Preliminary evaluation of a vacuum-induced concentrated-load sandwich tester

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Title:
Preliminary evaluation of a vacuum-induced concentrated-load sandwich tester
Physical Description:
Book
Creator:
Ericksen, W. S
Kuenzi, Edward W
Heebink, Bruce G
Forest Products Laboratory (U.S.)
University of Wisconsin
Publisher:
U.S. Dept. of Agriculture, Forest Service, Forest Products Laboratory ( Madison, Wis )
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Table of Contents
    Front Cover
        Front Cover 1
        Front Cover 2
    Summary and introduction
        Page 1
    Description and operation of the sandwich tester
        Page 2
        Page 3
        Page 4
    General observations and conclusions
        Page 5
    Appendix
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
    Tables and figures
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
        Page 21
        Page 22
    Back Cover
        Back Cover 1
        Back Cover 2
Full Text

5 : e--


PIELIAINAIRY EVALUATION OF A VACUUAtM-

INDUCED CONC[NTIRATID-LOAI)

SANDWICH TEST[EI
June 1952


- ,- ,-



~ ,


2~Y
'-


I I .


SER'
ENT


This Ieport is One of a Series
Issued in Cooperation with the
AII FOIPCCE-NAVY-CIVIL SUBCOMMITTEE
on
AIPCg AFT DESIGN CRITERPIA
Under the Supervision of the
AIICIRAFT COMMITTEE
of the
MUNITIONS BOARID


No. 1832-A


UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST SERVICE
FOREST PRODUCTS LABORATORY
Madison 5, Wisconsin
In Cooperation with the University of Wisconsin










PRELIMINARY EVLUATION OF A VACUU4-I11IDUCED

CONCENTRATVD-LOAD SA11DWIC H TESTER!


By

U. S. ERICIKSEN, Mathematician
E. W. iLTIIZI, Engineer
and
B. G. =BEINK, Engineer
Forest Products Laboratory,2 Forest Service
U. S. Department of Agriculture





Cuprnary


This progress report presents a description of a testing device designed
to aid in the inspection of aircraft sandwich constructions. Included is
a discussion of thle performance of the tester on a limited number of sand-
wich constructions. Results of a theoretical analysis for determining
deflections and maximum stresses are presented. Suggestions are given for
improving the performance of the device.


Introduction


Increasing use of sandwich construction, particularly for primary structural
elements, has created a greater demand for inspection devices to determine
the quality of sandwich constructions. Various methods have been suggested
and some successfully demonstrated to locate and explore the extent of
unbonded areas. Obviously, a testing device would be most useful if a
nondestructive test could be made that would not only locate but would also




1
-This progress report is one of a series prepared and distributed by the
Forest Products Laboratory under U. S. lTavy, Bureau of Aeronautics Order
No. NAer 01519 and U. S. Air Force No. USAF-l8(500)-70. Results here
reported are preliminary and my be revised as additional data become
available.
2
Maintained at Madison, Wis., in cooperation with the University of
Wisconsin.


Agriculture-Madison


Report No. 1852-A







evaluate the strength of poorly bonded areas. A vacuum tester was devised
and patented by an aircraft company.2 This vacuum tester was submitted to
the forest Products Laboratory for test at the request of the Air-Force-
Navy-Civil Panel 23.


Description and Operation of the Sandwich Tester


The tester consists of a dish-shaped casting, approximately 10 inches in
diameter, with a rubber gasket or washer around the outside rim. Figure 1
shows an external view of the device. The gasket is used to form a
pressure seal between the tester and a sandwich panel. An internal view
of the tester (fig. 2) shows a central rubber-covered steel foot that is
pressed against the panel as the dish-shaped cavity is evacuated. Foot
sizes of different diameters from 1 to 2 inches in steps of 1/4 inch are
supplied so that various ratios of compression and shear can be applied,
The foot is fastened by means of a convenient snap-on fitting to a threaded
bolt that extends through the casting. A vacuum gage is attached to the
casting to measure the applied load.

