t = 38 sec), respectively. The two 6.4 mm diameter defects were not detected Plots for Tmax, normalized Tmax, tmax, and t1/2 are provided in Figure 6-21. The general trends observed for Specimen A-1 we re also observed for Specimen A-2. The air-filled defects had consistently higher values for Tmax and normalized Tmax than the epoxy-filled defects. Three quantities that did not follo w the expected trends: tmax and

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126 t1/2 for Defect IB and tmax for Defect A75. These discrepancies are explained by the fact that small inter-lamina defect s developed between the first and second layers of the FRP composite. 100 101 102 0 0.5 1 1.5 2 2.5 Tdef (C)Time (sec) IB A75 A50 A25 100 101 102 0 0.2 0.4 0.6 0.8 1 Tdef (C)Time (sec) E75 E50 E25 A B Figure 6-19. Flash heating results for Specime n A-2: temperature vs. time data. A) Airfilled defects. B) Epoxy-filled defects. Table 6-7. Summary of Results for Specimen A-2: flash heating Defect Defect Mat. Tper (C) Tmax (C) tb (sec) tmax (sec) t1/2 (sec) SBR IB Air 7.3 2.4 0 7 55 19.1 A75 Air 7.1 1.1 8 20 47 12.4 A50 Air 7.1 0.7 7 23 35 16.4 A25 Air 5.8 E75 Epoxy 6.9 0.5 13 40 98 8.6 E50 Epoxy 6.5 0.3 17 38 50 6.7 E25 Epoxy 5.3 Figure 6-20. Unintentional defect s between layers in Specimen A-2 Defect A2 Defect A1 Defect A3 Unintentional Defects

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127 0 0.5 1 1.5 2 2.5 3 IBA/E-75A/E-50A/E-25Defect Tmax Air Epoxy 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 IBA/E-75A/E-50A/E-25Defect Tmax / Tper Air Epoxy A B 0 10 20 30 40 50 IBA/E-75A/E-50A/E-25Defecttmax Air Epoxy 0 20 40 60 80 100 120 IBA/E-75A/E-50A/E-25Defectt1/2 Air Epoxy C D Figure 6-21. Specimen A-2: Important paramete rs for defects with different diameters. A) Tmax. B) Normalized Tmax. C) tmax. D) t1/2 Specimen A-3 Specimen A-3 was constructed using thr ee layers of FRP co mposite. Thermal images and Tdef vs. time plots are provided in Figure 6-22. Only the air-filled defects IB, A75, and A50 developed a significant signal in the Tdef vs. time plots. The Tmax value recorded for these defects was 0.9C (t = 61 sec), 0.6C (t = 54 sec), and 0.3C (t = 36 sec), respectively. The signal that developed for Defect E75 ( 19 mm dia. epoxy-filled) was very weak. The thermal image in Figure 6-22B does indicate a defect and the Tdef vs. time plot (Figure 6-22D) also indicates a small signal. Based on a Tthresh value of 0.2C (Figure 6-6), this defect was not detected.

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128 IB A75 A50 A2561 23.6 23.8 24 24.2 24.4 E75 E50 E2540 23.6 23.7 23.8 23.9 24 24.1 A B 100 101 102 0 0.5 1 1.5 2 Tdef (C)Time (sec) IB A75 A50 A25 100 101 102 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Tdef (C)Time (sec) E75 E50 E25 C D Figure 6-22. Thermal images and Tdef vs. time plots for Specimen A-3. A) Thermal image scaled for air-filled defects (t = 61 sec). B) Thermal image scaled for epoxy-filled defect s (t = 40 sec). C) Tdef vs. time plot for air-filled defects. D) Tdef vs. time plot for epoxy-filled defects. Table 6-8. Summary of results for Specimen A-3: flash heating Defect Defect Mat. Tper (C) Tmax (C) tb (sec) tmax (sec) t1/2 (sec) SBR IB Air 6.1 0.9 25 61 136 12.7 A75 Air 6.0 0.6 27 54 91 9.5 A50 Air 5.9 0.3 18 36 60 7.3 A25 Air 4.9 E75 Epoxy 5.9 E50 Epoxy 5.4 E25 Epoxy 4.3 Important parameters from the Tdef vs. time plots are plotted against defect diameter in Figure 6-23. Since the epoxy-filled def ects were not detected, no data are presented for defects E75, E50, or E25. Tmax and normalized Tmax both increase as the diameter of the defect increases. A similar trend is observed for tmax and t1/2.

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129 0 0.2 0.4 0.6 0.8 1 IBA/E-75A/E-50A/E-25Defect Tmax Air Epoxy 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 IBA/E-75A/E-50A/E-25Defect Tmax / Tper Air Epoxy A B 0 10 20 30 40 50 60 70 IBA/E-75A/E-50A/E-25Defecttmax Air Epoxy 0 50 100 150 IBA/E-75A/E-50A/E-25Defectt1/2 Air Epoxy C D Figure 6-23. Specimen A-3: Important paramete rs for defects with different diameters. A) Tmax. B) Normalized Tmax. C) tmax. D) t1/2 Specimen A-4 Specimen A-4 was constructed using four layers of FRP composite. Thermal images and Tdef vs. time plots are provided in Figure 6-24. Only the air-filled defects IB and A75 developed a significant signal in the Tdef vs. time plots. The Tmax value recorded for these defects was 0.6C (t = 86 se c) and 0.4C (t = 72 sec). The half-life for Defect IB was not recorded since the valu e was not reached by the end of the data collection period. The half-life measured for Defect A75 was 146 sec. The gradient image that was generated for Defect IB was not sufficient to generate a suitable defect boundary.

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130 Once again, the signal for the epoxy-filled Defect E75 was questionable. Based on the criteria described for Specimen A-3, none of the epoxy-filled defects were assumed to have been detected. IB A75 A50 A2586 24 24.2 24.4 E75 E50 E2579 23.6 23.7 23.8 23.9 24 A Air-Filled Defects B Epoxy-Filled Defects 100 101 102 0 0.5 1 1.5 2 2.5 Tdef (C)Time (sec) IB A75 A50 A25 100 101 102 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Tdef (C)Time (sec) E75 E50 E25 C D Figure 6-24. Thermal images and Tdef vs. time plots for Specimen A-4. A) Thermal image scaled for air-filled defects (t = 86 sec). B) Thermal image scaled for epoxy-filled defect s (t = 79 sec). C) Tdef vs. time plot for air-filled defects. D) Tdef vs. time plot for epoxy-filled defects. General detectability Up to this point, the pulse heating method has been applied to all four specimens in series A. Tdef vs. time plots were constructed fo r the seven defects contained in each specimen resulting in a total of 28 unique plot s. The previous sections for each specimen highlighted the differences between defects on the same specimen. The objective of this section is to condense these results into a manageable format and develop a basis for determining detectability.

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131 The general detectability classification sy stem was applied to each defect for the pulse heating method. Re sults are summarized in Table 6-9. For the single-layer carbon FRP system, all of the implante d defects received a Level I or Level II classification. This indicates that a quantifiable signal was generated in the Tdef vs. time plots for each defect. For the two-layer FRP composite system, all of the defects at least 12.7 mm in diameter or larger were considered Level I or Level II. The 6.4 mm diameter defects did not develop a significant signal in the Tdef vs. time plots. For the three-layer specimen, only air-filled defects with 12. 7 mm diameters (or larger) developed a signal. Finally, only the IB and 19 mm diameter defect devel oped a significant signal in the four-layer specimen. Detectability will likely improve with the use of a flash system capable of generating a higher intensity heat flux. It may also be possible to improve upon the quality of the thermal images by moving the ca mera closer to the specimen. This could lead to improvement in the classificati on based on the COV of the defect boundary. Table 6-9. General detectabil ity results for flash heating Defect Flash Heating Detectability IB A75 A50 A25 E75 E50 E25 1 I-A I-A I-A II-B I-A II-A II-C 2 I-A II-B II-A IV-C II-C II-C IV-C 3 II-A II-C II-C IV-C IV-C IV-C IV-C Layers 4 II-B II-C IV-C IV-C IV-C IV-C IV-C Signal to boundary noise ratio (SBR) re sults for the pulse heating method are provided in Table 6-10. This quantity is calcul ated by dividing the maximum defect signal strength ( Tmax) by the standard deviation of the temperature profile around the boundary of the area used to define the def ect. Higher values of SBR indicate that a defect is well defined with respect to its surroundings. A low SBR indicates that the

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132 temperature fluctuation resulti ng from the defect is closer in magnitude to temperature fluctuations resulting fr om noise or natural vari ations in the composite. Table 6-10. Signal to boundary noise ratio (SBR) results for flash heating Defect Flash Heating SBR IB A75 A50 A25 E75 E50 E25 1 40.9 33.5 23.3 3.5 6.6 6.2 2.6 2 19.1 12.4 16.4 8.6 6.7 3 12.7 9.5 7.3 Layers 4 10.8 5.1 Defect characterization The objective of this analysis is to determine how well the parameters extracted from each Tdef vs. time plot can be used to characterize detected defects. Figure 6-25A provides a bar chart with the normalized Tmax plotted for each of the air-filled defects. The four bars grouped t ogether above the label on the x-axis were obtained for defects IB, A75, A50, and A25 in the single-la yer carbon specimen (A-1). Notice that there are only three bars provide d for the two-layer sp ecimen (defects IB, A75, and A50). This is because Defect A25 was not detected in the two-layer system. Results for the epoxy-filled defects are provided in Figure 6-25B. The general trend for all defects is that normalized Tmax decreases with FRP system thickness. Figure 6-25 also illustrates how normalized Tmax decreases as the size of the defect decreases for a given FRP system thickness. How can this information be used to char acterize a defect? A ssume that an IRT experiment was performed using the pulse method and 19 mm diameter defect was detected. The normalized Tmax for this defect was measured to be 0.15. These data alone represent two possible scenarios: (1) the defect is air-filled and lies below 2 layers of FRP composite or (2) the defect is epoxy-filled and lies be low 1 layer of FRP composite. A similar observation could be ma de for a 12.7 mm diameter defect with a

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133 normalized Tmax of 0.1. If the system thickness is known, there will be little question about the material composition of the defect If, however, the system thickness is not known a priori, additional information would be needed to characterize the defect. Air-Filled Defects0 0.1 0.2 0.3 0.4 0.5 1234Layers of FRP Tmax/ Tper IB A75 A50 A25 Epoxy-Filled Defects0 0.05 0.1 0.15 0.2 1234Layers of FRP Tmax/ Tper E75 E50 E25 A B Figure 6-25. Normalized Tmax for flash heating. A) Air-fil led defects. B) Epoxy-filled defects. Figure 6-26A provides a bar chart with tmax plotted for each of th e air-filled defects. Recall that tmax is the time between when the heat source was removed and the maximum defect signal strength was obtai ned. The number of FRP layers is labeled on the x-axis. The general trend for the single-layer system is that tmax decreases as the size of the defect decreases. This trend is also observed for the three and four-layer systems. The air-filled defects for the two-layer system do not follow this general trend. This is a result of the unintentional air-filled de fects that occurred between the first and second layer of the FRP system. Results for the epoxyfilled defects are provided in Figure 6-26B. These data are also potentially useful fo r characterizing detected defects. Recall the previous example of a 19 mm di ameter defect with a normalized Tmax of 0.15. Based on the measured value of normalized Tmax it was not possible to determine whether this represents De fect A75 on Specimen A-2 or Defect E75 on Specimen A-1 (both defects have the same size, but the composition and depth are unknown). Suppose

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134 the tmax recorded for this defect was 15 sec (close to tmax for Defect E75, Specimen A-1). This result would suggest that the unknown defect is epoxy-filled and lies beneath one layer of FRP composite. Air-Filled Defects0 20 40 60 80 100 1234Layers of FRPtmax (sec) IB A75 A50 A25 Epoxy-Filled Defects0 10 20 30 40 50 1234Layers of FRPtmax (sec) E75 E50 E25 A B Figure 6-26. Time to maximum signal for fl ash heating. A) Ai r-filled defects. B) Epoxy-filled defects Data for signal half-life (t1/2) displayed the same trend that was observed for tmax. Within each specimen, the values for t1/2 decreased as the size of the defect decreased. As the thickness of the composite increased, the values for t1/2 also increased. Data are provided in Figure 6-27 for the air and epoxy-filled defects. Air-Filled Defects0 50 100 150 200 1234Layers of FRPt1/2 (sec) IB A75 A50 A25 Epoxy-Filled Defects0 20 40 60 80 100 120 1234Layers of FRPt1/2 (sec) E75 E50 E25 A B Figure 6-27. Signal half-life for flash heati ng. A) Air-filled def ects. B) Epoxy-filled defects The bar charts provided above illustrate how defect size and FRP system thickness influence the parameters that were extracted from each Tdef vs. time plot. Table 6-11

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135 contains the ratio of normalized Tmax, tmax, and t1/2 for air-filled and epoxy-filled defects. Results are provided for the 19 mm diameter and the 12.7 mm diameter defects for the one and two layer composite systems. These ratios are omitted for the three and four layer specimens because the epoxy-filled defe cts were not detected using the pulse heating method. Table 6-11. Ratio of parameters for air and epoxy-filled defects NDSS (Air:Epoxy) tmax (Air:Epoxy) t1/2 (Air:Epoxy) Pulse Heating 19 mm 12.7 mm 19 mm 12.7 mm 19 mm 12.7 mm 1 2.21 3.23 0.71 0.67 0.62 0.67 2 2.09 2.15 0.50 0.61 0.48 0.70 3 Layers 4 Experimental Results: Sc an Heating (Series A) The scan heating procedure was outlined in Chapter 5. Since the entire surface area under consideration (all four specimens) was not heated at the same time, each defect was assigned a starting time (t=0) immediately after the heat source moved beyond the respective defect during the scan. Complete Tdef vs. time results for each specim en are provided in Appendix A. Results from the scan heating experiment are summarized in the following sections. Specimen A-1 All of the defects in Specimen A-1 were detected in the thermal images. The airfilled Defect IB generated a Tmax of 18.8C. The smallest air-filled defect (A25) generated a Tmax of 4.0C. These values are considerably higher than those observed in the pulse heating experiments: Tmax of 3.3C and Tmax of 0.8C for IB and A25, respectively. The three epoxy-filled def ects also generated significantly higher Tmax values than were generated in the pulse hea ting experiments. The most likely explanation

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136 for the increase in defect signal strength is the increase in backgr ound temperature due to the applied heat. Scan heating resulted in a Tper value of 17.2C for Defect IB while the pulse heating generated a Tper of only 7.1C. The signal to perimeter noise ratio (SBR) for Defect IB was 14.9. This is considerably less than the value computed fo r pulse heating (40.9). The difference in SBR values is due to a higher standard devi ation around the defect ar ea perimeter in the scan heating results. The value of the perime ter standard deviation is plotted against time in Figure 6-28. 0 5 10 15 20 0 1 2 3 4 perTime (sec) Pulse Heating Scan Heating Figure 6-28. Standard deviation of Defect IB perimeter for pulse and scan heating Area calculations were also performed fo r each defect using the gradient area method. The gradient images that were de veloped for defects A 25 and E25 (both with 6.4 mm diameter) did not generate a well-defi ned defect boundary. This was not a result of weak signal or noise in surrounding pixels. Both the Tdef vs. time plots and thermal images indicate a strong si gnal for both defects (see Figure 6-29). The inability to estimate the size of these def ects is a result of poor spat ial resolution in the thermal image. Recall that the camera distance fr om the surface of the specimens was 60 in. This separation results in a length ratio of 2.2 mm/pixel. If this is the case, only three

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137 pixels are required to span n early the entire diameter of defects A25 and E25. The gradient image computed under these circumstan ces is insufficient to extract a reasonable defect boundary. The gradient images generated for the larg er defects contained did generate welldefined defect boundaries. However, there was mo re error in these calc ulated values than was observed in the pulse heating experiment. The computed area for Defect A75 (19 mm diameter) was 2.1 cm2 (actual area = 2.8 cm2). This represents an error in the area computation of 27.3%. In terms of diamet er, however, the error is only 14.6%. The difference in the actual and computed diameter is 2.8 mm, which is less than the distance represented by two pixels in the thermal image. 0 50 100 150 200 250 0 1 2 3 4 T (C)Time (sec) A4 E3 33 34 35 36 37 A B Thermal image at t = 5 sec Figure 6-29. Data for defects A25 and E25 (6.4 mm diameter). A) Tdef vs. time plot. B) Thermal image at t = 5 sec. Specimen A-2 All of the defects in Specimen A-2 were detected in the ther mal images. Only defects A25 and E25 did not ge nerate sufficient gradient im ages for area computations. Another noteworthy observation for this specimen is in regards to Defect IB. Recall that this defect contained unintentional def ects between the top and second layer of composite. Figure 6-30A provides a thermal image that was collected 2 s ec after the heat

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138 source moved passed Defect IB. The two uni ntentional defects a bove IB are clearly distinguishable in the thermal image. The image taken at t = 18 sec, however, does not distinguish between the unintentional defects and the interface bubble (see Figure 6-30B). Complete results for Specimen A-2 are provided in the appendix. 30 35 40 45 50 55 30 35 40 A B Figure 6-30. Thermal images for Defect IB (Sp ecimen A-2). A) t = 2 sec. B) t = 18 sec. Specimen A-3 Only defects with a diameter of 6.4 mm (defects A25 and E25) did not develop a signal in the Tdef vs. time plots. All of the remaining defects were detected in the Tdef vs. time plots and the gradient images. Comp lete results for Specimen A-3 are provided in Appendix A. Specimen A-4 Defects A50, A25, E50, and E25 were not detected in the Tdef vs. time plots. The remaining defects were detected in the Tdef vs. time plots and the gradient images. Complete results for Specimen A-4 are provided in Appendix A. General Detectability Table 6-12 provides a summary of the genera l detectability results for all defects implanted in the one, two, three, and four-lay er specimens. These data indicate an overall increase in detectability over the flash hea ting method. The scan heating method also resulted in higher classification levels based on the quality of the defect boundary. In the

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139 pulse heating experiments, a to tal of 8 defects were classifi ed as Level A. The scan heating experiments resulted in a total of 13 Level A defects. The number of Level B defects also increased from three to six for the pulse and scan heating methods, respectively. Table 6-12. General detectabil ity results for scan heating Defect Scan Heating Detectability IB A75 A50 A25 E75 E50 E25 1 I-A I-A I-B I-C I-A I-A I-B 2 I-A I-A II-A II-B II-A II-B II-C 3 II-A II-A II-A IV-C III-A III-B IV-C Layers 4 II-A II-B IV-C IV-C III-B IV-C IV-C Signal to boundary noise ratio (SBR) results are provided in Table 6-13. For the one and two-layer specimens, it is interesti ng to note that the SBR for Defect IB is smaller than A75 even though the magnitude of Tmax is larger. This difference can be attributed to non-uniform heati ng resulting from moving the heat source from one side of the specimen to the other. When using the scan heating method, a temperature gradient will always be present on the surface of the specimen running pa rallel to the direction of heat source motion (x-direction). The eff ect that this gradient has on the SBR calculations is magnified for Defect IB since th e length of the rectangl e used to define the defect is larger in the x-direction. Table 6-13. Signal to boundary noise ratio (SBR) results for scan heating Defect Pulse Heating SBR IB A75 A50 A25 E75 E50 E25 1 14.9 22.8 10.5 4.8 13 7.5 5.5 2 7.2 13.7 11.4 3.1 10.7 6.2 3.5 3 14.1 6.1 5.4 2.3 3.3 Layers 4 10.4 3.8 3.0

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140 Defect characterization Normalized Tmax is presented in a bar chart for each defect in Figure 6-31. The same general trend that was observed in the pulse heating experime nts was also observed in the scan heating results: as the thickne ss of the FRP system increase, the normalized Tmax decreases. If the FRP system thickness is held constant, normalized Tmax decreases as the size of the defect decreases. Air-Filled Defects0 0.2 0.4 0.6 0.8 1 1.2 1234Layers of FRP Tmax/ Tper IB A75 A50 A25 Epoxy-Filled Defects0 0.05 0.1 0.15 0.2 0.25 0.3 1234Layers of FRP Tmax/ Tper E75 E50 E25 A B Figure 6-31. Normalized Tmax for scan heating. A) Air-fill ed defects. B) Epoxy-filled defects Figure 6-32 provides bar chart plots for tmax. In general, the time required for the maximum Tdef to develop increases with FRP syst em thickness. These data, however, do not provide consistent results for defects occurring in the same specimen. For the airfilled defects in Specimen A-1, there is no clearly defined relationship between defect size and tmax. The same is true for the air-filled def ects in Specimen A-2, but this is likely explained by the unintentional defects between the first and second layer of composite. For specimens A-3 and A-4, the air-filled def ects do exhibit a well-defined trend in which tmax decreases as the size of the defect decreases. There are insufficient data to compare these results with the pulse heating experi ments since none of these quantities were measured for the three and four-layer specimens.

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141 The epoxy-filled defects also show some interesting tr ends with regards to tmax. For specimens A-1 and A-2, the value of tmax tends to decrease as the area decreases. For Specimen A-3, however, this trend is not observed. The tmax value recorded for the 19 mm diameter defect (E75) was 34 sec while the tmax recorded for the 12.7 mm diameter defect was 38 sec. This discrepancy can be explained by a close examination of the Tdef vs. time plots. For both defects, a large si gnal develops due to nonuniform heating early in the cooling process. This signal gives wa y to the true defect signal very slowly resulting in a Tdef vs. time plot that is extremely flat over the region of the maximum value. Even though the scan heating method wa s sufficient to generate a signal for each of the defects, it is not possible to determine precisely when the maximum Tdef occurs. Air-Filled Defects0 20 40 60 80 100 1234Layers of FRPtmax (sec) IB A75 A50 A25 Epoxy-Filled Defects0 20 40 60 80 100 1234Layers of FRPtmax (sec) E75 E50 E25 A B Figure 6-32. Time to maximum signal for s can heating. A) Air-filled defects. B) Epoxy-filled defects Figure 6-33 provides ba r chart plots of t1/2. For the air-filled defects, a consistent trend is observed in which t1/2 increases with FRP system thickness. Smaller defects also result in consistently lower t1/2 values for a constant FRP system thickness. The same trend is observed for the epoxyfilled defects up to the f our-layer specimen. The t1/2 measured for Defect E75 in Specimen A-4 was 83 sec. The t1/2 measured for Defect E75 in the three-layer specimen was 84 sec.

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142 Air-Filled Defects0 20 40 60 80 100 120 140 1234Layers of FRPt1/2 (sec) IB A75 A50 A25 Epoxy-Filled Defects0 20 40 60 80 100 1234Layers of FRPt1/2 (sec) E75 E50 E25 A B Figure 6-33. Signal half-life for scan heati ng. A) Air-filled defects. B) Epoxy-filled defects s The ratios of Tdef vs. time parameters for epoxy a nd air-filled defects are provided in Table 6-14. Based on these data, the most promising parameter for distinguishing air and epoxy-filled defects is normalized Tmax (NDSS). These values are consistently higher for air-filled defects and the smallest recorded ratio for ai r:epoxy was 1.98. The quantities tmax and t1/2 are also useful para meters when consider ing one and two-layer FRP systems. These values are consistently higher for epoxy-filled defects and result in air:epoxy ratios less than one. For three a nd four-layer systems, however, extracting tmax and t1/2 from each Tdef vs. time plot is not possible. The ratios for ai r:epoxy defects of the same size are not consistent and in some cases have magnitudes greater than 1 (highlighted in Table 6-14). Table 6-14. Ratio of parameters for air and epoxy-filled defects (scan heating) NDSS (Air:Epoxy) tmax (Air:Epoxy) t1/2 (Air:Epoxy) Scan Heating 19 mm 12.7 mm 19 mm 12.7 mm 19 mm 12.7 mm 1 2.53 2.80 0.82 0.71 0.68 0.69 2 2.87 2.46 0.55 0.60 0.59 0.56 3 1.98 2.21 1.12 0.74 0.82 1.04 Layers 4 2.06 0.89 1.05

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143 Experimental Results: Long -Pulse Heating (Series A) Complete results for the l ong-pulse heating experiment s are provided in Appendix A. The following section on general detectability will highlight results from the three pulse durations that were investigated (30 sec, 45 sec, and 60 sec). The section on defect characterization will summarize the results obtained for the 30 sec pulse duration. Results from the 45 sec and 60 sec pulse duration experiments will be discussed in the following section that compares result s from all of the heating methods. General detectability Table 6-15 provides general detectability re sults for the three pulse durations that were considered. These results indicate that pulse duration has very little influence on general detectability. For the single-layer sp ecimen, all of the defects were detected in the Tdef vs. time plots. The gradient images also produced well-defined boundaries for all defects except A25 and E25. For the two-layer specimen, all of the air-filled defects 6.4 mm in diameter or larger were detected in the plots and gradient images. Only epoxy-filled Defect E75 was detected in the plots and gradient images. For Specimen A3, air-filled defects IB, A75 we re detected in the plots and gradient images. Defect A50 and E75 developed very weak signals with a Tmax of 1.4C and 1.1C, respectively. Defects A25, E50, and E75 were not detected Finally, only defects IB and A75 were detected in the four-layer specimen. Defect characterization Figure 6-34 provides data for normalized Tmax for each of the four specimens. The same general trend that was observed for the flash and scan heating experiments was also observed for the long-pulse heating method: Smaller defects have lower normalized Tmax

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144 As FRP system thickness increases, Tmax decreases Epoxy-filled defects have consistently lower normalized Tmax values than airfilled defects Table 6-15. General detectabilit y results for long-pulse heating Defect 30 Sec Pulse IB A75 A50 A25 E75 E50 E25 1 I-A I-A I-A I-C I-A I-A I-C 2 I-A I-A I-B IV-C II-B III-C IV-C 3 II-A II-A III-B IV-C III-C IV-C IV-C Layers 4 II-B II-C IV-C IV-C IV-C IV-C IV-C Defect 45 sec Pulse IB A75 A50 A25 E75 E50 E25 1 I-A I-A I-A I-C I-A I-A I-C 2 I-A I-A I-A IV-C II-A II-C IV-C 3 II-A II-A III-B IV-C III-B IV-C IV-C Layers 4 II-B II-B IV-C IV-C IV-C IV-C IV-C Defect 60 sec pulse IB A75 A50 A25 E75 E50 E25 1 I-A I-A I-A I-C I-A I-A I-C 2 I-B I-A I-B IV-C II-A IV-C IV-C 3 II-A II-A III-B IV-C III-B IV-C IV-C Layers 4 II-A II-C IV-C IV-C IV-C IV-C IV-C Air-Filled Defects0 0.5 1 1.5 1234Layers of FRP Tmax/ Tper IB A75 A50 A25 Epoxy-Filled Defects0 0.1 0.2 0.3 0.4 0.5 1234Layers of FRP Tmax/ Tper E75 E50 E25 A B Figure 6-34. Normalized Tmax for long-pulse heating (30 sec pulse). A) Air-filled defects. B) Epoxy-filled defects. Data for tmax are shown in Figure 6-35 For the air-fille d defects, these data do not indicate a consistent trend with respect to de fect size. On average, however, the values for tmax increase with FRP system thickness. Figure 6-36 provides a bar chart for defect half-life for each specimen. These results do provide consistent trends for the air-filled defects:

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145 Smaller defects have shorter half-lives As FRP system thickness increases, t1/2 increases Air-Filled Defects0 10 20 30 40 50 60 70 1234Layers of FRPtmax (sec) IB A75 A50 A25 Epoxy-Filled Defects0 10 20 30 40 50 1234Layers of FRPtmax (sec) E75 E50 E25 Figure 6-35. Time to maximum signal for longpulse heating. A) Air-filled defects. B) Epoxy-filled defects. Air-Filled Defects0 50 100 150 1234Layers of FRPt1/2 (sec) IB A75 A50 A25 Epoxy-Filled Defects0 20 40 60 80 1234Layers of FRPt1/2 (sec) E75 E50 E25 Figure 6-36. Signal half-life fo r long-pulse heating. A) Air-filled defects. B) Epoxyfilled defects. Comparison of Heating Methods Three of the heating methods described in Chapter 4 (flash, scan and long-pulse) were applied to the four specimens in Series A. Three pulse durati ons were investigated for the long-pulse heating method. Data coll ection for all heating methods consisted of capturing thermal images at a rate of 1 fram e per second for a total of 240 seconds. The start time for recording data occurred imme diately after the heat source was removed. These data were used to compare the performance of each heating method on specimens containing one, two, three or four layers of FRP composite. Each specimen

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146 contained implanted defects of varying size and material composition. The discussion of this analysis will be divided into two part s: (1) general detectability and (2) defect characterization. General Detectability Defect detectability is compared fo r the different heating methods in Figure 6-38 (legend is provided in Figure 6-37). Only eight of the 12 possible detectability levels were encountered. These eight levels were grouped into three cat egories representing high detectability, medium detectability, and not detected. High detectability was assigned to defects receiving the classificati on levels I-A, I-B, II-A, and II-B. This category contains defects that were well-defined in Tdef vs. time plots and at least moderately defined in the gradient images. Medium detectability was assigned to defects classified as I-C, II-C, or III-B These defects are approaching the limits of detection. The final category, not-detected, wa s assigned to level IV-C defects. Based on these criteria, th e scan heating method outperformed the flash and longpulse heating methods from the standpoint of general detect ability. For the one and twolayer specimens, all seven def ects were categorized as medi um or high. For the threelayer specimen, only the 6.4 mm diameter de fects were not detected, and for the fourlayer specimen only defects w ith 12.7 mm diameters or smaller were not detected. Figure 6-37. Legend for Figure 6-38 IB A75 A50 A25 E75 E50 E25 High (I-A, I-B, II-A, II-B) Medium (I-C, II-C, III-B) Not-Detected (IV-C) Legend for Detectability Categories (schematic of specimen provided for reference)

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147 Figure 6-38. Summary of gene ral detectability for flash, scan, and long-pulse heating IB A75 A50 A25 E75 E50 E25 4 1 2 3 La y ers Defect ID Step 60 sec Heating IB A75 A50 A25 E75 E50 E25 4 1 2 3 La y ers Defect ID Step 45 sec Heating IB A75 A50 A25 E75 E50 E25 4 1 2 3 La y ers Defect ID Step 30 sec Heating IB A75 A50 A25 E75 E50 E25 4 1 2 3 La y ers Defect ID Scan Heating IB A75 A50 A25 E75 E50 E25 4 1 2 3 La y ers Defect ID Pulse Heating

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148 Another important measure of detectability is Tmax. Defects with higher values of Tmax are more likely to be de tected for several reasons: Resolution of the camera Noise in the image Easier to see in color scale Figure 6-39 provides a series of bar charts for three of the defects that were examined: IB, A75, and E75. These defects were chosen since they provide a reasonable representation of the behavior that was observed for all defects. The x-axis of these charts denotes the heating method a nd the y-axis provides the measured Tmax for the defect. Results for Specimen A-1 are provided in Figure 6-39A. The highest value of Tmax observed for Defect IB was obtained usi ng the scan heating method (18.8C). The lowest value for Tmax was obtained with the pulse heat ing method (3.3C). The three different pulse durations th at were investigated for the long-pulse heating method provided different results. There is a noticeable tr end of increasing Tmax with increasing pulse duration ( Tmax = 12.7C, 16.9C, and 18.1C for the 30, 45, and 60 sec pulse durations, respectively). The 19 mm diamet er defects followed a similar trend. A similar trend was observed for the tw o, three, and four-layer specimens. The scan heating method produced the highest values for Tmax. The smaller diameter defects produced lower Tmax values Epoxy-filled defects produced lower Tmax values It should be mentioned again that the re sults for Defect IB on Specimen A-2 are influenced by the unintentional defects betw een the top and second layer of composite. The Tmax values presented in Figure 6-39B for Defect IB actually represent smaller airfilled defects that are closer to the surface.

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149 0 5 10 15 20 PulseScanS-30S-45S-60Heating Method Tmax IB A75 E75 0 5 10 15 20 PulseScanS-30S-45S-60Heating Method Tmax IB A75 E75 A B 0 1 2 3 4 5 6 PulseScanS-30S-45S-60Heating Method Tmax IB A75 E75 0 1 2 3 4 5 6 PulseScanS-30S-45S-60Heating Method Tmax IB A75 E75 C D Figure 6-39. Comparison of Tmax for different heating methods A) Specimen A-1. B) Specimen A-2. C) Specimen A-3. D) Specimen A-4. An interesting comparison can be made between the Tmax results for Specimen A1 and A-4. For Defect IB on Specimen A-1, the Tmax values for the scan and step 60 heating methods are very close (relative diffe rence = 3.8%). For Defect IB on Specimen A-4, the relative difference is much larger (62%). The potential significance of this observation is that the scan heating met hod provides an increasing advantage over the long-pulse heating method as the thickness of th e composite increases. This trend is not observed for Defect A75. The relative differe nce between the scan and step 60 values for Specimen A-1 was 35%. For Specimen A4, the relative difference was 40%. For a given defect, Tmax values are influenced by tw o factors: intensity of the applied heat flux and hea ting duration. The rela tively high values for Tmax that were

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150 observed for the scan heating method are due to the high intensity of the applied heat flux. This is an important result because it demonstrates that scan heating is the most effective means for raising the temperature on the surface of a composite in a short period of time. The higher increase in surface temperature then translates into a higher Tmax for a given defect. Normalized Tmax ( Tmax/ Tper) is a more useful parame ter for investigating the effects of pulse duration on def ect detectability. By dividing Tmax by the average temperature increase that was experienced by the perimeter of the defect boundary, the effects of heat flux intensity are removed. Normalized Tmax results for defects IB, A75, and E75 are provided in Figure 6-40. These results indicate that there is an adva ntage to using longer pulse durations. It is not clear from these data, however, at wh at point the system reaches steady-state. Consider what would happen if the pulse dura tion was increased to infinity. At some point in time, the system would achieve a steady-state condition and the normalized Tmax would reach a maximum value. There are insufficient data to identify how close the 60 sec pulse duration is to a steady-sate condition. For Defect IB on specimens A-1, A-2, A-3 and A-4, normalized Tmax appears to be leveling off as the pulse duration increases. These data are insufficient to develop a mathematical relationship for normalized Tmax as a function of pulse duration, defect depth, defect size, and defect material composition. However, it is possible to use these data to help determine how a particular FRP composite system should be inspected. Results from the following section on defect characterization suggest that a Tmax of 2.0C is a reasonable minimum value that is

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151 required to estimate the size and depth of a defect. The normalized Tmax plots indicate the required Tper that must be generated to develop this signal for a specific type of defect. For example, assume that an IRT inspection is going to be conducted on a threelayer carbon FRP system and the smallest defect of interest is 19 mm in diameter. If a long-pulse setup is going to be used with a 30 sec pulse duration, the minimum required temperature increase for the defect-free area ( Tper) is 14.8C. Once this quantity has been obtained, it becomes a matter of determ ining the appropriate lamp intensity and lamp configuration required to gene rate the 14.8C temperature increase. 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 FlashScanLP-30LP-45LP-60Heating Method Tmax/ Tper IB A75 E75 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 FlashScanLP-30LP-45LP-60Heating Method Tmax/ Tper IB A75 E75 A B 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 FlashScanLP-30LP-45LP-60Heating Method Tmax/ Tper IB A75 E75 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 FlashScanLP-30LP-45LP-60Heating Method Tmax/ Tper IB A75 E75 C D Figure 6-40. Comparison of normalized Tmax for different heating methods. A) Specimen A-1. B) Specimen A-2. C) Specimen A-3. D) Specimen A-4.

