Effect of unbonded joints in an aluminum honeycomb-core material for sandwich constructions

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Title:
Effect of unbonded joints in an aluminum honeycomb-core material for sandwich constructions
Physical Description:
Book
Creator:
Norris, C. B ( Charles Brazer ), b. 1894
Forest Products Laboratory (U.S.)
University of Wisconsin
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U.S. Dept. of Agriculture, Forest Service, Forest Products Laboratory ( Madison, Wis )
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aleph - 29522209
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Table of Contents
    Front Cover
        Front Cover 1
        Front Cover 2
    Summary and introduction
        Page 1
        Page 2
    Material and block shear tests of core materials
        Page 3
        Page 4
    Beam tests of sandwich panels
        Page 5
    Fatigue tests of core materials
        Page 6
    Compressive tests of sandwich constructions and of core materials
        Page 7
    Conclusions
        Page 8
    Tables and figures
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
    Back Cover
        Back Cover 1
        Back Cover 2
Full Text
I
/ .. C.. ~, ~ 4**.- F


EIlECT of UNI(NUDEl JOINTS IN AN

ALUMINUM eONLYCOMB-COIe

MATEIPIAL rOe SANIWICE

CONSTIUCTIONS

May 1952

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This Peport is One of a Series
Issued in Cooperation with the
AIl FOPRCE-NAVY-CIVIL SUBCOMMITTEE
on
AIIRCIPAFT DESIGN CIRITERIA
Under the Supervision of the
AIRICRIAIFT COMMITTEE
of the
MUNITIONS BOAIID

No. 1835

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


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EFFECT OF UMBODED JOINTS IN AN ALUMINUM HONEYCOMB-CORE

MATERIAL FOR SANDWICH CONSTRUCTIONS1


By

CHARLES B. NORRIS, Engineer

Forest Products Laboratory,2 Forest Service
U. S. Department of Agriculture





Summary


Static shear, fatigue shear, and compression tests were made on an aluminum
honeycomb-core material. Bending and compression tests were made on
sandwich panels having this core material and aluminum facings. The bonds
between the honeycomb cells of some of the core material were completely
removed. The modulus of rigidity and the shear strength of the unbonded
material were found to be about 70 percent of those of the well-bonded
material. Calculated values, taking into consideration the stress concen-
trations in the neighborhood of the unbonded joints, lead to substantially
the same value. Results of shear fatigue tests of the core material are
substantially consistent with the shear-strength values obtained from the
static shear tests. The compressive strength of the unbonded material is
about 50 percent of that of the well-bonded material, which is consistent
with the assumption that the compressive strength is proportional to the
critical compression stress of the cell walls.


Introduction


The structure of a honeycomb-core material for sandwich construction is
formed of sheets of material corrugated and bonded together as shown in
figure 1. The core material is orientated in the sandwich construction so
that the planes of the facings are perpendicular to the directions of the
cells formed by the corrugated sheets. Thus the thickness of the sandwich
construction is measured in the L direction of the core (the direction

1
-This progress report is one of a series prepared and distributed by the
Forest Products Laboratory under U. S. Navy, Bureau of Aeronautics,
Order No. NAer 01237 and U. S. Air Force No, USAF-(33-058) 51-4062E.
Results here reported are preliminary and may be revised as additional
data become available.
2i ined at adison, Wis., in cooperation with the University of Wisconsin.
-Mabintained at Ma~dison, Vis., in cooperation with the University of Wisconsin.


Agriculture-Madis on


Report No. 1835







-erper.'icular to the paper in figure 1), The other two directions in the
core rnAtc-rial are indicated by the letters R and T as shown in the figure.
7 .e shear stra.inn imposed on the core by the facings (when the sandwicil
ccrn3truction is bent) lie, t. erefore, in the UL and LT planes.

A q'icsticon las arisen reg.rding t.e necessary strength in the bcrnds between
the corrugated sheets of the core material. It is evident tiat if the
sh.car stress in th,.e core is so directed as to produce shcar strains in tl.e
L. plae, t' e cnds could be ca able of resisting s:.ar stress; so t at
the sear strength of the bonds should be at least:

m+ n
n

or about 3s for hexagonal core material: in wiich: s is t:.e c'e--r ztrenrth
of the core material that can be attained if the bonds do not fail, and
m and n are the distances indicated in figure 1. It does not follcv, row-
ever, that if the bonds have zero sh.ear strength, the sandwich construction
will he.ve zero shear streringtli. If tiLe bonds he.ve zero shear stren:ft.., t.e
shear stress will move front the core to the facings in the neigic.cr:Iod of
t e unbonded joints in the core Laterial, as described in Forest Frcd'.scts
Laboratory Report No. 1505-A,-2 and shear stress concentrations will occur
in the core material.