In use, the tester is operated by placing it on a sandwich panel, adjusting
the position of the foot by turning the threaded bolt until the panel
contacts both the foot and the rubber gasket, and drawing a vacuum on the
casting until failure occurs or until some desired proof load, determined
by the setting on the poppet valve, is reached.

Exploratory trials of ti.e tester showed that the rubber gasket may be
deformed so much, by the deflection of the panel, that the casting makes
contact with the sandwich. If this occurs, continued evacuation merely
places small additional uniform load on the sandwich with little further
deflection. This can be easily demonstrated by applying the tester to a
thin sheet of aluminum; the casting makes contact with the sheet at low
vacuum and then the sheet is slightly concave around the foot until maximum
vacuum is attained.

In order to indicate rim contact, a buzzer was introduced in a circuit
between the casting and sandwich facing. If a nonconductive facing material
is on the sandwich being tested, thin strips of metal foil can be placed on
the surface and used in the circuit.

If rim contact occurs at low loads, the vacuum can be partially released
and the foot extended toward the sandwich by turning the foot adjusting
screw.

It was conceivable that some sandwich constructions would deform consider-
ably under test with no visible or audible indication of failure.

Devised by R.M. I.tlock and patented by Lockheed Aircraft Corporation.
Licenses for manufacture and use granted to Aircraft Die Cutters,
Los Angeles, Calif., and Zenith Plastics, Gardena, Calif.


Report No. 1852-A


-2-







Indications of failure might appear on a load-deflection curve.
Accordingly, a deflection dial was arranged to read the central deflection
of the sandwich with reference to three points located opposite the rim of
the tester. The arrangement of this apparatus is shown in figure 5. It
was subsequently demonstrated that failure of a balsa core was apparent on
load-deflection curves (fig. 4), but after failure the load increased until
rim contact occurred and eventually increased to maximum vacuum with no
audible signs of failure.


Stresses Induced by the Tester

Of primary interest are compression and shear stresses induced in the core
of the sandwich construction by the concentrated load at the foot of the
tester. Failures that may occur in the bond between facings and core can
be interpreted in terms of the shearing stress developed in the core.

The area covered by the casting and gasket of the tester decreases
slightly as the vacuum is increased as a result of deformation of the
gasket. The area of the tester used at the Forest Products Laboratory was
computed to be 90.76 square inches. From this area the compressive and
shear stresses in the core were calculated according to the values in
table 1.

The expressions given in table 1 were obtained from approximations of the
formulas given in the appendix of this report. For unusual constructions
having either thick facings or extremely soft cores the more exact
expressions in the appendix should be used to compute the stresses.
Expressions for deflections and facing stresses are also included in the
appendix.


Experimental Work

In order to determine whether the tester would perform satisfactorily
in measuring the quality of sandwich constructions, a few panels were
tested to determine either core shear strengths, the location of unbonded
areas, or the strength of poorly bonded areas.

For more accurate load readings the dial vacuum gage was replaced with a
mercury column, shown in figure . A needle valve was placed in the
vacuum line to permit sensitive adjustment of load application.

A preliminary test on a sandwich having 0.012-inch 24ST clad aluminum
facings on a core of end-grain balsa wood 1/4 inch thick developed core
failures in shear at approximately the shear strength of the balsa as
evaluated in shear tests. Load-deflection curves for this construction
are shown in figure 4.

Preliminary testing on a sandwich panel mnowrn to have unbonded areas
demonstrated that the tester was capable of detecting the unbonded areas


Report No. 1832-A


-3-






regardless as to whether the unbonded facing was the one on which the
tester was applied or whether it was the opposite one,

As a further check on the performance of the tester, two flat panels were
tested of each of two sandwich constructions. One panel of each construction
was well bonded and one panel was poorly bonded on one side. Poor bonding
was obtained by using less adhesive than is required for good bonding. The
constructions were (1) 0.020-inch 24ST clad aluminum facings bonded with
adhesive 55 to a 1/2-inch-thick core of aluminum honeycomb of 0.005-inch
foil formed to 3/8-inch cell size (core 52), and (2) facings of 8 plies of
glass cloth 112-114 impregnated with resin 2, wet-laminated to a 1/2-inch-
thick core of glass-cloth honeycomb of 112-114 cloth formed to 1/4-inch
cell size (core 56).