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152 Defect Characterization Defect size The gradient area method was previously id entified as a means for determining the size of a detected defect. The coefficient of variation of the computed radii was also identified as a means for assessing the quali ty of the defect bo undary in the thermal image. Low COV values indicate a we ll-defined boundary while high COV values indicate a poorly defined boundary. C OV results for the three defects under consideration are provided in Figure 6-41. For the air-filled defects in the single-layer specimen (A-1), each of the heating methods produced a COV of less than 0.1 (indicating high detectability). There does not appear to be any strong advantage afforded to a particular heating method. The highest C OV value for the epoxy-filled defect (E75) was 0.2. This value occurred during the pulse heati ng experiment and is likely a result of the weak signal that developed for this defect ( Tmax = 1.2C). The general trend for the remaining specimens is that radii COV tends to increase with specimen thickness. This is logical since the overall signal strength for de fects tends to decrease with FRP thickness. Figure 6-42 provides a plot of Tmax vs. radii COV for each of the detected defects in series A. If all of the defects had been detected for each of the heating methods, this plot would contain a total of 140 data points (7 defects x 4 specimens x 5 heating methods). Since a number of defects were classified as undetected, Tmax and COV values were only recorded fo r 92 points. The plot in Figure 6-42 offers interesting insight into the relationship between Tmax and radii COV. These data indicate that high radii COV values are most likely to occur if Tmax is less than 2.0C. If Tmax is greater than 2.0C, the radii COV tends to be less than 0.2 (indicating a well-defined defect boundary). This observation has the poten tial of simplifying the detectability

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153 classification structure that wa s developed previously in th is chapter. Instead of 12 different detectability levels (four based on Tdef vs. time plot characteristics and three based on radii COV), it may be more effici ent to classify defects as follows: Well defined defects have a Tmax > 2.0C Poorly defined defects have a Tmax < 2.0C 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 PulseScanS-30S-45S-60Heating Methodradii COV IB A75 E75 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 PulseScanS-30S-45S-60Heating Methodradii COV IB A75 E75 A B 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 PulseScanS-30S-45S-60Heating Methodradii COV IB A75 E75 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 PulseScanS-30S-45S-60Heating Methodradii COV IB A75 E75 C D Figure 6-41. Coefficient of variation (COV) of computed radii for different heating methods. A) Specimen A-1. B) Speci men A-2. C) Specimen A-3. D) Specimen A-4. Using tmax to determine depth and material composition The first Tdef vs. time plot parameter that will be considered for estimating defect depth is tmax. A model for predicting defect depth based on tmax is provided in Shepard et al. (2003): max* 2 *t depth nce circumfere (6-2)

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154 0 2 4 6 8 10 12 14 16 18 20 00.20.40.60.81radii COV Tmax Figure 6-42. Maximum signal vs. radii COV for all detected defects in Series A In this equation, is the thermal diffusivity of the material above the defect. This model assumes that the volume of material above the defect acts like a heat trap. Excess heat that builds up in this volume can only escape through the curved surface of the imaginary cylinder above the defect. tmax corresponds to the amount of time required to fill-up this volume. As a result, smaller defects at the same depth will have a shorter time to Tmax. Similarly, if two defects of th e same size are considered, the one closer to the surface will have a smaller tmax. Since this model is based on filling a vol ume above a defect with heat, the time associated with the pulse duration will have an impact on the time at which Tmax occurs. As a result, the data generated using the di fferent pulse durations can not be analyzed using this model. This is illustrated by the bar charts in Figure 6-43. For the single-layer specimen (Figure 6-43A), the tmax recorded for Defect IB during the flash heating experiment was 12 sec. The same defect resulted in a tmax of 2 sec for the 60 sec pulse duration. A similar trend is observed for th e remaining defects in all four specimens.

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155 The material composition of the defect also may restrict the use of this model. The tmax values obtained for epoxy-filled defects were consistently larger than the values obtained for air-filled defects of the same size. This could be a result of the thin layer of thickened epoxy that was used during specimen construction. Even though this layer is relatively thin compared to the FRP, the ther mal diffusivity of the epoxy is much lower. This would result in a larger volume of material above the defect that must be filled (and hence more time). 0 10 20 30 40 50 FlashScanLP-30LP-45LP-60Heating Methodtmax IB A75 E75 0 10 20 30 40 50 FlashScanLP-30LP-45LP-60Heating Methodtmax IB A75 E75 A B 0 20 40 60 80 100 FlashScanLP-30LP-45LP-60Heating Methodtmax IB A75 E75 0 20 40 60 80 100 FlashScanLP-30LP-45LP-60Heating Methodtmax IB A75 E75 C D Figure 6-43. Time to maximum signal for di fferent pulse durations (selected defects only). A) Specimen A-1. B) Speci men A-2. C) Specimen A-3. D) Specimen A-4. The model outlined by Shepard et al (2003) was applied to the data from the flash heating experiments. Only the air-filled defect s that were detected in the one, two, three, and four-layer specimens were considered. De fect IB in Specimen A-2 was also removed

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156 from the data set since this defect containe d unintentional air voids between the first and second layer of FRP. Figure 6-44 provides a plot of de fect circumference C x defect depth D vs. tmax. A linear trend is evident, but there is considerable scatter in the data (R2 = 0.81). The slope of the trend line (4.96 mm2/sec with 95% confidence interval from 3.18 to 6.75 mm2/sec) corresponds to a thermal diffusivity ( ) of 7.02 mm2/sec. This value is 3.4 times greater than the value used by Starnes et al (2003) to model the response of an air-filled defect impl anted beneath a precured carbon-fiber epoxy laminate and 16.7 times great er than the diffusivity va lue for carbon FRP provided in Maldague (2001). y = 4.9619x 4.544 R2 = 0.81450 100 200 300 400 500 600 020406080100tmax (sec)C x d (mm2) Figure 6-44. Defect circumference (C) x depth (d) vs. tmax for flash experiments (airfilled defects only) It should be noted that the Tdef vs. time plots that were used to extract the tmax values in Figure 6-44 had relatively low Tmax values (weak signals). Only three of the 11 data points came from signals with Tmax greater than 2.0C. It was previously noted that it was not possible to obtain a precise value for tmax for these relatively weak signals. This could help to explain some of the scatter in the data.

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157 These data do not support using tmax as an indicator of defect depth and material composition. In order to eliminate the eff ects of pulse duration, the surface should be heated using a flash system. This type of heating resulted in the lowest levels of detectability and the weakest signals for defects. The m odel proposed by Shepard et al (2003) also fails to account for the material composition of a defect. According to this model, the tmax for an air and epoxy-filled defect w ith the same diameter at the same depth should be the same. Observations from the flash heating experiment do not support this conclusion. Using t1/2 for determining depth and material composition The second parameter considered for dete rmining depth and material composition was defect half-life (t1/2). Unlike tmax, t1/2 did not appear to be influenced significantly by pulse duration. Results for each of the specimens in Series A are provided in Figure 645. The model presented by Shepard et al (2003) can be modified as follows to accept t1/2 as an input: 2 / 1* *t A depth nce circumfere (6-3) In this equation, A represents an unknown constant In the previous model, tmax was described as being proportional to the time required to fill-up the volume above the defect. This explanation does not seem correct since the time required to fill-up the volume is independent of the size of the de fect (assuming that th e surface experiences uniform heating in the vicinity of the defect). If however, the half-lif e of the signal can be described as the time required for the exce ss heat retained above the defect to drain into the surrounding defect-free area, it is logical to include both the depth and size

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158 parameter in the model. In this case, th e size parameter is the defect circumference which is directly proportiona l to the defect diameter. 0 20 40 60 80 100 120 FlashScanLP-30LP-45LP-60Heating Methodt1/2 IB A75 E75 0 20 40 60 80 100 120 FlashScanLP-30LP-45LP-60Heating Methodt1/2 IB A75 E75 A B 0 20 40 60 80 100 120 140 160 FlashScanLP-30LP-45LP-60Heating Methodt1/2 IB A75 E75 0 20 40 60 80 100 120 140 160 FlashScanLP-30LP-45LP-60Heating Methodt1/2 IB A75 E75 C D Figure 6-45. Signal half-life for different pu lse durations (selected defects only). A) Specimen A-1. B) Specimen A-2. C) Specimen A-3. D) Specimen A-4. Figure 6-46 provides a plot of defect circumference C x defect depth D vs. t1/2. The points on this graph represent data from all of the heating methods for air-filled defects with Tmax values greater than 2.0C. The sample size for this analysis was 30. A linear regression analysis result ed in a slope of 4.33 mm2/sec (R2 = 0.97, 95% confidence interval = 4.03 to 4.63). This value is surprisingly close to slope obtained in the C x d vs. tmax regression analysis. The re sulting curve-fit for the t1/2 data, however, is much better as evidenced by the higher R2 value (0.81 vs. 0.97) and narrower 95% confidence interval for the slope. A second regression analysis was performed for the t1/2 data in which the

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159 y-intercept was forced to zero. The resulting slope was 3.6 mm2/sec (R2 = 0.92, 95% confidence interval = 3.35 to 3.85). y = 4.3301x 52.193 R2 = 0.96960 100 200 300 400 500 600 050100150t1/2 (sec)C x d (mm2) Figure 6-46. Plot of defect circumference C x depth D vs. t1/2 for all heating methods (air-filled defects with Tmax > 2.0C) A similar analysis was performed for the e poxy-filled defects in Series A. The plot of C x d vs. t1/2 is provided in Figure 6-47. Only defects with Tmax greater than 2.0C were considered in the analysis. Since Tmax values for epoxy-filled defects tend to be lower than air-filled defects, the sample size for this analysis was limited to 9 observations. The slope determined by th e linear regression analysis was 1.81 mm2/sec (R2 = 0.99, 95% confidence interval = 1.64 to 1.99). The physical explanation for why epoxy-fille d defects have a longer half-life than air-filled defects was not found in the literatur e. A possible explan ation is that epoxyfilled defects store more energy and hence the defect material maintains a higher temperature for longer periods of time than the air-filled defects. Under this scenario, the heated epoxy acts like a heat s ource after the heat stored in the material above the defect drains away to the perimeter.

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160 y = 1.8145x 9.9089 R2 = 0.98820 20 40 60 80 100 120 140 020406080t1/2 (sec)C x d (mm2) Figure 6-47. Plot of defect circumference C x depth D vs. t1/2 for all heating methods (epoxy-filled defects with Tmax > 2.0C) Determining Depth and Material Comp osition Using Pulse Analysis Data Data from the pulse analysis experime nts suggests that defect half-life (t1/2) is a strong indicator of defect depth and material composition. To apply the model described in Equation 5-3, the only qua ntities that must be known ar e the size (diameter) of the defect and half-life. It was shown that if a Tmax value of 2.0C or greater is obtained for a defect, the gradient area method will provide an accurate estimate of defect size. This minimum Tmax of 2.0C also ensures that the t1/2 measured for a defect is accurate. An example will be presented to illustrate how this model can be applied to IRT data. The thermal image provided in Figure 6-48 was collected during a long-pulse heating experiment (60 sec pulse duration) th at was conducted for specimens from Series B and Series C. Tdef vs. time plots were generated for the four defects labeled A1, A2, A3, and A4 using the procedure that was de veloped for the Series A specimens. The Tdef vs. time plot is provided in Figure 6-49 and the relevant parameters have been summarized in Table 6-16.

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161 Figure 6-48. Thermal image from long-pulse experiment (Series B and C specimens) 100 101 102 0 5 10 15 20 Tdef (C)Time (sec) A1 A2 A3 A4 Figure 6-49. Defect signal vs. time plot for defects shown in Figure 6-48 These are sufficient data to en ter the plots of C x d vs. t1/2. There are two possibilities that must be consid ered for each defect: (1) the de fect is air-filled and (2) the defect is epoxy-filled. To check the ai r-filled assumption, an estimate of C x d (circumference x depth) is computed using the best-fit line shown in Figure 6-46. Since the defect diameter was also measured from the thermal image, the depth of the defect can be estimated. The proce ss is repeated for the epoxyfilled assumption using the A1 A2 A3 A4

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162 equation shown in Figure 6-47. The results from bot h estimates are summarized in the columns labeled Air Depth and Epoxy Depth in Table 6-16. At this point, the air vs. epoxy question is still unanswered. Cons ider the results for Defect A1. Based on the t1/2 and defect diameter measured during the long-pulse experiment, this defect is either air-filled with a depth of 0.98 mm OR epoxy-filled at a depth of 0.50 mm. If information about the th ickness of each layer in the FRP system is known, it is relatively straightfo rward to deduce which estimate is accurate. In this case, where 1 mm thick layers were applied, the co rrect conclusion would be that the defect is air-filled and is 1 mm below the surface. A similar conclusion can be drawn about Defect A2. Table 6-16. Defect data and pred icted depth for defects shown in Figure 6-48 Defect ID Image Diameter (mm) Tper (C) Tmax (C) Tmax/ Tper t1/2 (sec) Air Depth (mm) Epoxy Depth (mm) A1 43 6.3 13.5 2.2 43 0.98 0.50 A2 38 9.0 19.3 2.2 43 1.1 0.56 A3 52 12.1 6.7 0.6 118 2.8 1.24 A4 36 15.0 12.1 0.8 48 1.4 0.68 If no additional information is known about the FRP system, other parameters can be used to assist in the ch aracterization process. It mu st be noted, however, that the remaining parameters that were extracted from the Tdef vs. time plots are dependent on pulse duration. The discussion becomes more interesting when Defect A3 is considered. The analysis procedure described above indicates that this defect is either air-filled with depth D = 2.8 mm or epoxy-filled with d = 1. 24 mm. For practical purposes, these data can be interpreted as air-filled beneath 3 mm of FRP or epoxy-filled beneath 1 mm of FRP.

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163 Normalized Tmax was used as an additional parameter to help determine the depth and material composition of the defect. Figure 6-50 provides a plot of normalized Tmax vs. defect diameter data that were collected during the long-pulse (60 sec) experiments conducted on specimens A-1 and A-3. Only data for the epoxy-filled defects are provided from Specimen A-1. Only data fr om the air-filled defects are provided for Specimen A-3. The unknown data point is also plotted on the graph as a separate series. The objective of this plot is to investigat e which data series the unknown point belongs to. A visual inspection of the graph indicat es that the unknown point follows the trend of the 3-layer air specimen. It must be not ed, however, that this conclusion requires extrapolation of data and is only based on th ree observations. The final conclusion then is that the defect is air-filled and is 3 mm below the surface. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 00.511.522.5Defect Diameter (in) Tmax/ Tper 3-Layer Air 1-Layer Epoxy Unknown Point Figure 6-50. Characterization of Defect A3 in Figure 6-48 Defect A4 presents another interesting problem. The C x d vs. t1/2 analysis indicates that this defect is either air-filled at a depth of 1.4 mm or epoxy-filled at a depth of 0.67 mm. Neither of these conclusions is consistent with the information known about the FRP system (1 mm thick layers). Once again, normalized Tmax was investigated to

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164 try and determine which of the following assumptions is correct: (1) the defect is airfilled beneath one layer of FRP or (2) the defect is epoxyfilled beneath one layer of FRP. Figure 6-51 provides a plot of normalized Tmax vs. defect diameter data collected for Specimen A-1 during the 60 sec long-pulse experiment. Data fo r the air-filled and epoxy-filled defects are provided. In this case, a visual inspection of the data indicates that the unknown point follows th e trend of the epoxy-f illed defects. Again, this requires extrapolation based on three observations. 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 00.511.5Defect Diameter (in) Tmax/ Tper 1-Layer Air 1-Layer Epoxy Unknown Point Figure 6-51. Characterization of Defect A4 in Figure 6-48 A comparison of the predicted and actual pr operties of the defects is provided in Table 6-17. All of the predictions turned out to be correct with the exception of Defect A4. First, defects A1, A2, and A3 were all located at the in terface of the FRP and concrete. This defect configur ation was identical to the defect configurations used in the development of the model (specimens A-1, A-2, A-3, and A-4). Defect A4, however, was located between the top a nd second layer of FRP in a th ree-layer FRP system (interlamina bubble on Specimen B-MC-3).

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165 Table 6-17. Predicted and actua l properties of defects in Figure 6-48 Predicted properties Actual properties Defect ID Depth (mm) Material Depth (mm) Material A1 1 Air 1 Air A2 1 Air 1 Air A3 3 Air 3 Air A4 1 Epoxy 1 Air This apparent breakdown of the model can be explained by examining the response of Tper while the specimen is being heated. The final value of Tper that was achieved due to heating is clearly different among all four defects. This difference is a result of two factors: (1) the location of the perimeter with respect to the h eat source and (2) the thickness of the FRP composite. This concept is illustrated in Figure 6-52. To eliminate the influence of the first fact or (proximity to th e heat source), four new areas were identified (A1, A2, A3, and A4 in Figure 6-52A). Areas A1, A2, and A3 were all chosen on a specimen with a singl e layer of FRP. Area A4 was placed on a specimen with three layers of FRP. The location of these ar eas was selected such that they were all located approximately the same distance from the heat source. Figure 652B provides a plot of the av erage temperature increase expe rienced by the perimeter of each area vs. the square root of time. A majo r distinction can be made between the areas that were placed on the single-layer compos ite (A1, A2, and A3) and the area placed on the three-layer composite (A4). For the three-layer composite, the temperature increase is nearly linear as a function of the square root of time (t1/2). For the single-layer systems, the temperature increase exhibits a non-lin ear trend in which the slope of the line decreases with increasing t1/2 (second derivative < 0). The cause of this change in slope will be investigated in greater detail in the following section on step analysis. For now, it is important to focus on the impact this observation has on the Tdef vs. time plot for the orig inal A4 defect shown in Figure 6-

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166 48. First, the relative increase in Tper experienced by Defect A4 will tend to decrease the computed value of Tdef and Tmax. The smaller Tmax combined with the larger Tper will also tend to push the normalized Tmax lower. The increase in Tper will also increase defect half-life sin ce the overall temperature gradie nt between the middle of the defect (hottest point) and the edge of the def ect will be lower (i.e. it will take more time for the excess heat stored above the defect to drain into the surroundi ngs). The resulting decrease in normalized Tmax and increase in defect half-lif e give the impression that the defect is either deeper than it actually is or that the defect is filled with something other than air. For the case of Defect A4, this effect was significant enough to result in a misinterpretation of the data. 1 2 3 4 5 6 7 8 0 5 10 15 Tper (C)Square Root of Time (sec1/2) A1 A2 A3 A4 Figure 6-52. Temperature increase for select areas Summary of Pulse Thermography Results Three of the heating methods described in Ch apter 4 were used to generate data for a pulse thermography analysis of the specimens in Series A. The distinguishing characteristic of the pulse analysis is that data are collected after the heat source is removed. Major conclusions can be summarized as follows: 58 A3 A4 A2 A1

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167 Of the three heating methods that were investigated, the scan heating approach resulted in the highest levels of detectability for the greatest number of defects. This heating method generates high temper ature increases over the surface of the composite which in turn lead to higher values of Tmax for detected defects. 2.0C was identified as the minimum value of Tmax required for defect characterization. Defects with Tmax values less than 2.0C did not generate welldefined boundaries when the gradient area method was applied. Defects with weak signals also generated Tdef vs. time with parameters that could not be extracted with precision (tmax and t1/2). Normalized Tmax and tmax are both dependent on pulse duration. t1/2 was shown to be relatively independent of pulse duration. The C x d vs. t1/2 model can be used as a good starting point in the process of defect characterization. This model generates tw o possible scenarios for the depth of each detected defect: (1) a depth assuming the defect is air-filled (2) a depth assuming the defect is epoxy-filled. The C x d vs. t1/2 model was calibrated using specimens in Series A. The implanted defects in these specimens were all located at the FRP/concrete interface. This model was applied to four defects in othe r specimens (Series B and Series C). For defects occurring at the FRP concrete inte rface, the predicted depth and material composition were accurate. When this model was applied to a defect that occurred between layers of FRP, the predicted mate rial composition did not match the actual composition. There was also a relatively large error in the predicted depth. Step Thermography Analysis Step thermography analysis involves monitoring the surface temperature of a sample during heating. This section contains results for Se ries A specimens that were heated for 60 seconds using the longpulse setup described in Chapter 4. Analysis Procedures The surface temperature increase ( T) as a function of time due to the application of a uniform heat flux is given by the following equation (Maldague 2001): t C Tc (6-4) t = time Cc is a constant that depends on the following: Thermal properties of the material

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168 Emissivity and reflect ivity of the surface Intensity of the applied heat flux For a sample containing two layers of material, the temperature increase T is given by the following (Osiander et al. 1996): 1 1 1 1 2 2exp ) ( 2 1n n ct nL erfc t nL t L n t C T (6-5) Cc = Constant t = time = Thermal mismatch factor L = Thickness of top layer 1 = Thermal diffusivity of top layer Thermal diffusivity, is defined as: c k (6-6) k = thermal conductivity = density c = specific heat The thermal mismatch factor, is defined as: 1 2 1 2e e e e (6-7) e1 = Thermal effusivity of top layer e2 = Thermal effusivity of bottom layer Thermal effusivity, e, is defined as: c k e (6-8) Typical thermal properties for materials us ed in the current study are provided in Table 6-18. It should be noted that different sources provide a range of properties for the materials under consideration. Carbon FR P represents a very broad category of composite materials. Different CFRP composites can have varying fiber volume

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169 fractions. CFRP composites that are a pplied using the wet layup method for field applications to concrete will tend to have lower fiber volume fractions (more epoxy and more voids) than CFRP composites cons tructed using vacuum bagging and other advanced techniques used in a factory set ting. The low thermal conductivity of epoxy will lead to a higher thermal diffusivity and thermal effusivity for CFRP composites applied to concrete than what would norma lly be expected for an aerospace quality composite. Table 6-18. Typical therma l properties for materials of interest (Maldague 2001) Material k (W/m-K) (kg/m3) c (J/kg-K) x 106 (m2/sec) e (J/m2-K) FRP Concrete (moist) 1.8 2500 1280 0.56 2400 0.38 Concrete (dry) 1.0 2400 800 0.53 1683 0.22 Carbon FRP ( to fibers) 0.8 1600 1200 0.8 1067 0 Epoxy .2 1300 1700 0.14 795 -0.15 Air 0.024 1.2 700 28 4.5 -1.0 From the standpoint of step thermography analysis, the most important physical property is thermal effusivity. Consider the one-dimensional m odel of a two-layer sample provided in Figure 6-53. If the two materials have the same thermal effusivity, the thermal mismatch factor, will be zero. In this cas e, Equation 6-5 reduces to Equation 6-4 (homogeneous case). Th e surface temperature increase ( T) is plotted as a function of time1/2 in Figure 6-53. For = 0, the resulting curve is linear. If is not equal to zero, the surface te mperature rise will diverge from the homogenous case at some transit time, tT, which is the time required for the thermal front to travel from the surface to the second layer. tT is proportional to the thic kness of the top layer squared divided by the thermal diffusivity (L2/ ).

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170 Figure 6-53. Surface temperature increase due to uniform heat flux applied to multi-layer specimen Based on the values provided in Table 6-18, the thermal mismatch factor for carbon FRP and concrete (assuming the FRP is on the surface) is 0.22. This indicates that once the thermal front reaches the FRP concrete inte rface, the rate of temperature rise on the surface will decrease (second deriva tive will become negative). Since the thermal effusivity of air is significantly smalle r than the effusivity of concrete, the thermal mismatch factor for an air-filled defect will always approach -1. This will result in a relative surface te mperature increase with respect to the homogeneous case. Epoxy-filled defects al so represent a case where the thermal mismatch factor is less than zero ( = -0.15). It is interesting to compare the relative magnitudes of for a CFRP/concrete interface and a CFRP/epoxy interf ace. According to the theoretical model (and assuming the material properties provided in Table 6-18 are correct), the concrete beneath the surface of the CFRP will have a larger effect on the surface temperature than the presence of epoxy. Epoxy results in a rela tive surface temperature increase ( = -0.15) and the Layer 1: 1 k1 c1 L Layer 2: 2 k2 c2 Note: Thickness of Layer 2 assumed to be infinite Uniform Applied Heat Flux Time1/2 T = 0 > 0 < 0 T measured at surface tT 1/2

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171 concrete results in a relative surface temp erature decrease ( = 0.22). The previous section on pulse thermography analysis was primarily concerned with monitoring the difference in temperature between a defect ar ea and a surrounding defect-free area. The fact that the defect-free ar ea actually consists of a multi-layer material that responds differently than a homogeneous material to thermal stimulation was not considered. Two factors must also be considered wh en applying the step heating model to results from the current study: two-dime nsional heat flow around defects and nonuniform heating. If the thermal front trav eling from the surface does encounter a defect, Equation 5-9 will only apply for as long as the heat flow remains one-dimensional (Osiander et al. 1996). If a thermal gradient is present between the defect area and the surrounding defect-free area, the backed-up heat that is stored above the defect drain off into the surroundings by traveling around the defect. Once this occurs, the magnitude of the divergence away from th e homogeneous case is reduced. Non-uniform heating can be addressed by applying a normalization procedure to each pixel in the series of thermal images (Osiander et al. 1996). Normalized T is computed with the following equation: 1 ) ( ) ( t C t T t Tc norm (6-9) Tnorm = Normalized change in temperature Cc = Initial slope of T vs. t1/2 curve (C/sec1/2) t = time Tnorm provides an indication of how far th e change in temperature for a single point has drifted away from what would be expected for a homogeneous material. Note that for a homogeneous material, Equati on 6-9 results in a value of zero for Tnorm at all values of time. For the case of a non-homogeneous material, Tnorm will equal zero up to

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172 a certain time (tT in Figure 6-53) and then diverge. The process is illustrated for two points on Specimen A-1 (one-layer FRP) in Figure 6-54. Point 1 is located very close to the heat source and experiences an overall T of 11.5C after 60 seconds of heating. Point 2 is located farther from the heat source, which results in a lower T of 6C after the same heating duration. Cc is computed for each curve by measuring the slope of the line through the points collected for t < tT. For both of the curves shown in Figure 6-54A, tT occurs at some time between t1/2 = 2 and t1/2 = 3. Once Cc is known, Tnorm can be computed using equation 5-9. The resulting curves for Tnorm are provided in Figure 654B. With the exception of some minor deviation that begins at t1/2 = 6, the two points display a similar Tnorm response. It was observed that the initial slope of T vs. t1/2 plots did not pass through the origin. This was likely a result of poor s ynchronization between the time that the lamps were turned on and the time the first image was recorded. If this were the case, the T response would actually be zero for some time in terval between t = 0 and t = 1 sec. This observation could also result from the fact th at some time is required for the lamps to reach their maximum output level. In this case, the initial slope of the T vs. t1/2 curve would start at a lower value and increase as the lamps reach a constant output level. In order to apply this method to the da ta collected during ea ch experiment, the initial slope and correspondi ng y-intercept was determined using a least-squares regression for the data points obtained between t = 1 sec and t = 4 sec. Since the top layer of FRP was essentially the same for al l of the specimens, this initial window proved adequate to capture the trend of th e initial response for each pixel.

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173 0 1 2 3 4 5 6 7 8 0 2 4 6 8 10 12 T (oC)Square Root of Time (sec1/2) Point 1 Point 2 m1 = Cc1 m2 = Cc2 0 1 2 3 4 5 6 7 8 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 Temperature vs. Time DataTnormSquare Root of Time (sec1/2) Point 1 Point 2 A B Figure 6-54. Normalizing T for two points on Specimen A-1 for 60 sec pulse duration. A) Raw data. B) Normalized Figure 6-55. T image for Series A specimens (t = 60 sec, long-pulse heating) IB A75 A50 E75 E50 IB A75 A50 E75 E50 IB A75 A50 E75 E50 IB A75 A50 E75 E5058 8 9 10 11 12 13 A-1 A-2 A-3 A-4

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174 Figure 6-56. Normalized T image for Series A specimens (t = 60 sec, long-pulse heating) Defect-free areas One defect-free area was identified on each Series A specimen (labeled DF-1, DF2, DF-3, and DF-4 in Figure 6-57). The term defect-free is used to describe these areas only because there were no implanted defect s inside the areas or in the immediate vicinity. Careful inspection of each area, howev er, did reveal small unintentional defects. The average value of Tnorm was computed for each area at each time step (NP = total number of pixels inside the area). Outl ier pixels due to unintentional defects were removed at each time step. The criterion for discarding a pixel was +/2.5 standard deviations away from the mean value. The standard deviation was recomputed after each outlier was removed. After the outlier pixe ls were removed, the modified mean and standard deviation were computed for each ar ea at each time step. The mean was plotted for each area along with error bars representing +/2 (Figure 6-58). The single-layer specimen (A-1) produced similar results to the two points shown in Figure 6-54B: Tnorm stays close to zero up to t1/2 = 2 and then decreases as the thermal IB A75 A50 E75 E50 IB A75 A50 E75 E50 IB A75 A50 E75 E50 IB A75 A50 E75 E5058 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 A-1 A-2 A-3 A-4

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175 front is influenced by the concrete. Ideally one would expect similar behavior for the remaining specimens with the divergence from zero occurring at a later time depending on the thickness of the FRP composite. This trend, however, was not observed in the two, three, or four-layer sp ecimens. At approximately t1/2 = 2, the remaining specimens experienced a rise in Tnorm. At t1/2 = 4.25, the curve for Specimen A-2 diverges from the trend and the slopes become negative. This curve crosses the Tnorm = 0 line at t1/2 = 5.75. The curve for Specimen A-3 assumes a negative slope somewhere between t1/2 = 4 and t1/2 = 5. Specimen A-4 does not a ssume a negative slope until t1/2 is approximately 6.75. One possible explanation fo r the slight rise in Tnorm experienced by the multi-layer specimens is contact resistance between layers of FRP composite. This minor thermal resistance could be a result of small air bubble s that may have formed in the thin glass veil present on the bottom side of each layer. This would have the effect of making < 0 at the interface of two layers (see Figure 6-59). Eventually, the thermal front moves through to the concrete and the normalized T curve assume a negative slope. This behavior might be expected to occur at th e interface between the si ngle-layer system and concrete. There are two possibilities as to w hy this was not observed in the data: (1) the image save rate is insufficient to capture this eff ect (2) there is adequate epoxy present at the interface of the FRP and concrete. Table 6-19. Summary statistics fo r defect-free areas (Series A) Area ID Specimen NP # of Outliers @ t = 60 sec Mean @ t=60 sec Standard Deviation @ t = 60 sec DF-1 A-1 770 66 -0.29 0.014 DF-2 A-2 780 17 -0.09 0.022 DF-3 A-3 864 32 +0.02 0.024 DF-4 A-4 851 29 +0.08 0.024

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176 Figure 6-57. Defect-free areas for Series A specimens Figure 6-58. Mean value of Tnorm for defect-free areas on Series A specimens (error bars represent +/2 ) L158 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 A-1 A-2 A-3 A-4 DF-3 DF-4 DF-1 DF-2 -1 0 1 2 3 4 5 6 7 8 -0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 TnormSquare Root of Time (sec1/2) DF-4 DF-3 DF-2 DF-1

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177 A B Figure 6-59. One-dimensional model of FRP sy stems. A) single-layer FRP system. B) multi-layer FRP system with contact resistance These data indicate that l ong-pulse heating combined wi th an analysis of the normalized temperature increase is potentially useful for estimating the thickness of the FRP composite. By the end of heating (t = 60 sec, t1/2 = 7.75), the curves for specimens A-1 and A-2 have diverged significantly. The curves for the three and four layer specimens also show signs of divergence; howev er, there is still substantial overlap in the +/2 confidence intervals at t1/2 = 7.75. It is hypothesized that if the samples were heated for a longer duration, th e curves for the three and f our-layer systems would also exhibit a statistically si gnificant divergence. Defect area analysis The next step in the analysis was to examine the normalized T response of the defect areas (shown in Figure 6-56). Tdef was computed for each area at each time step using the following equation: ) ( (max) avg per norm norm defT T T (6-10) Tdef = Signal strength of defect Tnorm(max) = Maximum Tnorm bounded by area Tnorm(per_avg) = Average Tnorm on the perimeter Concrete FRP L > 0 Concrete Contact resistance develops between layers L < 0

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178 The process is illustrate d for an ideal case in Figure 6-60A and B. Two important parameters that can be used to charac terize defects are the time required for Tdef to become visible, tT 1/2, and the rate of increase of the Tdef curve. tT 1/2 is related to the depth of the defect, and ra te of increase of the Tdef curve provides an indication of the material composition of the defect (Maldague 2001). The signal fo r air-filled defects should increase faster than the signal for epoxy-filled defects. Figure 6-60C and D provide results from Defect A75 on Specimen A-2. Small, unintentional defects in the FRP system generate a signal for Tdef for all t1/2 > 2. This trend was observed for all of the defects th at were examined regardless of their actual depth. The ultimate significance of the unintentional defects is that tT 1/2 is not a distinct parameter that is easily extracted from a Tdef vs. t1/2 plot. As a result, defining the time at which a defect first becomes visible in a series of normalized thermal images requires additional consideration. Two methods were investigated to assist in defining the time that a defect is first detected. This first method involves establishing a threshold value for Tdef. The objective of this analysis is to establish a criterion for determining when the signal resulting from the implanted defect is more prominent than the ex traneous signal that develops due to background noise. The bac kground noise can be a result of unintentional defects or the natural texture of the composite It was determined that a plot of the perimeter average +/2 standard deviations serves as a good indicator of the overall noise level present in an area. The threshold for de tection was then established as the point at which Tdef exceeds the Tnorm(per_avg) + 2

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179 A B C D Figure 6-60. Computation of Tdef from normalized temperature data. A) Tnorm for idealized case. B) corresponding Tdef C) Tnorm for Defect A75, Specimen A-2. D) corresponding Tdef The procedure is illustrated for Defect A50 on Specimen A-2 in Figure 6-61. The hatched area in the figure represents the average Tnorm on the perimeter of the area with a range of +/2 At t1/2 = 4.2 (t = 18 sec), Tdef exceeds the upper boundary defined by Tnorm(per_avg) + 2 This point corresponds to tT 1/2. It is important to note that the extracted value for tT 1/2 represents a best-estimate. For the defect described in Figure 6-61, it was determined that the signal for the defect diverged from the surrounding noi se at t = 18 sec. Close inspection of the area at t = 13 sec (five seconds before tT 1/2) reveals a signal has developed above the implanted defect (Figure 6-62A), but the magnitude is on the sa me scale as the surr ounding noise. At t = Tnorm(max) Tnorm(per_avg) Tnorm t1/2 tT 1/2 Tdef t1/2 tT 1/2 0 1 2 3 4 5 6 7 8 -0.05 0 0.05 0.1 0.15 0.2 0.25 0.3 TdefSquare Root of Time (sec1/2) 0 1 2 3 4 5 6 7 8 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25 TnormSquare Root of Time (sec1/2) Tdef Tnorm ( max ) Tnorm(per_avg)

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180 18 sec, the defect is better defined with re spect to the surroundings and by t = 23 sec the defect has risen substantially higher than the surroundings. Figure 6-61. Determining point at which defect is detected in Tdef plots The second method that was investigated involves computing a two-dimensional cross-correlation coefficient for each defect area at each time step. The two-dimensional cross-correlation coefficient, R, is defi ned as follows (Matlab Users Guide 2002): mn mn mn mn mn mn mnB mean B t A mean t A B mean B t A mean t A t R2 2) ( )) ( ( ) ( ) ( )) ( ( ) ( ) ( (6-11) R(t) = 2-D correlation coefficient at time = t A(t) = mxn matrix of defect area pixels at time = t B = mxn matrix of defect area pixels @ time = 60 R provides an indication of similarity be tween two matrices. The matrix B in Equation 5-11 is populated with the pixel values of the rect angle defining the defect area at t = 60 sec. This matrix represents the bes t image available for the defect of interest. The two-dimensional correlation coefficient is computed at each time step by applying Equation 5-11 to the matrix for the defect area at time t (A(t)) and B. The general 0 1 2 3 4 5 6 7 8 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25 TdefSquare Root of Time (sec1/2) Tdef > Tnorm(per_avg) + 2 tT 1/2 Tnorm(per_avg) +/2 Tdef

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181 motivation behind performing this computation is that defects close to the surface will correlate more rapidly than deeper defects. Consider the surface plots for Defect A50 (Specimen A-2) provided in Figure 6-62. The R computed at t = 13 sec was 73%. R increases to 87% and 95% for t = 18 sec and t = 23 sec, respec tively. R is plotted as a function of time for this defect in Figure 6-63. As long as the de fect area at t = 60 sec is used as the reference matrix in Equation 5-11, the resulting R value for all areas will be 1 at t = 60. A B C D Figure 6-62. Surface plots of Defect A50 (S pecimen A-2). A) t = 13 sec. B) t = tT 1/2. C) t = 23 sec. D) t = 60 sec.

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182 0 1 2 3 4 5 6 7 8 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 RSquare Root of Time (sec1/2) Figure 6-63. Two-dimensional correlation coe fficient, R, for Defect A50 (Specimen A-2) Specimen A-1 Only defects with diameters 12.7 mm or larger were considered in the analysis. A plot of Tdef vs. t1/2 is provided in Figure 6-64. There is a di stinct divergence between Tdef and Tnorm for the perimeter of each defect area at t1/2 = 2. Ideally, the initial slopes for the IB, A75, and A50 def ects would be the same since at the FRP/def ect interface is the same. Defect A50 does not line up with the others. It is also interesting to note how the Tdef curves begin to level off as time increas es. This is a result of two-dimensional effects as the heat flows from the center of the defect towa rds the edges. The divergence is most pronounced for the smaller defects. The epoxy-filled defects, E75 and E50, follow a similar trend, but the initial slopes of the respective Tdef curves is less than what was observed for the air-filled defects. This is a result of at the FRP/defect interface being closer to 0 fo r the epoxy-filled defects.

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183 0 1 2 3 4 5 6 7 8 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 TdefSquare Root of Time (sec1/2) IB A75 A50 E75 E50 Figure 6-64. Defect signal vs. t1/2 for Specimen A-1 defects The two-dimensional correlation coefficien t also provides intere sting insight into the behavior of each defect. The time at which each defect reaches a 95% correlation varies from t1/2 = 2.25 to t1/2 = 3.5. Defects IB and A75 produce very similar R vs. t1/2 curves (Figure 6-65) and Defects A50, E75, and E50 appear to correlate at a slightly lower rate. Careful inspection of the normalized T images generated from t1/2 = 2.25 to 3.5 confirmed this observation. The Tnorm image provided on the graph in Figure 6-65 was generated for t1/2 = 3. First, this image demonstrat es that the defects are well defined at this level of correlation (all five def ects are between 90% and 98% correlated with their respective defect area at the end of heating). Second, if one was asked to offer a qualitative assessment of the image and rank th e defects in order from most-defined to least-defined, it is likely that the three ai r-filled defects would rank the highest and the smallest epoxy-filled defect would rank the lowest. The computed R for each of the defects at t1/2 = 3 indicates a similar order.

