FATIGUE CF SANDWICF1 CCNSTRUCTUCN S
MR AIRCRAFT
(Glass-fabric-laminate Facinc and Waffle-type Cor e
Sandwich Material Tested in Shear )
Cctober 195 0
This Report is One of a Serie s
Issued In Cooperation with th e
AIR LORCE-NAVY-CIVIL SUCCOMMITTL E
on
AIRCRAFT DESIGN CRITLRI A
Under the Supervision of th e
AII2CI?AFT COMMITTE E
of th e
MUNITIONS BOAR D
No . 1559-1
UNITED STATES DEPARTMENT OF AGRICULTUR E
- ~FOWEST SERVIC E
LFGRE5T~"PRGDUCTS LABORATOR Y
Madison 5, Wisconsi n
In Cooperation with the University of Wisconsin
FATIGUE OF SANDWICH CONSTRUCTIONS FOR .AIRCRAFT1
(Glass -fabric.-laminate Facing and Waffle-tyype Core .
'Sandwich Materia l. Tested in Shear) ,
I
By FRED WE1 REN, Enginee r
Forest Products Laboratory,3 Forest .Servic e
U . .S .. Department of Agriculture - ;r
Summary and Conclusions ,
A limited number .of tests have been made at .the Forest-Products .Labo
ratory to determine the shear fatigue properties of an assembled sandwic h
panel with glass-fabric-laminate facings andl waffle-type core material .
Repeated tests were made at a ratio of minimum to maximum loading of 0 .1 .
The results of the tests indicate a'fatigue strength at 30 million cycles o f
approximately 64+ percent of the static strength for the condition of loadin g
used . - .
This progress report is one of a series prepared and distributed by the
Forest Products Laboratory under U . S . Nary, Bureau of Aeronautics No .
NBA--PO-NAer 00619, Amendment No . - 2, and U.. S .' Air Force No USAF-PO ' (33 1-038)49-4696E . Results here reported iate preliminary and may be'
revised as additional data become available .
2
--This is the ninth of a series of reports intended to offer a comparison o f
the shear fatigue properties of different sandwich materials, . The-follow ing FYI reports discuss the shear fatigue properties of ;
1559
" Cellular Cellulose Acetate Core Material "
1559-A " Aluminum Face and Paper HoneycomblCore Sandwich Material " .
155.9-B "Aluminum Face and End-grain Balsal~ICore Sandwich Material" .
1559-C "Aluminum or Fiberglas-laminate FaC e ,and Fiberglas Honeycomb' Core Sandwich-Material"
- 1559-D "Fiberglas-laminate Face and End-gain Balsa Core Sandwic h
Material "
1559-E "Aluminum or Fiberglas-laminate Face and Cellular-hard-rubbe r
Core Sandwich Material "
1559-F " Cellular Cellulose. Acetate Core terial with . Aluminum or
Fiberglas-laminate Facings "
1559-G "Fiberglas-laminate Facing and Paper Honeycomb Core Sandwic h
Material "
1559-H "Aluminum Facing and Aluminum Honeycomb Core Sandwich Material
Tested in Sheat""
2Maintained at Madison, Wis ., in cooperation with the University o f
Wisconsin .
'
Report No . 1559-I
-1 -
, Agriculture-Madison
The results or tests from a.single panel were erratic and
weakness in the core-to-facing glue Brie . Thus the S-N curve is
indicative of th e ' shear fatigue 5rtperties of the core material',
applies only to a aandwieh panel maws by the sgtde method's as the
indicated a
probably no t
but instead
panel tested .
Introduction '
If plates of sandwQ h snsbrgetion are designed so that their facing s
are elastically stable, the most critical stress to which the core is subjecte d
is shear . The cona<tderation.of the effect of repeated shear stresseaon the
material of the cores and oh the bonds between the core and facings i,a, there fore, importan t
The general testing praC titres and the nomenclature applied te thes e
tests are similar to those use$ by the Forest Products Laboratory ip . testing
sandwich material with aluminum facings and . paper honeycomb cores, ?
Description of Material an
Specimen s
The waffle-typo core material, core 31, used in this study was purchase d
from t}•e-manufacturer_ in-1949, and is considered-to be representative of th e
material in that status of development . It bras- , -composed o f , a polyester-resin impregnated glass-fiber mat formed into a coniguration . resemblirag a waffle
(fig . 1) . The 1/4.;inch-square tapered indentations, were .spacedeapproximatel y
on 7/16-inch .. centers, . The material used in these tests was r furnished ' in,' sec ti,ons 10 inches square and 0 .300 inch thick, and . cf a specific -gravity of about
0 .175 (11 .pounds per cubic foot) .