Each panel was large enough to permit four tests with the tester (two from
each side) without overlapping the test areas. Load-deflection curves for
the aluminum panel are given for each test in figure 5 and for the glass-
cloth panel in figure 6. Values of the shear stresses developed by the
tester at failure are given in table 2. The average strength values as
measured by the tester show that the poorly bonded aluminum panel had
approximately 80 percent of the strength of the well-bonded panel and that
the poorly bonded glass-cloth panel bad approximately 55 percent of the
strength of the well-bonded panel. Figure 7 shows a cross section through
an aluminum control panel (the two halves laid face to face) that illustrates
a typical failure under the foot of the tester, and figure 8 shows a typical
bond failure in a control glass-cloth panel. Poorly bonded panels of both
types failed in a similar manner, respectively, except that the aluminum
panels failed in the poor bond as well as in shear in the core, and the bond
failures in the poorly bonded glass-cloth panels were more extensive.

After the tests had been made with the tester, the panels were cut into
minor coupons to be tested in bending to determine shear strength, and in
tension normal to the facings to determine bond strength. The bending speci-
mens were 1 inch wide and were tested under loads applied at two-third points
on a total span of 6 inches for the aluminum sandwich and 4-1/2 inches for
the glass-cloth sandwich. The strong direction of the core was placed
parallel to the span length. Tensile specimens were 1 by 1 inch in cross
section. The results of these tests are also given in table 2.

The shear strengths as determined for the bending-test coupons were from
30 percent to 70 percent of the shear strengths as determined by the tester.
The bending-test coupons were cut from portions of the panel adjacent to the
area tested by the tester, and, therefore, this reduction in shear strength
may have been due to damage caused by the tester load or may also have been
due to stresses caused by the saw when the minor coupons were cut.
Shear strengths as determined from bending tests showed the poorly bonded
aluminum panel to have 88 percent of the strength of the well-bonded panel,
and the poorly bonded glass-cloth panel to have 19 percent of the strength
of the well-bonded panel.
Tensile strengths of the poorly bonded aluminum panel were 27 percent of the
strengths of the well-bonded panel, and strengths of the poorly bonded glass-
cloth panel were 11 percent of the strengths of the well-bonded panel.


Report No. 1332-A


-4-







General Observations


In the evaluation work on the tester to date, it appears that the device
has considerable promise of providing a practical means of proof-loading
flat, and possibly curved, sandwich panels to a precalculated stress.
The addition of the spring loaded relief valve provides a means for
controlling the load to any desired level within the range of 1 to 24
inches of mercury. The accuracy of the load application appears to be
about 1/2 inch of mercury. The trials showed that tests can be made at
the rate of 12 to 15 per minute if load-deflection data are not obtained.


Conclusions


The tester can be used to determine the location of unbonded or poorly
bonded areas in panels of sandwich construction.

Although the data of this report do not positively show direct correlation,
the tester may be used to determine shear strengths of sandwich constructions.

In the realm of nondestructive testing the tester should find use in careful
application of certain proof loads as an aid in inspection of the quality of
sandwich panels.


Report No. 1852-A






APPEITDI:.L


The deflection and the stresses induced in the facings and in the core by
the tester have been analyzed by the use of. formulas derived in U. S.
Forest Products Laboratory Report No. 1828., In that report the core
and facing materials are assumed to be isotropic and, consequently, the
present results are limited to this special case.