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184 In summary, the two-dimensional correl ation coefficient provides a quantitative measure for how well a defect is defined with respect to some baseline value. In this case, the baseline value was chosen at the end of heating since this represents a point at which the defect is well-defined with resp ect to the surroundings. For the single-layer specimen, correlation with the baseline imag e occurs relatively qui ckly since the time required for the thermal front to reach the defect is small. Figure 6-65. Two-dimensional correlati on coefficient for Specimen A-1 defects Specimen A-2 Applying the same analysis procedure to Specimen A-2 generated markedly different results. The threshold value for Tdef was achieved between t1/2 = 4.3 and t1/2 = 5.8 for defects A75, A50, E75, and E50. Data are provided in Figure 6-66. Defect IB achieved the threshold value much earlier at t1/2 = 2.25. Recall that Defect IB on Specimen A-2 contained rather large uninten tional defects between the top and bottom layer of composite. The results contained in the Tdef vs. t1/2 plot are significant for two reasons. First, Defect IB generated a signal at a t1/2 value very close to what was 0 1 2 3 4 5 6 7 8 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 RSquare Root of Time (sec1/2) IB A75 A50 E75 E50 Tnorm image @ t1/2 = 3 IB A75 A50 E75 E50

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185 observed for the defects in Specimen A-1. Second, the Tnorm curve for the perimeter of Defect IBs area is consistent with that of a two-layer FRP composite system. This would suggest that the initial signal generated by Defect IB is a resu lt of a delamination 0 1 2 3 4 5 6 7 8 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 TdefSquare Root of Time (sec1/2) IB A75 A50 E75 E50 Figure 6-66. Defect signal vs. t1/2 for Specimen A-2 defects The computed two-dimensional correlation coefficient is provided for Specimen A2 defects in Figure 6-67. For the two layer specime n, a 95% correlation is achieved for all defects between t1/2 = 4.0 and t1/2 = 5.5. It is interesting to note how the behavior of R for Defect IB is consistent with the others even though the Tdef curve for Defect IB diverges. Figure 6-68A provides a Tnorm image for Defect IB at t1/2 = 2.4. The unintentional defects ar e clearly visible in this image and the Tdef curve has identified the defects at this time. Figure 6-68B provides a Tnorm image for Defect IB at the end of heating (t1/2 = 7.7 or t = 60 sec). This image was used as the baseline value in the twodimensional correlation coefficient computation. At this point, the shape of the defect is consistent with the actual interface bubble. When R is computed at early time steps, the two images are not well correlated (75% at t1/2 = 2.4). This illust rates the usefulness of

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186 the two-dimensional correlation coeffici ent for removing the effects of small, unintentional defects that corrupt the Tdef vs. t1/2 curves. 0 1 2 3 4 5 6 7 8 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 RSquare Root of Time (sec1/2) IB A75 A50 E75 E50 Figure 6-67. Two-dimensional correlation coefficient vs. t1/2 for Specimen A-2 defects A B Figure 6-68. Normalized T images for IB defect (Specimen A-2) at A) t1/2 = 2.4 (t = 6 sec). B) t1/2 = 7.7 (t = 60 sec) Specimen A-3 and Specimen A-4 Figure 6-69 provides Tdef vs. t1/2 results for Specimen A-3. The general trend of Tnorm for the perimeters defining the defect boun daries is consistent with the findings from the defect-free area analysis. Based on th ese data it is reasonabl e to conclude that the specimen has three or four layers of composite material. If the total thickness of the

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187 composite was less than three or four layers the normalized temper ature response of each defect boundary should be less than zero at the end of heating. The Tdef curves indicate that IB was the only detected defect. The threshold value for Tdef was reached at t1/2 = 6. A 95% correlation factor was also achieved at t1/2 = 6 (see Figure 6-70). This value is suspect, howev er, since a strong signal for Defect IB never developed. The R vs. t1/2 curves for the remaining defects illustrate how an R of 1 will always be achieved at the end of heati ng even if the defect area contains mostly noise. Based on these data it is not possible to determine if the computed R values for IB reflect the properties of the defect. 0 1 2 3 4 5 6 7 8 -0.05 0 0.05 0.1 0.15 0.2 0.25 0.3 TdefSquare Root of Time (sec1/2) IB A75 A50 E75 E50 Figure 6-69. Defect signal vs. t1/2 for Specimen A-3 defects Finally, none of the implanted defects were detected for Specimen A-4. This is to be expected since the 60 sec pulse duration was not adequate to reveal all of the defects in the three-layer specimen. The Tnorm image provided in Figure 6-71 was generated for Specimen A-4 at the end of heating. Ev en though the implanted defects are not discernable, it is still interesting to note th e high degree of variation in the image. The

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188 precise cause of this variati on is not known, but it is like ly the lack of consistent saturation of the composite during installati on or the presence of small, unintentional defects between layers. 0 1 2 3 4 5 6 7 8 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 RSquare Root of Time (sec1/2) IB A75 A50 E75 E50 Figure 6-70. Two-dimensional correlation coefficient vs. t1/2 for Specimen A-3 defects Figure 6-71. Tnorm image for Specimen A-4 (t = 60 sec) Summary of Step Thermography Results: Series A Specimens Thermal images were collected at a rate of 1 frame per second for 60 seconds while specimens in Series A were heated using the long-pulse experimental setup. Data collected during the first 4 seconds of heati ng were used to normalize each pixel in the series of thermal images. Important fi ndings may be summarized as follows:

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189 The normalization procedure is useful for reducing the effects of non-uniform heating A 60 second pulse duration is adequate for the one and two-layer FRP systems Additional heating time is required for three and four-layer FRP systems. The overall effectiveness for three and f our layer systems is currently unknown. This analysis provides an indication of the thickness of the FRP composite It is possible to differentiate defects that occur between layers of FRP and defects that occur at the FRP/concrete interface There are insufficient data available to develop a mathematical model that estimates defect depth. It is, however, po ssible to summarize the observations from the analysis of the Series A specimens and identi fy several parameters that can be used as guidelines for characterizing defects. The first parameter of interest is the time at which Tdef exceeds a threshold value of Tnorm. A convenient threshold value was determined as the average of the Tnorm experienced by the perimeter + 2 This value (tT 1/2) is provided for each defect that was detected in Table 6-20. If tT 1/2 is on the order of 2 3, the defect is likely beneath a sing le layer of composite. For tT 1/2 on the order of 4 5.5, the defect is likely beneath two layers of composite. Th ere are no data available to support a claim for tT 1/2 > 6. The second parameter is the time at whic h the defect area becomes 95% correlated with a baseline value for the defect that wa s collected at the end of heating. This parameter is labeled t1/2 @ R95% in Table 6-20. For defects ben eath a single layer of FRP, t1/2 @ R95% is between 2 and 3.5, and for def ects beneath a two layers of FRP t1/2 @ R95% lies between 4 and 6.

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190 Table 6-20. Parameters of interest for ch aracterizing defects from step thermography data Specimen Defect tT1/2 (sec1/2) t1/2 @ R95% (sec1/2) A-1 IB 2 2.4 A-1 A75 2 2.4 A-1 A50 2 2.8 A-1 E75 2 3 A-1 E50 2 3.5 A-2 IB 2.25 4.3 A-2 A75 4.3 4 A-2 A50 4.2 4.8 A-2 E75 5.8 5.3 A-2 E50 5.8 5 A-3 IB 6 6 Frequency Domain Analysis: Series A Data from the pulse thermography analysis and the step heati ng analysis indicate that IRT can be used confidently to detect and characterize defects in one and two-layer carbon FRP systems. Furthermore, larger defects (> 10 cm2) can be detected in the three and four-layer systems, but char acterization of these defects is less certain. The objective of this section is to examine two additional heating and analysis te chniques that can be used for thicker composite systems. The first method involves heating each specimen using the sinusoidal heating setup descri bed in Chapter 4. Results from these experiments will be processed to generate a series of phase images for each specimen. The second method will utilize the data collect ed during the pulse heating experiments. This method will also generate a series of phase images by applying a discrete Fourier transform to the cooling cu rve obtained for each pixel. Sinusoidal Heating (Lock-In IRT) Data collection A total of 10 pulse durations were inve stigated during the sinusoidal heating experiments. Each pulse duration represents one frequency value (fmod) since the shape

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191 of the pulse was sinusoidal. The specimens were allowed to cool for 10 minutes between pulse durations, and no images were recorded during this cooling pe riod. A summary of the pulse durations that were i nvestigated and the number of im ages that were collected is provided in Table 6-21. Table 6-21. Frequencies investigated during sinusoidal heating experiments Pulse Duration (sec) fmod (1/sec) # of images 5 0.2 10 6.25 0.16 10 8.3 0.12 10 12.5 0.08 20 25 0.04 20 100 0.01 20 125 0.008 40 167 0.006 40 250 0.004 40 500 0.002 40 Analysis procedures and results The general concept behind sinusoidal heati ng is that defects closer to the surface will appear when the pulse duration is short (f requency is high) and deeper defects will appear when the pulse duration is long (fre quency is low). A least squares sinusoidal curve fit was applied to the temperature vs time response for each pulse duration that was investigated. The quantity of interest in this analysis is the phase shift experienced by each pixel. By focusing on phase (as opposed to amplitude), the effects of nonuniform heating are minimized and defect s with weak signals are accentuated. Consider the three points shown in the thermal image provided in Figure 6-72A. These points are all located on Specimen A-4 with Point 1 positioned above Defect IB, and Point 2 and Point 3 positioned above def ect-free areas. The temperature vs. time results for the 500 sec pulse duration indica tes that the overall amplitude of the temperature response is highly dependent on the location of th e point with respect to the

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192 heat source. The general form of the sinusoida l curve fit that was applied to each point is as follows: ) 2 cos( ) ( ft B A t T 6-12) T = Temperature t = Time f = Frequency of heat source modulation (1/pulse duration) = Phase shift (measured in radians) A = Average offset B = Amplitude of sine wave Note that when the phase shift is zero, th e temperature response is a scaled function of the modulated heat input. For the case of a 500 second pulse duration, the peak output of the heat source occurs at 250 sec. The resulting phase shift for Point 1 (above the defect area) was -0.86 rad. This quantity indicates that the peak value of the sinusoidal curve-fit occurs 68 seconds later at t = 318 sec. The phase shift for each of the defect free pixels was -0.73 rad which corresponds to a lag in the peak response of 58 sec. The important thing to recognize is that the comput ed phase shift is independent of amplitude. In the example provided, the defect-fr ee region described by Point 3 has greater amplitude than the defect ar ea throughout the entire pulse duration. The phase shift, however, is less for the defect-free area since the flow of heat from the surface into the concrete is not interrupted (slowed down) by the defect. By applyi ng this procedure to each pixel in a series of thermal images, it is possible to generate a single phase image that is independent of amplitude.

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193 A B Figure 6-72. Data analysis for sinusoida l heating (pulse duration = 500 sec). A) Temperature vs. time response for three points on Specimen A-4. B) Results of sinusoidal curve-fit Results for the 8.33 sec pulse duration are provided in Figure 6-73. During this short pulse duration, the thermal front is onl y affected by features that occur near the surface. Most of the implanted defects in Sp ecimen A-1 are visible in the surface plot as 0 100 200 300 400 500 22 24 26 28 30 32 34 36 38 T (C)Time (sec) Point 1 Point 2 Point 3 Point 1 Point 3 Point 2 0 100 200 300 400 500 22 24 26 28 30 32 34 36 38 T (C)Time (sec) Point 1 Point 2 Point 3 )) 855 0 ( 500 / 2 cos( 4 6 4 29 ) ( t t T )) 733 0 ( 500 / 2 cos( 4 4 0 27 ) ( t t T )) 731 0 ( 500 / 2 cos( 5 6 3 30 ) ( t t T

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194 well as the large unintentional defects pres ent above Defect IB in Specimen A-2. Significant non-uniformities are al so observed in the region of Defect IB in specimens A3 and A-4. Ideally, the phase images for speci mens A-2, A-3, and A-4 would show little or no variation. A B C D Figure 6-73. Sinusoidal heati ng results for Series A specimens (Pulse Duration = 8.33 sec). A) Specimen A-1. B) Specimen A-2. C) Specimen A-3. D) Specimen A-4 For the 25 sec pulse duration (shown in Figure 6-74), all of the implanted defects in Specimen A-1 develop a significan t signal. Only the air-filled defects (IB, A75, and A50) are clearly detected in Specimen A-2. It is interesting to compare these results with the Tdef vs. t1/2 plot that was generated for Speci men A-2 during the step thermography analysis (shown in Figure 6-66). At t1/2 = 5 (t = 25 sec), the Tdef plot indicates that defects IB, A75, and A50 have exceeded th e threshold value for detectability. The epoxy-filled defects, however, have not yet developed a signifi cant signal with respect to

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195 the surrounding areas. A similar conclusion can be drawn from the sinusoidal heating results. A B C D Figure 6-74. Sinusoidal heating results for Series A specimens (Pulse Duration = 25 sec) A) Specimen A-1. B) Specimen A-2. C) Specimen A-3. D) Specimen A4. Results for the 100 sec pulse duration experiment are shown in Figure 6-75. All implanted defects in Specimen A-1 and A-2 ar e clearly defined, and the interface defects in Specimen A-3 are beginning to dominate the phase response. The fact that the IB and A75 defects appear to blend t ogether is an indication that these defects were placed too close together. The overall pattern of the phase shift response for the three-layer specimen does indicate that the thermal front is influenced by the implanted defects. Without a destructive inspecti on of the specimen, it is no t possible to determine how much of the signal that has developed for the three and four-layer sp ecimens is due to the implanted defects and how much results from non-uniformities in the composite above the defect.

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196 A B C D Figure 6-75. Sinusoidal hea ting results for Series A specimens (Pulse Duration = 125 sec) A) Specimen A-1. B) Speci men A-2. C) Specimen A-3. D) Specimen A-4. Results for the 500 second pulse duration experiment are provided in Figure 6-76. Specimen A-1 exhibited a very interesting phase response. For this pulse duration, the phase shift above the air-fille d defect areas was actually smaller in magnitude than the phase shift above the defect-free areas. The epoxy-filled defects are can not be distinguished. For the two-layer specimen, th e phase response of the air-filled defects was essentially the same as the surrounding areas (no signa l developed) and the epoxyfilled defects display a slight ly larger phase shift than the surroundings. Bai and Wong (2001) reported this phenomenon and referre d to the frequency (inverse of pulse duration) at which defects bl end in with the surroundings as the blind frequency. Bai and Wong do not offer a physical explanati on for this phenomenon but they do suggest that this frequency be avoided.

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197 A B C D4 Figure 6-76. Sinusoidal hea ting results for Series A specimens (Pulse Duration = 500 sec) A) Specimen A-1. B) Speci men A-2. C) Specimen A-3. D) Specimen A-4. Summary of sinusoidal heating results Results from this investigation into sinus oidal heating highlight the major downside to method: multiple inspections must be perfor med in order to detect defects at different depths in an FRP system. On the other hand, the pulse duration at wh ich a defect is first detected can be correlated to the defects dept h. The original intent was to investigate a large number of pulse durations and establis h a relationship between defect depth and blind pulse duration. This was accomplished to a certai n extent for the one and twolayer specimens, but unintentional defects a nd other non-homogeneities in the composite system do not support establishing a precise relationship for the three and four-layer systems. Nonetheless, results from this st udy can be used to make recommendations for suitable pulse durations depending on the th ickness of the FRP composite and/or the desired depth of the inspection.

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198 The proposed guidelines are summarized in Table 6-22. These guidelines cover a range of depths from 1 mm to 4 mm. The second column in the table represents the recommended pulse duration for inspecting a de sired depth. The third column provides an acceptable range of values for pulse durat ion. The idea behind the acceptable range for pulse duration is to id entify an upper and lower bounda ry for a specific inspection depth. The lower boundary represents the mi nimum required pulse dur ation for a defect at that depth to become visible in the pha se image. The upper boundary is intended to provide some guidance in avoiding the blind pulse duration at which the phase shift for the defect and defect-free regions are equal. These values were chosen based on an inspection of def vs. frequency plots that were genera ted for the five defects (IB, A75, A50, E75, and E50) in each of the four specimens. def was computed the same way that Tdef was computed in the time domain anal ysis. These results are provided in Appendix B. Table 6-22. Recommended pulse durations and detection li mits for sinusoidal heating (carbon-FRP systems) Depth of inspection (mm) Recommended pulse duration (sec) Acceptable range for pulse duration (sec) Dimension of smallest detectable defect (mm) 1 30 10 100 12.7 2 60 45 150 12.7 3 120 100 250 19 4 240 200 500 25.4 It must be noted that these recommendati ons are made based on results from four specimens. It remains to be seen whether the effects of fiber satura tion levels, fiber type (glass vs. carbon), and surface preparation me thods have any significant impact on IRT results. These effects will be investigated with the specimens contained in Series B, Series C, and Series D.

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199 Pulse Phase Thermography Pulse phase thermography (PPT) is a heati ng and data analysis method that can be used to reduce the effects of non-uniform heatin g (Maldague et al. 2002). In this method, a high energy heat pulse is applied usi ng a photography flash and thermal images are recorded while the surface cools. The discre te Fourier transform (D FT) is computed for the temperature vs. time response of each pixel in the series of thermal images using the following: N n n N kN kn i N kn T N F0 1 02 sin 2 cos 1 (6-13) N = Number of time steps Tn = Temperature value of pixel at each time step The output of the DFT includes real and imaginary components that vary as a function of frequency. The phase angle is de termined at each frequency interval using the following: k k N kF F Re Im arctan1 0 (6-14) Im(Fk) = Imaginary component of DFT output Re(Fk) = Real component of DFT output This procedure generates a new series of phase images in the frequency domain. The first value in the DFT output, F0, corresponds to the average value of the temperature response (dc-component). The corresponding phase angle, 0, is zero. The second value in the DFT, F1, corresponds to the lowest frequency. The lowest frequency value can be determined as follows: Nt f 11 (6-15)

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200 tN = duration of the record The final value in the DFT output corr esponds to the highest frequency, fmax, and is equal to the image save rate. The DFT ope ration generates a unique set of values for frequencies between 0 and fmax/2. The values generated by the DFT for frequencies between fmax/2 and fmax are the complex conjugates of th e values generated between 0 and fmax/2. Data collection The data used to investigate the PPT method were collected during the pulse analysis experiments (time domain) that were described previously. Each of the four specimens in Series A was heated using th e photography flash set up and the long-pulse setup. Three pulse durations were consider ed from the long-pulse data set: 30 sec, 45 sec, and 60 sec. Only data recorded after the heat source was removed were considered. This section will only consider result s from Specimen A-3 and Specimen A-4 (three and four-layer FRP systems). Specimen A-1 and Specimen A-2 were omitted based on the findings of the time domain anal ysis and the lock-in analysis. The heating methods and analysis procedures previously described were shown to be effective for detecting defects in one and two-layer FRP systems. Previous research by others (Ibarra-Cast anedo and Maldague 2004, Avdelidas et al. 2004, Woolard and Cramer 2005, Ibarra-C astanedo and Maldague 2005) involved heating the surface of FRP composites using a high powered photography flash. This approach was attempted in the current study but was only moderately successful for Specimen A-3 and Specimen A-4. The flash system used in the current study was not powerful enough to develop strong signals fo r defects in three and four-layer FRP systems. When the PPT method was applied to the data collected using the photography

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201 flash, the resulting phase images were very noisy. There was no observable advantage gained by applying the PPT method to flash heating data. The output of the DFT operation depends on the image save rate and the duration for which data are recorded. Several experiments were c onducted where thermal images were collected for up to 15 minutes after th e heat source was removed. Results from these trials indicated that after 240 sec the surface temp erature of each specimen was uniform and had returned to its initial value before the heat was applied. No additional information was gained by recording data for longer than 240 sec. Experiments were also conducted in which th e image save rate was varied from the maximum possible (5 frames per second = 5 Hz) to 1 frame per 2 seconds (0.5 Hz). Results from these trials indicated that a capture rate of 1 Hz is adequate. Phase images that were generated for frequencies higher th an 0.5 Hz contained mostly noise and were not considered. Analysis procedures and results The general concept behind PPT is simila r to the lock-in analysis procedure: defects close to the surface appear at high fr equencies while deeper defects appear at lower frequencies. The fundamental differen ce between the two methods is that lock-in analysis requires multiple experiments for each frequency of interest. PPT examines all frequencies based on results from one experiment. The objective of this analysis for Specimen A-3 and Specimen A-4 is to investigate using PPT to remove the effects of non-uniform heating. Data were collected using the long-pulse setup with a 60 sec pulse duration. Temperatures vs. time results for three points on Specimen A-3 are shown in Figure 6-77 A. Point 1 was selected directly above Defect IB. Point 2 and Point 3 were sele cted above defect-free areas. Point 2 was

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202 located relatively close to the defect and expe rienced similar heat intensity. Point 3 was located very close to the heat s ource. The thermal image shown in Figure 6-77 A was collected at the time of maximum defect signal strength. This image shows the effects of non-uniform heating. The phase response of each point is provided in Figure 6-77 B. Between 0.05 and 0.5 Hz, It is not possible to distinguish th e phase response of each point. Between 0 and 0.05 Hz, the phase shift for the point above the de fect is larger than the phase shift for the defect-free regions. The important thing to notice about these curves is that the phase response for the defect-free points is nearly id entical for all frequenc ies. The effects of non-uniform heating have been removed. Figure 6-78 A contains a surface plot of the phase shift for Specimen A-3. The maximum phase difference occurred at a frequency value of 0.0042 Hz. This was the lowest frequency generated by the DFT since the total duration of the record was 240 sec (1/240 sec = 0.0042 Hz). All of the defects 12.7 mm in diameter or larger can be seen in the surface plot. A surface plot of the temperature profile at time = 41 sec (tmax for Defect IB) is shown in Figure 6-78 B. The same defects that can be seen in the phase profile are also visible in the temperature pr ofile. The only major advantage of the phase image is that the response of th e defect-free region is uniform. A similar analysis was conducted for three points on Specimen A-4. The frequency at which the point above the defect dive rged with the def ect-free regions was approximately 0.05 Hz. This is consistent with the overall concept of PPT analysis: deeper defects appear at lowe r frequencies while defects clos er to the surface appear at

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203 higher frequencies. The increased detectability over the time domain results is shown in Figure 6-79. The next step in the an alysis was to construct def vs. frequency plots (Figure 680). The rectangular areas used to define th e defects for the pulse analysis (time domain) were also used in the PPT analysis (frequency domain). def was calculated by taking the difference of the maximum value for phase shift inside the area and the average value of phase shift measured along the perimeter. A rigorous analysis of the resulting def vs. frequency plots was not performed in th e current study. General observations were made for the air-filled defects in Sp ecimens A-1, A-2, A-3, and A-4: The maximum value of def ( max) decreases as the size of the defect decreases There was no observable trend between defect depth and max for Defect IB. This is likely a result of the unintentional def ects that occurred in the multilayer systems max decreased as depth increased for defects of the same size. A visual inspection of the def vs. frequency plots i ndicated that the blind frequency (frequency at which a defect firs t becomes visible in a series of phase images) for air-filled defect s in Specimen A-1 occurred at approximately 0.15 Hz. The blind frequency for Specimen A-2 defect s occurred at approximately 0.075 Hz. The blind frequency for Specimens A-3 a nd A-4 occurred at 0.035 Hz and 0.015 Hz, respectively.

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204 A B Figure 6-77. Application of PPT method to Specimen A-3. A) Time domain results and thermal image. B) Frequency domain results and phase image 0 0.1 0.2 0.3 0.4 0.5 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 (rad)Frequency (Hz) Point 1 Point 2 Point 3 1 Point 1 Point 2 Point 3 0 50 100 150 200 250 20 25 30 35 40 45 T (C)Time (sec) Point 1 Point 2 Point 3 89 Point 1 Point 2 Point 3

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205 A B Figure 6-78. Comparison of time domain and frequency domain (PPT) results for Specimen A-3. A) Phase angle su rface plot at f = 0.0042 Hz. B) Temperature surface plot at t = 41 sec. A B Figure 6-79. Comparison of time domain and frequency domain (PPT) results for Specimen A-4. A) Phase angle su rface plot at f = 0.0042 Hz. B) Temperature surface plot at t = 75 sec. Summary of PPT results It was shown in the previous section on sinusoidal heating that computing phase angle from a series thermal of images can reduce the effects of non-uniform heating. Similar results were obtained with the PPT method. The primary advantage of the PPT method is that phase angle can be computed for multiple frequencies using data obtained from one experiment. The long-pulse experi mental setup was used with a 60 sec pulse duration and data were recorded for 240 sec onds while the specimens cooled. A visual comparison of the phase response for Specimen A-3 using the PPT method (Figure 6-81

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206 A) and the sinusoidal heating method (Figure 6-81 B) indicates that both methods are suitable for detecting defects. From a qualitative perspective, the two results are identical. 0 0.05 0.1 0.15 0.2 0.25 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 (rad)Frequency (Hz) Area 1 Area 2 Area 3 0 0.05 0.1 0.15 0.2 0.25 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 (rad)Frequency (Hz) Area 1 Area 2 Area 3 A B 0 0.05 0.1 0.15 0.2 0.25 0 0.1 0.2 0.3 0.4 (rad)Frequency (Hz) Area 1 Area 2 Area 3 0 0.05 0.1 0.15 0.2 0.25 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 (rad)Frequency (Hz) Area 1 Area 2 Area 3 C D Figure 6-80. Defect signal (phase) vs. frequency plots for air-filled defects. A) Specimen A-1. B) Specime n A-2. C) Specimen A-3. D) Specimen A-4 This issue of determining material composition based on PPT results was not addressed in the current study. Additional work is also needed to investigate the relationship between blind fre quency and defect depth.

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207 A B Figure 6-81. Frequency domain results for Specimen A-3. A) PPT method @ f = 0.0083 Hz. B) Sinusoidal heating @ pulse duration = 125 sec (f = .008 Hz) Comparison of Heating Methods and Analysis Techniques General Detectability Four heating methods were investigated: Flash heating Scan heating Long-pulse heating Sinusoidal heating From the standpoint of general detectabil ity, the scan heating method was the most effective. This method generated a Tmax of nearly 3.8C for Defect IB in Specimen A-4 (four-layer carbon FRP). This value is near ly double the quantity obtained from the longpulse heating setup with a 60 sec pulse duration ( Tmax = 2.0C). The flash heating method only generated a Tmax of 0.6C for Defect IB in Specimen A-4. Scan heating was also the most efficien t means for heating the surface of the FRP composite. It was possible to adequately heat an area of 1858 cm2 in 25 sec using two 500 W halogen lamps. To generate the Tmax of 2.0C in Specimen A-4, the long-pulse heating method required 60 sec and four 500 W halogen lamps (total area = 2787 cm2). The flash heating method was not efficient. Only 0.5 ft2 could be heated each time the flash was fired. It may be possible to im prove the efficiency of flash heating by using a higher powered flash system. It should be noted, however, that the system used in the

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208 current study (two Godard 3.6 kJ power pack s for a total of 6.4 kJ) was valued at over $6000. Increasing the total power supplied by the flash heads will only increase the cost of the system. The flash system also may not be suitable for field applications. Each lamp head requires its own power pack and associated cabling. Gaining access to the FRP composite may be difficult if all of this equipment must be repositioned to heat a small area. The flash bulbs themselves are also quite delicate and can easily be broken ($150 each). The major disadvantage of the scan heating me thod is that it is difficult for a person to control the rate at which the heat source is moved. The final scanning rate that was adopted in the current study was approximate ly 2.2 cm/sec. This corresponded to a heating duration of 12 sec for each specimen. The targeted heating duration was 15 sec, but after multiple attempts to achieve this duration it was determined that 12 sec would have to suffice. Without employing some t ype of mechanical means for moving the heat source across the surface it would be very di fficult to reproduce the same results from one inspection to the next. The final justification for choosing the s can heating method as the best approach for general detectability is based on a side by side comparison of results from the four heating methods (Figure 6-82). A defect must be identi fied in a series of thermal images before a quantitative analysis can be perfor med. This identification entails a certain degree of subjectivity. A defect is more likely to be recognized if the difference between the defect temperature and the b ackground temperature is large. It is also desirable for the background temperature to be uniform. Thermal images are formed by assigning a color or grayscale intensity to a pixel based on temperature

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209 measurements. The range of temperatures wi thin an image is typically divided into 64 (Matlab standard color maps) or 120 bins (Thermacam Researcher 2001 color maps). Ideally, the temperature above the defects will be the hottest areas in an image and the temperature above the background will be the coolest. The thermal images generated with the long-pulse heating set up do not display this feature (Figure 6-82 C). Defect IB in Specimen A-4 is detectable using th e long-pulse setup; however, the maximum temperature in the thermal image appears in th e corner closest to the heat source. There is the potential for misinterpretation of this feature. A B C D Figure 6-82. Comparison of h eating methods for Specimen A-4. A) Flash heating. B) Scan heating. C) Long-pulse he ating. D) Sinusoidal heating

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210 Defect Characterization Four data analysis technique s were also investigated: Pulse (time domain) Step (time domain) Lock-in (frequency domain) Pulse-phase (frequency domain) The pulse analysis procedure utilized da ta that were collec ted while the surface cooled. Five different pulse durations we re investigated. The signal half-life (t1/2) was the most useful parameter for determining de fect depth. Two m odels were calibrated using data from the Series A specimens. Th e following relationshi p was established for air-filled defects: 2 52 33 42 / 1 t d C (6-16) For epoxy-filled defects: 9 9 81 12 / 1 t d C (6-17) C = Circumference of defect (mm) d = Depth of defect (mm) t1/2 = Defect signal half-life The size of the defect and the signal ha lf-life are obtained fr om the series of thermal images. Once these parameters are es tablished, two possible values for defect depth can be determined. One value assumes that the defect is air-filled. The second value assumes the defect is epoxy-filled. This model was calibrated using specimen s containing fabricated defects at the FRP/concrete interface. When the model wa s applied to data co llected from three different specimens, it was shown that good agre ement exists for other defects that occur at the FRP/concrete interface. The model was shown to be ineffective when applied to a defect located between layers of FRP in a multi-layer system.

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211 Step analysis involved analyz ing data that were collected while the specimens were heated. A normalization process was applied to each pixel in the thermal images and the effects of non-uniform heating were reduced. This method also demonstrated that the normalized temperature response on the surface of the FRP is strongly influenced by the thickness of the FRP system. The concrete substrate acts like a large heat sink and reduces the rate of surf ace temperature increase. The heating time for step analysis was limite d to 60 sec in the current study. This pulse duration was shown to be sufficient fo r detecting defects in the one and two-layer FRP systems. It was possible to different iate between defects that occurred at the FRP/concrete interface and defects that occu rred between layers. This was accomplished by examining the Tdef signal generated by a defect and the normalized temperature response of the adjacent defect-free region. The 60 sec pulse duration was not sufficient to detect and characterize defects in the three and four-layer specimens. It wa s later determined that the normalized temperature response ( Tnorm) could also be computed for each pixel while the specimen cooled. Even though the absolute magnitude of Tnorm decreases rapidly after the heat source is removed. The relativ e rate of cooling between defe ct and defect-free areas is still different. A sample surface is for Specimen A-3 is shown in Figure 6-83 A. This surface plot of Tnorm was generated 100 sec after the 60 sec heat pulse was terminated. The effects of non-uniform heating have been removed The two remaining data analysis technique s, lock-in and pulse phase, were also shown to limit the effects of non-uniform heating. It is interesting to make a side by side comparison of the Tnorm results (time domain) and the frequency domain results (Figure

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212 6-83 B and C). The first result shown in Figure 6-83 A was obtai ned by normalizing the series of thermal images using the in itial slope of the temperature vs. t1/2 response. The second result (Figure 6-83 B) was obtained by applyi ng a least squares sinusoidal curve fit to temperature vs. time data and examin ing the phase response. The final result, Figure 6-83 C, was obtained by applying a di screte Fourier transform to the data collected during cooling. All three results are very similar. Most importantly, the results are independent of the intensity of the heat flux that was used to heat the surface. A B C Figure 6-83. Comparison of da ta analysis techniques for Specimen A-3. A) Normalized temperature result at t = 160 sec. B) Lock-in results for pulse duration = 250 sec. C) Pulse phase result at f = 0.0083 Hz. Series B, C, D, and E Specimens The specimens in Series B, C, D, and E we re constructed to i nvestigate the effects of the following FRP system properties on IRT results:

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213 Matrix saturation levels Fiber type Surface preparation methods Fabric saturation methods Lap splices The original intent was to evaluate the response of implanted defects that were placed in specimens with different FRP system properties. All of the implanted defects in Series B and C were created as inte rface bubbles using the procedure outlined in Chapter 5. This defect confi guration produced poor results for the specimens in Series A. The large number of unintentional defect s and other non homogene ities in the FRP composite made the characterization process difficult. Initial investigations into the Series B specimens produced similar results. It was not possible to compare results for the defects in multi-layer FRP systems. Ra ther than focus on the IRT results for the different defects in Series B and Series C, it was determined that an investigation into the response of the defect-free areas would provide more useful information. The step analysis procedure was shown to provide an indication of the thickness of the FRP composite. This is the most important parameter to consider when trying to decide how an FRP composite should be inspec ted using IRT. If th e composite is thin (i.e. the effects of the concrete are observed at early times in th e heating process), the duration of heating and the observation time can be short. Thicker composites require longer heating and observation times. Thick and thin are relative terms. Decisions regarding h eating and observation times should be based on the observed response of the FRP composite to applied heating. In this section, the step anal ysis procedure was used to inve stigate the response of defectfree regions in Series B to E specimens. The objective of this investigation was to

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214 determine if the parameters that were varied between each specimen in these series have an impact on the normalized temperature response. Data Collection Each specimen in Series B, C, D, and E was heated for 60 sec using the long-pulse experimental setup described in Chapter 5. Thermal images were collected at a rate of one frame per second while the specimens were heated. The data were normalized based on the slope of the temperature vs. t1/2 response for 1 t1/2 2. A defect-free area was identified near the center of each specimen. The exact size and orientation of the defectfree areas were not the same on each specimen because of the implanted defects. The size of each area varied between 22.6 cm2 and 55.5 cm2. The mean and standard deviation ( ) of Tnorm were computed for each area at each time step. These results were plotte d against the square root of time (t1/2) with error bars representing +/2 Tnorm vs. t1/2 plots and summary statistics for each curve at t = 60 sec are provided in Appendix C. The following sections contain a summary of these results. Series B Series B contained a total of 18 specimens with different matrix saturation levels (low, medium, and high). Twelve specimens were constructed using carbon-fibers and six specimens were constr ucted using glass-fibers. Low saturation (carbon-fibers) Under saturation of the carbon-fibers was expected to result in small air voids throughout the FRP composite. These air void s should have the following effects on the normalized temperature response:

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215 The overall rate of heat transfer through the FRP composite and into the concrete will be less than for a properly saturated composite. The effects of the concrete substrate should be obser ved at a later time. The normalized temperature distribution wi ll have greater variation across the surface of the composite. Defect-free areas on the one and twolayer specimens (B-LC-1 and B-LC-2) responded similarly to the one and two-layer sp ecimens in Series A. The mean value of Tnorm at the end of heating was -0.27 and -0. 10 for B-LC-1 and B-LC-2, respectively. The corresponding values for the properly saturated systems in Series A were -0.29 and 0.9. Specimens B-LC-3 and B-LC-4 behave d differently than the corresponding three and four-layer specimens in Series A. The mean values for the defect-free areas at the end of heating were higher in the Series B specimens (+0.14 and +0.23 for B-LC-3 and B-LC-4, respectively). This w ould indicate that the thermal front requires more time to reach the FRP/concrete interface. The observations made for the three and f our-layer specimens are consistent with the expected behavior. The standard deviati ons computed for each area were also higher than what was observed for Series A. Careful examination of surface plots for normalized temperature did indicate some vari ability in the temper ature response across the three and four-layer specimens but not nearly to the extent that was expected. It is not possible to ascertain the cause of this va riability without a dest ructive inspection of the FRP composite thro ugh the cross-section. Medium saturation (carbon-fibers) The four specimens contained in the medi um saturation subset were expected to behave the same as the four specimens in Series A. The Series A specimens and the

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216 medium saturation specimens in Series B we re constructed using the amount of epoxy recommended by the FRP system manufacturer. Normalized temperature results for the si ngle-layer specimens appear to be very similar. The mean value of Tnorm for Specimen B-MC-1 was -0.277 ( = 0.023). The mean value of Tnorm for Specimen A-1 was -0.283 ( = 0.018). A Student t-test was performed to validate the hypothesis that the two means are equal (Table 6-23). The corresponding t-test value was 6.66. This value exceeds the tabulated t-value for p = 0.001 (degrees of freedom = ), which indicates that the means are significantly different. The t-test value obtained when comparing the two means for the two-layer specimens was 0.99, which indicates that the means are essentially the same. Table 6-23. Normalized temperature re sponse @ t = 60 sec for properly saturated specimens Series B: medium saturation Series A: medium saturation Layers NP Mean @ t=60 sec Standard deviation @ t = 60 sec NP Mean @ t=60 sec Standard deviation @ t = 60 sec t-test value 1 1350 -0.277 0.023 770 -0.283 .018 6.66 1a 50 -0.277 0.022 50 -0.279 .019 0.49 2 1196 -0.093 0.023 780 -0.092 .021 0.99 3 1092 +0.008 0.017 864 +0.023 .019 18.16 4 990 +0.027 0.025 851 +0.084 .020 54.31 a Defect free areas on Specimen A-1 and Specimen B-MC-1 were randomly sampled for 50 data points The requirements for claiming significance based on Students t-test are very stringent if a large number of data points are considered. A better test for significance may be to randomly sample each of the areas for 50 data points and then compute the mean and standard deviation of the sample. This operation was performed for the defectfree areas on Specimen A-1 and Specimen B-MC-1. The resulting means and standard deviations were essentially the same but the t-value was reduced to 0.48. This t-value indicates that the means are not significantly di fferent. Additional investigation is needed

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217 to determine an appropriate method for samp ling the defect-free regions in a specimen and comparing the normalized temperature response. High saturation (carbon-fibers) Applying excess epoxy during fiber saturation was expected to slow down the rate of heat transfer though the composite. This behavior was observed. The single-layer specimen (B-HC-1) generated a Tnorm response that was similar to the two-layer specimens that were properly saturated. The Tnorm at t = 60 sec for Specimen B-HC-1 was only -0.15 ( = 0.042). The two-layer specimen with heavy saturation displayed similar behavior to a three-layer specimen th at was properly saturated. At the end of heating, the two, three, and four-layer specimens were still grouped together with Tnorm values greater than 0.05. These data s upport the finding from the Phase I study that highly saturated FRP composites will requir e longer heating and observation times than properly saturated composites. Glass-fiber specimens Analysis of the defect-free area in each glass-fiber speci men showed that heat is transferred from the surface of the composite to the concrete more slowly than for carbon-fiber FRP systems. This suggests th at glass-fiber FRP systems require longer heating durations and longer observation times than carbon-fiber FRP systems of the same thickness. For the two-layer specimen, the maximum value of Tnorm (+0.38) occurred at t1/2 = 6.5. At the end of heating, Tnorm was still positive (+0.35). The fourlayer specimen did not achive a local maximum and the Tnorm at the end of heating was +0.55. This indicates that the thermal front had not yet reached the FRP/concrete interface.