If the shear stress in the core is so directed as to produce shear strains
in the LT plane, the bond strength between the sheets in thle core material
-1ould not be i:uortant unless failure in the core n-ateral involves
buckling of the cell walls. The bond strength need be sufficient oncly to
cause the double walls to act as a unit. Quite weak bonds should le suffi-
cient for this vurpcse and, therefore, the required stren-,th Lrcvicusly
given for the LE plane is probably quite sufficient.

The Glenn L. Martin Complany has made a nut.ber of tests of an aluminum
honeycor.-.o-core material and of a sandirich construction emLodyin,. th.is core
material and aluminum facings, to determine tie effect on ..hear strength
of comri.lete lack of bond in the core material. T.e following tcsts were
mar.de on both well-bonded and completely unbonded core materials:

1. Block shear tests of the core materials to determine
modulus of rigidity and shear strength.

2. Beam tests on tDle sandwich constructions to determine
the shear strength of thie core materials.

3.5, Fatigue tests in sear on the core materials.

4. Cor.-ressive tests of t.he sandwich constructions and
th;e core materials.

-C. .'rris, W. S. .ricksen, and W. J. ior.ners, Flexural iCigidity of a
Bectanj-ular Strip of 5andwicl. Construction. -uj ple-,icntar.,. ha.eratical
analysiss and Comrarison with th.e Results of Tests, Forest Froducts
labIoratory Report i.'o. 15'A-...


FRerort Ito. 1i`5


-2-







It is the purpose of this report to present the results of these tests and
an analysis of them.


Material


The honeycomb material was made of aluminum foil 0.005 inch thick, per-
forated in the usual way. The cells were substantially hexagonal, and
the cell size (C in fig. l) was 3/8 inch. Two different foils were used.
Foil A had a tensile strength of 3355,400 pounds per square inch, and foil B
of 26,000 pounds per square inch. The core materials made from these
foils had slightly different densities. The densities were: Core material A
(made from foil A), 4.48 pounds per cubic foot; and core material B, 4.45
pounds per cubic foot. The bond strength of these two core materials was
roughly determined by applying tensile forces to the core material in the R
direction. The tensile strengths (peel strengths) of the two materials were
found to be 7.8 and 11.0 pounds per square inch, respectively.

Some of the core material of each kind was made with a bonding agent that
could be leached out after the test specimens were made, so that zero bond
strength was obtained; that is, the adjacent corrugated sheets of thie core
material were not bonded to each other at all in the completed sandwich
panels.

Sandwich panels were made from each of these four core materials (one of
each kind of foil, both well-bonded and leached) by bonding aluminum
(75S-T6) facings 0.02 inch thick to each side of a sheet of the core material
1/2 inch thick. The adhesive used for bonding the core material to the
facings or to the test apparatus was not affected by the liquid employed in
the leaching process to remove the bonds in the core material.


Block Shear Tests of Core Materials


Description of Tests and Results

The test method used was similar to that described in Forest Products
Laboratory Report No. 1555,- paragraphs 36-40. The specimens were 5/4 inch
thick, 3 inches wide and 9 inches long. The thicmkness of the shear plates
was 3/8 inch. The loads and displacements during the early part of the
test were obtained, and the test was continued until failure occurred, so
that the modulus of rigidity and shear strength could both be computed.
The results of these tests are given in table 1. The ratios given in
table 1 are the ratios of the average values obtained from the unbonded
cores to those obtained from the well-bonded cores.

Methods of Test for Determining Strength Properties of Core Naterial for
Sandwich Construction at Normal Temperatures. Forest Products Laboratory
Report No. 1555,. (Revised Oct. 1948.)


Report No. 1835


-3-







.'.nelysis of resultss


T.Ic val'ics in table 1 show th.at the modulus of rigidity of the unbonded
core ntj.rial is less than that of the well-bonded core m-aterial. If the
s .e^r strains are in the LT plane, th.e ratio of the two moduli is C',750;
and if t ey are in t:.e LR plane, tne ratio is 0.655. TVe latter reduction
is :.-roblably due, at least in rart, to the reduction of the modulus of
rigidity to zero at the locations of the discontinuities in the core material
f.r.:c1.' by the lack of bond. The former reduction, which is nearly as reat
as the latter, is not explained. An adequate analysis of the action o0 a
honeycoijb core within a sandwich panel has not been made.
Table 1 shows that the average shear strength of the unbonded cores
less than that of the bonded cores. If the shear strains are in the LT
,'l.kne, the ratio of the two strengths is 0.661; and if they are in the L
plane, th.e ratio is 0.752, The latter reduction occurs because of t.he
shear stress concentration in the core due to the discontinuities in the
core forced by the lack of bond, as subsequently set forth. The former
reduction, which is slightly greater, is not explained.