In order to carry out the analysis, the distribution of load over the foot
of the tester must be specified. An attempt has been made to keep the
distribution uniform by covering the foot with a rubber gasket, and it is
possible that at small vacuum loads it actually is quite uniform. At
large vacuum loads, however, the tester forces the panel to bend, and the
load is possibly concentrated more heavily at the rim of the foot than at
the center. On the basis of these considerations, the analysis has been
carried out for two assumed distributions, namely, (1) a load uniformly
distributed over the foot, and (2) a load concentrated at the rim of the
foot. For a given vacuum a uniform distribution yields higher predicted
shear stress in the core and lower bending stresses in the facings than a
load concentrated at the rim of the foot. It is expected that if the true
distribution of load over the foot of the tester could be determined, the
results would be intermediate between those of the two extreme cases
considered.

The formulas given below are derived on the assumptions that the test panel
is of infinite extent and that the facings are of equal thickness. More-
over, the formulas are given in forms applicable to a panel with thin
facings and with a core that, like end-grain balsa, has a relatively high
shear modulus. Specifically, it is assumed that aa and ab, defined below,
are so large that the Bessel functions that appear in formulas taken from
Report No. 1828R can be expressed in the forms (64) and (65) of that

report, and that the quantity e' a'b) can be neglected in comparison

with unity. It is believed that these conditions will be fulfilled for
most constructions that are likely to be of practical interest.

The deflection and the components of stress in the facings and in the core
are given in terms of the following symbols:

a: radius of the tester gasket

b: radius of the foot

c: thickness of the core


S rE f


-W. S. Ericksen. The Bending of a Circular Sandwfich Panel under Normal Load.


Report No. 1852-A


-6-





Ef: Young's modulus of the facing material

f: thiclmess of the facings

G: shear modulus of the core material

I =M + If

S= f(c + f)2
_m 2
f5
_if Z -

q: applied vacuum (p.s.i.)

_2 2GI
SEcflf

7: Poissons ratio of facing material

For a load distributed uniformly over the foot of the tester, the deflec-
tion of the sandwich, wrU, at the center of the tester relative to the
outer rim of the tester"is given by the formula


wU = WbU + "sU


(1)


where,


WbU = a6.


2fqa
2ETfa2


5(a b2) 4b2 log I

loga 2
l b 2b2


For a load concentrated at the rim of the foot, the central deflection is
given by


Wc = -bc + sc


(4)


where,


-b q~a2

qa2
sc =2EI 2


5a2 8b2 (1 + log a)
b

I a l (1+ L)
10 :o U a


The shear stress in the core evaluated at the rim of the foot is given by
the formula


Report No. 1852-A


(2)


(5)


(5)

(6)


-7-






U 2(c q+ f)
ub = 2(c T


a2 b2 a2
b "-ab2


(7)


for a load distributed uniformly over the foot, and by the formula


r =
c 2c+f


!-I
a2
a -
'2b
L.-, "


(8)


for a load concentrated at the rim of the foot.

The stress at the outer surface of the facing upon which the tester is
placed is evaluated at the rim of the foot by the formula


= q(c + 2f)
321


16(c + f) (3c + 2f)
tbUc + f(c + 2f) sU, c


where the subscripts U and c again designate quantities associated with a
uniform foot load anda load concentrated at the rim of the foot,
respectively.

For a load distributed uniformly over the foot,


7 2 log a (3 +7y ) (a2 b2)
mbU = 4(1 + y) a2 log -


(1 + 7) (a2- b2)
msU= 22
2i b


(10)


(11)


a2
22 I
2 b t


and for a load concentrated at the rim of the foot,

mbc = 4(1 + ) a2 log a 2(1 +Y) a2 + (3 +7) b2


a2
Dsc = 4-b


11 ....J


1 +7
" -S2


At the present time a reliable means of estimating the transverse pressure
on the core is not available. However, if the load can be taken to be
uniformly distributed over the foot, it is 6stinm->ed that the pressure
exerted by the loaded facing upon the core is about one-half the pressure
on the foot.