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218 Specimens with low and high matrix saturation levels produced different Tnorm results than the properly saturated specimens. The Tnorm was influenced by the concrete at an earlier time for the undersaturated specimens. The Tnorm response of the oversaturated specimens was not influenced by the concrete. Series C Series C contained six specimens. Two variab les were investigated with this series: Surface preparation methods: none, light blast, or heavy blast Use of thickened-epoxy tack-coat No significant difference was observed in the normalized temperature response based on the surface preparation method. A c onsistent trend was observed with respect to specimens constructed with and without thickened-epoxy tack-coat. The three specimens constructed without tack-coat move d heat away from the surface faster than the three specimens constructed with tack-coat. The mean value of Tnorm at t = 60 sec for the three specimens containi ng the thickened epoxy was 0.296 ( = 0.006). These values are consistent with the other single-la yer specimens that were properly saturated and constructed with tack-coat. The mean value for the three specimens without tackcoat was 0.329 ( = 0.003). The two means were comp ared using Students t-test. The resulting t-value of 9.31 shows that the mean s are significantly different (degrees of freedom = 4, p = 0.001, tsig = 8.61). Series D Series D contained three specimens. Th ree different fiber saturation techniques were used during specimen construction: Surface saturation (D-1) Heavy pressure with roller (D-2) Light pressure with roller (D-3)

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219 The first two cases, surface saturation and h eavy pressure with the roller, resulted in composites with fiber weight fractions of 0.50 and 0.51, respectively. The final layer of epoxy top-coat was omitted from Specimen D-1. The third saturation method, light roller, resulted in a fiber weight fraction of 0.40 (i.e. a larger percentage of the composite was epoxy). The total amount of epoxy used to construct each specimen (including saturant, tack-coat and top coat) was: D-1: 43.7 g D-2: 50.6 g D-3: 72.5 g The Tnorm response for this series of specime ns followed a trend that consistent with the total amount of epoxy th at was used to construct each specimen. The composite containing the least amount of epoxy (D-1) was shown to move heat away from the surface more rapidly than the other three specimens with more epoxy. Series E The final series contained three specimens The objective of these specimens was to investigate whether or not IRT could be used to determine the location of lap splices in FRP systems. All three specimens were constructed using properly proportioned composites. The lap splices were designed su ch that the middle third of each specimen contained the thickest portion of the lap splice. It was shown in the previous sections that the step analysis method was capable of distinguishing between FRP systems with different thicknesses. The only thing unique ab out this set of specimens is that the FRP thickness varies within each specimen. In all cases the lap splices were detected. The normalized temperature response for each area was also consistent with results from other

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220 specimens having the same thickness. Surface plots of normalized temperature at t = 60 sec are provided in Appendix C.

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221 CHAPTER 7 SUMMARY AND RECOMMENDATIONS FOR FUTURE RESEARCH Summary The use of fiber-reinforced polymer (F RP) composites for strengthening existing civil infrastructure is expanding rapidly. This research investigated using infrared thermography (IRT) to evaluate bond in FRP systems. Current guidelines provided by ACI, NCHRP, and ICBO specify allowable def ect sizes in FRP systems, and IRT is cited as a possible nondestructive evaluation ( NDE) technique. There are currently no standardized inspection procedures for a pplying IRT to FRP composites bonded to concrete. Results from the current study indicate that IRT is a potentially useful tool for evaluating bond in FRP systems applied to conc rete. It was shown that defects as small as 6.4 mm in diameter can be detected in 1 mm thick carbon FRP composites. For the thickest carbon FRP composite systems investigated in the current study (4 mm), defects as small as 19 mm in diameter were detect ed. The experimental data generated by this research is an important first step toward s the development of st andardized inspection procedures. This chapter contains a summary of the findings from Phase I and Phase II of the current study as well as recommendations for future research. Finally, recommendations are provided for the deployment of IRT for inspecting FRP composites applied to concrete.

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222 Phase I In Phase I of this research, IRT was used to inspect full-scale AASHTO pretensioned concrete bridge girders that were strengthene d with FRP composites. The first part of this phase involved an FDOT study on using FRP composites to strengthen bridges that had been hit by over height vehicles. Four full-scale AASHTO girders with simulated vehicle impact damage were repair ed and strengthened with FRP composites in a laboratory environment. Each FRP syst em was designed and installed by the FRP system manufacturer. In the second part of Phase I, an in service bridge that had been damaged by vehicle impact and repaired with FRP composites was inspected using IRT. Significant findings and conc lusions are highlighted in the following sections. Laboratory Study IRT results were found to vary considerab ly with FRP composite characteristics. The measured thickness of the installed FRP systems, on average, was two times the manufacturers reported thickness for the carbon-fiber systems. Infrared thermography was not effectiv e for the FRP system that contained polyurethane matrix material. This type of matrix material acts as a strong thermal insulator that prevents heat from traveli ng through the FRP composite. IRT is not recommended for use with these types of FRP systems. Thermal images collected using a handpropelled cart result ed in non-uniform heating of the FRP composite due to the variation in cart speed. As a result, quantitative data that was collected for defects could not be compared. The average IRT data collection period was 20 sec, which was found to be too short to identify defects that occurred at depths greater than 1 mm. The scanning procedure, however, did detect defect s located within 1mm of the surface. Constant motion of the camera created images in which the relative position of the subject was constantly changing. Conseque ntly it was necessary to extract the data for each defect frame by frame from a seri es of thermal images. If a quantitative analysis is desired, the IR camera shoul d remain stationary throughout the duration of heating and cooling.

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223 Field Study Applying IRT in the field presented seve ral unanticipated challenges. In the current study, a scissor lift was used to gain access to the FRP composite. The IR camera, the IR camera operator, the heat source, and the heat source operator were all confined to a 6 ft. x 12 ft. area th at was lifted 12 ft above the ground, which inhibited both heating and image collection activity. Furthermore, noise from the generators and adjacent highway traffi c inhibited verbal communication. Positioning the camera with respect to the surface being inspected was also challenging. Installation defects were detected using IRT in the original FRP system used to strengthen the bridge. The IRT results c ould be verified for large defects (on the order of 25 cm2) with a combination of visual inspection and acoustic sounding. Defects smaller than 10 cm2, however, were not detected visually or with acoustic sounding. Damage sustained by the FRP composite syst em due to vehicle impact was limited to the immediate vicinity of the impact. This was verified with acoustic sounding (coin-tap) and IRT. Phase II Results from Phase I indicated that addi tional research was needed to develop a standardized approach for inspecting FRP co mposites applied to concrete. Thirty-four small-scale specimens were constructed. Th ese specimens contained fabricated defects of varying size and depth. The effects of matr ix saturation level were also investigated. The overall objective of Phase II was to i nvestigate different heating methods and quantitative IRT analysis techniques that can be used detect and ch aracterize defects (i.e. determine the size, depth, and material composition). Heating Methods Flash heating was effective for detecting ai r-filled defects in single-layer (1 mm) carbon FRP systems. Defects larger than 12.8 mm in diameter developed a strong signal ( Tmax > 2.0C) that could be used to es timate size. Flash heating produced weak signals for implanted defects in carbon FRP systems 2 mm thick or greater. Scan heating was effective for detecti ng air-filled and epoxy-filled defects in carbon FRP systems up to 4 mm thick. Scan heating also produced higher values for Tmax compared to flash and long-pulse heating.

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224 Long-pulse heating was also effective for detecting air and epoxyfilled defects that occurred up to 4 mm beneath the surface. One disadvantage of the long-pulse method was that the surface was not heated un iformly. It was shown, however, that several data analysis techniques can be employed to reduce the effects of nonuniform heating. Data Analysis Methods Pulse analysis The defect signal strength ( Tdef) vs. time response for implanted defects depended on the following parameters: heat source intens ity; duration of h eating; thickness of the FRP composite system; and the size, depth, and material composition of the defect. The pulse analysis model based on signal half-life (t1/2) provided a good estimate of defect depth for carbon FRP systems up to 3 mm thick. Signal half-life could also be used to de termine if a defect was air-filled or epoxyfilled for carbon FRP composite systems up to 2 mm thick. A model was developed to estimate dept h and material composition of defects based on data collected during cooling. Th is model was calibrated using data from defects that occurred at the FRP/concrete interface. When this model was applied to defects that occurred between layers of FRP, the resulting estimates for depth and composition were not accurate. Step analysis The normalized surface temperature increase ( Tnorm) that was measured during heating was strongly influenced by the th ickness of the FRP composite system. These data could be used to provide an estimate of the thickness and matrix saturation level of the FRP composite system. A 60 sec pulse duration was sufficient to de tect implanted def ects to a depth of 2 mm in properly saturated carbon FRP co mposites. Deeper defects were not detected with the 60 sec pulse duration. It was possible to distinguish between de fects that occurred between layers and defects that occurred at th e FRP/concrete interface. Lock-in analysis Lock-in analysis requires sinusoidal h eating. Unique experiments involving different pulse durations ar e required to inspect FRP composites with different thicknesses.

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225 Lock-in analysis was shown to reduce the effects of non-uniform heating and increase detectability for defects in three and four-layer FRP systems. Pulse phase analysis The pulse phase analysis procedure was us ed on data that were collected during cooling. A discrete Fourier transform wa s applied to each pixel in a series of thermal images to generate a series of phase images. The phase response of defect-free areas wa s not affected by non-uniform heating. This increased detectability for defects in three and four-layer FRP systems. Defects that are close to th e surface appear over a wide range of frequencies while deeper defects only appear in low-frequency phase images. FRP System Properties and IRT Results Oversaturated composites increase the amount of time required for the thermal front to reach the concrete substrate. A 1 mm thick carbon FRP lamina that was oversaturated by 50% displayed a normali zed temperature response similar to a 2 mm thick carbon laminate that was properly saturated. On average, the standard deviation of the normalized temperature response was larger for undersaturated composites ( = 0.035) than for properly saturated composites ( = 0.022). Concrete surface preparation levels had no noticeable affect on the normalized temperature response of specimens in Series C. It was possible to identify the locati on and length of lap splices using the normalized temperature response of defect-free areas. Recommendations for Deployment of IRT Guidelines for Qualit ative IRT Inspections The scan heating method (described in Ch apter 5) is recommended for qualitative IRT inspections. The IR camera should be positioned with re spect to the surfac e under consideration such that at least 10 pixels occupy the dime nsion of the smallest defect of interest in the thermal images. The heat source configuration should be ad justed such that an average temperature increase of 20 C is obtaine d after 15 sec of exposure. A reflective shield should be used to limit the amount of heat energy reflected by the surface of the composite back to the IR camera.

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226 The heat source should be moved from one side of the cameras field of view (FOV) to the other at a constant velocity (same speed and same direction). The heat source should not be waved ba ck and forth across an area (Figure 7-1). If multiple passes are required to heat th e entire area within the IR cameras FOV, the surface temperature of the heated area should return to the initial conditions before the next pass is made. This will avoid overlap of heated areas. Thermal images should be stored at a mi nimum rate of one frame per second for carbon FRP systems less than 2 mm thick. The image save rate may be decreased to one frame per two seconds for carbon FRP systems greater than 2 mm thick. The observation time for carbon FRP systems less than 2 mm thick should be from 3 to 4 minutes. The observation time fo r FRP systems greater than 2 mm thick should be from 4 to 6 minutes. Longer obs ervation times increase certainty that all defects have been detected. Defect size can be estimated using the gradient area method. The temperature difference between the defect and defect -free area should be at least 2.0 C Figure 7-1. Field inspection (scan heating method) of FR P system applied to AASHTO girder. IR Camera was located on the ground (9 feet from surface being heated). A 2x telephoto lens was used to narrow the IR cameras field of view. Quantitative Analysis The long-pulse heating method (described in Chapter 5) is recommended for quantitative IRT inspections. The heat source and camera should remain fixed throughout the duration of heating and cooling. The heat lamps should be configured such that a minimum temperature increase of 5C is experienced by all areas within the IR cameras FOV.

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227 Thermal images should be collected at a minimum rate of one frame per second during heating and cooling. Additiona l research may support increasing the minimum image save rate during the initial stages of heating. This increase in image save rate may improve the estimate of the initial slope used in the step analysis procedure. The duration of the heat pulse and requi red observation time is dictated by the thickness of the FRP system and the amount of matrix material that was used to saturate the composite. A minimum pulse duration of 60 sec is recommended for carbon FRP systems less than 2 mm thick. Future Research The guidelines for performing qualitative IRT inspections were based primarily on experience. A rational method for dete rmining the required heating duration, heating intensity and observation time is still needed. The step analysis method generated norma lized temperature resu lts that could be related to the thickness of the FRP compos ite. It may be possible to relate these data to heating intensity and duration requirements for thicker FRP systems. Additional research is needed to refine the data collection procedure for the step analysis method. Experiments should be conducted using homogenous samples, such as acrylic or polyethylene. Variab les to be considered include the image acquisition rate during the initial stages of heating and the uniformity of the heat pulse as a function of time. Additional research is needed to evaluate the effect of FRP system properties on the response of defect areas. Finite element modeling would be useful for describing the fundamental heat transfer mechanisms that generate temper ature differences above defects. These models would help to provi de a better understand ing of the temperature distribution below the surface of the FRP composite. Further research is needed into th e effects of convection on IRT results. Further research is needed to describe the physical meaning of frequency domain results. Of particular interest is the case where near surfac e defects generate a relative phase response that is smaller than what wa s observed for defect-free regions. The implanted defects that were discussed in the current study all occurred between layers of FRP or at the FRP concrete inte rface. Additional research is required to establish inspection procedures and detecti on limits for defects that occur below the FRP /concrete bond line.

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228 All of the implanted defect s used in the current study were round or elliptical. Further experimental and/or analytical work is needed to investigate the effects of long defects that can not be reas onably contained by a rectangle. Further investigation is needed to verify that the temperature gradients induced by IRT inspections are not harmful to FRP composites.

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229 APPENDIX A TIME DOMAIN RESULTS: SERIES A

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230 IB A75 A50 A2512 25 26 27 E75 E50 E2512 24 24.5 25 (A) A-1 (air-filled) (B) A-1 (epoxy-filled) IB A75 A50 A257 26 26.5 27 27.5 28 E75 E50 E2540 23.4 23.6 23.8 24 (C) A-2 (air-filled) (D) A-2 (epoxy-filled) IB A75 A50 A2561 23.6 23.8 24 24.2 24.4 E75 E50 E2540 23.6 23.7 23.8 23.9 24 24.1 (E) A-3 (air-filled) (F) A-3 (epoxy-filled) IB A75 A50 A2586 24 24.2 24.4 E75 E50 E2579 23.6 23.7 23.8 23.9 24 (G) A-4 (air-filled) (H) A-4 (epoxy-filled) Figure A-1. Flash heating results: Thermal images for Series A.

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231 100 101 102 0 0.5 1 1.5 2 2.5 3 3.5 Tdef (C)Time (sec) IB A75 A50 A25 100 101 102 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Tdef (C)Time (sec) E75 E50 E25 (A) A-1 (air-filled) (B) A-1 (epoxy-filled) 100 101 102 0 0.5 1 1.5 2 2.5 Tdef (C)Time (sec) IB A75 A50 A25 100 101 102 0 0.2 0.4 0.6 0.8 1 Tdef (C)Time (sec) E75 E50 E25 (C) A-2 (air-filled) (D) A-2 (epoxy-filled) 100 101 102 0 0.5 1 1.5 2 Tdef (C)Time (sec) IB A75 A50 A25 100 101 102 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Tdef (C)Time (sec) E75 E50 E25 (E) A-3 (air-filled) (F) A-3 (epoxy-filled) Figure A-2. Flash heating results: Tdef vs. time plots for Series A.

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232 100 101 102 0 0.5 1 1.5 2 2.5 Tdef (C)Time (sec) IB A75 A50 A25 100 101 102 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Tdef (C)Time (sec) E75 E50 E25 (G) A-4 (air-filled) (H) A-4 (epoxy-filled) Figure A-2. Continued

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233 Table A-1. Flash heatin g results for Series A Defect Defect material Tper (C) Tmax (C) tb (sec) tmax (sec) t1/2 (sec)SBR Actual diameter (in) Image diameter (in) COV IB Air 7.1 3.3 0 12 38 40.9 1.34 1.15 0.33 A75 Air 7.0 2.8 1 10 28 33.5 0.75 0.72 0.10 A50 Air 7.0 2.5 0 8 18 23.3 0.50 0.48 0.11 A25 Air 5.8 0.8 2 6 12 3.5 0.25 0.27 0.28 E75 Epoxy 6.7 1.2 1 12 47 6.6 0.75 0.84 0.20 E50 Epoxy 6.4 0.7 3 14 25 6.2 0.50 0.49 0.08 Specimen A-1 E25 Epoxy 5.3 0.4 6 5 20 1.9 0.25 0.34 0.79 Defect Defect material Tper (C) Tmax (C) tb (sec) tmax (sec) t1/2 (sec)SBR Actual diameter (in) Image diameter (in) COV IB Air 7.3 2.4 0 7 55 19.1 1.69 1.46 0.40 A75 Air 7.1 1.1 8 20 47 12.4 0.75 0.74 0.23 A50 Air 7.1 0.7 7 23 35 16.4 0.50 0.56 0.12 A25 Air 5.8 0.25 E75 Epoxy 6.9 0.5 13 40 98 8.6 0.75 0.72 0.66 E50 Epoxy 6.5 0.3 17 38 50 6.7 0.50 0.83 0.62 Specimen A-2 E25 Epoxy 5.3 0.25 Defect Defect material Tper (C) Tmax (C) tb (sec) tmax (sec) t1/2 (sec) SBR Actual diameter (in) Image diameter (in) COV IB Air 6.1 0.9 25 61 136 12.7 1.63 1.56 0.33 A75 Air 6.0 0.6 27 54 91 9.5 0.75 0.75 0.49 A50 Air 5.9 0.3 18 36 60 7.3 0.50 0.85 0.62 A25 Air 4.9 0.25 E75 Epoxy 5.9 0.75 E50 Epoxy 5.4 0.50 Specimen A-3 E25 Epoxy 4.3 0.25 Defect Defect material Tper (C) Tmax (C) tb (sec) tmax (sec) t1/2 (sec)SBR Actual diameter (in) Image diameter (in) COV IB Air 8.1 0.6 37 86 10.8 1.63 1.69 0.47 A75 Air 7.9 0.4 25 72 146 5.1 0.75 0.87 0.65 A50 Air 7.9 0.50 A25 Air 6.4 0.25 E75 Epoxy 7.4 0.75 E50 Epoxy 7.0 0.50 Specimen A-4 E25 Epoxy 5.6 0.25

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234 IB A75 A50 A2521 30 35 40 45 50 E75 E50 E2519 32 34 36 (A) A-1 (air-filled) (B) A-1 (epoxy-filled) IB A75 A50 A2520 35 40 45 50 E75 E50 E2541 27 27.5 28 28.5 29 (C) A-2 (air-filled) (D) A-2 (epoxy-filled) IB A75 A50 A2571 27 28 29 30 31 32 E75 E50 E2552 28.5 29 29.5 30 (E) A-3 (air-filled) (F) A-3 (epoxy-filled) IB A75 A50 A2593 27 28 29 30 31 E75 E50 E2589 25.8 26 26.2 26.4 26.6 26.8 (G) A-4 (air-filled) (H) A-4 (epoxy-filled) Figure A-3. Scan heating results: Thermal images for Series A.

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235 100 101 102 0 5 10 15 20 Tdef (C)Time (sec) IB A75 A50 A25 100 101 102 0 1 2 3 4 5 6 7 Tdef (C)Time (sec) E75 E50 E25 (A) A-1 (air-filled) (B) A-1 (epoxy-filled) 100 101 102 0 2 4 6 8 10 12 14 Tdef (C)Time (sec) IB A75 A50 A25 100 101 102 0 2 4 6 8 Tdef (C)Time (sec) E75 E50 E25 (C) A-2 (air-filled) (D) A-2 (epoxy-filled) 100 101 102 0 2 4 6 8 10 12 Tdef (C)Time (sec) IB A75 A50 A25 100 101 102 0 2 4 6 8 10 Tdef (C)Time (sec) E75 E50 E25 (E) A-3 (air-filled) (F) A-3 (epoxy-filled) Figure A-4. Scan heating results: Tdef vs. time plots for Series A.

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236 100 101 102 0 2 4 6 8 Tdef (C)Time (sec) IB A75 A50 A25 100 101 102 0 1 2 3 4 5 6 Tdef (C)Time (sec) E75 E50 E25 (G) A-4 (air-filled) (H) A-4 (epoxy-filled) Figure A-4. Continued

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237 Table A-2. Scan heating results for Series A Defect Defect material Tper (C) Tmax (C) tb (sec) tmax (sec) t1/2 (sec)SBR Actual diameter (in) Image diameter (in) COV IB Air 17.2 18.8 0 8 36 14.9 1.34 1.08 0.28 A75 Air 21.8 15.7 0 9 25 22.8 0.75 0.69 0.10 A50 Air 19.1 12.3 0 5 18 10.5 0.50 0.47 0.24 A25 Air 22.3 4.0 0 4 12 4.8 0.25 0.29 0.82 E75 Epoxy 23.5 6.7 1 11 37 13.0 0.75 0.79 0.09 E50 Epoxy 19.6 4.5 0 7 26 7.5 0.50 0.50 0.15 Specimen A-1 E25 Epoxy 22.8 2.3 1 6 16 5.5 0.25 0.39 0.33 Defect Defect material Tper (C) Tmax (C) tb (sec) tmax (sec) t1/2 (sec)SBR Actual diameter (in) Image diameter (in) COV IB Air 23.8 13.0 0 7 61 7.2 1.69 1.51 0.25 A75 Air 26.4 6.5 1 18 41 13.7 0.75 0.71 0.13 A50 Air 22.9 4.1 3 18 32 11.4 0.50 0.54 0.13 A25 Air 25.6 0.9 9 15 25 3.1 0.25 0.36 0.31 E75 Epoxy 23.3 2.0 14 33 70 10.7 0.75 0.78 0.15 E50 Epoxy 20.6 1.5 12 30 57 6.2 0.50 0.56 0.21 Specimen A-2 E25 Epoxy 23.1 0.7 11 20 34 3.5 0.25 0.45 0.69 Defect Defect material Tper (C) Tmax (C) tb (sec) tmax (sec) t1/2 (sec) SBR Actual diameter (in) Image diameter (in) COV IB Air 25.4 5.5 20 58 104 14.1 1.63 1.51 0.27 A75 Air 26.5 2.9 23 38 69 6.1 0.75 0.80 0.12 A50 Air 25.2 1.7 14 31 56 5.4 0.50 0.57 0.20 A25 Air 28.0 0.25 E75 Epoxy 30.8 1.6 21 44 73 5.2 0.75 0.77 0.16 E50 Epoxy 26.2 0.8 30 42 54 3.3 0.50 0.49 0.22 Specimen A-3 E25 Epoxy 29.0 0.25 Defect Defect material Tper (C) Tmax (C) tb (sec) tmax (sec) t1/2 (sec)SBR Actual diameter (in) Image diameter (in) COV IB Air 26.3 3.8 40 80 129 10.4 1.63 1.55 0.24 A75 Air 29.2 1.8 30 62 91 3.8 0.75 0.86 0.22 A50 Air 24.1 0.50 0.98 0.46 A25 Air 0.25 E75 Epoxy 22.1 0.7 52 80 86 3.5 0.75 0.79 0.39 E50 Epoxy 0.50 Specimen A-4 E25 Epoxy 0.25

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238 IB A75 A50 A254 30 35 40 E75 E50 E258 28 29 30 31 (A) A-1 (air-filled) (B) A-1 (epoxy-filled) IB A75 A50 A252 32 34 36 38 40 42 E75 E50 E2537 24.6 24.8 25 25.2 25.4 25.6 (C) A-2 (air-filled) (D) A-2 (epoxy-filled) IB A75 A50 A2550 25.5 26 26.5 27 27.5 28 E75 E50 E2544 26.4 26.6 26.8 27 27.2 (E) A-3 (air-filled) (F) A-3 (epoxy-filled) IB A75 A50 A2566 25 25.5 26 E75 E50 E2517 26 26.5 2 7 (G) A-4 (air-filled) (H) A-4 (epoxy-filled) Figure A-5. Long-pulse (30 s ec) heating results : Thermal images for Series A.

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239 100 101 102 0 2 4 6 8 10 12 14 Tdef (C)Time (sec) IB A75 A50 A25 100 101 102 0 0.5 1 1.5 2 2.5 3 3.5 Tdef (C)Time (sec) E75 E50 E25 (A) A-1 (air-filled) (B) A-1 (epoxy-filled) 100 101 102 0 2 4 6 8 10 Tdef (C)Time (sec) IB A75 A50 A25 100 101 102 0 0.5 1 1.5 2 Tdef (C)Time (sec) E75 E50 E25 (C) A-2 (air-filled) (D) A-2 (epoxy-filled) 100 101 102 0 0.5 1 1.5 2 2.5 3 Tdef (C)Time (sec) IB A75 A50 A25 100 101 102 0 0.5 1 1.5 2 Tdef (C)Time (sec) E75 E50 E25 (E) A-3 (air-filled) (F) A-3 (epoxy-filled) Figure A-6. Long-pulse ( 30 sec) heating results: Tdef vs. time plots for Series A.

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240 100 101 102 0 0.5 1 1.5 2 2.5 Tdef (C)Time (sec) IB A75 A50 A25 100 101 102 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Tdef (C)Time (sec) E75 E50 E25 (G) A-4 (air-filled) (H) A-4 (epoxy-filled) Figure A-6. Continued

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241 Table A-3. Long-pulse (30 sec) heating results for Series A Defect Defect material Tper (C) Tmax (C) tb (sec) tmax (sec) t1/2 (sec)SBR Actual diameter (in) Image diameter (in) COV IB Air 9.0 12.7 0 4 33 17.7 1.34 1.07 0.30 A75 Air 7.2 8.1 0 3 26 12.5 0.75 0.72 0.08 A50 Air 5.6 4.6 0 1 18 11.1 0.50 0.48 0.15 A25 Air 5.0 1.2 3 0 13 5.0 0.25 0.25 1.00 E75 Epoxy 7.1 3.2 0 8 38 7.4 0.75 0.80 0.09 E50 Epoxy 5.7 1.7 0 5 29 7.5 0.50 0.47 0.16 Specimen A-1 E25 Epoxy 4.9 0.8 0 1 18 5.0 0.25 0.33 0.86 Defect Defect material Tper (C) Tmax (C) tb (sec) tmax (sec) t1/2 (sec)SBR Actual diameter (in) Image diameter (in) COV IB Air 10.4 8.6 0 2 51 5.8 1.69 1.55 0.37 A75 Air 7.6 2.9 0 7 47 4.2 0.75 0.74 0.12 A50 Air 5.8 1.1 2 12 34 3.4 0.50 0.56 0.25 A25 Air 5.0 0.25 E75 Epoxy 6.0 0.7 20 37 62 2.7 0.75 0.81 0.21 E50 Epoxy 5.0 0.50 Specimen A-2 E25 Epoxy 4.3 0.25 Defect Defect material Tper (C) Tmax (C) tb (sec) tmax (sec) t1/2 (sec) SBR Actual diameter (in) Image diameter (in) COV IB Air 7.9 2.2 15 50 100 5.7 1.63 1.52 0.30 A75 Air 9.6 1.3 16 31 68 3.9 0.75 0.83 0.17 A50 Air 11.9 0.7 19 39 50 2.4 0.50 0.57 0.24 A25 Air 13.9 0.25 E75 Epoxy 9.5 0.6 30 44 59 2.2 0.75 0.78 0.47 E50 Epoxy 11.4 0.50 Specimen A-3 E25 Epoxy 13.3 0.25 Defect Defect material Tper (C) Tmax (C) tb (sec) tmax (sec) t1/2 (sec)SBR Actual diameter (in) Image diameter (in) COV IB Air 6.8 1.2 30 66 129 3.4 1.63 1.58 0.41 A75 Air 8.0 0.7 33 62 82 2.2 0.75 0.94 0.46 A50 Air 7.9 0.50 A25 Air 9.0 0.25 E75 Epoxy 5.6 0.75 E50 Epoxy 6.4 0.50 Specimen A-4 E25 Epoxy 7.0 0.25

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242 IB A75 A50 A254 35 40 45 50 E75 E50 E256 31 32 33 34 35 (A) A-1 (air-filled) (B) A-1 (epoxy-filled) IB A75 A50 A253 35 40 45 E75 E50 E2522 27 27.5 28 (C) A-2 (air-filled) (D) A-2 (epoxy-filled) IB A75 A50 A2539 28 29 30 31 E75 E50 E2545 28 28.5 29 (E) A-3 (air-filled) (F) A-3 (epoxy-filled) IB A75 A50 A2556 26.5 27 27.5 28 28.5 E75 E50 E2519 27.5 28 28.5 29 (G) A-4 (air-filled) (H) A-4 (epoxy-filled) Figure A-7. Long-pulse (45 s ec) heating results : Thermal images for Series A.

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243 100 101 102 0 5 10 15 20 Tdef (C)Time (sec) IB A75 A50 A25 100 101 102 0 1 2 3 4 5 Tdef (C)Time (sec) E75 E50 E25 (A) A-1 (air-filled) (B) A-1 (epoxy-filled) 100 101 102 0 2 4 6 8 10 12 Tdef (C)Time (sec) IB A75 A50 A25 100 101 102 0 0.5 1 1.5 2 Tdef (C)Time (sec) E75 E50 E25 (C) A-2 (air-filled) (D) A-2 (epoxy-filled) 100 101 102 0 0.5 1 1.5 2 2.5 3 3.5 Tdef (C)Time (sec) IB A75 A50 A25 100 101 102 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Tdef (C)Time (sec) E75 E50 E25 (E) A-3 (air-filled) (F) A-3 (epoxy-filled) Figure A-8. Long-pulse ( 45 sec) heating results: Tdef vs. time plots for Series A.

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244 100 101 102 0 0.5 1 1.5 2 2.5 Tdef (C)Time (sec) IB A75 A50 A25 100 101 102 0 0.5 1 1.5 2 Tdef (C)Time (sec) E75 E50 E25 (G) A-4 (air-filled) (H) A-4 (epoxy-filled) Figure A-8. Continued

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245 Table A-4. Long-pulse (45 sec) heating results for Series A Defect Defect material Tper (C) Tmax (C) tb (sec) tmax (sec) t1/2 (sec)SBR Actual diameter (in) Image diameter (in) COV IB Air 10.6 16.9 0 4 32 18.6 1.34 1.07 0.28 A75 Air 8.5 10.1 0 4 24 14.5 0.75 0.70 0.10 A50 Air 6.5 5.5 0 1 19 10.3 0.50 0.48 0.16 A25 Air 5.7 1.3 0 0 14 5.1 0.25 0.31 0.79 E75 Epoxy 8.2 4.3 0 6 40 7.1 0.75 0.77 0.15 E50 Epoxy 6.5 2.1 0 4 28 5.3 0.50 0.49 0.16 Specimen A-1 E25 Epoxy 5.6 0.9 0 4 19 4.8 0.25 0.30 0.93 Defect Defect material Tper (C) Tmax (C) tb (sec) tmax (sec) t1/2 (sec)SBR Actual diameter (in) Image diameter (in) COV IB Air 12.4 11.6 0 3 53 6.4 1.69 1.54 0.34 A75 Air 9.3 4.1 0 4 44 4.5 0.75 0.73 0.19 A50 Air 6.7 1.7 0 11 33 4.7 0.50 0.58 0.20 A25 Air 5.8 0.25 E75 Epoxy 7.2 1.2 11 22 69 2.9 0.75 0.80 0.19 E50 Epoxy 6.0 0.7 14 19 57 2.2 0.50 0.63 0.62 Specimen A-2 E25 Epoxy 5.1 0.25 Defect Defect material Tper (C) Tmax (C) tb (sec) tmax (sec) t1/2 (sec) SBR Actual diameter (in) Image diameter (in) COV IB Air 10 3.4 12 39 105 5.9 1.63 1.51 0.28 A75 Air 12.1 2 16 28 66 4.4 0.75 0.81 0.19 A50 Air 14.4 1.2 7 30 50 2.5 0.50 0.61 0.22 A25 Air 17.1 0.25 E75 Epoxy 11.7 0.9 11 45 67 2.5 0.75 0.76 0.29 E50 Epoxy 14.1 0.50 Specimen A-3 E25 Epoxy 16.4 0.25 Defect Defect material Tper (C) Tmax (C) tb (sec) tmax (sec) t1/2 (sec)SBR Actual diameter (in) Image diameter (in) COV IB Air 8.6 1.8 41 56 133 3.4 1.63 1.66 0.44 A75 Air 10.1 1.0 27 48 93 2.3 0.75 0.86 0.28 A50 Air 10.3 0.50 A25 Air 11.6 0.25 E75 Epoxy 7.2 0.75 E50 Epoxy 8.3 0.50 Specimen A-4 E25 Epoxy 9.2 0.25

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246 IB A75 A50 A252 35 40 45 50 E75 E50 E255 30 31 32 33 34 35 (A) A-1 (air-filled) (B) A-1 (epoxy-filled) IB A75 A50 A252 35 40 45 E75 E50 E2518 27 27.5 28 28.5 (C) A-2 (air-filled) (D) A-2 (epoxy-filled) IB A75 A50 A2541 28 29 30 31 E75 E50 E2543 28 28.5 29 (E) A-3 (air-filled) (F) A-3 (epoxy-filled) IB A75 A50 A2575 25.5 26 26.5 27 27.5 28 E75 E50 E2523 27 27.5 28 (G) A-4 (air-filled) (H) A-4 (epoxy-filled) Figure A-9. Long-pulse (60 s ec) heating results : Thermal images for Series A.

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247 100 101 102 0 5 10 15 20 Tdef (C)Time (sec) IB A75 A50 A25 100 101 102 0 1 2 3 4 5 Tdef (C)Time (sec) E75 E50 E25 (A) A-1 (air-filled) (B) A-1 (epoxy-filled) 100 101 102 0 2 4 6 8 10 12 14 Tdef (C)Time (sec) IB A75 A50 A25 100 101 102 0 0.5 1 1.5 2 Tdef (C)Time (sec) E75 E50 E25 (C) A-2 (air-filled) (D) A-2 (epoxy-filled) 100 101 102 0 1 2 3 4 Tdef (C)Time (sec) IB A75 A50 A25 100 101 102 0 0.5 1 1.5 2 Tdef (C)Time (sec) E75 E50 E25 (E) A-3 (air-filled) (F) A-3 (epoxy-filled) Figure A-10. Long-pulse (60 sec) heating results: Tdef vs. time plots for Series A.