T'he shear stress concentration in the core in the neighborhood of the
unbonded surfaces can be estimated by means of figure 5 of NI.CA Tech.nical
I'.te 2152.5 For use in this figure:

t = 0.375 thickness of shear plate or skin

w = 0.75 thickness of core or cap strip

L = 0.575 distance between discontinuities -- cell
size or width of cap strip

E = 10,000,000 modulus of elasticity of facing or skin

G = 22,250 modulus of rigidity of core or cap strip
(average from table 1 for the well-boneed
cores)

Thus t = 0.5
w

L 9 = 0.0334


By using these values in figure r of NACA Tecimical I-Tote 2152,- the stress
concentrc.tion is found to be 25.7 percent. Ti e ratio of the two strength
values is, t",erefore, 1 or 0.7)9, which, compares favorably within th.e
1.257
experimental value of 0.732.

-1)Sear "tress Distribution Alcnr Glue Line Between Skin and Cap-strip of an
Aircr".f*t .Uin. National Adv'isory Conmsttee for Aeronautics Technical
ILte 2152.


Report -o. 1i ,


-4-







If the modulus of rigidity of the unbonded core (14,500) is used in the
same manner, a ratio of 0.798 is obtained.


Beam Tests of Sandwich Panels


Description of Tests and Results


The test method used was similar to that described in Forest Products
Laboratory Report No. 1556,2 paragraphs 20-24. The specimens were 3 inches
wide and tested over a span of 6 inches. All of them failed due to s".ear
in the core. The maximum loads were read and the shear stress at failure
was computed. The results of these computations are given in table 2. The
ratios given in the table are the ratios of the average values obtained
from the unbonded cores to those obtained from the well-bonded cores.


Analysis of Results

The values in table 2 show that the shear strengths obtained from the
unbonded cores were less than those obtained from the bonded cores. The
values of the ratios are similar to those obtained from the block shear
tests, being 0.694 when the shear strains are in the LT plane and 0.777
when they are in the LR plane.


The shear stress concentration
calculation:

t = 0.020

w = 0.5

L = 0.575


E = 10,000,000

G = 22,250


can be determined as before. For this


thickness of facing or skin

thickness of core or cap strip

distance between discontinuities --
cell size or width of cap strip

modulus of elasticity of facing or skin

modulus of rigidity of core or cap
strip (average from table 1 for well-
bonded cores)


Thus t = 0.04
W


L/i 0.179


6
Methods for Conducting 11echanical Tests of Sandwich Cons-'u.-: -,n '. "Tcrmal
Temperatures. Forest Products Laboratory Report No. 1556o (y2vised
Feb. 1950.)


Report No. 1835


-5-







By using t;.cse values in figure 5 of W.CA Tecmhnical Note 2152,2 the stress
concentration iz fund to be 29.5 percent. The ratio of the two strengths
is, t.ere"orc, 1 or 0.772, which compares favorably ritli the experimental
!. c95
value of 0.777.

If t;.e modulus of rigidity of the unbonded core (14,500) is used in thie same
manner, a ratio of 0.778 is obtained.


Fatigue Tests of Core Materials


rDescrirtion of Tests and Results

The tect specimens were identical to those used in the bloc, shear tests.
They were mounted in a fatigue mrachine similar to that described in -"orest
rrodu.::ts Laboratory Report No. 1559.1 Three groups of tests were i.:a,.e. In
the first rrou) tie shear strain was in the LT plane and the stress level
was 100 :ounds per square inch. In t'.,e second and third Groups the sh'ear
strain Mas in the LB plane. The stress levels in these two groups were 75
and 50 no-nds per square inch, respectively. Each cycle consisted of
raising the stress from 10 percent of the stress level to the stress level
and back to 10 percent of the stress level. The numbers of cycles to
failure for each specimen are given in table 3.

In this table the averages given are the antiloga.-ithms of the averages of
the logarithms of the individual values. Thus these averages agree with the
usual way of plotting stress level vs. cycles to failure cul-ves. The stress
levels given in percent in table 3 were deter ined by the use of figure 5 of
Forest Products Laboratory Report INo. 1559-H.a- This curve was obtained from
tests on an aluminum honeycomb-core material similar to 'that used in the
present tests. This material }has a 5/8-inc. cell size and a wall thickmess
of 0.004 inc'. and was perforated in the usual way. The strengths given in
table 3 were obtained by dividing the stress level in pounds per square inch
by the stress level in percent and multiplying by 100. The ratios given are
the ratios of the strengths, which, of course, are equal to the ratios of
the stress levels in percent.