Application of formulas

Figure 4 sho,.s the load-central deflection curve for a test panel to which
the preceding formLulas are applicable. The results given below were


Deport No. 1852-A


0*
15c


(9)


(12)


(13)






obtained from these formulas for a foot diameter of 1-3/4 inches. In the
computations, the shear modulus of the core material, which was not
measured, was taken as 20,000 pounds per square inch. The following is a
complete list of the dimensions and elastic properties used.

a = 5.38 inches

b = 0.875 inch

c = 0.25 inch

Ef = 107 pounds per square inch

f = 0.012 inch
4
G = 2 x 104 pounds per square inch

y = 0.5

These yield

E = 1.1 x 107 pounds per square inch

Im = 4.12 x 10"4 inches3

if = 2.88 x l0"7 inches3

I = 4.12 x 10"4 inches3

a = 41.6

From (2) and (5) it is found that the deflections are given by

WbU = 0.0135 q

and (14)
Wsu = 0.0049 q

Therefore, if the load is uniformly distributed over the foot, the central
deflection obtained from (1) is

wu = 0.018 (p.s.i.) = 0.0090 q(in. G) (15)

Similarly, from (5) and (6)

wbc = 0.0127 q

and (16)
Wsc = 0.0034 q


Report No. 1832-A


-9-






so that for a load concentrated at the rim of the foot, (4) yields

wc = o.o0161 q(p.s.i.) = 0.0079 q(in. HG) (17)

Formulas (15) and (17) yield, respectively,

q(in. HG) = 111 wU and q(in. HG) = 127 (18)w

The slopes of the lines represented by these equations would change
slightly if the shear modulus of the core was changed and if the radius
of the tester, which presumably decreases with increasing load, was
varied. They are, however, of the sar.e order of magnitude as the slopes
of the linear portions of the two curves in figure 4 for the foot
diameter of 1-5/4 inches, which are 125 and 152. It is of interest to
observe that the deflections due to shear represented by WsU and Ws.
are about one-third of the deflections due to bending represented by
wbU and wbc. These relatively large shear deformations for an aluminum-
balsa panel are attributed to the small dimensions of the area of the panel
over which deflection takes place.

The shear stress in the core obtained from (7) and (8) are, respectively,
U = 60 q(p.s.i.) = 29 q(in. iG) (19)

and
c& = 0 15 n(0
S= 3 q(p.s.i.) = 1(in. ::G) (20)

The stress predicted on the basis of a uniform load on the foot is thus
about twice as great as that obtained by taking the load to be concentrated
at the rim of the foot.

For the present core and facing thicknesses, formula (9) for determining
facing stresses takes the form

OUc = -20.8 qbU c 20,500 qmsUJc (21)
From formulas (10) to (13)

20.8 rbUq = 5760 q (22)

20,300 nmsUyq = 267 q (23)

20.8 mbcq = 4170 q (24)

20,500 mscq = 5950 q (25)


Report No. 1832-A


"-l-






Therefore, for the load distributed uniformly over the foot, the stress
at the surface of the facing at the rim of the foot is

U= -3927 q(p.s.i.) = -1930 q(in. HG) (26)

and for the load concentrated at the rim of the foot the corresponding
stress is

c = -8120 q = -5990 q(in. HG) (27)

The minus sign in these equations indicates a compressive stress.
Formula (27) indicates that if the load on the foot was concentrated at
the rim, the proportional-limit stress of 27,000 pounds per square inch
would be reached at 6.6 inches of mercury.


Report Ho. 1832-A


-11-






Table 1.--Stresses induced by thile tester

Diameter Core stresses
of foot ---------------------. -. ------
.1 2
Compression= Shearl

n--------- m--------.------..-- .--- --- ---
In. P.s.i. P.s.i.