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248 100 101 102 0 0.5 1 1.5 2 2.5 3 Tdef (C)Time (sec) IB A75 A50 A25 100 101 102 0 0.5 1 1.5 2 Tdef (C)Time (sec) E75 E50 E25 (G) A-4 (air-filled) (H) A-4 (epoxy-filled) Figure A-10. Continued

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249 Table A-5. Long-pulse (60 sec) heating results for Series A Defect Defect material Tper (C) Tmax (C) tb (sec) tmax (sec) t1/2 (sec)SBR Actual diameter (in) Image diameter (in) COV IB Air 10.6 18.1 0 2 33 20.6 1.34 1.05 0.29 A75 Air 8.6 11.0 0 2 25 13.6 0.75 0.71 0.09 A50 Air 6.6 5.7 2 0 18 10.5 0.50 0.49 0.17 A25 Air 5.9 1.3 2 0 12 4.8 0.25 0.21 0.94 E75 Epoxy 8.5 4.8 0 5 42 8.2 0.75 0.79 0.08 E50 Epoxy 6.7 2.4 0 5 27 5.8 0.50 0.47 0.17 Specimen A-1 E25 Epoxy 5.8 0.9 2 3 17 3.9 0.25 0.22 1.43 Defect Defect material Tper (C) Tmax (C) tb (sec) tmax (sec) t1/2 (sec)SBR Actual diameter (in) Image diameter (in) COV IB Air 12.7 12.9 0 2 53 8.0 1.69 1.57 0.43 A75 Air 9.6 4.8 0 2 44 4.9 0.75 0.73 0.11 A50 Air 7.0 1.9 0 5 37 4.1 0.50 0.56 0.23 A25 Air 0.25 E75 Epoxy 7.5 1.3 9 18 63 2.8 0.75 0.83 0.15 E50 Epoxy 0.50 Specimen A-2 E25 Epoxy 0.25 Defect Defect material Tper (C) Tmax (C) tb (sec) tmax (sec) t1/2 (sec) SBR Actual diameter (in) Image diameter (in) COV IB Air 10.4 3.9 8 41 100 6.5 1.63 1.49 0.30 A75 Air 12.4 2.4 11 29 61 5.0 0.75 0.82 0.19 A50 Air 15.0 1.4 14 26 53 2.6 0.50 0.61 0.29 A25 Air 18.2 0.25 E75 Epoxy 12.1 1.1 20 43 70 2.9 0.75 0.77 0.22 E50 Epoxy 14.4 0.50 Specimen A-3 E25 Epoxy 17.1 0.25 Defect Defect material Tper (C) Tmax (C) tb (sec) tmax (sec) t1/2 (sec)SBR Actual diameter (in) Image diameter (in) COV IB Air 9.2 2.0 27 75 114 4.1 1.63 1.52 0.32 A75 Air 10.6 1.2 17 37 97 2.5 0.75 0.83 0.48 A50 Air 10.5 0.50 A25 Air 12.4 0.25 E75 Epoxy 7.6 0.75 E50 Epoxy 8.6 0.50 Specimen A-4 E25 Epoxy 9.5 0.25

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250 APPENDIX B SINUSOIDAL HEATING RESULTS: SERIES A

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251 A1 A2 A3 0.7 0.8 0.9 1 1.1 A1 A2 0.75 0.8 0.85 0.9 0.95 1 (A) A-1 (air-filled) (B) A-1 (epoxy-filled) A1 A2 A3 0.8 0.9 1 1.1 A1 A2 0.8 0.85 0.9 0.95 (C) A-2 (air-filled) (D) A-2 (epoxy-filled) A1 A2 A3 0.8 0.9 1 1.1 A1 A2 0.77 0.78 0.79 0.8 0.81 0.82 (E) A-3 (air-filled) (F) A-3 (epoxy-filled) A1 A2 A3 0.85 0.9 0.95 1 1.05 A1 A2 0.86 0.88 0.9 (G) A-4 (air-filled) (H) A-4 (epoxy-filled) Figure B-1. Sinusoidal heating resu lts: Phase images for Series A.

PAGE 272

252 0 0.05 0.1 0.15 0.2 0 0.1 0.2 0.3 0.4 0.5 Phase vs. Frequency Data Air-Filled Defects (rad)Frequency (Hz) Area 1 Area 2 Area 3 0 0.05 0.1 0.15 0.2 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Phase vs. Frequency Data Epoxy-Filled Defects (rad)Frequency (Hz) Area 1 Area 2 (A) A-1 (air-filled) (B) A-1 (epoxy-filled) 0 0.05 0.1 0.15 0.2 0 0.1 0.2 0.3 0.4 0.5 Phase vs. Frequency Data Air-Filled Defects (rad)Frequency (Hz) Area 1 Area 2 Area 3 0 0.05 0.1 0.15 0.2 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Phase vs. Frequency Data Epoxy-Filled Defects (rad)Frequency (Hz) Area 1 Area 2 (C) A-2 (air-filled) (D) A-2 (epoxy-filled) 0 0.05 0.1 0.15 0.2 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Phase vs. Frequency Data Air-Filled Defects (rad)Frequency (Hz) Area 1 Area 2 Area 3 0 0.05 0.1 0.15 0.2 0.02 0.03 0.04 0.05 0.06 Phase vs. Frequency Data Epoxy-Filled Defects (rad)Frequency (Hz) Area 1 Area 2 (E) A-3 (air-filled) (F) A-3 (epoxy-filled) Figure B-2. Sinusoida l heating results: vs. frequency plots for Series A.

PAGE 273

253 0 0.05 0.1 0.15 0.2 0 0.05 0.1 0.15 0.2 0.25 Phase vs. Frequency Data Air-Filled Defects (rad)Frequency (Hz) Area 1 Area 2 Area 3 0 0.05 0.1 0.15 0.2 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Phase vs. Frequency Data Epoxy-Filled Defects (rad)Frequency (Hz) Area 1 Area 2 (G) A-4 (air-filled) (H) A-4 (epoxy-filled) Figure B-2. Continued

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254 Table B-1. Sinusoida l heating results: vs. frequency plot para meters for Series A. Defect Defect material Actual size (in2) Image size (Pixels) Image size (in2) max (rad) SBR IB Air 1.1 173 1.16 0.41 24.6 A75 Air 0.44 84 0.56 0.31 22.8 A50 Air 0.20 37 0.25 0.25 15.5 E75 Epoxy 0.44 80 0.54 0.25 23.5 Specimen A-1 E50 Epoxy 0.20 36 0.24 0.16 29.0 Defect Defect material Actual size (in2) Image size (Pixels) Image size (in2) max (rad) SBR IB Air 2.0 143 0.96 0.36 24.2 A75 Air 0.44 99 0.66 0.22 13.6 A50 Air 0.20 45 0.30 0.15 17.1 E75 Epoxy 0.44 76 0.51 0.14 18.8 Specimen A-2 E50 Epoxy 0.20 50 0.34 0.10 11.9 Defect Defect material Actual size (in2) Image size (Pixels) Image size (in2) max (rad) SBR A1 Air 2.0 243 1.63 0.31 13.1 A2 Air 0.44 74 0.50 0.12 5.4 A3 Air 0.20 66 0.44 0.06 9.8 E1 Epoxy 0.44 109 0.73 0.06 7.0 Specimen A-3 E2 Epoxy 55 0.37 0.04 7.2 Defect Defect material Actual size (in2) Image size (Pixels) Image size (in2) max (rad) SBR A1 Air 2.0 226 1.52 0.21 8.0 A2 Air 0.44 96 0.64 0.09 4.2 A3 Air 0.20 98 0.66 0.04 5.0 E1 Epoxy 0.44 88 0.59 0.05 7.6 Specimen A-4 E2 Epoxy 0.20

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255 APPENDIX C SERIES B, C, D, AND E RESULTS

PAGE 276

256 0 1 2 3 4 5 6 7 8 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 TnormSquare Root of Time (sec1/2) 1-Layer 2-Layer 3-Layer 4-Layer Figure C-1. Low matrix satu ration (Series B carbon-fibers) 0 1 2 3 4 5 6 7 8 -0.35 -0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 TnormSquare Root of Time (sec1/2) 1-Layer 2-Layer 3-Layer 4-Layer Figure C-2. Medium matrix sa turation (Series B carbon-fibers)

PAGE 277

257 0 1 2 3 4 5 6 7 8 -0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25 TnormSquare Root of Time (sec1/2) 1-Layer 2-Layer 3-Layer 4-Layer Figure C-3. High matrix satu ration (Series B carbon-fibers) 0 1 2 3 4 5 6 7 8 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 TnormSquare Root of Time (sec1/2) 2-Layer 4-Layer Figure C-4. Low matrix saturation (Series B glass-fibers)

PAGE 278

258 0 1 2 3 4 5 6 7 8 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 TnormSquare Root of Time (sec1/2) 2-Layer 4-Layer Figure C-5. Medium matrix satu ration (Series B glass-fibers) 0 1 2 3 4 5 6 7 8 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 TnormSquare Root of Time (sec1/2) 2-Layer 4-Layer Figure C-6. High matrix satu ration (Series B glass-fibers)

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259 Table C-1. Series B (low saturation) summary statistics for defect-free areas Specimen NP Mean @ t=60 sec Standard deviation @ t = 60 sec B-LC-1 1350 -.27 0.030 B-LC-2 1196 -0.10 0.028 B-LC-3 1092 +0.14 0.036 Carbon FRP (low saturation) B-LC-4 990 +0.23 0.047 Specimen NP Mean @ t=60 sec Standard Deviation @ t = 60 sec B-MC-1 1350 -.28 0.023 B-MC-2 1196 -0.09 0.023 B-MC-3 1092 +0.01 0.017 Carbon FRP (med saturation) B-MC-4 990 +0.03 0.025 Specimen NP Mean @ t=60 sec Standard Deviation @ t = 60 sec B-HC-1 780 -.15 0.042 B-HC-2 1196 +0.09 0.025 B-HC-3 1092 +0.13 0.028 Carbon FRP (high saturation) B-HC-4 990 +0.17 0.028 Specimen NP Mean @ t=60 sec Standard Deviation @ t = 60 sec B-LG-2 1088 +.236 .048 B-MG-2 621 +.350 .044 B-HG-2 621 +.525 .048 B-LG-4 1036 +.406 .065 B-MG-4 575 +.507 .044 Glass FRP B-HG-4 575 +.634 .056

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260 0 1 2 3 4 5 6 7 8 -0.35 -0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 TnormSquare Root of Time (sec1/2) C-1 (Tack-Coat) C-2 (No Tack-Coat) Figure C-7. No surface prepar ation (Series C carbon-fibers) 0 1 2 3 4 5 6 7 8 -0.35 -0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 TnormSquare Root of Time (sec1/2) C-3 (Tack-Coat) C-4 (No Tack-Coat) Figure C-8. Light blast surface pr eparation (Series C carbon-fibers)

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261 0 1 2 3 4 5 6 7 8 -0.4 -0.35 -0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 TnormSquare Root of Time (sec1/2) C-5 (Tack-Coat) C-6 (No Tack-Coat) Figure C-9. Heavy blast surface pr eparation (Series C carbon-fibers) Table C-2. Series C (surface prep) su mmary statistics for defect-free areas Specimen TackCoat NP Mean @ t=60 sec Standard Deviation @ t = 60 sec C-1 Yes 621 -.296 .012 C-3 Yes 621 -.291 .020 C-5 Yes 621 -.302 .013 C-2 No 575 -.327 .016 C-4 No 575 -.332 .011 C-6 No 575 -.328 .014

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262 0 1 2 3 4 5 6 7 8 -0.4 -0.35 -0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 TnormSquare Root of Time (sec1/2) D-1 D-2 D-3 Figure C-10. Different fiber satura tion methods (Series D carbon-fibers) Table C-3. Series D (fiber saturation methods) summary stat istics for defect-free areas Specimen NP Mean @ t=60 sec Standard Deviation @ t = 60 sec D-1 1088 -.343 .018 D-2 621 -.271 .024 D-3 621 -.212 .038

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263 Figure C-11. Specimen E-1 (1-layer/3-layer/2-layer) Figure C-12. Specimen E-2 (2-layer/3-layer/2-layer)

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264 Figure C-13. Specimen E-3 (3-layer/4-layer/2-layer)

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265 APPENDIX D COMPOSITE PROPERTIES FO R SMALL-SCALE SPECIMENS

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266 Table D-1. Series A composite properties Specimen ID Layers Surface prep Saturant (g) Tack coat (g) Fibers (g) Matrix (g) Top coat (g) wf a A-1 1 LB 6 11 31 31 9 0.50 A-2 2 LB 5 11 62 62 11 0.50 A-3 3 LB 6 10 93 93 9 0.50 A-4 4 LB 6 12 124 124 11 0.50 a wf is weight fraction of fibers Table D-2. Series B composite properties Specimen ID Layers Surface prep Saturant (g) Tack coat (g) Fibers (g) Matrix (g) Top coat (g) wf B-GL-2 1 LB 8 9 88 43 7 0.67 B-GL-4 2 LB 8 10 181 85 10 0.68 B-CL-1 1 LB 7 7 32 16 9 0.67 B-CL-2 2 LB 7 11 63 30 7 0.68 B-CL-3 3 LB 7 9 97 48 11 0.67 B-CL-4 4 LB 7 10 127 70 8 0.64 B-GM-2 1 LB 6 7 88 74 9 0.54 B-GM-4 2 LB 5 10 179 159 9 0.53 B-CM-1 1 LB 6 9 33 31 9 0.52 B-CM-2 2 LB 6 7 63 67 7 0.48 B-CM-3 3 LB 7 11 96 96 9 0.50 B-CM-4 4 LB 8 8 125 125 8 0.50 B-GH-2 1 LB 6 7 90 106 9 0.44 B-GH-4 2 LB 5 10 176 236 9 0.43 B-CH-1 1 LB 6 9 33 44 9 0.43 B-CH-2 2 LB 6 7 63 109 7 0.37 B-CH-3 3 LB 7 11 92 127 9 0.42 B-CH-4 4 LB 8 8 124 180 8 0.41 Table D-3. Series C composite properties Specimen ID Layers Surface prep Saturant (g) Tack coat (g) Fibers (g) Matrix (g) Top coat (g) wf C-1 1 None 6.3 11.0 28.7 36.7 0 0.44 C-2 1 None 6.1 NA 29.4 40.1 0 0.42 C-3 1 LB 6.5 8.0 29.1 36.3 0 0.45 C-4 1 LB 7.9 NA 28.8 36.4 0 0.44 C-5 1 HB 7.0 12.8 28.3 33.8 0 0.46 C-6 1 HB 6.5 NA 28.2 37.1 0 0.43

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267 Table D-4. Series D composite properties Specimen ID Layers Surface prep Saturant (g) Tack coat (g) Fibers (g) Matrix (g) Top coat (g) wf D-1 1 LB 5.7 10.0 28.3 28.0 0 0.50 D-2 1 LB 6.5 11.0 29.8 28.4 11.2 0.51 D-3 1 LB 7.1 10.5 28.7 43.4 11.5 0.40 Table D-5. Series E composite properties Specimen ID Layers Surface prep Saturant (g) Tack coat (g) Fibers (g) Matrix (g) Top coat (g) wf E-1 2/3/1 LB 7 8 65 51 7 0.56 E-2 2/3/2 LB 9 7 78 74 11 0.51 E-3 2/4/3 LB 6 11 94 111 9 0.46

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268 LIST OF REFERENCES American Concrete Institute (ACI) Comm ittee 440 (2002). Guide for the design and construction of externally bonded FRP systems for strengthening concrete structures. American Concrete Instit ute, ACI 440.2R-02, Farmington Hills, Michigan. American Society for Testing and Material s (ASTM) (1997). Standard practice for location of wet insulation in roofing sy stems using infrared imaging. C 1153-97. ASTM, West Conshohocken, Pennsylvania. American Society for Testing and Materials (ASTM) (2003). Standard test method for detecting delaminations in bridge deck s using infrared thermography. D 4788. ASTM, West Conshohocken, Pennsylvania. ASM Materials Handbook (2005). ASM Handbooks Online. ASM International, 9639 Kinsman Road, Materials Park, OH. h ttp://products.asmi nternational.org. Avdelidis, N. P., D. P. Almond, Dobbinson, A., Hawtin, B., Ibarra -Castanedo, Clemente, Maldague, Xavier P. (2004). Thermal transient thermographic NDT and E of composites. Thermosense XXVI, Edited by D.D. Burleigh, Eliot Cramer, G. Raymond, Peacock. Proceedings of SPIE, Vol. 5405, pp. 403-413. Bai, W. and B. S. Wong (2001). Phototh ermal models for lock-in thermographic evaluation of plates with finite thickness under convection conditions. Journal of Applied Physics 89(6): 3275. Carlomagno, G. M. and C. Meola (2002) Comparison between thermographic techniques for frescoes NDT. ND T & E International 35(8): 559-565. CERF (2001). Gap analysis for durability of fiber reinforced polymer composites in civil infrastructure. Civ il Engineering Research F oundation, Reston, Virginia. Cytec Industries (2005). http://www.cyt ec.com/business/EngineeredMaterials/ CFInternet/cfthornelpitch.shtm. Cytec Industries Inc., 5 Garret Mountain Plaza, West Paterson, NJ Edge Structural Composites (2005). http ://www.edgest.com/edgedataintro.html. 145 Park Place Point, Richmond, CA. Fyfe Co. LLC (2005). http://www.fyfeco.com/products/compositesystems.html. Nancy Ridge Technology Center, 6310 Nancy Ridge Drive, Suite 103,San Diego, CA.

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269 Gibson, R. F. (1994). Principles of compos ite material mechanics. McGraw-Hill, Inc., 1994. Hexcel (2005). http://www.hexcel.com/Pr oducts. Hexcel, 6400 West 5400 South, Salt Lake City, UT ICC Evaluation Services (2003) Acceptance criteria for concre te and unreinforced masonry strengthening using fiber-rein forced composite systems. AC125, Whittier, California. Ibarra-Castanedo, C. and X. P. Maldague (2004). Defect depth re trieval from pulsed phase thermographic data on Plexiglas and aluminum samples. Thermosense XXVI, Edited by D.D. Burleigh, Eliot Cramer, G. Raymond, Peacock. Proceedings of SPIE, Vol. 5405, pp. 348-356. Ibarra-Castanedo, C. and X. P. Maldague (2005). Pulsed phase thermography inversion procedure using normalized parameters to account for defect size variations. Thermosense XXVII, Edited by D.D. Burleigh, Eliot Cramer, J. Miles. Proceedings of SPIE, Vol. 5782, pp. 334-341. Kaiser, H. and Kharbari, V. (2001a). Q uality and Monitoring of Structural Rehabilitation Measures (Part I: Descri ption of Potential Defects). Report Submitted to the Oregon Department of Transportation under Contract Number 18347. Kaiser, H. and Kharbari, V. (2001b). Q uality and Monitoring of Structural Rehabilitation Measures (Part II: Revi ew and Assessment of Non-destructive Testing (NDT) Techniques). Report Subm itted to the Oregon Department of Transportation under Contract Number 18347. Kharbari, V. M., J. W. Chin, Hunston, D., Be nmokrane, B., Juska, T., Morgan, R., Lesko, J. J., Sorathia, U., Reynaud, D. (2003). Dur ability gap analysis for fiber-reinforced polymer composites in civil infrastructure . Journal of Composites for Construction 7(3): 238-247. Kulowitch, P. J., I. M. Perez, Granata, D., (1995). Flash infrared thermography for nondestructive testing (NDT) I/E of naval aircraft. Thermosense XVI, Edited by Sharon A. Semanovich, Proceedings of SPIE, Vol. 2473, pp. 252-262. Lammert, K. A. (2003). Structural evaluation of impact damaged prestressed concrete I girders repaired with fiber reinforced polymer (FRP) materials. Masters Thesis, University of Florida, Gainesville, FL. Levar, J. M. and H. R. Hamilton (2003) Nondestructive evaluation of carbon fiberreinforced polymer-concrete bond using in frared thermography. ACI Materials Journal 100(1): 63-72.

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270 Maierhofer, C., A. Brink, Rollig, M., Wigge nhauser, H. (2003). Detection of shallow voids in concrete structures with im pulse thermography and radar. NDT & E International 36(4): 257-263. Maldague, X., F. Galmiche, Ziadi, A. (2002) Advances in pulsed phase thermography. Infrared Physics & Technology 43(3-5): 175-181. Maldague, X. P. V. (2001). Theory a nd Practice of Infrared Technology for Nondestructive Testing. John Wiley & Sons, Inc. Matlab User's Guide. (2002). On-line reference manual. The MathWorks, Inc., 3 Apple Hill Drive, Natick, MA. Nanni, A. (2003). North American design gui delines for concrete reinforcement and strengthening using FRP: principles, applications and unresolved issues. Construction and Building Materials 17(6-7): 439-446. Mirmiran, A., Shawhawy, M., Nanni, A., a nd Kharbari, V. (2004). NCHRP Report 514. Bonded repair and retrofit of concrete st ructures using FRP composites. National Cooperative Highway Research Program, Washington, D.C. Osiander, R., J. W. M. Spicer, Maclachlan J.W., and Murphy, J. C. (1996). Analysis methods for full-field time-resolved in frared radiometry. Thermosense XVI, Edited by Douglas D. Burleigh, Jane W. M aclachlan Spicer, Proceedings of SPIE, Vol. 2766, pp. 218-227. Sebastian, W. M. (2001). S ignificance of midspan debondi ng failure in FRP-plated concrete beams. Journal of Structur al Engineering-ASCE 127(7): 792-798. Shepard, S. M., J. R. Lhota, Hou, Y. L ., Ahmed, T. (2004). Bli nd characterization of materials using single-sided thermogra phy. Thermosense XXVI, Edited by D.D. Burleigh, Eliot Cramer, G. Raymond, Peaco ck. Proceedings of SPIE, Vol. 5405, pp. 442-446. Starnes, M. A., N. J. Carino, and Kausel, E.A. (2003). Preliminary thermography studies for quality control of concrete structur es strengthened with fiber-reinforced polymer composites. Journal of Materi als in Civil Engine ering 15(3): 266-273. VSL (2005). http://www.vsl.net/strengthen ing_products/vsl_frp_composites.html. 7455 New Ridge Road, Suite T, Hanover, MD. William D. Callister, J. (1997). Materials science and engi neering an introduction. John Wiley & Sons, Inc. Woolard, D. F. and K. E. Cramer (2005) Line scan versus flash thermography: comparative study on reinforced carboncarbon. Thermosense XXVII, Edited by D.D. Burleigh, Eliot Cramer, J. Miles. Proceedings of SPIE, Vol. 5782, pp. 334341.

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271 BIOGRAPHICAL SKETCH Jeff Brown completed high school in Daytona Beach, Florida. In August 1996, he was awarded the degree of Bach elor of Science in Civil Engineering from the University of Central Florida and receive d top honors from the College of Engineering. He began his graduate studies at the University of Central Florida that same year and was awarded a masters degree in Structures and Foundations Engineering in May 1998. Upon completion of his masters degree, he served for two years as a Peace Corps volunteer in the East African nation of Tanzania. In Janua ry 2002, he entered th e Graduate School of the University of Florida.


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Permanent Link: http://ufdc.ufl.edu/UFE0011621/00001

Material Information

Title: Infrared Thermography Inspection of Fiber-Reinforced Polymer Composites Bonded to Concrete
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0011621:00001

Permanent Link: http://ufdc.ufl.edu/UFE0011621/00001

Material Information

Title: Infrared Thermography Inspection of Fiber-Reinforced Polymer Composites Bonded to Concrete
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0011621:00001


This item has the following downloads:


Full Text












INFRARED THERMOGRAPHY INSPECTION OF
FIBER-REINFORCED POLYMER COMPOSITES
BONDED TO CONCRETE













By

JEFF ROBERT BROWN


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


2005


































Copyright 2005

by

Jeff Robert Brown


































To my wife, Heather,
and daughter, Zoe















ACKNOWLEDGMENTS

This work would not have been possible without the help and contributions of

others. First, I would like to thank my committee chair and advisor, Dr. Trey Hamilton,

for his guidance and support throughout this effort. It was a truly an honor to work on

this project and I will always appreciate the experience. Members of my dissertation

advisory committee are also acknowledged for their efforts: Dr. Andrew Boyd (Cochair),

Dr. Gary Consolazio, Dr. Ron Cook, and Dr. Elliot Douglas. Additional thanks goes to

Dr. Kurt Gurley for his assistance with my research and teaching here at UF.

Chuck Broward provided essential support during the laboratory phase of this

study. He has also been a wonderful friend. I can safely say that if it were not for Chuck

and his passion for astronomy, I would never have seen the rings of Saturn or the transit

of Venus. I would also like to thank Tony Murphy for providing excellent computer

support.

My master's thesis advisor, Dr. Sashi Kunnath, was responsible for sparking my

interest in research and also been a wonderful influence on my career. I cannot thank

him enough for all of his help. Another colleague from UCF, Dr. Mark Williams, was

also a major influence and helped to get me established here at the University of Florida.

I would like to thank a number of my friends and colleagues for their help and

support. Tony Michael and Markus Kutarba were a tremendous help both in the lab and

out in the field. We will always have some amazing stories to tell about two bridges in

Jacksonville. Amber Paul assisted in the specimen construction and data collection for









Phase II of this study. This work would never have been completed on-time without her

help. Finally, I would like to thank Gustavo Alvarez for his help in the early stages of

this project.

My parents have also provided tremendous support over the years, and their

contributions are gratefully acknowledged.

I have also been blessed with a family of my own, and none of this would have

been possible without the support of my wife, Heather. Our daughter, Zoe, also

contributed in more ways than can be mentioned. The only way I would do this again

would be if we could do it together.

Finally, this material is based upon work supported under a National Science

Foundation Graduate Research Fellowship.















TABLE OF CONTENTS

page

A C K N O W L E D G M E N T S ...................................................................... .....................iv

LIST OF TABLES ............ ................. ..................... .... ............... ix

LIST O F FIG U R E S .............................................. .. .. ........... ..... .. ....xii

A B STR A C T .............................. ........................................................ xix

CHAPTER

1 IN T R O D U C T IO N ............................................................................. .............. ...

2 FIBER-REINFORCED POLYMER COMPOSITES USED TO STRENGTHEN
REINFORCED CON CRETE ............................................. ..... ....................... 9

Constituent M materials ............................................................ .................... 9
F ib e r s ............................................................ ................ .. 9
M atrix M materials .......................... .. ............ ... .............. .. ............. .... 11
Construction Methods and Application Procedures for FRP Composites..................12
Composites Used in the Aerospace Industry ............................. ............... 12
Composites Used to Strengthen RC ................ .......................... ............... 13
Locations of Defects in FRP Systems Bonded to Concrete ......................................14
Q quality C control Standards ................................................ .............................. 15
Research Significance............ ............................. 17

3 NONDESTRUCTIVE EVALUATION USING INFRARED
T H E R M O G R A PH Y ......................................................................... ........ ........... 18

Infrared Therm ography Fundam entals ............................................ ............... 18
Detection of EM Radiation with an IR Camera........ ....... ............... 22
Thermal Imaging System Used in Current Study......... ......................................24
Infrared Thermography Methods for NDE of Materials ................. ............... 25
H eating M methods .................. .......... ....................... .. ........ .... 26
Im age A acquisition ........ .................................................................. .. .... ... ... 28
D ata A n aly sis .................................................. ................ 2 9
O objectives of Current R research ...............................................................................35









4 PHASE I EXPERIMENTAL WORK AND FIELD STUDY............................... 36

In tro d u ctio n ................... ...................................... ............. ................ 3 6
Full-Scale A A SH TO G irders................................................. ..................37
Description of AASHTO Girders and FRP Systems .......................... .........37
Infrared Inspection Procedures ....... ..................... ..................41
Initial IR Inspections ........................... ..... .................. 44
IR Inspections Performed During Load Testing.............................................49
IR Inspections of Known Debonded Areas After Failure .............. .....................49
Summary of IR Inspection Results for Each FRP System ................................54
Field Inspection: Chaffee Road ............................................................................ 57
Summary of Findings for Phase I ..... ............................................................ 61

5 PHASE II: EXPERIMENTAL SETUP..... ................. ............... 65

Intro du action ............. ................. .................................................................... 6 5
Specim en C on struction ...................................................................... ...................66
FR P C om posite M aterials.......................................................... ............... 67
C concrete Substrate .......... .......... .................... ........... .............. .... 68
Surface Preparation........... .................................... .. ...... .. ..... ......... 70
Surface Saturation and Tack-Coat...... ................. ...............70
Application of FRP Composite to Concrete............... ....................................73
Construction Details for Each Series.......... ........ ........ ................. ......... 74
Heating Methods and Thermal Imaging ........... ................................. ...............82
F la sh H e atin g .................................................................... .............. 8 3
Scan H eating .................................................................. ......... 86
Long-Pulse H eating ...................................... ................... ..... .... 90
Sinu soidal H eating ................................................................ ......... .......93
Comparison of Heating Configurations...............................................94

6 PHASE II: DATA COLLECTION AND ANALYSIS ...........................................97

In tro d u ctio n ................. ......... .................... ........................................................ 9 7
Pulse Therm ography: Series A ................................ ............... ............... 98
Specimen Heating and Data Collection............... .........................................98
Im age Preprocessing .............................................. ...... ..... ........ .... 99
D effect A nalysis................................................................ ............... 10 1
Proposed Method for Characterizing Detectability ................. ................116
Experim ental Results: Flash Heating (Series A) ...............................................118
Experimental Results: Scan Heating (Series A)............................................... 135
Experimental Results: Long-Pulse Heating (Series A) .....................................143
Com prison of Heating M ethods............................................... .................. 145
General Detectability .................. ........................... ...... ................146
D effect C characterization ............................................. ............................. 152
Summary of Pulse Thermography Results .................................... ............... 166
Step Therm ography Analysis ............................................................................167
A analysis Procedures......... .................................................... ... .. .... ..... 167









Summary of Step Thermography Results: Series A Specimens.......................188
Frequency Dom ain Analysis: Series A .......................................... ............... 190
Sinusoidal Heating (Lock-In IRT)...................................... ......... ...............190
Pulse Phase Therm ography................. .......................... ........................... 199
Comparison of Heating Methods and Analysis Techniques..................................207
G general D etectability .......................................................... ............... 207
D effect C haracterization ........................................................... .....................2 10
Series B, C, D and E Specim ens ................................................... .................212
D ata C collection .....................................................................2 14
Series B ..................................... ............................ ..... ... ...... 2 14
S erie s C ..........................................................................2 18
S erie s D ..........................................................................2 18
Series E ................................................................. ..... ... ...... 2 19

7 SUMMARY AND RECOMMENDATIONS FOR FUTURE RESEARCH...........221

S u m m a ry ............................................................................................................. 2 2 1
P hase I........................................................ 222
L laboratory Study .......................... ............ ............... .... ....... 222
F field Stu dy ................................................................................................ 223
P h a se II ..............................................................2 2 3
H eating M eth od s ............................................................................. 22 3
Data Analysis M methods ................................................................................224
FRP System Properties and IRT Results .................................... .................225
Recommendations for Deployment of IRT ..................... ............ ..........225
Guidelines for Qualitative IRT Inspections................................ ... ..................225
Q u antitativ e A n aly sis............................................................... .....................22 6
Future Research .................... ..................................... 227

APPENDIX

A TIME DOMAIN RESULTS: SERIES A ...................................... ............... 229

B SINUSOIDAL HEATING RESULTS: SERIES A............................................250

C SERIES B, C, D, AND E RESULTS ............................................ ............... 255

D COMPOSITE PROPERTIES FOR SMALL-SCALE SPECIMENS.....................265

L IST O F R E F E R E N C E S ...................................................................... .....................268

B IO G R A PH IC A L SK E T C H ........................................ ............................................271
















LIST OF TABLES


Table page

2-1 Dry carbon-fiber properties used in aerospace industry .....................................10

2-2 Properties of dry fibers for commercially available fiber-reinforced
polymer systems used to strengthen reinforced concrete .............. ...................11

2-3 Properties of epoxies used in commercially available fiber-reinforced
polymer systems for strengthening reinforced concrete.............. ... .............12

4-1 Fiber-reinforced polymer system properties for full-scale AASHTO
g ird ers ............................................................................. 3 8

4-2 Summary of scanning speed and uniformity of heating ............................. 47

5-1 Overview of Specimen M atrix....... ... ........... .................... .. ......... 67

5-2 Material properties for fibers, epoxy, and lamina.................. ...................68

5-3 Concrete mix proportions used for Series A to E ...............................................70

5-4 Series A details ......................................................................76

5-5 S series B d etails................................................ ................ 7 9

5-6 S series C d etails................................................ ................ 80

5-7 Series D details ......................................................................8 1

5-8 S series E d details ................................................ ................ 82

5-9 Surface temperature increase results for different heating methods...................96

6-1 Summary of data collected for pulse analysis study............................................99

6-2 Parameters computed for defect area at each time step ............ ... ................104

6-3 Parameters extracted from ATdef VS. time plot for each defect............................ 109

6-4 Detectability classification based on ACOV of computed radii ..........................118









6-5 Flash heating results for Specimen A- .................................... ............... 120

6-6 Gradient area method results for Specimen A-1: flash heating ...........................122

6-7 Summary of Results for Specimen A-2: flash heating ............... ..................126

6-8 Summary of results for Specimen A-3: flash heating ........................................128

6-9 General detectability results for flash heating ............... .................... ..........131

6-10 Signal to boundary noise ratio (SBR) results for flash heating .........................132

6-11 Ratio of parameters for air and epoxy-filled defects .................................. 135

6-12 General detectability results for scan heating ................................... ................139

6-13 Signal to boundary noise ratio (SBR) results for scan heating ............................139

6-14 Ratio of parameters for air and epoxy-filled defects (scan heating)....................142

6-15 General detectability results for long-pulse heating .........................................144

6-16 Defect data and predicted depth for defects shown in Figure 6-48 ...................162

6-17 Predicted and actual properties of defects in Figure 6-48.................................. 165

6-18 Typical thermal properties for materials of interest ..........................................169

6-19 Summary statistics for defect-free areas (Series A)..........................................175

6-20 Parameters of interest for characterizing defects from step
therm ography data ......................... ....... ..... .. ...... .............. 190

6-21 Frequencies investigated during sinusoidal heating experiments ........................191

6-22 Recommended pulse durations and detection limits for sinusoidal heating
(carbon-FRP systems) ........... .. ......... ......................... 198

6-23 Normalized temperature response @ t = 60 sec for properly saturated
specim ens ............. ........... ................ .....................................2 16

A -i Flash heating results for Series A ............................................. .....................233

A -2 Scan heating results for Series A ........................................................ .............. 237

A-3 Long-pulse (30 sec) heating results for Series A............................................241

A-4 Long-pulse (45 sec) heating results for Series A............................................245









A-5 Long-pulse (60 sec) heating results for Series A............................................249

B-l Sinusoidal heating results: AQ vs. frequency plot parameters ..........................254

C-1 Series B (low saturation) summary statistics for defect-free areas......................259

C-2 Series C (surface prep) summary statistics for defect-free areas.........................261

C-3 Series D (fiber saturation methods) summary statistics
for defect-free areas ................................ .............. ................ ............. 262

D -1 Series A com posite properties ........................................ ......................... 266

D -2 Series B com posite properties........................................ .......................... 266

D -3 Series C com posite properties........................................ .......................... 266

D -4 Series D com posite properties ........................................ ......................... 267

D -5 Series E com posite properties.................................... ........................... ......... 267
















LIST OF FIGURES


Figure pge

1-1 Strengthening reinforced concrete beams ....................................... .............2

1-2 Prestressed AASHTO girder damaged by over height vehicle .............................