Anel-,sis of Fesults

The strength ratios obtained from the fatigue tests agree reasonably well
with tiose obtained from the block shear tests and the bending tests, as
can be seen ,y comparing the ratios in table 3 with those in tables 1 and 2.


7
.1. C. Lewis. fatigue of Sandwich Constructions for Aircraft. Forest
Products Lahoratory Report lo. 1559.
0
-frred Werren. Fatigue of Iandwicl. Constructions for Aircraft. Forest
Products Laboratory Recort NIo. 1559-H.


1-
-Q


Report -t io...,.







Better agreement is obtained from the groups of fatigue tests that contain
four tests tian is obtained from the groups having a lesser number.

The fact that these ratios do agree indicates that the fatigue strength is
not affected by the lack of bonding in the core material other than the
effect that might be expected due to the reduction of the shear strength.
Fatigue curves in which the stress level is plotted as a ratio of the shear
stress to the shear strength may be used for unbonded as well as for well-
bonded honeycomb core materials.

The strengths obtained from the fatigue tests are, in general, less than those
obtained from the block shear and the bending tests. This may be seen by
comparing the strength values in table 5 with those in tables 1 and 2. The
difference may be due to the use, in computing the strengths of the fatigue
curve given in Forest Products Laboratory Report No. 1559-H, which was
obtained from a slightly different core material.


Compressive Tests of Sandwich Constructions
and of Core Materials


Descriptions of Tests and Results

The sandwich constructions were tested by a method similar to that described
in Forest Products Laboratory Report No. 1556,' paragraphs 12-14. The speci-
mens were 3 inches square and the load was applied over their facings. The
core materials were similarly tested. The specimens were 3 inches square and
1/2-inch thick, with the lengths of the cells being orientated in the direction
of the 1/2-inch dimension. The strengths computed from the values of the
maximum loads are given in table 4.


Analysis of Results

When a honeycomb-core material is compressed in the direction of the length
of the cells, the compression is resisted by the walls of the cells acting as
plates subjected to edgewise compression. If the cell walls are thin with
respect to their other dimension, their edgewise compressive strengths will be
only slightly greater than their critical stresses. If the double walls are not
bonded together, the critical load for a single unit of the structure (shown in
figure l) is given by 4P where P is the critical load of a single wall. The
critical load of a single wall is proportional to the cube of its thickness;
thus if two walls are bonded together, the critical load of the double wall is
8P. If the double walls in a single unit of the structure (shown in figure 1)
are all bonded together, each unit consists of two single walls and one double
wall. Thus the critical load for a single unit of the structure is 2P plus
8P or lOP. The critical load of an unbonded cell is, therefore, roughly
0.4 of the critical load of a well-bonded cell. Each cell wall is subjected
to the same compressive deformation, so that, if it is assumed that each
does not greatly exceed its critical load as the deformation is increased,


Report No, 1835


-7-







the unbonded cells will have about 0.4 the compressive strengths of the
well-bonded cells.

This conclusion agrees approximately with the results of tests given in
table 4 for the core materials. T7,e cores of the sandwich panels were
probably strengthened by the bonds between the core and facings.


Conclus ions


1. The tests indicate that the modulus of rigidity of the unbonded
core material is approximately 75 percent of that of the well-bonded core
material when the shear strain is in the LT plane, and about 65 percent
when tlhe shlear strain is in the LR plane. This is due to the redistribution
of shear strains in the neighborhood of the unbonded joints. The experi-
mentally determined reduction of the modulus of rigidity in the LT plane
due to the lack of bonding in the core material cannot readily be explained.
It may be due to deflections of the unbonded cell walls.

2. The tests indicate that the shear strength of the unbonded core
material is about 68 percent of that of the well-bonded core material when
the shear strain is in the LT plane, and about 75 percent when the shear
strain is in the LR plane. The latter reduction can be quantitatively
accounted for by consideration of the stress concentrations due to the dis-
continuities in the core material created by the unbonded joints. The
former reduction is not understood.

3. The results of the shear fatigue tests of both the unbonded and
the rell-bonded core materials are substantially consistent with the
results of the static shear-strength tests.

4. The tests indicate that the compressive strength of the unbonded
core material is about 53 percent of that of the well-bonded core material.
This is substantially consistent with tl-e assumption that the comrpressive
strengths of the cell walls are proportional to their critical stresses.