1.00 57.2q : 57.2 --
h+c
.h + c

1.25 356.5q 45.6 --
h + c
h+c

1.50 25.2 : 57.8 -1
h + c
h+c
1.75 18.4(q 52.2 -q
h + c

2.00 13.9q 27.8 q

1
S= applied vacuum (p.s.i.), h = total sandwich
thiclmess (in.), c = core thickness (in.)

2
See Appendix, page 12.


Report ITo. 1852-A








Table 2.--Strengths of panels as determined by the tester


Shear strengths : Tensile strength
em -- ------------ -- --- --- -- -------e- - -- -- - -
Well-bonded : Poorly bonded : Well- : Poorly
panel panel : bonded : bonded
.-.....--.------- --- . ---- : panel : panel
Tester : Bending : Tester : Bending
: tt :: test

(1) : (2) : (3) : (4) : (5) : (6)
.. .. .. : . ..:. .....- .......... inn .... .... m -...--......
P.Sqi. P.s.i. P.s.i.: P.s.i. P.S.i. P.s.i.

Sandwich Construction: Facings -- 0.020-inch 24ST Clad
Alumi-num, Core m- 1/2-inch-thick aluminum Ioneycorib
of 0.003-inch Foil Formed to 3/8-inch Cell Size-L


191 : 78 : 2165 : 76 :
242 : 95 : 2170 : 156 :
211 : 179 : 163 : 142 :
204 : 196 : 170 : 56 :
......: 86 : .........: 1653
....@ e175 .. ..o.....

A. 212135e6611
*.o. eee e see... 555o* B oo5 oo5
see...... sos*o..os. oeo.o.o.. .s.,s....eo
eO e 5@* *o 50ooo0oo 5o 5oo e oooo *oeo o e.
*oooeoe.o ..e...ooe*eese.o. 0 -ooeeseee.
* sssesoe .eO ee 50* *55eO Oe s*es.osoo .
* e e o ee so 5500o.o 5 55500005*.

B S .. *. O~* * 6 0 0*. 65 0 0*
S.. 0CCee 0 55* Os Co *oso
*es*Bs..s. se e eees ees...@ e *eeB..e.


O******B** *@**@ 550e. *OO*O*O. 505OOOO *O

9 5
Av. 212 : 155 : 166 119 :


570
330
550
520
510
250
230
250
270
280
280
270
500
270
250
540
340
500
310
510

296


110
110
100
70
70
60
70
60
50
6o
50
80
110
100
100
O0OOO ss es
ssseO@@O@
cee see. se
Oee..esesO
seeOee..e s


(Sheet 1 of 2)


Report No. 1832-A







Table 2.--Strengtlhs of panels as determined by the tester (Cont.)



Shear strengths : Tensile strength

Well-bonded : Poorly bonded : Well- : Poorly
panel panel : bonded : bonded
-.---..-.-.... --- -. ..--....- .. : panel : panel
Tester : Bending : Tester : Bending :
: test: : test :
... .. .. e.... .. : .. .-. ..i.n..in: ..........----------- ....... -
n - - -- ---- - --- - - - -5 5 5 5
(1) : (2) (3) : (4) : (5) : (6)


P.s.i. : P.s.i. : P.s.i, : P.s.i. : P.s.i. : P.s.i.

Sandwich Construction: Facings -- 8 plies of Glass
Cloth 112, Core -- 172-inch-thick Glass-cloth Honey.-
comb of 112 Cloth Formed to 14-inch Cell Size.


451
544
521
515
ao0 e00005.



..o....*0*
0000OOO.0:
**699640*
0 0 00 6 09 0 *


180
252
34o
354
226
282

.SO0..0S.e.

.0005000 6.-


2146 :
E213 :
151 :
185 :
00..09..0*




.t0.000000


Soo0 00 0 e, *. &ooo a o0#00ooam


38 :
66 :
50
52 :



S.@.O.S.....

@050000050.