1-3 Application of FRP composite to strengthen existing structure ............................4

1-4 Reinforced concrete column wrapped with FRP ............................................. 5

1-5 Vehicle impact damage to FRP composites that occurred after installation .......... 5

1-6 Damage to FRP composite due to corrosion of internal reinforcing steel..............6

1-7 Surface temperature response due to external radiant heating..............................7

1-8 Infrared thermography inspection of FRP composite system .............................. 8

2-1 Location of potential unbonded, debonded, and delaminated areas in FRP
systems ............. .... .. .. ................... ........... ....... 15

3-1 Incident radiation (Oi) is reflected, transmitted or absorbed ..............................20

3-2 Electromagnetic emission curves for objects at different temperatures ...............21

3-3 Atmospheric emission in the MWIR and LWIR spectral bands..........................23

3-4 General schematic of a focal plane array (FPA) and associated optics ...............24

3-5 Application of IR thermography to FRP composite bonded to concrete ..............25

3-6 Surface heating and defect detection for pulse thermography...............................31

3-7 Defect detection with lock-in thermography .............. ............ .....................34

4-1 Full-scale AASHTO type II girder and load test setup.......................................37

4-2 Cross-section views of FRP system s ....................................... ......... ............... 39

4-3 Data collection for full-scale AASHTO girders ......................................... 43









4-4 Subsurface defect found on Girder 3 .......................................... ............... 44

4-5 Subsurface defects found on Girder 3.................... ............................ ............ 45

4-6 Non-uniform surface heating of Girder 4 ................................... .................46

4-7 Thermal images collected for full-scale AASHTO girders .................................47

4-8 Background temperature increase vs. position along length of girder ..................48

4-9 Failure modes for full-scale AASHTO girders ................ ............. ...............50

4-10 Defect signal strength (ATdefect) vs. time for known debonded area...................51

4-11 Debonded area after failure for Girder 6..................................... ............... 53

4-12 Series of thermal images for air and epoxy filled defects................. ......... 54

4-13 Polyurethane matrix shown after debonding from concrete (Girder 4).................56

4-14 Vehicle impact damage sustained after FRP strengthening............................. 58

4-15 Visual and thermal images of vehicle impact damage ................ ..................59

4-16 Infrared thermography inspection of undamaged girder .................. ................60

4-17 Damaged girder before new FRP system was applied..........................................61

5-1 TYFO SCH-41 carbon-fibers...................... ...... ............................ 69

5-2 TYFO SEH-51 glass-fibers ......... .................................... ......................... 69

5-3 Surface preparation before FRP placement ................................. ............... 70

5-4 Application of epoxy saturant and tack-coat ............................... ............... .71

5-5 Fiber saturation ......................... ......... .. .. ..... .. ............. 73

5-6 C om pleted specim en ........................ ....... .................................... ............... 74

5-7 Defect configuration for Series A specimens ................................ ............... 76

5-8 Defect configuration for Series B specimens............................................. 80

5-9 Lap-splice configuration for Series E ........................................ ............... 82

5-10 Heat source and camera configuration for pulse heating experiments ..................84

5-11 Typical thermal image collected during pulse heating experiment .....................84









5-12 Surface temperature profile due to pulse heating ...............................................87

5-13 Heat source used in scan heating experiments............................................... 88

5-14 Thermal images collected during scan heating experiment ...................................88

5-15 Thermal image collected during scan heating experiment ....................................89

5-16 Surface temperature profile for scan heating ...................................................... 89

5-17 Heat source and camera configuration for long-pulse heating ...........................91

5-18 Thermal image collected at t = 1 sec during long-pulse heating .........................91

5-19 Laboratory setup for long-pulse heating experiments .......................................92

5-20 Surface temperature profile for long-pulse heating (30 sec pulse).....................92

5-21 Surface temperature profile for long-pulse heating (60 sec pulse).....................93

5-22 Diagram for sinusoidal heating control and data acquisition ..............................96

6-1 Application of 3x3 averaging filter applied to each pixel in
therm al im age ................................................ ................. 102

6-2 Area identification for defect analysis ........ ............ ....................104

6-3 Constructing ATdef VS. time plots from area parameters............... ...............105

6-4 Thermal images and ATdef VS. time plot for Defect IB ....................................... 106

6-5 Non-uniform heating and weak signals for defects ....................................... 107

6-6 Identification of important parameters for weak signals ............................. 108

6-7 Signal for undetected defect.......................................... ........................... 109

6-8 Defect area computations using boundary trace method .................................. 111

6-9 Surface temperature profile and gradient used to approximate the
boundary of detected defects ........ ....................... ...... ... ............... 112

6-10 Reduced accuracy in area computations due to a weak signal ..........................114

6-11 Coefficient of variation for ellipse radii (computed with NP = 250)................. 115

6-12 Reduced accuracy in area computations due to non-uniform heating ...............115

6-13 Reduced accuracy in area computations due to low image resolution ..............116









6-14 Detectability classification based on ATdef VS. time plot ...................................118

6-15 Flash heating results for Specimen A- .................................... ............... 120

6-16 Gradient im ages for defects. ...........................................................................122

6-17 Specimen A-1: Important parameters for defects ...............................................123

6-18 Thermal images for Specimen A-2: flash heating ............................................124

6-19 Flash heating results for Specimen A-2 : temperature vs. time data ...................126

6-20 Unintentional defects between layers in Specimen A-2 .....................................126

6-21 Specimen A-2: important parameters for defects .............................................127

6-22 Thermal images and ATdefVS. time plots for Specimen A-3 .............................128

6-23 Specimen A-3: important parameters for defects ...........................................129

6-24 Thermal images and ATdefVS. time plots for Specimen A-4 .............................130

6-25 Normalized ATmax for flash heating..............................................................133

6-26 Tim e to m aximum signal for flash heating ....................................................... 134

6-27 Signal half-life for flash heating ................................. ...................................... 134

6-28 Standard deviation of Defect IB perim eter ........................................................ 136

6-29 Data for Defects A25 and E25 (6.4 mm diameter).............................................137

6-30 Thermal images for Defect IB (Specimen A-2)..................................................138

6-31 Norm alized ATmax for scan heating ........................................ ............... 140

6-32 Time to maximum signal for scan heating................................ ...............141

6-33 Signal half-life for scan heating............................................... ......... ...... 142

6-34 Normalized ATmax for long-pulse heating ............................... ................144

6-35 Time to maximum signal for long-pulse heating................................................145

6-36 Signal half-life for long-pulse heating .......................... ................... 145

6-37 L egend for F igure 6-38 ........... .................................................. ..................... 146

6-38 Summary of general detectability for flash, scan, and
long-pulse heating .................. .......................... ................... .. 147









6-39 Comparison of ATmax for different heating methods. ........................................ 149

6-40 Comparison of normalized ATmax for different heating methods. .......................151

6-41 Coefficient of variation (COV) of computed radii for different
heating m methods. ................................................ .............. 153

6-42 Maximum signal vs. radii COV for all detected defects in Series A.................154

6-43 Time to maximum signal for different pulse durations ......................................155

6-44 Defect circumference (C) x depth (d) vs. tmax for flash experiments .................156

6-45 Signal half-life for different pulse durations....................................................... 158

6-46 Plot of defect circumference C x depth D vs. tl/2 for all heating methods...........159

6-47 Plot of defect circumference C x depth D vs. tl/2 for all heating methods........... 160

6-48 Thermal image from long-pulse experiment (Series B and C specimens) ..........161

6-49 Defect signal vs. time plot for defects shown in Figure 6-48 ...........................161

6-50 Characterization of Defect A3 ............................................... ........ ......... 163

6-51 Characterization of Defect A4 ............................. ...... ............. .................. 164

6-52 Temperature increase for select areas......... ........ .... .....................166

6-53 Surface temperature increase due to uniform heat flux ................................. 170

6-54 Normalizing AT for two points on Specimen A-1 ..............................................173

6-55 AT image for Series A specimens...............................................173

6-56 Normalized AT image for Series A specimens ..........................................174

6-57 D efect-free areas for Series A specim ens .......... ..............................................176

6-58 Mean value of ATnorm for defect-free areas on Series A..................................... 176

6-59 One-dimensional model of FRP systems...........................................177

6-60 Computation of ATdef from normalized temperature data .................................179

6-61 Determining point at which defect is detected in ATdef plots .............................. 180

6-62 Surface plots of Defect A50 (Specimen A-2).................... .................. ................ 181

6-63 Two-dimensional correlation coefficient, R, for Defect A50...........................182









6-64 Defect signal vs. t1/2 for Specimen A- ...... ......... ....................................... 183

6-65 Two-dimensional correlation coefficient vs. t1/2 for Specimen A-i ..................184

6-66 Defect signal vs. t1/2 for Specimen A-2..................................... ............... 185

6-67 Two-dimensional correlation coefficient vs. t1/2 for Specimen A-2 ..................186

6-68 Normalized AT images for Defect IB (Specimen A-2) ......................................186

6-69 Defect signal vs. t1/2 for Specimen A-3............... ............................................ 187

6-70 Two-dimensional correlation coefficient vs. t1/2 for Specimen A-3 ....................188

6-71 Normalized temperature image for Specimen A-4 (t = 60 sec)........................188

6-72 Data analysis for sinusoidal heating (pulse duration = 500 sec)........................193

6-73 Sinusoidal heating results for Series A specimens
(Pulse D uration = 8.33 sec) ........... ... .. ............................. ............... 194

6-74 Sinusoidal heating results for Series A specimens
(Pulse D uration = 25 sec)............................................. ............................ 195

6-75 Sinusoidal heating results for Series A specimens
(Pulse Duration = 125 sec) .......... ... ................ ................. .. ............ ... 196

6-76 Sinusoidal heating results for Series A specimens
(Pulse Duration = 500 sec) .......... ... ................ ................. .. ............ ... 197

6-77 Application of PPT method to Specimen A-3 ...............................................204

6-78 Comparison of time domain and frequency domain (PPT)
results for Specim en A -3 ............................................. ............................ 205

6-79 Comparison of time domain and frequency domain (PPT)
results for Specim en A -4 ............................................. ............................ 205

6-80 Defect signal (phase) vs. frequency plots for air-filled defects .........................206

6-81 Frequency domain results for Specimen A-3.................... .................. ................ 207

6-82 Comparison of heating methods for Specimen A-4.................. ..................209

6-83 Comparison of data analysis techniques for Specimen A-3 .............................212

7-1 Field inspection (scan heating method) of FRP system .........................226

A-1 Flash heating results: Thermal images for Series A. ........................................230









A-2 Flash heating results: ATdef VS. time plots for Series A. ..................................... 231

A-3 Scan heating results: Thermal images for Series A. ...........................................234

A-4 Scan heating results: ATdef VS. time plots for Series A. .......................................235

A-5 Long-pulse (30 sec) heating results: Thermal images for Series A...................238

A-6 Long-pulse (30 sec) heating results: ATdef VS. time plots for Series A..............239

A-7 Long-pulse (45 sec) heating results: Thermal images for Series A...................242

A-8 Long-pulse (45 sec) heating results: ATdef VS. time plots for Series A..............243

A-9 Long-pulse (60 sec) heating results: Thermal images for Series A...................246

A-10 Long-pulse (60 sec) heating results: ATdefVS. time plots for Series A..............247

B-l Sinusoidal heating results: Phase images for Series A. ........... ...............251

B-2 Sinusoidal heating results: A
C-l Low matrix saturation (Series B carbon-fibers)................... .................256

C-2 Medium matrix saturation (Series B carbon-fibers) ........................................256

C-3 High matrix saturation (Series B carbon-fibers)................................................257

C-4 Low matrix saturation (Series B glass-fibers) ............................................. 257

C-5 Medium matrix saturation (Series B glass-fibers) ............................................258

C-6 High matrix saturation (Series B glass-fibers)...............................................258

C-7 No surface preparation (Series C carbon-fibers) .............................................260

C-8 Light blast surface preparation (Series C carbon-fibers) ........... .. ............ 260

C-9 Heavy blast surface preparation (Series C carbon-fibers) ................................261

C-10 Different fiber saturation methods (Series D carbon-fibers) .............................262

C-11 Specimen E-l (l-layer/3-layer/2-layer) ........... ........ .... ..............263

C-12 Specimen E-2 (2-layer/3-layer/2-layer) .................................... ................263

C-13 Specimen E-3 (3-layer/4-layer/2-layer) .................................... ...............264


xviii















Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

INFRARED THERMOGRAPHY INSPECTION OF
FIBER-REINFORCED POLYMER COMPOSITES
BONDED TO CONCRETE

By

Jeff Robert Brown

August 2005

Chair: H.R. Hamilton III
Cochair: Andrew J. Boyd
Major Department: Civil and Coastal Engineering

The use of fiber-reinforced polymer (FRP) composites to strengthen existing civil

infrastructure is expanding rapidly. Many FRP systems used to strengthen reinforced

concrete are applied using a "wet layup" method in which dry fibers are saturated on-site

and then applied to the surface. Air voids entrapped between the FRP system and

concrete as a result of improper installation reduce the integrity of the repair.

The objective of this study was to investigate the use of infrared thermography

(IRT) for evaluating bond in FRP composites applied to reinforced concrete. Phase I of

this study examined FRP strengthening systems that were applied to full-scale bridge

girders. IRT inspections were performed on four AASHTO type II girders with

simulated impact damage that were loaded to failure. Phase I also contained a field

inspection of an in-service bridge that was strengthened with FRP composites. The

results of the field studies indicated that as the overall thickness of the FRP system









increased the detectability of defects was diminished. In addition the installation

procedures influenced IRT results. The use of excessive epoxy tack-coat was shown to

reduce detectability and increase the required observation time.

A second experimental study (Phase II) was conducted in which 34 small-scale

specimens (15 cm x 30 cm) containing fabricated defects were inspected in a laboratory

environment. These specimens were constructed using different FRP composite

thicknesses (1mm to 4 mm) and matrix saturation levels. Four heating methods were

investigated (flash, scan, long-pulse, and sinusoidal), and quantitative analyses were

performed on the thermal data using currently available techniques.

Data were used to establish detection limits for air and epoxy-filled voids in carbon

FRP composites. It was shown that IRT is capable of detecting 19 mm diameter and

larger defects in carbon FRP composites up to 4 mm thick. Quantitative data analysis

techniques were also used to estimate the depth and material composition of defects up to

2 mm below the surface. These data analysis techniques were also effective for

enhancing detection of defects up to 4 mm below the surface.














CHAPTER 1
INTRODUCTION

Fiber reinforced polymer (FRP) composites are currently used to repair and

strengthen existing reinforced concrete structures. Several types of FRP repair systems

are commonly encountered: surface bonded, near surface mounted (NSM), and FRP bars.

Surface mounted systems consist of dry fibers that are saturated on-site with a matrix

material and applied directly to the surface before the composite cures. NSM systems

involve pre-cured FRP laminates that are bonded to the structure using an intermediate

bonding agent. One advantage of pre-cured laminates is that they are manufactured in a

factory setting. The resulting laminates are typically of high quality and possess uniform

material properties. FRP bars are also pre-cured composites that are intended to serve as

additional reinforcement in reinforced concrete (RC) structures. These systems are

installed by cutting a groove in the surface of the concrete just large enough to

accommodate the FRP bar. An intermediate bonding agent, typically a thickened epoxy,

is then used to grout the bar in place.

The advantages of using FRP composites for strengthening and repair include the

high strength to weight ratio of FRP materials, ease of installation, and resistance to

corrosion. Prior to the availability of composite materials, traditional repair and retrofit

techniques involved attaching steel plates to members or outright replacement. This type

of repair is often difficult to implement and requires the mobilization of heavy equipment

simply to handle the repair materials.









The American Concrete Institute (ACI) committee 440 has produced a document

that engineers can use to specify FRP system requirements based on design objectives

(ACI 440-R02 2002).

FRP composites are used primarily to:

* Increase flexural capacity of a member
* Increase shear capacity
* Provide additional confinement to increase concrete ductility

For flexural strengthening, the fibers are oriented along the length of the beam axis

to serve as added tensile reinforcement (Figure 1-1A). The FRP composite is typically

applied to the tension face. To increase the shear capacity of a member, unidirectional

FRP composite is applied to the web with the fibers oriented transverse to the beam axis

(Figure 1-1B). Bi-directional FRP composites are also used with the goal of providing

additional reinforcement in areas of high diagonal tension.

Flexural
Stxraln Shear Strengthening
Strengthening


77~7-


Externally Bonded FRP
Composite


A B
Figure 1-1. Strengthening reinforced concrete beams. A) For flexure. B) For shear.

A common thread with these two approaches is that they rely entirely on bond to

transfer stresses between the concrete beam and fibers. These applications are considered

"bond-critical". The adhesive and concrete substrate must be sound and of sufficient

strength to transfer stress to the fibers. The bond is critical because there are no

redundant load paths for stress to follow should the bond fail.


I _









A practical example of these types of repairs is shown in Figure 1-2. In this case,

an AASHTO girder was hit by an over height vehicle and was subsequently repaired with

FRP composites. In addition to the heavy concrete spelling, a number of the girder's

prestressing strands were cut. The FRP composite was applied to restore both flexural

and shear capacity. Figure 1-3(a) demonstrates the relative ease with which FRP

composites can be applied to existing structures. FRP composites are easily conformed

to the member and the orientation of fibers can be adjusted depending on the particular

strengthening application.





,- -, ,








Figure 1-2. Prestressed AASHTO girder damaged by over height vehicle. Girder was
repaired with FRP composites.

The third approach, improving concrete confinement, is popular in seismically

active regions and is generally referred to as "column-wrapping". Figure 1-4 shows the

use of column-wrapping at the base of a column or pier where a plastic hinge is expected

to form during extreme ground shaking. The FRP composite wrap confines the concrete,

which improves the ductility and apparent strength as well as improving lap splice

performance. Bond is not considered critical in this application since the fibers are

continuously wound around the member. Column wrapping is commonly referred to as

"contact-critical".























A B
Figure 1-3. Application of FRP composite to strengthen existing structure. A) Workers
applying carbon-fiber composite. B) completed project.

If the composite is not installed properly and air-bubbles are present at the

FRP/concrete interface, the system may not perform as desired. Figure 1-4 shows the

severity of installation defects that can occur in FRP composite systems. Another issue

that can affect the overall performance of an FRP system is durability. Numerous

researchers have cited durability of FRP composite systems as a major challenge

confronting the industry (CERF 2001, Kharbari et al 2003, Nanni 2003). A number of

factors can contribute to the degradation of an FRP composite system during its service

life:

* Environmental exposure (moisture, temperature cycles)
* Overloading resulting in partial debonding
* Vehicle impact
* Corrosion of internal reinforcing steel

A number of the FRP repairs initiated by the Florida Department of Transportation

(FDOT) to mitigate vehicle impact damage have also been struck and damaged by over

height vehicles (Lammert 2003). Figure 1-5 provides two examples of damage to FRP

systems resulting from vehicle impact.
























t I- I
Figure 1-4. Reinforced concrete column wrapped with FRP. The large air bubbles are a
result of installation defects.










A B
Figure 1-5. Vehicle impact damage to FRP composites that occurred after installation.
A) Chaffee Rd./I-10 overpass near Jacksonville, Florida. B) 45th St./Florida
Turnpike overpass in West Palm Beach, Florida.

The expansive nature of corrosion byproducts result in cracking of the concrete

substrate. If an FRP system is applied to a member experiencing active corrosion,

subsequent cracking of the concrete substrate can lead to debonding or rupture of the FRP

composite. This scenario was observed on an FRP repair that was applied to a US









Highway 100 bridge spanning the intracoastal waterway in Melbourne, Florida (Figure 1-

6). This damage led to the removal and replacement of the entire FRP system

..Licknw,_ ,d dcbondm,_ du, to










Figure 1-6. Damage to FRP composite due to corrosion of internal reinforcing steel

After an extensive survey on defects in FRP composites, Kaiser and Kharbari

(2001 a) concluded that the performance and expected lifetime of FRP repairs are largely

dependent on the quality of installation and the presence of defects. This work also

highlights a need for long-term monitoring of performance and durability. Kaiser and

Kharbari (2001b) also provide a description of NDE techniques that can be used to

evaluate FRP composites.

It is apparent that FRP composites are increasingly used to repair and strengthen

reinforced concrete structures. Because this is a relatively new construction technique,

methods of evaluating the installation quality and long-term efficacy are needed. ACI

440 cites acoustic sounding, ultrasonics, laser shearography, and infrared thermography

(IRT) as methods that can be used to evaluate these critical aspects of FRP composites.

The objective of this research is to develop IRT techniques to evaluate bond quality

in FRP composites applied to concrete. IRT is a non-contact remote sensing technique

that can be used to measure the surface temperature of an object. The fundamental

approach is shown in Figure 1-7. If the surface of a homogeneous material is heated









using an external heat source, the increase in temperature on the surface will be uniform.

If, however, the thermal front traveling from the surface into the material encounters an

air-void or other discontinuity (defect), the relative rate of surface temperature increase

above the defect will change. Depending on the size and material characteristics of the

subsurface defect, it may be possible to detect this change in temperature using an

infrared camera. A sample application of IRT is shown in Figure 1-8. In this example,

the surface of a carbon-fiber/epoxy FRP system was heated using a 500 W halogen lamp.

The thermal image provided in Figure 1-8(a) indicates that a portion of the FRP is not

bonded to the concrete substrate. The light colors in the thermal image indicate higher

temperatures. Portions of the FRP composite that are well bonded to the concrete

substrate appear darker in the thermal image.


i i ,+ Surface heating with
t t + external heat source



material w/
defect I
-3 -^ A i ,,l
-4.f



homogeneous
material

Tamblent


tp timel/2

Figure 1-7. Surface temperature response due to external radiant heating for
homogeneous materials and materials with subsurface imperfections.









This research investigated the use of IRT for detecting defects in FRP systems

bonded to concrete. Specifically, a major goal was to use IRT data to provide the

following information about detected defects:

* Size
* Depth below the surface
* Material composition

Other items that are addressed include:

* Detection limits
* Heating methods
* Data analysis procedures

Previous research in this field has focused on the inspection of FRP composites that

are commonly used in the aerospace industry. A number of data analysis techniques have

been developed that can assist in using IRT results to characterize defects. None of these

methods have been calibrated for use on FRP systems bonded to concrete.


A B
Figure 1-8. Infrared thermography inspection of FRP composite system. A) Thermal
image. B) Visual image.















CHAPTER 2
FIBER-REINFORCED POLYMER COMPOSITES USED
TO STRENGTHEN REINFORCED CONCRETE

Constituent Materials

The term composite is used to describe any material that is created by combining

two or more materials on a macroscopic level. In the current study, the term composite

will refer to a combination of a polymer matrix and fibers. The primary function of the

fiber material is to carry load. The matrix serves as a binder that holds the composite

together and transfers stress between fibers.

Fibers

The following fiber materials are commonly used in FRP composites (Gibson

1994):

* Glass
* Carbon
* Aramid
* Boron

The type of fiber chosen for a specific application depends on the specific

requirements for strength, toughness, stiffness, and service temperature. Glass (E-glass)

and carbon-fibers are widely used for strengthening reinforced concrete.

It is interesting to note the wide array of material properties that are found amongst

different types of carbon-fibers. The strength, stiffness and thermal conductivity of

carbon-fiber materials are highly dependent on the manufacturing process and on the base

material from which the fibers are extruded. Carbon-fibers can be divided into two

general categories: PAN based fibers, which are extruded from a polyacrlyonitrile









precursor, and pitch based fibers, which are extruded from a petroleum based pitch

precursor. PAN based fibers tend to have a lower modulus of elasticity (207 to 310 GPa)

and a higher ultimate tensile strength (3.8 to 5.2 GPa) than pitch based fibers (Table 2-1).

Pitch based fibers are noted for their relatively high stiffness and can have a modulus of

elasticity of ranging 379 to 965 GPa.

A wide range of thermal conductivity values is associated with different carbon-

fiber types. Callister (1997) provides a thermal conductivity for low modulus PAN based

fibers of 8.5 W/m-k. The ASM Materials Handbook (Vol. 21, 2005) provides a thermal

conductivity of 20 W/m-K for standard modulus PAN based carbon-fibers and values as

high as 1100 W/m-K for ultra-high modulus pitch based carbon-fibers. All quantities

cited for thermal conductivity represent values in the longitudinal direction of the fibers.

The glass-fibers used in structural engineering applications are typically E-glass,

with a thermal conductivity of 1.3 W/m-K which is lower than all forms of carbon-fibers.

Table 2-1. Dry carbon-fiber properties used in aerospace industry
Tensile Modulus of
Organic strength elasticity
Manufacturer Designation precursor (GPa) (GPa)
Hexcela AS4 PAN 4.27 228
Hexcel IM7 PAN 5.17 276
Cytecb Thomel-P55s Pitch 1.90 379
Cytec Thomel-P120s Pitch 2.41 827
Cytec K-1100 Pitch 3.10 965
a Hexcel (2005). http://www.hexcel.com/Products. 6400 West 5400 South, Salt Lake City, UT
bCytec Industries (2005) lLhp \ \\ \ .cytec.com/business/EngineeredMaterials/
CFInternet/cfthornelpitch.shtm. Cytec Industries Inc., 5 Garret Mountain Plaza, West Paterson, NJ

Mechanical properties of dry carbon and glass-fibers commonly used in structural

engineering applications are provided in Table 2-2. Only one of the FRP system

manufacturers listed in the table (VSL) explicitly identifies their carbon-fibers as PAN

based. The modulus of elasticity of all carbon-fibers provided in Table 2-2 is relatively










consistent between the different FRP system manufacturers. These modulus values

suggest that a PAN precursor is common amongst the different carbon-fiber systems.

Table 2-2. Properties of dry fibers for commercially available fiber-reinforced polymer
systems used to strengthen reinforced concrete
Modulus
Tensile of Area Percent
strength elasticity density elongation
Manufacturer Designation Fiber type (GPa) (GPa) (g/m2) @break
Fyfe Co.a SCH-41 Carbon 3.79 230 644 1.7
Fyfe Co. SCH-41S Carbon 3.79 230 644 1.7
Edge VelaCarb
Compositesb 335 Carbon 4.48 234 335 1.9
Edge VelaCarb
Composites 600 Carbon 4.48 234 600 1.9
V-Wrap
VSLc C100 Carbon 3.79 228 300 1.5
V-Wrap
VSL C150 Carbon 3.79 228 440 1.0
V-Wrap
VSL C200 Carbon 3.79 228 600 1.5
Fyfe Co. SEH-51A E-Glass 3.24 72 915 4.5
Edge
Composites Vela-Glass E-Glass 2.28 72 875 4.0
V-Wrap EG-
VSL 50 E-Glass 2.28 72 900 4.0
a Fyfe Co. LLC (2005). http://www.fyfeco.com/products/compositesystems.html. Nancy Ridge
Technology Center, 6310 Nancy Ridge Drive, Suite 103,San Diego, CA
b Edge Structural Composites (2005). http://www.edgest.com/edgedataintro.html. 145 Park Place Point,
Richmond, CA
c VSL (2005). http://www.vsl.net/strengthening_products/vsl frpcomposites.html. 7455 New Ridge
Road, Suite T, Hanover, MD.

Matrix Materials

A variety of matrix materials (resins) are commonly used in structural engineering

applications: epoxy, polyester, vinylester, and polyurethane. Epoxies are commonly used

in wet layup systems due to the relatively long pot-life (usually on the order of several

hours depending on the temperature). Typical mechanical properties of epoxies

commonly used in structural engineering applications are provided in Table 2-3.

Polyester resins and vinylester resins are used in spray-up applications where chopped

glass-fibers and matrix material are sprayed onto the surface. These materials tend to









cure more rapidly than epoxies. Polyurethane resin can be found in certain pre-

impregnated (prepreg) systems. An interesting feature of this matrix material is that

water can be used to activate the curing process.

Table 2-3. Properties of epoxies used in commercially available fiber-reinforced
polymer systems for strengthening reinforced concrete
Tensile Modulus of Glass transition
strength elasticity Density temperature
Manufacturer Designation (MPa) (GPa) (g/cm3) (C)
Fyfe Co.a Tyfo S 72.4 3.2 1.1 82
Edge
Compositesb Veloxx LR 44.8 2.1 NA 63
V-Wrap
VSLc C100 55.2 3.4 NA NA
a Fyfe Co. LLC (2005)
b Edge Structural Composites (2005)
c VSL (2005)

The thermal conductivity associated with the different matrix materials ranges from

0.15 W/m-K to 0.2 W/m-K (Callister 1994).

Construction Methods and Application Procedures for FRP Composites

Composites Used in the Aerospace Industry

Major developments in the field of FRP composites occurred in the 1960s around

the growth of the aerospace industry (Gibson 1994). A wide variety of fiber and matrix

materials were developed along with a number of advanced manufacturing procedures.

Today, most composites used in the aerospace industry consist of carbon-fibers and an

epoxy matrix. Composite parts are typically constructed by placing layers of carbon-

fibers that have been pre-impregnated with the matrix material onto a mold with the

desired fiber orientation. Parts are then placed in a vacuum bag and cured in an autoclave

under high pressures at an elevated temperature. The resulting parts have a high fiber

volume fraction (typically from 0.5 to 0.8) and a low void content (0.001 to 0.01 by

volume).









Composites Used to Strengthen RC

FRP composites applied to RC are typically installed using a wet layup method.

This procedure involves saturating dry fibers on-site and then applying the wet composite

directly to the surface being strengthened. The composite is then allowed to cure in-situ.

The resulting composites typically have low fiber volume fractions (high matrix content)

and a higher percentage of air voids than aerospace composites.

The concrete substrate must be properly cleaned and contain no sharp protrusions

before the saturated composite is applied to the surface. The level of surface preparation

that is performed can vary significantly between different applications. In some cases the

surface will be sandblasted while in other cases the surface might be ground smooth with

a grinding wheel.

Large imperfections in the concrete substrate must be repaired by backfilling the

damaged area with a cementitious material. Smaller imperfections, such as "bug holes"

and formwork joints, can be repaired by filling the void with putty or thickened epoxy. It

is important that any remaining sharp edges are removed before the FRP composite is

applied.

The next step in surface preparation involves saturating the surface with matrix

material (epoxy). Concrete is a naturally porous material that can absorb epoxy. If the

saturated fabric was applied directly to dry concrete, there would be a tendency for the

concrete to pull matrix out of the fibers. This can result in air voids at the FRP/concrete

interface. If the composite is being applied to an overhead surface, an additional layer of

thickened epoxy tack-coat can be used to ensure that the saturated fabric does not fall

down before the matrix material cures.









After the saturated fibers have been applied to the surface, a squeegee or roller is

used to remove air bubbles and any excess matrix material from the FRP composite. If

the specific application calls for more than one layer of composite, the layers are applied

one at a time. Once the composite has cured, a final top-coat of epoxy is applied to

provide an additional layer of protection for the composite.

Another common application involves bonding precured FRP laminates to the

structure. The FRP laminates are manufactured in a controlled environment using similar

procedures to those described for the aerospace composites. These precured laminates

are then bonded to the structure using a thickened epoxy paste. Even though the

composite material is not likely to contain defects such as air voids, there is a possibility

that imperfections will exist along the thickened epoxy bond line if the material is not

applied properly.

The wet layup method provides the most flexibility for RC strengthening

applications. Different thicknesses can be obtained over critical areas and the fiber

orientation can be easily adjusted depending on the strengthening requirements. It is also

possible to span long distances across beams by splicing shorter pieces together.

Locations of Defects in FRP Systems Bonded to Concrete

Defects in FRP systems can result from improper installation or long-term

degradation due to environmental factors. Defects in FRP systems can be classified in

three ways: unbonded areas, debonded areas, and delaminated areas (adopted from Levar

and Hamilton (2003)). The term unbondedd" refers to areas of the FRP system that were

not properly bonded when the system first cured. The most common causes of unbonded

areas are improper surface preparation of the concrete and attempting to apply material

across sharp angles or re-entrant corners. Debonded areas are locations in which bond










that previously existed between the concrete and FRP has been destroyed. Debonded

areas can occur at several locations in the composite/concrete interface region (Figure 2-

1) and are usually a result of excessive loading or impact. If the debonded area occurs

due to excessive loading, it is common for the failure plane to occur a few millimeters

below the adhesive concrete interface. It is also possible for the entire layer of concrete

cover to separate from the beam at the level of the reinforcing steel (Sebastian 2001).

Delaminations are a lack of bond between different layers in a multi-layer FRP system.

Delaminations can be a result of improper installation or excessive loading.


SDelamination of
R 1 c 1 Concrete Cover




Delamination 1-5 mm
Conc'irc C'c'rr c" from concrete surface

adhesive Layer


FRP Layers FRP/Adhesive Interface
Adhesive/Concrete
Delamination Interface
e 2-1. Location of potential unbonded, debonded, and delaminated areas in FRP
systems


Quality Control Standards

Installation defects are likely to occur in FRP systems bonded to concrete. ACI

document 440.2R-02 provides acceptance criteria for the allowable debonded area in wet

layup FRP systems. These guidelines are intended to be applied to the installation of new

FRP systems and may be summarized as follows:


A


Figure









* Small delaminations less than 12.9 cm2 each are permissible as long as the
delaminated area is less than 5% of the total laminate area and there are no more
than 10 such delaminations per 0.93 m2

* Large delaminations, greater than 161 cm2,can affect the performance of the
installed FRP and should be repaired by selectively cutting away the affected sheet
and applying an overlapping sheet patch of equivalent plies

* Delaminations less than 12.9 cm2 may be repaired by resin injection or ply
replacement, depending on the size and number of delaminations and their
locations.

The ACI document also identifies three NDE techniques that can be used to

evaluate bond: acoustic sounding (hammer sounding or coin-tap), ultrasonics, and

thermography. No additional information is given regarding the deployment of a

particular technique or the interpretation of results. In addition, the document does not

provide any references or cite specific data to justify these guidelines.

NCHRP Report 514 (Mirmiran et al. 2004) also provides guidelines for the

allowable debonded area in wet layup FRP systems. These requirements are more

stringent than those prescribed by ACI. According to the NCHRP report, small debonded

areas less than 6.4 mm in diameter are acceptable so long as there are less than five such

defects in a 0.93 m2 area. Debonded areas with diameters between 6.4 mm and 32 mm

should be repaired by injecting the void with epoxy. Debonded areas with diameters

between 32mm and 152 mm should be repaired by cutting out the defective area and

replacing the removed material with a new FRP composite patch that extends a distance

of one inch beyond the borders of the original defect. Larger defects (greater than 152

mm in diameter) are to be repaired in a similar manner except that the replacement patch

should extend a distance of 152 mm beyond the defect area.

The NCHRP report also recommends acoustic sounding as the primary NDE

technique. The report states that "if an air-pocket is suspected, an acoustic tap test will be









carried out with a hard object to identify delaminated areas by sound with at least one

strike per 929 cm2." This report also acknowledges infrared thermography, microwave

detection, and ultrasonics as additional NDE testing that may be performed.

Guidelines provided by the International Conference of Building Officials (ICBO)

in document AC125 (ICC Evaluation Services 2003) are similar to the standards laid out

by ACI. This document recognizes that small diameter defects (1.6 to 3.2 mm in

diameter) are naturally occurring and do not require any attention. Defects smaller than

12.9 cm2 are acceptable so long as there are fewer than 10 per 0.93 m2. Specific

requirements for repair procedures are not provided, but the document does describe

backfilling with epoxy and replacement of small areas as acceptable. The AC125

document also recommends a visual inspection of the cured FRP system in combination

with acoustic sounding using a ball peen hammer to identify debonded areas.

Research Significance

The overall effect that defects have on the short and long-term performance of FRP

systems bonded to concrete is not well understood. ACI, NCHRP, and ICBO have all

recognized that defects are an important issue that must be addressed. The most common

NDE method that is currently used to inspect FRP systems is acoustic sounding (coin

tapping). This method is subjective and may not accurately identify or characterize

defects. The focus of the current research effort was to develop a NDE technique for

evaluating FRP composites bonded to concrete using infrared thermography (IRT). This

technique can be used to evaluate bond in FRP systems immediately after installation and

throughout the service life of the repair.














CHAPTER 3
NONDESTRUCTIVE EVALUATION USING
INFRARED THERMOGRAPHY

This chapter contains background information about infrared thermography (IRT)

and a review of previous research. The first section deals with the fundamentals of IRT

and describes some of the basic technology used in thermal imaging systems. The

following section addresses IRT as a nondestructive evaluation (NDE) technique.

Infrared Thermography Fundamentals

All objects at a temperature greater than 0 K emit electromagnetic (EM) radiation.

Furthermore, this radiation is emitted across a range of wavelengths. Max Planck

formally quantified the EM emissions of a blackbody (perfect emitter) in 1900 with the

following relationship describing intensity of the emitted radiation as a function of

wavelength and temperature of the object (Maldague 2001):

2he
I(A, T) = (3-1)
'[exp(hc/A DKT)-1l]

I = spectral radiance (W m2 sr-1 am)
S= wavelength of emitted radiation (itm)
T = temperature of the object (K)
h = Planck's constant (6.63 x 10-34 J s)
K = Boltzmann constant (1.38 x 10-23 J/K)
c = speed of light in a vacuum (m/s)

The wavelength at which the peak intensity occurs is given by the Wien

displacement law:


Amax = (3-2)
T









Xmax = wavelength of peak intensity (tm)
T = Temperature of the object (K)
C3 = a radiation constant (2898 [tm K)

This formula is obtained by taking the derivative of Plank's law (Equation 3-1)

with respect to wavelength, X, and setting the result equal to zero. Another useful

formula when considering EM emissions is the Stefan-Boltzmann Law. This law states

that the total amount of radiation per unit area, M, emitted by an object can be described

by:

M = go-T4 (3-3)

M = total radiant power emitted by object (W/m2)
S= emissivity of objects' surface
T = Temperature of the object (K)
c = Stefan-Boltzmann constant (5.67 x 10-8 W/m2-K4)

The Stefan-Boltzmann law is simply the integration of Planck's law over all

wavelengths. This relationship also contains a factor to account for the surface

characteristic of the object: emissivity. Emissivity can be summarized by the following

relationship:


(A, T) IAT) (3-4)
Ib (A, T)

S= emissivity of the objects' surface
Io = Intensity of the radiation emitted by the surface
Ib = Intensity of radiation emitted by a black body (perfect emitter)

Another useful relationship describes what happens to the total radiation flux

incident on an object. The total incident flux is the sum of the reflected, transmitted, and

absorbed radiation. The behavior is illustrated in Figure 3-1 and is defined as:

0, =0, +O +0+D (3-5)

i = Total incident flux
Or = Total flux reflected by the surface









(t = Total flux transmitted
Da= Total flux absorbed

Equation 3-5 is often expressed as a fraction of the total incident flux and can be re-

written as follows:

l=p+r+a (3-6)

p = % of flux reflected by the surface (reflectivity)
T = % of total flux transmitted (transmissivity)
a = % of total flux absorbed absorptivityy)








1)1



Figure 3-1. Incident radiation ((i) is reflected, transmitted or absorbed (Maldague
2001)

For simplicity, the discussion will now be limited to opaque objects that do not

transmit incident radiation. If this is the case, Equation 3-6 can be simplified as:

1=p+a (3-7)

p = % of flux reflected by the surface (reflectivity)
a = % of total flux absorbed absorptivityy)

Finally, the relationship between absorptivity and emissivity can be expressed by

Kirchoff s law which states that the two quantities are equal.