Report 'o. 11.35


-6-







Table l.--Moduli of rigidity and shear strengths obtained from block
shear tests

Core material A Core material B

Well bonded Unbonded : Well.1 bonded : Unbonded

Modulus : Shear :Modulus : Shear :Modulus : Shear :Modulus : Shear
of :strength: of :strength: of :strength: of :strength
rigidity : :rigidity: :rigidity: :rigidity:
......C fin c... : ne .. .. :- c -nc c e- .. .:. . en ...ce cn. -: ...ee : ... ... : ... ....n


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


Shear strain in LT plane


68,700 :

69,4o00 :


258

2735 :


70,000 : 269

Av...69,400 : 267
Ratios........


46,400 : 174 : 61,400 : 200

48,700 : 172 : 68,400 : 221

58,600 : 165 : 63,100 : 210 :

51,200 : 170 : 64,300 : 210 :

0.738 : 0.636 ........ ........


Shear strain in LR plane


25,000

20,600


134 :
135
155 :


19,200 : 133 :

Av...21,600 : 134 :
R
Ratios......: ........:


14,600 : 99 : 22,200 : 103

15,150 : 96 : 21,800 : 104 :

15,250 : 101 : 24,800 : 110 :

15,000 99 : 22,900 : 106 :

0.694 0.739 ........ ........


Report No. 1835


47,700

46,400

52,600

48,900

0.761


144

148

139

144

0.686


14,800

15,200

13,900 :

14,000

0.612


75

82

76

77

0.726


P.s.i. : P.s.i. : P-sBi. : P.s.i. : Pcs.i. : P.s.i. :







Table 2.--Shear strength values of core material
obtained from beam tests of sandwich
cons truck t] on


Core material A : Core imterial B

Well : Unbonded : Well : Unbonded
bonded : bonded :
....... ...... -------...... : ... .. ...--.. ...--..


P.s.i.


: P.s.i.

h-.ear strain


289


205


500 221

295 195

Av.......,294 : 207

Patios.........: 0.704


: P.s.i.

in LT plane


254

251.

257

247


Shear strain in L. plane


152 124

154 116

147 118

Av........ 151 119

Ratios......... 0.736


i report No. 1335


P.s.i.


162

167

177

169

O.664,


120

125

117

120


0.766







Table 3.--Cycles to failure obtained from shear fatigue tests of
core material


Core material A


: Core material B


--- **--- ----W M W WW M W W WM1 WW----- ^---------*---M --------W-- - --- ---- -- --
Well :Unbonded : Well : Unbonded
bonded : : bonded :


Shear strain in LT pla.ne
Stress level 100 pounds per square inch


1,311,000 : 85,o000oo : 54,000 :
789,000 : 70,000 : 554,oo000o :
968,000 : 328,000 : 219,000 :
2,107,000 : 96,000 366,000 :

Av...................... l,205,000 116,500: 197,850

Stress level (percent).........35 51 : 49

Strength (p.s.i.).............286 : 196 204

atio............................. 0.686 0...........


116,000
3,000



18,660

69

145

0.711


Shear strain in LP plane
Stress level 75 pounds per square inch


250,000 :
437,000 :
81, C00 :
174,000 :
Av........................198,100:

Stress level (percent).........66 :

Strength (p.s.i. )............. 1135

Ratio...............0....... ...... :


24,000 : 30,000
12,000 : 27,000
23,000 : 31,000
16,000 : 12,000

18,040 : 23,430

82 : 80.5

91.5 : 95.1

0.810 ...........


2,000
Ce"..l....

0 0 @0 a,

2,000

90

83.4

0.856


Shear strain in
Stress level 50 pounds


LR plane
per square inch


10,000,000 : 555,000 : 1, 55333,000 :


Stress level (percent).........58 :
Strength (p.s.i. ) .......... .132
Ratio.


Report Uo. 1855


57 :
87.7 :


47:
106 :


0.664 :


87,000
72
69.5
0.656






Table 4.--Compressive strengths obtained from tests
made in tihe L direction

Core material .' Core material B

Vel : Un'conded : Well : Unbonded
bonded : bonded :
---------------------- -----
Ps.i. : P.s.i. P.s.!. : P.s.i.

Sandwich panels

582 530 : 441 318

540 323 440 o

534 358 : 447 269

v33.......552 57 445 295

Ratios. ........ 0.611 ........ 0.666

Core materials

418 : 137 357

395 : 205 355 144

452 199 361 153

Av.......422 197 3538 141

Ratios.*..* .* 0.467 0.......


Pe-nort ilo. 1835




















































7 M 89875 F


Figure l.--Sketch of cross section of honeycomb material.




























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