00.00..000.
..@.. .@@ *


* 0oo* *0e oooaoo# 0* OoooeoSo0*O
** 0 5 S O aSe## ,00 a0 0 0 e0oo e 0000a0 000#
0oe05eee550..0eo0o0.5000e5eoee eeeeeeeeeeS
0 505000 *.sa ese eo a 0. 0e0 0e o 0 0e00
a 0 & 0 0 5 0 0 0 0 05 5000000 0 0
0 a 0 00 000a00o0 a S00#00# 0 0
*v.56 2 a 1 e 52

Av. 506 : 272 : 174 : 52 :


510
270
250
240
420
220
340
290
300
230
290
240
340
280
290
300
280
320
290
300

290


S 0
40
60
20
350
S 0
20
50
50
50













52
* SS.eoo@.....@

*O.0..00000500.

* .O......SOo.O

S0@06 00.0 0000@0
* 000.06 *0.0 00

3 2


1
Tested with a 2-inch-diameter foot.
2
-Tester on poor-bond side of sandwich.


ATested with a 1-1/2-inch-diameter foot.


(Sheet 2 of 2)


Report No. 1832-A




































Figure 1.--External view of the sandwich tester, showing
rubber gasket, attached vacuum line, and vacuum gage.
The center bolt extends to the interior foot. An auxiliary
poppet valve has been installed (immediately below the gage on
the photograph) for control of vacuum during use as a tester.


z M 88o41 F


f/~


7)


00'W."_


































/


E9


Figure 2.--Internal view of the tester, showing the central foot
and alternate foot sizes available for use. The cantilever spring
actuating the poppet valve for control of vacuum is also shown.
z M 88040 F





















































Figure 3.--The tester in use during evaluation tests, showing the addition of
a deflection-measuring device on the opposite facing.

z M 88058 F










/6



/4
RIM C ON TAC 7\
R/M CONTAC 1 \ f


ko 1? I^ ,CORE FAIL URE
-INCHES- / SHEAR STRESS
DIAMETER FOOT F 362 P S. I
LIO





d^ ^^ P RI CONTACT-



-J ^ ---- ^ --f^ ---- SHEAR STRESS =388 P.S.I. -
41






















0 0.02 0.04 0.06 0.08 0.10 0.12
FOOT DEFL ECTION (INCH)ER FOT
8 0.2 0C006 008 0.I 01
FOOTek1 DCFL E7TO (N










7. M 90070 V

Figure 4.--Load-deflection curves produced by applying tester
to a sandwich having 0.012-inch 24ST clad aluminum facings on
a 1/4-inch-thick core of end-grain balsa.











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ao TEST 2
Q6 v TEST 3
/ A TEST 4
^ -CONTROL PANEL
POORLY BONDED PANEL
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0 0.0/ 0.02 0.03 0.04 0.05 0.06
FOOT DEFLECTION (INCH)
Z M 90071 F
Figure 5.--Load-deflection curves produced by applying tester
with a 2-inch-diameter foot to a sandwich having 0.020-inch
24ST clad aluminum facings on a 1/2-inch-thick core of 0.003-
inch aluminum foil formed to 5/8-inch hexagonal cells.










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^ CT0 TEST!
o TES T 2
41 _________ V TEST 3
A TEST 4
r CONTROL PANEL
--- POORLY BONDED
PA NEL

0 0.02 0.04 0.06 0.08 0.1/0 0.1/
FOOT DEFL EC TION (INCH)
Z M 90072 F

Figure 6.--Load-deflection curves produced by applying tester with
a 1-1/2-inch-diameter foot to a sandwich having facings of eight
plids of glass cloth 112-114 on a 1/2-inch-thick core of glass
cloth 112-114 formed to a honeycomb of 1/4-inch cell size.














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Figure 7.--Cross section through failed portion of an aluminum honeycomb control
panel (the two halves laid face to face), showing distortion and shear failures
in core. There was no evidence of bond failure in any of the aluminum
control panels.
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Figure 8.--Typical failure in a control glass-cloth panel. Failure is confined to
the bond between the facing and the core on the side opposite the tester.
z M 88863 F


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