Figure 3-2 A shows the electromagnetic (EM) radiation emission curves for several

common objects at different temperatures. The curves illustrate that the intensity

(brightness) of the EM emissions increase with the object's temperature and that the

wavelength containing the peak intensity increases as temperature decreases.
















r 5000C






UV visib Near IR Far IR
Wavelength
Figure 3-2. Electromagnetic emission curves for objects at different temperatures

Equation 3-1 to Equation 3-6 serve as the foundation for both visible light imaging

(photography) and thermal imaging (IR thermography). The importance of these

relationships is best demonstrated with several examples. First, consider the largest

emitter of EM radiation that humans on earth are likely to experience: the sun. The

surface temperature of the sun is approximately 5800 K, which, when substituted into

Equation 3-1 results in relatively high intensities across the entire EM spectrum. The

wavelength at which the peak intensity occurs, 0.5 am, can be determined using the Wien

Displacement law. This turns out to be very convenient for humans since 0.5 [am (500

nanometers) happens to fall very close to the center of the visible spectrum. When

visible light from the sun strikes an object, a portion of that light is absorbed and a

portion is reflected (assuming the object is opaque). Visual imaging devices, including

the human eye, are designed to capture this reflected energy and measure the intensity of

the radiation that occurs within the visible spectrum.

Next, consider an object with a temperature of 300 K (270C or 800F). The EM

radiation emitted by an object at this temperature is limited to longer wavelengths outside









of the visible spectrum. Using the Wien displacement law, the peak intensity is found to

occur at 9.7 atm (9700 nanometers). This value falls within the infrared (IR) region of the

EM spectrum. Thermal imaging devices are designed to capture and record the radiation

emittedby an object in the IR region of the EM spectrum. Additional information about

thermal imaging systems is provided in the following section.

Detection of EM Radiation with an IR Camera

The IR region is commonly divided into five categories based on wavelength:

* Near IR (NIR) (0.7 1.4 [am)
* Short wavelength IR (SWIR) (1.4 3 [am)
* Mid wavelength IR (MWIR) (3 8 [am)
* Long wavelength IR (LWIR) (8 15 [am)
* Far IR (FIR) (15 1000 [m)

IR cameras measure surface temperature using EM radiation emitted by an object.

The two primary regions of interest of the EM spectrum for IR cameras are referred to as

mid-wavelength IR (MWIR) and long-wavelength IR (LWIR). MWIR cameras are

sensitive to wavelengths between 3 and 5 tm (this range can vary slightly depending on

the particular detector and optics used) while LWIR cameras are primarily sensitive to

wavelengths between 8 and 13 tm. Figure 3-3 shows why sensors are banded in this

manner. The figure plots the EM emissions of the earth's atmosphere, which shows very

high levels between 5 and 8 tm. Based on Kirchoffs law, this translates into very high

absorption. IR radiation emitted by the subject is effectively blocked by the atmosphere.

MWIR cameras are typically more sensitive than LWIR cameras. However, both

MWIR and LWIR cameras can accurately measure surface temperatures within the range

of interest for IR inspections of composites bonded to concrete. A fundamental

difference between the two types of cameras is that MWIR detectors often require some









type of cryogenic cooling to avoid signal noise due to the EM emissions from the

detector and surrounding electronics. This adds to the overall complexity of the thermal

imaging system and requires an additional level of maintenance as compared to uncooled

detectors.

High






3 gm 5 Am 8 gm 12 gm
MWIR Wh LWIR
Wavelength
Figure 3-3. Atmospheric emission in the MWIR and LWIR spectral bands

Many IR cameras made today operate in the LWIR region and use microbolometer

focal plane array (FPA) technology. A bolometer is a type of thermal detector made of a

material whose electrical conductivity varies with temperature change due to incident

radiation. A microbolometer FPA is simply an array of extremely small bolometers (50

[m x 50 [m) onto which an image is projected (similar to a CCD digital camera).

Typical FPA detectors might include a 320x240 array of microbolometers.

The electrical signal that is developed by each bolometer is converted to a single

pixel containing temperature data by applying an appropriate calibration factor. The

electrical signal must be corrected to ensure that the temperature determined from the

incident EM radiation matches the actual surface temperature of the object. Factors that

must be corrected for include:

* Emissivity of the object's surface
* Background temperature of any objects that might reflect off the surface of interest
* Distance to the object
* Atmospheric temperature
* Relative humidity










The optical lenses for LWIR cameras are typically made of germanium because of

its high index of refraction (around 4.0) for wavelengths between 2 and 12 [tm and its

high opacity to wavelengths outside of the 2 to 20 [tm band. This allows the lens to serve

as a filter for the visible and UV radiation that would otherwise be incident on the

detector (resulting in noise). The remaining wavelengths outside of the 8-12 [tm band are

removed using in-line spectral filters. This is important since EM radiation emissions by

the atmosphere in the 5-8 tm band would result in additional background noise. A

general schematic of an FPA camera and associated optics is provided in Figure 3-4.

j ) UV and Visible
Spectral Filter

SResulting
Microbolometer Thermal Image
Focal Plane Array

c" fitted by HHUM
MENOMONEE
IR Radiation mmmm

subject" un m mmmmm



IR Radiation
emitted by
atmosphere Germanium
Lens
Figure 3-4. General schematic of a focal plane array (FPA) and associated optics

Thermal Imaging System Used in Current Study

A FLIR ThermaCAM PM 695 infrared camera was used in this study. This

thermal imaging system operates in the 8 12 atm (LWIR) wavelength band of the

electromagnetic spectrum. An important feature of this camera is the ability to save

thermal images digitally. Each pixel in the thermal image (320x240) is stored as a

temperature value. This allows for easy post-processing of collected images using









proprietary software. The maximum image save rate for this thermal imaging system is 5

frames per sec (5 Hz).

Infrared Thermography Methods for NDE of Materials

The fundamental concept behind using IRT as an NDE technique is to apply heat to

the surface of an object and generate a thermal front that travels into the material. The

increase in surface temperature should be uniform if the material is homogeneous. If the

material contains defects below the surface, such as air voids, "hot-spots" will develop

since the flow of heat from the surface to the substrate is interrupted. These "hot-spots"

can be detected with an IR camera. The thermal image provided in Figure 3-5A

demonstrates this concept for an FRP system applied to RC. The "hot-spots" in the

image result from very small air voids at the FRP concrete interface. The visual image

provided in Figure 3-5B was taken after a saw-cut was made through the composite to

examine the cross-section. The thickness of the FRP layer in this system was

approximately 1 mm. The sizes of the air voids detected in this FRP system were

extremely small (less than 6.4 mm across for the smallest dimension and less than 0.25

mm thick).

ii .i Void at FRP-
Imm fluckl, FRP
composite *






A B
Figure 3-5. Application of IR thermography to FRP composite bonded to concrete A
Thermal image of FRP surface showing defects and B cross-section view
showing air voids









This technique has been applied by numerous researchers to a wide variety of

materials. There are currently ASTM standards available that describe procedures for

detecting pavement delaminations in bridge-decks due to corrosion (ASTM 2003) and for

identifying wet insulation in roofing systems (ASTM 1997). A recent search of the

ASTM standards database indicated that a new document is currently being drafted to

address the NDE of polymer matrix composites used in aerospace applications. Work

item summary WK8211 cites IRT as an "... emerging NDT [method] that [has] yet to be

validated..."

Considerable work does exist in the literature that investigates the use of IRT for

identifying subsurface defects in materials. Much of the work on composites has focused

on aerospace applications, though several researchers have addressed the issue of FRP

composites applied to concrete. This work will be discussed in greater detail in the

following sections.

There are three fundamental issues that must be addressed when using IRT as an

NDE tool:

* Heating methods
* Image acquisition
* Data analysis

Heating Methods

A wide variety of heating methods can be used when inspecting FRP composites.

Short duration heat pulses can be applied using a photography flash. Longer duration

heat pulses can be generated with halogen or IR heating lamps. These heat sources

transfer energy to the surface being inspected by radiation. The resulting surface









temperature increase is dependent on the intensity of the heat source and the

configuration of the heat source with respect to the surface.

Photography flashes offer a great deal of flexibility with regards to controlling the

intensity of the heat pulse. Different f-stop settings can be used to regulate the amount of

energy released during each flash. A wide variety of models are also available with

different maximum output capabilities. Flash systems are typically rated in terms of the

amount of energy that can be stored in the system's capacitors. This quantity is measured

in Joules or Watt-seconds. Models with relatively low output for IRT applications are

typically rated from 500 to 1000 W-s. Models with relatively high output can be rated as

high as 6400 W-s. As a general rule, higher intensity will translate into better IRT results

since the difference in temperature between defects and defect free regions is

proportional to the intensity of the applied heat. The cost of photography flash systems,

however, is proportional to their maximum output capabilities. The overall cost of these

systems can range from $1000 to $10000 depending on the maximum output and time

required to recharge the capacitors after each flash.

Infrared heating lamps and halogen lamps are also efficient means for heating

samples during IRT inspections. The output of these lamps is measured in Watts and can

range from 250 to 1000 W. A wide variety of lamp configurations are available. Most

IR heating lamps are designed to project a narrow beam of energy. Halogen lamps are

usually designed to illuminate large areas and tend to disperse the energy over a wider

field.

The specific requirements for surface temperature increase depend on the thermal

properties of the material under consideration and the depth below the surface that









defects occur. Heating methods that work well for one material may not be appropriate

for another. A major focus of the current study is to determine the required heat source

intensity and configuration for inspecting FRP composites bonded to concrete.

Image Acquisition

IR cameras capture and record data in two basic formats:

* Intensity images
* Radiometric images

Intensity images provide information about the relative temperature difference

between objects or areas within the IR camera's field of view. Before intensity images

are collected, the user is required to specify the level and span of temperatures that will

be encountered. The total span is typically divided into 255 bins. In a standard grayscale

image, the highest temperatures will appear as white and the coolest temperatures will

appear as black. Depending on the sophistication of the thermal imaging system, the

intensity image may or may not contain a temperature scale in physical units. Another

important thing to note about intensity images is that any intensity values that are greater

than the specified level and span will be assigned a value of 255, and any values less than

the preset level and span will be assigned 0. If the span and level are not set properly, the

image may appear underdeveloped (excessively dark) or overdeveloped (washed out).

Radiometric images involve storing a temperature value for each pixel in the

thermal image, eliminating any requirements for presetting the span and level. This

format facilitates post processing since thermal images can be viewed with any desired

grayscale or color scale limits. It is also possible to access a specific temperature value in

an image by specifying the coordinates of the point in terms of the row and column









number. If a series of images are saved at a specified time interval, the temperature vs.

time history for a single point (or series of points) can be extracted and analyzed.

The image save rate is also a distinguishing feature of thermal imaging systems.

The most sophisticated research grade cameras can save thermal images at rates up to 120

frames per second. Specific image save rate requirements depend on the nature of the

material being inspected. If the thermal diffusivity of the material is high and defects are

located very close to the surface, a higher image save rate is required. Conversely, a

lower image save rate is sufficient if the thermal diffusivity is low and defects occur deep

beneath the surface.

Data Analysis

There are two primary types of IRT analysis techniques: qualitative and

quantitative. Qualitative inspections involve collecting thermal images and searching for

any signs of non-uniformity in the resulting images. The thermal image provided in

Figure 3-5 represents a qualitative analysis in that the thermal image indicates something

of interest is occurring in the composite. Without additional information or an

accompanying destructive test to reveal the source of the hot-spots, very little can be said

about the true nature of the defects.

Levar and Hamilton (2003) conducted a study involving qualitative IRT

inspections of FRP composites bonded to RC. In this study, small-scale RC beams were

strengthened in flexure and shear using CFRPs and loaded to failure in laboratory testing.

IRT inspections were performed after the FRP systems were installed and areas that

appeared unbonded in the thermal images were recorded directly on the specimen. IRT

inspections were also performed at various stages of loading and patterns of debonding

were monitored. Important observations from these experiments were as follows: the









total debonded area increased as the load was increased up to failure; and certain

debonded areas appeared to have different thermal signal strengths.

The objective of quantitative IRT analysis is to use time dependent temperature

data to assess defect characteristics. The properties of interest in the current study

include:

* Defect size (in physical units)
* Depth of the defect below the surface
* Material composition of the defect

During a quantitative IRT experiment, thermal images are collected at a

predetermined interval while the surface of the object is being heated and then while the

surface cools. Specific points of interest can then be identified in these images and the

temperature variation can be monitored as a function of time. Careful analysis of the

results can help to establish where the defect is located in the FRP system.

No standard test methods currently exist for performing quantitative IRT

inspections. The following sections will highlight the basic principles behind some of the

existing methods. Specific details regarding the implementation of each method for the

current study will be presented in a later chapter.

Pulse IRT

Pulse IRT involves the application of a short burst of high intensity heat onto the

surface of an object. The most common heat source is a photography flash apparatus.

After the heat is applied to the surface, cooling will proceed as shown in Figure 3-6. If

the thermal front encounters a defect as it travels into the object, the area above the defect

will not cool as quickly. An important parameter to note is the time required for the

perturbation to begin (tp). This value is proportional to the defect depth (Zd). Another









important parameter is the observed thermal contrast (ATdef). This value is proportional to

the size, depth, and thermal properties of the defect.

Pulse IRT is commonly used in the NDE of materials with high thermal

conductivities containing defects near the surface. A good example of this application is

the NDE of aerospace structures made from FRP composites and/or metals (Kulowitch et

al. 1995). The required heating and observation time is short, which results in the ability

to inspect large areas very quickly. This, however, requires very high image acquisition

rates that can translate into higher equipment costs. Another disadvantage is that the

small amount of heat deposited on the surface may not reveal deeper defects.




Sperturbation in
g surface cooling
c curve due to defect

ATdef


Tamblent


tpulse time
Figure 3-6. Surface heating and defect detection for pulse thermography (Maldague
2001)

Step heating

Step heating IRT involves a longer duration and lower energy heat input than pulse

IRT. The temperature response on the surface of the specimen is monitored during

heating and also after the heat source is removed. Materials with lower thermal

diffusivities and deeper defects can often be evaluated with step heating IRT. Some

advantages of this method include a low image acquisition rate and low cost heat sources

(IR or halogen lamps). A disadvantage of this method is that it is sometimes difficult to









apply heat uniformly to the surface being inspected. An interesting study was reported

by Maierhofer et al (2003) in which a large block of concrete (1.5 m x 1.5 m x 0.5 m)

containing fabricated defects up to 3" below the surface was inspected using step heating

IRT. The surface was heated using an array of three 2400 Watt IR radiators for up to 60

minutes. A computer controlled arm was required to move the radiators across the

surface in a manner that resulted in uniform heating.

Starnes et al. (2003) used step heating IRT to identify and characterize defects in

FRP systems bonded to concrete. Experimental results were presented for a small-scale

specimen containing fabricated defects. The FRP system was comprised of a single 1.3

mm thick pre-cured carbon/epoxy lamina bonded to a concrete substrate with an epoxy

adhesive. A total of 8 simulated defects were created by placing different materials

between the lamina and the concrete. The first step in the experiment was to simply

detect the subsurface defects. This was accomplished by passing a 250 Watt IR lamp

across the surface at a rate of 15 cm/sec. The lamp was held at a distance of 5 cm from

the surface. This technique easily revealed all of the implanted defects. Once the

location of each defect was established, a quantitative step heating experiment was

performed to characterize the defect. A single lamp was aimed toward the defect at a

distance of 33 cm and heat was applied to the surface for 10 sec. This configuration

resulted in a defect signal strength of 2.70C for an air void.

Starnes et al. (2003) also used the finite element method to simulate the heat

transfer process involved in step heating. It was difficult to establish an appropriate

thermal conductivity (k) value for the carbon/epoxy lamina used in the experiment. This

was due to the large range of fiber volume fractions that are commonly encountered in









FRP composites. For pre-cured laminates, fiber volume fractions are typically greater

than 70%. Wet layup composites typically have much lower values (<50%). The matter

is further complicated by the wide range of published k values for plain carbon-fibers.

Textbook values range from 8 to 500 W/m-K depending on the modulus of the fiber and

the type of precursor used (Pitch vs. PAN) (Callister 1997). A thermal conductivity of

2.9 W/m-k (perpendicular to the main fiber direction) was ultimately used in the finite

element model. The finite element results were compared to experimental results from

one fabricated defect and good agreement was observed.

Lock-in IRT

Both pulse and step heating IRT rely on the ability of the IR camera to detect

temperature differences on the surface above defect and defect-free regions. In lock-in

IRT, thermal images are recorded while a modulated heat source is used to heat the

surface. Rather than monitor the temperature data at each pixel in a thermograph, lock-in

thermography focuses on the phase shift of each pixel (see Figure 3-7). This technique

results in "cleaner" thermal images and is also capable of detecting subsurface defects at

greater depths.

The major advantage of lock-in IRT is that phase images are not as sensitive to

non-uniform surface heating. Different values of surface emissivities and reflection of

the heat source also have limited effect on phase images (Maldague 2001). An

interesting study was presented by Carlomagno et al. (2002) which compared pulsed and

lock-in IRT in the NDE of historic frescoes. An important finding was that lower net

surface temperature increases are required for lock-in IRT than pulse or step-heating.

This has implications for the current study since it is important to avoid heating the










surface beyond the glass transition temperature, Tg, of the FRP matrix (Kharbari et al

2003).

Pulse phase IRT

The experimental setup and data acquisition procedure used for pulse phase IRT

(PPT) is similar to the pulse thermography procedure described above. After the series of

thermal images is collected, a discrete Fourier transform operation is performed on each

pixel of the images in the time domain. This operation results in a series of images in the

frequency domain with each pixel consisting of an imaginary number. Phase images are

obtained for each frequency by computing the inverse tangent of the imaginary part

divided by the real part.


/f dulated Heat Source



Note: Max
SIdealized temp response above A)defect for #1
Sdeect free region #1 occurs at lower
S\/ A"" n f\-, than #2
/~~ ~~~ / \ -


-- --Adefect #2
;\ / \ / Idealized temp response above P
subsurface defect Phase Image

Figure 3-7. Defect detection with lock-in thermography

The advantage of this method is that the resulting phase images are relatively

independent of non-uniform heating. There is also a strong relationship between the

frequency at which a defect first appears in the phase images and the depth of the defect.

Defects that are closer to the surface appear in higher frequency phase images while

deeper defects only appear at lower frequencies. A major disadvantage of this method is


I \


- I









that the amplitude of the defect signal strength is significantly less than for long-pulse

heating.

Objectives of Current Research

The overall objective of the current research was to develop IRT methods that

could be used to detect defects in FRP systems bonded to concrete. Specifically, a major

goal was to use IRT data to provide the following information about detected defects:

* Size
* Depth below the surface
* Material composition

Other items that are addressed include:

* Detection limits
* Heating methods
* Data analysis procedures

Previous research in this field has focused on the inspection of FRP composites that

are commonly used in the aerospace industry. A number of data analysis techniques have

been developed that can assist in using IRT results to characterize defects. None of these

methods have been calibrated for use on FRP systems bonded to concrete.

The remainder of this dissertation is divided into two main sections: Phase I and

Phase II. Results from Phase I of the current study are presented in Chapter 4. During

Phase I, IRT was used to inspect FRP systems that were applied to full-scale AASHTO

girders. Phase I contains a laboratory study and a field study. Findings from Phase I

were used to develop a second laboratory study that was conducted in Phase II. Details

of the Phase II experimental work are provided in Chapter 5 and Chapter 6.














CHAPTER 4
PHASE I EXPERIMENTAL WORK AND FIELD STUDY

Introduction

This chapter describes experimental work that was performed in conjunction with a

Florida Department of Transportation (FDOT) project investigating the performance of

FRP strengthening systems. The FDOT currently uses FRP composites to repair impact-

damaged bridge girders. The objective of the FDOT study was to develop a "quality

products list" (QPL) for FRP systems that are suitable for repairing impact damage

suffered by bridges.

The FDOT study involved full-scale load testing of six AASHTO Type-II bridge

girders at the FDOT's structural research facility in Tallahassee, Florida. Impact damage

was simulated at the midspan of each girder by removing a section of concrete and

cutting four prestressing strands. Four different FRP system manufacturers then had the

opportunity to design and install an FRP system to restore the capacity of the damaged

girder. These repairs were then validated by load testing each girder to failure.

This project represented an excellent opportunity to investigate the use of IRT for

evaluating the installation and performance of FRP systems. Each FRP system was

inspected prior to the load test and then again at various stages of loading. The first part

of this chapter results from this research. It should be noted, however, that the objective

is to describe the IRT results and not the results from the load testing. This information is

available in Lammert (2003).










Full-Scale AASHTO Girders

Description of AASHTO Girders and FRP Systems

A typical AASHTO type II bridge girder that was used in this series of experiments

is shown in Figure 4-1. The total depth of each girder was 122 cm (48 in.), which

includes a 30.5 cm (12 in.) cast-in-place slab. The distance between supports for each

load test was 12.2 m (40 ft.). The critical dimensions with regards to the infrared

inspections were the width of the girder's tension face, 45.7 cm (18 in.), and the

clearance between the girder and the laboratory floor, 50.8 cm (20 in.).


T P/2
30.5 C/L
152

Prestressing 121.9
Strand Cut
at Midspon
(typ.)

609
S- Concrete Removed
FRP Strengthening Over Middle 152 cm
S5 50.8 System Applied to
45.7 Tension Face Over
Middle 609 cm
I Note: All Dimensions in CM
Figure 4-1. Full-scale AASHTO type II girder and load test setup

Before the installation of each FRP strengthening system, vehicle impact damage

was simulated by removing a section of concrete and cutting four prestressing strands at

midspan. This area was then patched with concrete to restore the original cross-section

of the girder.

Four different FRP strengthening systems were evaluated in this study (applied to

Girder 3, 4, 5, and 6). The properties of each system are shown in Table 1. Each FRP

system was independently designed by the system manufacturer to restore the flexural

capacity provided by the cut strands. The FRP system manufacturers also installed each









system. During installation, each girder was raised to a height of 122 cm above the

laboratory floor. This provided a challenge for the FRP installers by limiting access to

the girder's tension face.

Table 4-1. Fiber-reinforced polymer system properties for full-scale AASHTO girders
Width of
FRP system Thickness laminate
Girder Fiber Matrix Layers (mm)a (cm) Anchorage
3 Carbon Epoxy 4 4/7 40.6 None
4 Carbon Polyurethane 4 3.1 / 6 30.5 2-ply carbon
5 E-Glass Polyester Resin 1 12.7 / 3.5 9.8 45.7 12.7 mm
6 Carbon Epoxy 3 1.75 / 4.34 45.7 2-ply carbon
a Data Sheet thickness / As-Built thickness

Girder 3

The FRP system applied to Girder 3 consisted of four layers of unidirectional

carbon-fiber fabric (with aramid cross-fiber) and an epoxy matrix. Each layer extended

over the entire middle 6.1 m of the girder. A tack-coat (epoxy thickened with silica

fume) was first applied to the concrete surface followed by the first layer of saturated

carbon-fiber fabric. During installation, there was a tendency for the saturated carbon

sheets to fall from the tension face. This prompted the installers to apply an additional

coat of thickened epoxy between each layer of fabric. The final step was the application

of an epoxy gel coat to the surface of the system. The thick layer of gel coat combined

with the overhead application resulted in drips forming before the matrix cured. These

thickened areas affected the infrared inspections. There were also areas where the gel

coat was thin, but no exposed unsaturated fibers were observed. Acoustic sounding (coin

tap) indicated that the system was well bonded to the concrete substrate and there were

no visible abnormalities that would indicate debonded areas.

The material data sheet (MDS) for this FRP system indicated a 1mm ply thickness

resulting in a total laminate thickness of 4 mm. In order to verify this thickness, a small









area of the strengthening system and concrete substrate (2 cm x 7 cm x 1.5 cm thick) was

removed from the girder after load testing. The total thickness of the laminate varied

between 6 mm and 7 mm (62.5% thicker than the MDS thickness). A 2 mm layer of

thickened epoxy was observed between the second and third layers of carbon-fiber

(Figure 4-2).


Principle Fiber Direction:

A


B
Figure 4-2. Cross-section views of FRP systems. A) Girder 3. B) Girder 4 (note:
principle fiber direction is out of page for Girder 4)

Girder 4

The FRP system applied to Girder 4 consisted of multiple layers of unidirectional

carbon-fiber fabric that was pre-impregnated with a water-activated polyurethane matrix.

Four layers of carbon-fiber were applied to the middle 4.9 m of the girder; three layers


Layer 4
I r









extended over the middle 7.3 m; 2 layers extended over the middle 9.75 m; and a single

layer was applied over the entire length of 12.2 m. A polyurethane tack-coat was first

applied to the concrete followed by the two longest layers of the pre-impregnated fabric.

These layers were then sprayed with water to initiate the curing process. Finally, the two

remaining layers were applied and sprayed with water. Anchorage for the FRP system

was provided by two FRP stirrups (each was 2 plies oriented at 0 and 90 degrees) located

at 12 feet on either side of midspan.

A coin tap inspection of the installed system did not indicate any debonded areas.

The MDS thickness for this system was 0.78 mm per layer, which resulted in a total

thickness at midspan of 3.1 mm. The measured thickness of the 4-ply laminate varied

between 5 and 7 mm (Figure 4-2 B).

Girder 5

The FRP system applied to Girder 5 was a sprayed-on mixture of chopped E-glass-

fibers and polyester resin. This process requires highly specialized equipment and is

commonly employed in the fabrication of boat hulls. The application method worked

extremely well on vertical surfaces (sides of the beam); however, it was difficult to apply

material to the bottom of the girder. After a thin layer of glass and resin were applied

with the spray gun, the material was pressed with a roller to condense the laminate. If too

much glass and resin were sprayed onto the bottom, large sections tended to fall down.

Sometimes this material would separate entirely and fall to the floor, and other times it

would simply cure as small draped areas. This resulted in a large number of visible

surface and subsurface defects in the laminate.

The laminate was extended over the middle 6.1 m of the girder and stirrups were

also sprayed onto the sides of the girder where the laminate was terminated. The final









measured thickness of the FRP system on the girder's tension face varied between 3.5

mm and 9.8 mm. Additional material was also sprayed on the sides of the girder's bulb

to an average thickness of 12.7 mm.

Girder 6

The FRP system applied to Girder 6 consisted of three layers of unidirectional

carbon-fiber fabric and an epoxy matrix. All layers extended over the middle 6.1 m of

the girder. The data sheet indicated 0.58 mm ply thickness resulting in a total laminate

thickness of 1.75 mm. Two additional plies of unidirectional fabric were used to anchor

the FRP system at the termination points. This resulted in a total laminate thickness of

2.9 mm on the tension face at the termination points. The as-built thickness of this FRP

system was not verified.

Little or no excess matrix material was present on the surface of the installed

system. A coin tap inspection indicated that the system was well bonded to the concrete

substrate and there were no visible abnormalities.

Infrared Inspection Procedures

Thermal images were collected using the thermal imaging equipment described in

Chapter 3. Heat sources used in this study included 125 Watt IR heating lamps and a 500

Watt halogen lamp. Limited access to the tension face of each girder along with the need

for efficiency in evaluating the relatively large area prompted the development of two

novel scanning procedures. In both procedures, the heat source and IR camera were

mounted to a rolling cart. The heat source was positioned on the leading edge of the cart

and placed a distance of 7.6 cm from the FRP surface. The camera was positioned to

view the FRP surface just behind the area being heated. As the cart was pushed along the

floor, the IR camera recorded a series of images as the surface cooled.









The cart configuration for the first procedure is shown in Figure 4-3 A. This

resulted in a camera field of view (FOV) of only 22.9 cm x 17.1 cm. Consequently, two

passes were required to inspect the entire 45.7 cm width of the girder's tension face. This

image was also slightly distorted since the angle of incidence for the camera was not 900.

The cart configuration for the second procedure (shown in Figure 4-3 B) utilized first-

surface mirrors located near the ground to increase the camera's FOV to 56.7 cm x 42.5

cm.

The image save rate for all inspections was set to one frame per two seconds (0.5

Hz). The fastest image save rate to the on-board PCMCIA storage card is approximately

1 Hz. This rate, however, produces an unmanageable amount of data (each thermal

image is 158 Kb). An even faster rate of up to seven frames per second is possible, but

this requires a direct link to a laptop computer. For the scanning speed used in these

inspections, the rate of 0.5 Hz was found to be adequate. A typical series of thermal

images containing a subsurface defect is shown in Figure 4-3 C. This particular series

was recorded using the cart configuration shown in Figure 4-3 A.

To characterize defects detected during each inspection, the defect signal strength,

ATdef, was calculated as follows:

ATdef =Tdef Tbackground (4-1)

Tdef = temperature above the defect
Tbackground = temperature of adjacent defect free area

The magnitude of Tdef was determined by identifying an appropriately sized area

above the brightest portion of the defect and using the average temperature measured

within that area. The standard deviation of temperature values within each area was









typically less than 0.50C. A similar technique was used to determine the corresponding

Background.


A B


Figure 4-3. Data collection for full-scale AASHTO girders. A) Scanning cart
configuration for Girder 3. B) Girders 4 to 6. C) Typical thermal images

To make a valid comparison between defect signal strengths, the amount of heat

applied to the surface should be consistent during each inspection. Heating consistency

for each scan was evaluated by monitoring ATbackground along the leading edge (edge

closest to the heat source, as shown in Figure 4-4) of each thermal image in a series:

ATbackground Tbackground -Tambient (4-2)

Tambient = ambient temperature of the girder prior to heating









This quantity was also monitored along the trailing edge (farthest away from the

heat source) of each image in a series in order to evaluate the average cooling rate on the

surface of the FRP.















Figure 4-4. Subsurface defect found on Girder 3

Initial IR Inspections

An initial infrared inspection was performed on each girder prior to load testing.

The objective was to identify any defects formed during the installation. Girder 3 was

inspected using the cart configuration shown in Figure 4-3 A. Girders 4-6 were inspected

using the configuration shown in Figure 4-3 B.

The inspection of Girder 3 revealed 11 minor subsurface defects (< 12.9 cm2) and

three moderate subsurface defects (> 12.9 cm2 but less than 161 cm2). Thermal images

for two of these defects are shown in Figure 4-5. These images were recorded

approximately six seconds after the area was heated.

The computed signal strength for defect 1 and defect 2 was 7.50C and 15. 1C,

respectively. The difference in signal strengths could be a result of several factors:

defect depth, amount of heat applied to the surface, and the size of the defect. Stronger

signal strengths are expected for defects that are closer to the surface (signal strength is









inversely proportional to defect depth). Applying more heat to the surface will also result

in higher signal strengths. Finally, a larger surface area will result in higher defect signal

strengths since the heat applied above the defect must travel farther before it is absorbed

by the concrete.











Figure 4-5. Subsurface defects found on Girder 3

The initial infrared inspection performed on Girder 4 did not reveal any defects

similar to those observed in Girder 3. There were, however, two interesting observations

made regarding the polyurethane matrix material and the uniformity of heating

perpendicular to the girder's length. Some areas of the FRP surface were covered with

excess polyurethane matrix. This excess matrix material had the appearance of a thin

layer of foam. The color of this layer was also much lighter than adjacent areas, which

appeared black. An example of this occurrence is shown in Figure 4-6. The resulting

ATbackground for the light colored area was 5.40C while the ATbackground for the dark color

was 7.80C. Another source of non-uniform heating was streaking due to the narrow

beam width of the IR heat lamps. The resulting ATbackground for the area directly in-line

with the heat lamp was 9.7C while the ATbackground in the area between lamps was 7.70C.











AU,-
= 5.4 OC^^^


Figure 4-6. Non-uniform surface heating of Girder 4

A visual inspection of Girder 5 revealed numerous defects on the surface of the

FRP system. These were a result chopped fibers falling down before the system fully

cured. There were also a large number of defects visible just below the surface of the

FRP, which were the result of improper saturation of the chopped glass-fibers. These

large imperfections near the surface interfered with the IRT inspection. The thermal

images were crowded with these imperfections and it was difficult to distinguish defects

that occurred near the surface and defects that occurred below the surface. A typical

thermal image is shown in Figure 4-7 A. All of the defects that were visible in the

thermal image were also visible to the naked eye.

Only one subsurface defect was detected during the initial scan of Girder 6 (shown

in Figure 4-7 B. The recorded defect signal strength was 7.40C and ATbackground was

8.50C. This defect occurred on the edge of the laminate and was not considered to be

significant.

A summary of the scanning speed for each initial inspection is presented in Table 2.

ATbackground was computed along the leading edge (closest to heat source) and trailing

edge (farthest from heat source) of the series of thermal images that were collected as the

cart was pushed along the beam. The average speed was computed by dividing the total









distance scanned by the total time required. These scanning rates are much slower than

those reported by Starnes et al (2003). Their basic procedure for identifying subsurface

defects involved passing a single 250 Watt IR heat lamp held a distance of 5 cm from the

FRP surface at a speed of approximately 15 cm/s. This approach was adequate to detect

defects beneath a 1.3 mm thick pre-cured CFRP laminate.















A B
Figure 4-7. Thermal images collected for full-scale AASHTO girders. A) Girder 5. B)
Girder 6

Table 4-2. Summary of scanning speed and uniformity of heating
Avg. Leading Edgea Trailing Edge Avg.
Scan Speed ATbackground Std. Dev. ATbackground Std. Dev. Cooling
Girder Config. (cm/s) (0C) (0C) (0C) (0C) (OC/s)
3 Fig. 3 A 1.2 13 1.4 9.5 1.1 0.25
4 Fig. 3 B 2.77 11.3 2.4 6.0 1.8 0.35
5 Fig. 3 B 1.25 7.2 1.5 4.26 0.98 0.09
6 Fig. 3 B 2.1 10.4 1.9 1.67 0.88 0.44
a Leading Edge of image is closest to heat source

For the current series of inspections, the average cooling rate (ACR) on the surface

of the FRP was computed as follows:


ACR = ATackground (leading) ATacgrod (trailing)
A L.K = --------------------

where: (4-3)

t' FOVSD
Speed










FOVsD is the camera's field of view in the direction of scanning and speed is the

average speed. This calculation assumes that the surface temperature cooling profile at

every point is linear, which is not the case. The results are reported in this format for

ease of comparison between FRP systems.

Controlling the speed of the cart during each scan was difficult. Figure 4-8 shows

the resulting ATbackground along the leading and trailing edge of each thermal image vs.

position for Girder 3 and Girder 6. The leading edge curve represents ATbackground

measured just after an area enters the thermal image. The trailing edge curve represents

ATbackground measured just before the same area leaves the image. The average time

between these two curves can be calculated as t' in equation 4. The significant

fluctuation observed in each curve demonstrates the sensitivity of ATbackground to cart

speed. In areas where the cart was pushed slowly, there was an increase in ATbackground

while areas in which the cart was moved more quickly experienced a decrease in

ATbackground. The standard deviation of ATbackground along the leading edge for Girder 3, 4,

5, and 6 was 1.4, 2.4, 1.5, and 1.90C, respectively.

16 T 18 -- Leading Edge
14- Leading Edge 16 --
a 12- 14-
o 12
10
0 8 0
STrailg 6 6 Trailing Edge
S4 Tiling Edge 4--
2 2
0I I 0 I I
0 200 400 600 0 200 400 600
Position (cm) Position (cm)

A B
Figure 4-8. Background temperature increase vs. position along length of girder. A)
Girder 6. B) Girder 3









IR Inspections Performed During Load Testing

Additional IR inspections were performed during the load test of each girder. For

Girder 3, the load was removed during each inspection. Girders 4-6 were inspected while

the specimen was under loading. Table 3 contains a summary of the load levels at which

each IR inspection was performed. The purpose of these inspections was to monitor the

subsurface defects detected in the initial scan as well as to detect any new debonded areas

resulting from the applied load.

None of the additional inspections that were performed prior to failure revealed

new defects or subsurface defect growth due to loading.

IR Inspections of Known Debonded Areas After Failure

The failure mode for Girder 3 was delamination of the concrete cover at the level of

the girder's pre-stressing tendons (see Figure 4-9). There were no visual or audible

indications ofFRP debond during the loading. A 90 cm x 45 cm piece of the delaminated

cover was recovered after load testing to perform a thorough IR inspection. This section

did not contain any defects that were identified in the initial inspection, however,

debonded areas were formed as a result of the delaminated cover concrete and FRP

striking the floor after the girder failed.

A series of IR inspections were performed on this section using the same amount of

heat that was applied during initial inspections prior to load testing. These inspections

did not reveal any debonded areas even though coin-tap testing did indicate that large

areas of the FRP had separated from the concrete. A very important observation was

made regarding a large section of FRP on the edge of the sample that was no longer

attached to any concrete (effectively an overhang). This area was expected to appear









extremely bright after being heated with the halogen lamp to a ATbackground of 150C.

However, no thermal signal was detected.























Figure 4-9. Failure modes for full-scale AASHTO girders. A) Girder 3. B) Girder 4.
C) Girder 5. D) Girder 6.

Figure 4-10 shows results from an experiment in which a ATbackground of 33.30C was

generated above the bonded area. Immediately following the removal of the heat source,

the temperature increase above the debonded (non-bonded / overhanging) area was

28.60C. This resulted in a thermal signal of- 4.610C at t = 0 seconds. This initial

negative temperature difference was likely due to improper lamp positioning that resulted

in non-uniform heating of the surface. After 282 seconds of cooling, the thermal signal

achieved its maximum value of 2.120C. Measurements were terminated after 594

seconds with a thermal signal of 1.690C.

If the sample had been heated uniformly, the maximum signal would likely have

approached 6.70C (2.120C (-4.610C)). It is important to note that this would occur after










4.7 minutes of cooling. These results stand in sharp contrast to the measurements

obtained during the initial IR inspections (defect signal strengths between 100C and 15C

that were visible after less than 2 seconds of cooling).

3 --
2-- C r M

0
-1 100 200 300 400 500 600
S-2 Time (sec.)
H-3
-4 -4
-5- -d,
-6-
Figure 4-10. Defect signal strength (ATdefect) vs. time for known debonded area

The failure mode for Girder 4 was debonding of the FRP laminate. Debonding

began in the middle of the specimen and progressed outward towards both ends of the

girder. At the north end of the girder, debonding caused the anchorage FRP to rupture

and then continued to end of the laminate. On the south end of the girder, the FRP

system ruptured in tension before the debonding reached the anchorage point.

After the specimen failed, the majority of the FRP system was no longer bonded to

the girder. There was, however, a short section on the south end that remained partially

bonded. The line of demarcation between the bonded and debonded area of this section

was easily recognized with a coin-tap inspection. An IR inspection of this debonded area

was made using the same procedure outlined above (cart speed of approximately 3

cm/sec). This inspection did not reveal the debonded area. Another inspection was

performed in which the lamp was passed over the debond line for 120 sec and the area

was observed while cooling for 3 minutes. Again, the debonded area was not detected.









The failure mode for Girder 5 was tensile rupture of the FRP laminate at the

girder's midspan. There were no audible indicators during loading that the FRP system

ever debonded from the surface of the concrete.

The area around the rupture point of the FRP system was thoroughly inspected after

the girder failed. A large debonded area (approx. 15.2 x 45.7 cm) was identified adjacent

to the rupture point on the bottom of the girder. This area could not be identified with the

scanning procedure that was used during the initial inspection of this girder.

The failure mode for Girder 6 was debonding of the FRP system. This debonding

began at midspan and progressed outward towards the anchorage points (very similar to

Girder 4). Audible indicators of the debonding were also present; however no IR scans

were performed between the time they were first heard and failure of the specimen. At

the ultimate load, a portion of the FRP slipped at the north anchorage point resulting in

failure.

Results from an IR inspection performed on the tension face of the girder at the

north anchorage point are shown in Figure 4-11. A thin strip (approx. 15 cm wide) in the

center of the beam remained bonded to the concrete at the anchorage point. The adjacent

debonded/delaminated areas are clearly distinguishable in the thermal image. The defect

signal strength for the delaminated area varied from 5C to 90C.

A small FRP test patch (21.6 cm x 45.7 cm) was constructed on the side of Girder 6

near the support. This test patch consisted of a single layer of carbon-fiber fabric. The

area chosen for the test patch contained numerous bug-holes and other surface

imperfections. A single bug-hole near the center of the area was identified and filled with

thickened epoxy paste prior to placement of the carbon-fiber. The remaining bug-holes









were left unfilled. The test patch area was heated for 18 seconds with an array of four

125 Watt IR heating bulbs. The resulting ATbackground above the defect free area was 50C.

The bug-holes were visible immediately after the heat source was removed. The

reference hole (unfilled), shown in Figure 4-12, had a defect signal strength of 9.5C.

The signal strength above the epoxy filled hole was only 5.250C immediately after the

heat was removed. After 8 seconds of cooling, the defect signal strengths above the filled

and unfilled holes were equal at 5.00C. As the area continued to cool, the signal strength

above the unfilled hole decayed rapidly, and after 20 seconds only the epoxy filled hole

continued to possess a significant thermal signal.














A B
Figure 4-11. Debonded area after failure for Girder 6. A) IR image. B) Visual image

This finding has several implications. Filling the hole with epoxy will ensure that

the FRP is bonded to the concrete; however there is still a difference in thermal

conductivity between the epoxy filler and concrete substrate. This results in the

appearance of a debonded area in thermal images. Careful scrutiny of the thermal signal

vs. time can differentiate the epoxy filled void from an air filled defect; however this

requires a series of thermal images to be recorded with the camera in a static position.










Also, it might be difficult to differentiate between these two thermal signals if a particular

image does not contain both types for reference.


12
Figu 210 Unfilled
Epoxy-
S t filled
6--6


2--
0 I I I
0 10 20 30
installat als Time (sec.)

C (d)
Figure 4-12. Serieths of thermal images for air and epoxy filled defects. A) t = 0 sec. B) t
= 8 sec. C) t = 20 sec. D) ATdefect vs. time plot.

Summary of IR Inspection Results for Each FRP System

The FRP system applied to Girder 3 consisted of four layers of unidirectional

carbon-fiber fabric with an epoxy matrix. Initial IR inspections performed after

installation revealed three subsurface defects having an area greater than 12.9 cm2

Defect signal strengths for these defects were greater than 10C and resulted from a

ATbackground of approximately 13'C. These defects were visible immediately after the heat

source was removed. Additional IR inspections performed on a section of the FRP









system with known debonded areas produced different results. A ATbackground of 330C

resulted in Tdefect measurements of only 2.10C after 282 sec of cooling. The defects found

during the initial inspection were very close to the surface signifying delaminations rather

than debonded areas. The more important finding is that the initial scanning technique

would not have detected debonded areas since the amount of heat applied to the surface

was relatively low and the camera was not positioned to record images when the defect's

maximum signal strength was reached.

The FRP system applied to Girder 4 consisted of four layers (gradually tapering

down to a single layer) of unidirectional carbon-fiber fabric pre-impregnated with a

polyurethane matrix. No subsurface defects were detected during IR inspections

performed after the installation of the FRP system. An IR inspection was also performed

on a known debonded area. This debonded area was located on a portion of the FRP

system that was partially attached to the girder after failure. Results indicated that this

particular FRP strengthening system is not well-suited to inspection with IRT. A closer

inspection of this system after failure revealed a thin layer of polyurethane matrix

between the FRP and concrete that resembled insulating foam (as shown in Figure 4-13).

If this particular type of matrix material is effectively insulating the carbon-fibers from

the concrete, subsurface defects will not result in hot spots on the surface after heating.

Additional experiments under controlled laboratory conditions are needed to determine

the limits of detection.

The FRP system applied to Girder 5 was a chopped glass / polyester resin mixture

that was sprayed on the surface. Numerous surface and subsurface defects (also close to

the surface) were clearly visible with the naked eye. IR inspections of this system clearly









revealed these defects. The thickness of the system, however, and possibly the insulating

characteristic of the glass-fibers made the detection of debonded areas difficult.


r i ^'" w gia -, .










Figure 4-13. Polyurethane matrix shown after debonding from concrete (Girder 4)

The FRP system applied to Girder 6 consisted of three layers of unidirectional

carbon-fiber fabric and an epoxy matrix. Initial IR inspections revealed only one

subsurface defect. Additional IR inspections performed on a known delaminated area

indicated that IR thermography was capable of detecting defects beneath at least two

layers of the FRP system. It should be noted that the installation procedure for this girder

was very different from Girder 3 even though the system specifications are similar.

Excess matrix material that was present in the laminate on Girder 3 that was not observed

on Girder 6. This reduction in matrix volume increased the effectiveness of the IR

inspections. An IR inspection performed on a small test patch (single layer of carbon-

fiber) containing numerous unfilled bug-holes demonstrated that IR thermography can be

very effective at detecting defects under a single layer of FRP. This inspection also

showed that epoxy-filled holes still possess a defect signal strength; however the rate of

decay of this signal is much slower than a simple air void.









Field Inspection: Chaffee Road

The Chaffee Road/Interstate 10 overpass (located in Jacksonville, Florida) suffered

severe vehicle impact damage in July of 2001 (see Figure 4-14). The impact dislodged

large sections of concrete and ruptured a number of prestressing strands. The most

severe damage occurred on the fascia girder that was hit first (east side of bridge). The

exterior girder on the west side of the bridge also experienced similar damage. The

interior girders were not significantly affected. Rather than replace these girders, the

FDOT decided to repair the damaged concrete and then apply an externally bonded FRP

strengthening system. This system was comprised of multiple layers of 00-90 carbon

fiber fabric and an epoxy matrix that fully encapsulated the middle 9 m of both exterior

girders. The exact configuration of the FRP system was not available at the time of this

study. Samples removed from the girder, however, contained two layers of the

bidirectional fabric. There were no signs that excess epoxy was applied during the

installation of the system.

Chaffee Road has the unfortunate distinction of being the lowest overpass on I-10

westbound out of Jacksonville. As a result, a number of minor vehicle impact events

occurred between the time the FRP system was installed and July of 2002. In June of

2003, another serious vehicle impact occurred (shown in Figure 4-14). Clearly the FRP

system was in need of repair and the strategy adopted by the FDOT was to completely

remove the existing FRP and restore the cross-section of the girder with concrete. After

this was completed, a new FRP system was applied to strengthen the girder.

Before the original system was removed, the author inspected the system using

IRT. The primary goal of this inspection was to assess the affect of the vehicle impact

damage on the FRP system (beyond what was clearly destroyed). This was also an









excellent opportunity to apply the IR inspection techniques developed during the full-

scale AASHTO girder tests in a field situation.












A B
Figure 4-14. Vehicle impact damage sustained after FRP strengthening. A) July 2002.
B) June 2003.

Areas of the FRP system that were damaged by the vehicle impact were heated

using four 125 Watt IR heat lamps. The inspection procedure required two people: one to

operate the camera and one to heat the surface. The camera operator and the surface

heater were lifted up to the girder in a mobile scissor lift positioned directly below the

area being inspected. The surface was heated by passing the lamps over the surface at a

distance of approximately 10 cm. The rate of motion of the heat lamps varied between

scans, but the average ATbackground generated by the heat lamps was 100C. As the

specimen was heated, the camera operator viewed the surface through the IR camera and

directed the heat lamp operator.

While there was some evidence of debonding, thermal images indicated that

significant damage was limited to the immediate area surrounding the point of impact

(Figure 4-15). The debonded areas visible in the thermal images were verified with a

coin tap inspection. This coin tap inspection also verified that areas which appeared

bonded in the thermal images in fact were.






















A B


C D
Figure 4-15. Visual and thermal images of vehicle impact damage. A) Damage to side
of girder. B) Thermal image. C) Damage to tension face. D) Thermal
image

While work was being done to apply the new FRP system to the east girder, the IR

inspection team was able to evaluate the FRP system that was originally applied to the

west girder. Access to the girder was achieved with a 2 m x 4 m scissor lift. The scanner

cart configuration shown in Figure 4-3 B was deployed on the scissor lift in an attempt to

duplicate the experiments performed on the full-scale girders in the laboratory.

Unfortunately, this met with little success. Unevenness of the scissor lift platform meant

that the height of the lamps were in constant need of adjustment as the cart was pushed









along the girder. Also the cart was not properly configured to account for the increased

distance between the platform and the girder that was mandated by the platform's railing.

As an alternative to the scanner cart, the camera was placed on a tripod and the

camera operator applied heat to the surface as the thermal images were recorded. This

was effective at revealing subsurface defects in the FRP system; however this method

required a significant amount of time for setup between shots. A typical thermal image

collected during this inspection is shown in Figure 4-16. A number of small defects were

detected throughout the inspected area. One area that was particularly prone to

debonding was the re-entrant corer where the bulb intersects the shear face.














Figure 4-16. Infrared thermography inspection of undamaged girder

The IR inspection technique worked very well in this field inspection. A number of

subsurface defects were identified in the original FRP system as well as a portion of the

system that suffered additional vehicle impact damage. Overall, the IR inspection

indicated that much of the FRP strengthening system was still bonded after the major

impact damage. This was verified as the workers attempted to remove the existing FRP

system with pneumatic jackhammers and encountered tremendous difficulty. Most of the

system was so well bonded that it was left in place and the new system was installed over

it (shown in Figure 4-17). An alternative repair procedure that might be considered is to









remove the debonded laminate around the damaged areas and patch the damaged

concrete. Once the patch is cured, apply new FRP composite over the repaired area with

an appropriate overlap onto the existing FRP system. It is not known, however, how this

repair technique would affect the strengthened flexural capacity of the girder.














Figure 4-17. Damaged girder before new FRP system was applied

Summary of Findings for Phase I

Results from the full-scale AASHTO girder experiments and the field study

conducted on the bridge at Interstate 10 and Chaffee Road provide insight into how IRT

can and cannot be used to inspect FRP strengthening systems applied to civil

infrastructure. The most important observation is that an IRT inspection procedure that is

effective for one FRP system may not be applicable to another.

In the case of the AASHTO girder experiments, four independent FRP system

manufacturers were given identical strengthening requirements for a damaged girder.

The FRP systems that were installed on each girder varied significantly: different fiber

types, different matrix materials, different thicknesses, and different installation

procedures. In general, thicker FRP systems that contained more matrix material

required longer heating times and longer observation times during cooling.









The IRT inspection performed on the bridge at Interstate 10 and Chaffee Road also

raised some important issues. The IRT inspection procedure used for the original FRP

system applied in 2001 did detect defects. These data, however, were not collected in a

uniform manner and the heating process varied widely from one portion of the FRP

system to the next. The general procedure involved heating an area with four IR heating

lamps and watching the surface cool with the IR camera. If no defects were observed in

the thermal images, the process was repeated by heating the area for a longer duration.

This approach gave the inspectors a general idea of the required heating times for this

specific FRP system. However, there was no rational basis for the heating time or the

observation time with the IR camera.

The radiometric data were reported for select defects by identifying the relative

surface temperature increase for the defect and the adjacent defect free areas. These data

represent an improvement over basic intensity images. At the very least it is possible to

state what the temperature difference is for the defect as opposed to stating the defect is

"hotter" than the surroundings. Without a rational model or an extensive database of

results to compare with these numbers, the radiometric temperature data do not provide

additional information about the defect. It is not possible to determine the depth below

the surface or the material composition of the defect.

Another finding of the Chafee Road study supports a conclusion made from the

AASHTO girder experiments: the installation methods and resulting FRP thickness varies

considerably and is not always known. The replacement system applied to the Chafee

Road bridge was applied on top of significant portions of the original system. A large

amount of thickened epoxy paste was also used to fill surface imperfections. There is









also a question about the final thickness of the replacement FRP system. The design for

this system specified two layers of carbon/epoxy in the longitudinal direction and one

layer of carbon/epoxy in the transverse direction for shear. However, installation of this

system required a large number of lap splices and overlaps between successive layers.

This installation procedure resulted in a composite thickness as high as five layers in

certain locations. These five layers were in addition to any portions of the original

system that were not removed.

Based on the findings of the Phase I experimental work, it was determined that

additional laboratory work was needed. This work is described in the following chapters

as Phase II experimental work. The objective of this work was to further investigate the

effects of FRP system properties on IRT results. The properties that were investigated

included thickness of the FRP system, fiber type, and matrix saturation levels.

Phase II experimental work also investigated different methods for applying heat to

the surface of the FRP system. The objective of this work was to develop a standardized

heating procedure that can be used to inspect FRP composites in the field. Phase I results

highlighted the fact that FRP systems applied to concrete cover a large surface area. The

heating methods that were investigated in Phase II were designed such that they could be

practically implemented in the field.

Another significant finding from the Phase I experimental work was that thicker

FRP systems require substantially longer heating times to reveal defects. From a

practical standpoint, the heat flux applied to the surface of the FRP is not uniform.

Longer heating times translate into large thermal gradients across the area of the

composite. If the overall temperature difference that develops for a defect is small, the






64


defect signal may become lost in the thermal gradient that develops from non-uniform

heating.

The data analysis techniques described in Chapter 3 were developed by other

researchers to address the issue of non-uniform heating. These data analysis techniques

were also developed to assist in using IRT data to characterize defects. The Phase II

experimental work in the current study investigated some of these data analysis

techniques and focused on calibrating the different methods for use with FRP composites

bonded to concrete.














CHAPTER 5
PHASE II: EXPERIMENTAL SETUP

Introduction

The overall objective of the current research is to develop IRT methods that can be

used to detect defects in FRP systems bonded to concrete. Specifically, a major goal is to

use IRT data to provide the following information about detected defects:

* Size
* Depth below the surface
* Material composition

The results presented in Chapter 4 indicate that a standardized approach for

collecting IRT data is needed. Although general heating methods and image capture

techniques are available, the application of IRT to composites bonded to concrete has yet

to be fully developed. Consequently, the following issues must be addressed before IRT

can be used in the field environment:

* Heat intensity and duration
* Timing and duration of image capture
* Image capture rate
* Subject size in field of view of IR camera

The results presented in Chapter 4 also indicate that FRP system properties can

vary significantly from one application to the next. A primary objective of the Phase II

research was to investigate how the following FRP system properties can affect IRT

results:

* Thickness of FRP composite
* Matrix saturation levels
* Fiber types









* Thickened epoxy tack-coat
* Lap splices

Small-scale specimens containing fabricated defects were constructed in a

laboratory environment. Four experimental procedures were designed and implemented

for heating the surface of the composite under consideration:

* Flash heating with a photographer's flash
* Scan heating with two 500 W halogen lamps
* Long-Pulse heating with four 500 W halogen lamps
* Sinusoidal heating with four 500 W halogen lamps

This chapter contains a description of the specimen matrix as well as the details of

each heating method. Information regarding the noise characteristics of the IR camera

used in the current study is also presented. Results and analysis are provided in

Chapter 6.

Specimen Construction

A total of 34 small-scale specimens were constructed and grouped into the five

categories shown in Table 5-1. Series A, B, and C contain fabricated defects of known

size and location. Series A seeks to investigate the relationship between the smallest

detectable defect and the thickness of the applied FRP composite system. Series B

examines how different levels of fiber saturation and fiber type affect IRT results. Series

C includes specimens that were prepared using different degrees of surface preparation

(sandblasting) and tack-coat.

Series D and E were used to determine whether or not IRT is capable of providing

any additional information about how an FRP composite system was constructed. Series

D consists of three specimens that were constructed using different saturation methods

for the fibers. Series E was used to investigate the effect of lap splices on IRT results.









Specific details regarding how each specimen was constructed are provided in a later

section.

Table 5-1. Overview of specimen matrix
Number of
Series ID specimens Variable investigated
A 4 Defect size and detection limits
B 18 Saturation levels and fiber type
C 6 Surface prep and tack coat
D 3 Saturation methods
E 3 Lap splices

A number of the steps and procedures that were used to construct each specimen

are common to all of the specimens in each series. These general steps and procedures

will be described first. Specific details for each series will be discussed after the general

information has been provided.

FRP Composite Materials

All of the fiber and matrix materials used in this study were provided by Fyfe Co.,

LLC. These materials were chosen to represent common matrix and fiber types used in

wet layup FRP strengthening systems. Pre-cured laminates were not addressed in this

study. Two types of fiber materials were used in this study:

* Carbon (TYFO SCH-41)
* Glass (TYFO SEH-51A)

TYFO SCH-41 carbon fiber is a unidirectional, stitched carbon-fabric. The dry

fibers are shipped in a large roll that is 61 cm wide and 91 m long. The surface of the

fabric that is bonded to the concrete is covered with a very thin veil of multi-directional

glass-fibers. These glass-fibers are held in place by the fabric's cross-stitching. The

purpose of this veil is to keep the fabric intact during installation. These fibers also help

to reinforce the matrix material at the FRP/concrete bond line and aid in shear transfer

from the concrete into the composite. Figure 5-1 shows both the top and bottom surface









of the dry carbon fibers. TYFO SEH-51A glass fiber is a uni-directional, woven glass

fabric (shown in Figure 5-2). The dry fibers are shipped in a large roll that is 137 cm

wide and 46 m long. Important values are summarized in Table 5-2.

Table 5-2. Material properties for fibers, epoxy, and lamina
Dry Fiber Properties Epoxy Properties Lamina Properties
Area Tensile Tensile Tensile Tensile
Fiber density strength Tg strength Thickness modulus strength
type (g/m2) (MPa) (C) (MPa) (mm) (MPa) (MPa)
Carbon 645 3790 82 72.4 1 82.0 834
Glass 915 3240 82 72.4 1.3 26.1 575

TYFO S epoxy was used as the matrix material for all of the FRP composites used

in this study. This epoxy is a two-part system (component A and component B) that is

shipped in two 19 L buckets. These buckets are typically pre-proportioned such that all

of component B can be added directly into the bucket containing component A.

Component B is non-viscous (similar to water) and can be moved from one container to

another very easily while component A is very viscous. For this study, components A

and B were proportioned by weight according to the manufacturer's guidelines (A:B =

100:34.5). Mixing was performed in a 1 L plastic mixing cup using a drill-powered

mixing blade for a minimum of 3 minutes. Care was taken not to mix the epoxy too fast

to prevent the formation of air-bubbles. Typical mixes were approximately 500 mL in

total volume.

Concrete Substrate

The first step in specimen construction involved casting five 61 cm x 61 cm x 5 cm

concrete slabs. Concrete mix proportions were taken directly from PCA's Design and

Control of Concrete Mixtures (1994). This was a non air-entrained mix with a target

slump of 7.6 to 10 cm. Mix proportions are provided in Table 5-3. Steel plates were

used as the bottom surface of the concrete formwork and a thin layer of form-release oil









was applied to the surface of the steel before the concrete was placed. Prior to finishing,

the concrete was consolidated in the formwork using a standard concrete vibrator. This

was probably not the most effective means for consolidation considering the shallow

depth of the slab (5 cm).


1 2 3 4 1 2 3 4
A B
Figure 5-1. TYFO SCH-41 carbon-fibers (scale shown in inches). A) Top surface. B)
Surface bonded to concrete.


Figure 5-2. TYFO SEH-51 glass-fibers (scale shown in inches)

The concrete was allowed to cure in the forms for two days. No additional curing

was provided after the forms were removed. The next step was to cut each of the large

slabs into 30.5 cm x 15 cm x 5 cm concrete blocks. These blocks served as the base

material for all 34 specimens that were constructed. The final thing to note about each









block was that the FRP composite was applied to the surface of the block that had been in

direct contact with the steel formwork

Table 5-3. Concrete mix proportions used for Series A to E specimens
Concrete Mix Proportions
Water/Cement Ratio 0.45
Water (kg/m3) 217
Cement (kg/m3) 481
Fine Aggregate (kg/m3) 700
Coarse Aggregate (kg/m3) 902
Max Aggregate Size (mm) 13

Surface Preparation

Each specimen received a light sandblasting prior to placement of the FRP

composite. Figure 5-3A provides a visual reference for the level of sandblasting that was

achieved. The objective of Series C specimens was to investigate the affect that level of

surface preparation has on IR thermography results. Two samples of this series (C-5 and

C-6) received additional sandblasting up to the level shown in Figure 5-3B. This will be

referred to as "Heavy Blast" in the section describing Series C.












A B
Figure 5-3. Surface preparation before FRP placement. A) Light blast. B) Heavy blast.

Surface Saturation and Tack-Coat

After sandblasting, a 10 cm wide velour paint roller was used to apply a thin layer

of epoxy saturant to the concrete surface. The amount of epoxy that was applied to each

specimen was determined by weighing the paint tray containing the epoxy before and









after the surface was saturated. This epoxy (TYFO S) is the same epoxy that would later

be used to saturate the fibers during composite construction.

The epoxy was allowed to sit on the surface for approximately one hour before a

layer of thickened epoxy tack-coat was applied (TYFO TC). The same procedure that

was used to apply the saturant epoxy was also used for the tack-coat (see Figure 5-4).


















Figure 5-4. Application of epoxy saturant and tack-coat

There are two basic options available for saturating fibers in a wet layup

application: (1) machine saturation and (2) hand saturation. Machine saturation involves

a large piece of equipment that consists of an epoxy bath and two heavy rollers. The dry

fabric is first passed through the epoxy bath and then pressed between the rollers in order

to fully impregnate the fabric. This process also removes any excess epoxy from the

composite. The gap distance between the two rollers can be controlled such that the

resulting composite contains the desired amount of matrix material. The installation

guidelines provided by the FRP system manufacturer (Fyfe Co. LLC 2001) include

specific fiber to matrix proportions if a machine saturator is used: 1.0 lb. of fibers to 1.0

lb. of matrix for carbon-fiber systems and 1.0 lb. of fibers to 0.8 lb of matrix for glass-









fiber systems (allowable tolerance is +/- 10 %). It is common to use a machine saturator

for large jobs in which hand saturation of the fibers using a roller would be impractical.

The primary advantage of machine saturation is that the resulting composite is relatively

uniform and contains the correct proportion of fibers and matrix.

For smallerjobs, it is very common to saturate the fibers using a hand-roller

method. To give the reader some perspective on what might be considered a "large" or

"small" job, it should be noted that the FRP system installed on the Chaffee Rd. bridge in

2003 was saturated by hand. Another project that the author was involved in required the

application of 790 m of 15 cm wide by 4.6 m long carbon fiber strips. Again, all of the

fibers were saturated by hand. Unfortunately, the hand-roller method is a very subjective

procedure that may result in over/under/non-uniform saturation of the composite. The

following steps are provided in the manufacturer's (Fyfe Co. LLC 2001) specification for

hand saturation of fibers:

* Make a saturation bath frame out of a sheet of plywood and two-by-fours (or
similar materials creating the same effect) using the two-by-fours for the sides of
the bath and the plywood for the floor.

* Line the bath with plastic sheeting to create a non-permeable membrane for the
epoxy.

* Pre-cut lengths and widths of fabric necessary for application.

* Place dry fabric sheets in the bath and add epoxy. Work epoxy into the fabrics
using gloved hands, a trowel, paint roller, or similar.

* After the fabric has been completely saturated (both sides), remove excess epoxy
by squeegying it out with the trowel or by blotting the excess resin with the next
dry fabric sheet to be saturated. (NOTE: Properly saturated fabric is completely
saturated with no visible dry fibers and minimal excess epoxy)

The hand-roller method was used to saturate all of the fibers in the current study.

Each pre-cut 15 cm x 30.5 cm piece of dry fibers was laid on a piece of visqueen with the









surface to be bonded to the concrete facing up. TYFO S epoxy was then poured evenly

over the surface. The amount of epoxy that was applied varied depending on the

specimen being constructed (details for each specimen are provided below), but the 1:1

fiber to matrix ratio for carbon and 1.0:0.8 ratio for glass were chosen as the standard for

a properly saturated composite. The same 10 cm velour roller (Figure 5-5B) that was

used to saturate the surface and apply the tack coat was used to distribute the epoxy

evenly throughout the fibers (Figure 5-5A). The total amount of epoxy that was used to

saturate each layer was measured by weighing the dry fibers and then weighing the

saturated composite.














A B
Figure 5-5. Fiber saturation. A) The hand-roller method. B) 10 cm velour roller (scale
shown in inches)

Application of FRP Composite to Concrete

After each layer of FRP composite was saturated on the visqueen, both the

saturated fibers and the visqueen were placed on the surface of the specimen (visqueen

side up). The visqueen was then peeled off leaving the saturated fibers attached to the

specimen. Next, the piece of visqueen was weighed to account for any residual epoxy

that did not become part of the final composite. The 10 cm velour roller was then used to

smooth the composite onto the specimen and remove air bubbles. For multi-layer









systems, the process was repeated for each layer. After all of the layers were applied, the

composite was allowed to cure for 24 hours. A final coat of TYFO S epoxy was then

applied as a top-coat in accordance with the manufacturer's installation guidelines. The

final step in the specimen preparation phase involved trimming the sharp edges where the

composite extended beyond the edge of the concrete substrate. A finished specimen is

provided in Figure 5-6.















Figure 5-6. Completed specimen

Construction Details for Each Series

Series A

The objective of this subset of specimens is to investigate how the following

parameters affect IRT results:

* Composite Thickness
* Defect Size
* Defect Material Composition

A total of four specimens containing fabricated defects were constructed for this

series. Specimens A-i, A-2, A-3, and A-4 were constructed using one, two, three, and

four layers of TYFO SCH-41 carbon fiber composite, respectively. The target fiber to

matrix saturation level for each layer of composite was 1:1. This level was achieved by









carefully adding epoxy and weighing the composite before and after the roller was used

to saturate the fibers. The resulting weight volume fraction for layer of the composite

was 0.50. Appendix C contains specific details regarding all quantities of saturant, tack-

coat, composite matrix, fibers, and top-coat that was used for each specimen.

The fabricated defect configuration was the same for all four specimens. Defects

were created by drilling a series of holes (3 @ 6.4 mm, 3 @ 12.7 mm, and 2 @ 19 mm)

to a depth of 6.4 mm into the concrete substrate on the surface receiving the FRP

composite. Several of the holes were backfilled with epoxy or insulating foam and the

remaining holes were left empty. A detailed layout of the defects is provided in Figure 5-

7.

An interface bubble was also implanted by inserting a small nylon machine screw

(#8) into the surface of the concrete. The machine screw was cut such that it protruded

3 mm above the surface of the concrete before the FRP composite was applied. The

exact size of the interface bubble was difficult to control while the composite was being

rolled onto the surface of the concrete. It was also difficult to apply successive layers of

composite material over the interface bubble and ensure that no air voids developed

between layers. The final dimensions for each interface bubble were obtained by

measuring the size on the surface of the cured composite. Two measurements were

made: one parallel (d ) and one perpendicular (d-) to the principle fiber direction.

Results are summarized in Table 5-4.










Table 5-4. Series A details


Weight
vol. of
fibers
0.50
0.50
0.50
0.50


Interface bubble
d (mm) d- (mm)
51 18
57 29
57 25
51 32


Specimen
ID
A-1
A-2
A-3
A-4







15 cm


(size varies) L\ Drawing Not
#8-Nylon to Scale
Machine Screw

B
Defect configuration for Series A specimens. A) Plan view. B) Profile.


Series B


The objective of this subset of specimens was to investigate how the following

parameters affect IRT results:

* Epoxy saturation levels
* Composite thickness
* Fiber type (carbon vs. glass)
* Inter-lamina defects vs. interface defects


Fiber
type
Carbon
Carbon
Carbon
Carbon


Surface
prep
LB
LB
LB
LB


E Air-Filled
Epoxy-Filled
Foam-Filled

Specimen Layers
A-i 1
A-2 2
A-3 3
A-4 4


Figure 5-7.









A total of 18 specimens containing fabricated defects were constructed for this

series. These 18 specimens are subdivided into three groups based on the amount of

epoxy that was used to saturate the fibers: low saturation (6 specimens designated "L"),

medium saturation (6 specimens designated "M"), and high saturation (6 specimens -

designated "H"). Each of the six-specimen sub-groups contains four carbon-fiber

systems (designated "C") and two glass-fiber systems (designated "G"). The final

distinction to be made between each specimen of a sub-group is the number layers. The

four carbon-fiber specimens each have 1, 2, 3, or 4 layers, and the two glass-fiber

specimens have either 2 or 4 layers. To summarize, the specimen designated B-MC-3 is

a three-layer carbon-fiber composite with medium epoxy saturation. B-LG-4 is a four-

layer glass-fiber composite with low epoxy saturation.

The six specimens constructed using the low saturation level each had a fiber

weight fraction (wf) of approximately 0.67. For a carbon-fiber system, the 0.67 value

represents one-half of the recommended amount of epoxy needed to saturate the fibers.

For the glass-fiber system, one-half of the recommended epoxy would result in a wf of

0.71. Attempts to saturate the glass fabric at this saturation level were unsuccessful, and

it was clear that additional epoxy would be needed in order to create a composite that

appeared close to saturated. The final value ofwf used for the glass-fiber composite was

0.67.

The medium saturation level was achieved using a target wfof 0.5. This represents

a properly saturated composite for the carbon-fiber systems and a slightly oversaturated

composite for the glass-fiber systems.









The high saturation level specimens were constructed with a target wf of 0.4. This

wf is obtained when the amount of epoxy used to saturate the composite exceeds the

recommended level by 50%. Several trial specimens were constructed in which this

excess epoxy was applied directly to the dry fibers during the saturation process. The

resulting composites were too saturated and it became difficult to keep all of the epoxy

on the visqueen as the fibers were being wetted-out. A more effective strategy was to

first saturate the fibers to the medium saturation level and then apply the composite to the

specimen. Next, additional thickened epoxy tack-coat (TYFO-TC) was applied to each

layer using a roller. The amount of tack-coat applied was measured by weighing the pan

and roller before and after each application. This quantity was assumed to be matrix

material that was incorporated into the composite and was used in the wf calculations.

All specimens in Series B contained at least one fabricated defect. This defect

always occurred at the FRP/concrete interface and was created using the same procedure

described for the interface bubble in Series A. After each system had cured, an estimate

of the defect size was made by measuring the dimensions of the bubble with a ruler. For

the carbon-fiber systems, each of the bubbles assumed an elliptical shape. The dimension

of the ellipse in the direction of the fibers, d was typically larger than the dimension

perpendicular to the fibers, di. For the glass-fiber systems, the shape of the interface

bubble tended to be more round than elliptical.

For FRP systems containing more than 1 layer, an additional inter-lamina bubble

was created by placing a #8 nylon nut beneath the top layer of composite. This nut

resulted in a defect similar in size and shape to the interface bubble. Measured










dimensions for all of the fabricated defects are provided in Table 5-5. A schematic

drawing of the defect configuration for this series is provided in Figure 5-8.


Table 5-5. Series B details


Specimen
ID
B-LG-2
B-LG-4
B-LC-1
B-LC-2
B-LC-3
B-LC-4
B-MG-2
B-MG-4
B-MC-1
B-MC-2
B-MC-3
B-MC-4
B-HG-2
B-HG-4
B-HC-1
B-HC-2
B-HC-3
B-HC-4

Series C


Fiber
type
Glass
Glass
Carbon
Carbon
Carbon
Carbon
Glass
Glass
Carbon
Carbon
Carbon
Carbon
Glass
Glass
Carbon
Carbon
Carbon
Carbon


Saturation
level
Low
Low
Low
Low
Low
Low
Med
Med
Med
Med
Med
Med
High
High
High
High
High
High


Inter-lamina
bubble


wf
0.67
0.68
0.67
0.68
0.67
0.64
0.54
0.53
0.52
0.48
0.50
0.50
0.44
0.43
0.43
0.37
0.42
0.41


Interface
bubble


di d- di d-


(mm)



38
51
51
51
51

44
51
44



76
44
38


(mm)



19
29
25
38
38

22
22
25



25
19
19


(mm)
57
57
57
51
57
51
51
51
76
57
57
57
57
57
57
83
64
51


(mm)
51
44
19
25
19
22
44
51
25
38
35
32
57
57
29
38
25
25


Series C contained a total of six specimens. The objective of this series was to

investigate the effects of concrete surface preparation and the use of thickened epoxy

tack-coat. Three different levels of surface preparation were used (two specimens for

each method): none, light sandblasting, and heavy sandblasting. A visual reference for

the light and heavy sandblasting was provided in Figure 5-3. For each of the surface

preparation methods, one of the specimens received a layer of thickened epoxy tack-coat

and the other did not. The specimen matrix for this series is summarized in Table 5-6.

Each of these specimens was constructed using one layer of carbon-fiber

composite. One fabricated defected was implanted in each specimen using the interface










bubble procedure that was described above. In addition to investigating how surface

preparation affects the IRT results for each defect, these specimens were also used to

examine how surface preparation affects IRT results for non-defect areas.



Inter-lamina Bubble Interface Bubble



15 cmA2 Al



Principle Fiber
Direction
30 cm
A

Inter-lamina Bubble


Figure 5-8. Defect configuration for Series B specimens. A) Plan view. B) Profile


Table 5-6. Series C details


Specimen
ID
C-1
C-2
C-3
C-4
C-5
C-6


Surface Tack-


prep
None
None
LB
LB
HB
HB


coat
Yes
No
Yes
No
Yes
No


Wf
0.44
0.42
0.45
0.44
0.46
0.43


Int. Bubble


d (mm)
57
64
57
51
64
57


d- (mm)
38
32
32
25
32
32