• . -The sandwich material tae assembled in accordance with therecommene
dations of the mantfacturer-o f, the core'material . Nine sections of core ' ina terial were, used in making a sandwich-pane] : 30 inches square . Prior to-eeeembly, the 'core was dipped in s tliin . solution of resin 2 and dried, to pie vide e. spread, of about 2 .5 grams of resin per-square foot . Each facing
consisted` : ef &ix- i5lies of,crbss-laminated'--glass' cloth, _
4 impregnated with 50
percent of resin 2 by weight . Facings were'wet laminated to the Core material ,
and the panel as bag molded in an air-cir-culating . ove•n at 250° F : and a
pressure of 14 pounds per square inch for 1 hour .
The specimens were cut from the assemble& panel, parallel to the pane l
edge, with a high-speed steel circular saw to a width- and length of 2 and
4 .22 inches, respectively . They were glued to shear plates with primar y
adhesive25 and secondary adhesive 29C . Adhesive 29C was cured at 15 pound s
of pressure per square inch at 220° F . for 1 hour .
_Resins, adhesives, and material referred to In this report are describe d
further in .appendix 1 .
Report 'No . 1559-1 .
-2-
Specimens were similar to the frame shear specimen described i n
Forest Products Laboratory Report No . 1559-A,2 except that each specime n
was 4 .22 inches long and the core material 0 .300 inch thick . The line o f
action of the load passed through diagonally opposite corners of the core .
Results of 16 fatigue tests and 8 control tests are presented in
this report .
Testing .
si ,
All tests were made by methods similar to those described in,Fores t
Products Laboratory Report No . 1559-A, except that specimens were condi tioned and tested in an atmosphere of 73° F . and a relative humidity of 50
percent .
It was mentioned previously that specimens were cut parallel to th e
panel edge, and therefore parallel to the edge of a section of core material .
Since the corowa .a made of a glass mat having random orientation of fibers ,
it is expected that the shear properties parallel to either edge of the cor e
section are similar . Therefore, tests were not made in two directions ,
although this had been done for most of the other sandwich materials
previously tested .2
The failure of both control and fatigue specimens was rapid once i t
began . All specimens failed in the core-to-facing glue bond . In all but
one specimen, this failure occurred on what might be called the female sid e
of the core . Referring to figure 1, this would be the side that is showin g
on the lower core section . There was no positive evidence of any failur e
of the core material itself .
Presentation of Dat a
Table I presents the results of the individual control and fatigu e
tests . Six specimens withstood more than 30 million cycles without failure .
In table 1 the number of cycles is followed with a plus sign to indicate the
specimen was capable of resisting additional repeated stresses .
The results of the fatigue tests are plotted in figure 2, and an S- N
curve has been drawn through the plotted points .
Analysis of Data
The results of eight control tests show a large variation in shea r
strength for this sandwich material ; from 85 .9 to 1+2 .1 pounds per squar e
inch . Examination of the core material itself indicates considerable non uniformity in the material . For example, an indentation may be made up
Report No . 1559-I
-3 -
largely p! f fhers
qne-place , while a section- .adjacent to it= is entirely
resin . Despite these irregular,ities, the sandwich did. nit fail in the core,.
but in all cases , failed in the, glue bond between the core and .facing .
.
Because of the random nonuniformity of both the material and the con trol, .test, result s, the average. .shea'r strength was used its-the control valu e
for all fatigue specimens . Thus one would expect considerablescatter• o . '
the fatigue data and some difficulty in establishing the S-N curve (fig, 2) .
It had been previously agreed to discontinue testing any specimen after i t
had withstood 3 0 . million cycles . -m examination of previous reports : shows
that the shear fatigue strength at 30 million cycles for this particular
sandwich construction (about 64 percent of the static strength) is highe r
than . .hat. : of any of the other .1 sandwich constructions-tested .
'.
T10:e rI S-N curve as•,drawn through bheeplotted data shows :the `-trend of
shear fatigue strength for this sandwich panel . If another method .of bonding
the core and facings were used, the curve might be entirely different . A
poorer •bond, would . result in lower she eae strengtht-and• probubiy a different S- N
.curve . A better bond, on the other hand, would result
'increas'ed shifter:
strength (unto a point where the core itself would fail) and probably-stil l
another S -N curve . .Thus, in a sandwich material--where failure 'occurs in .th e
core-to-facing glue bond, .,-.it is di f.fihul t- to predict what the properties-o f
a similar panel will be becat e gluing techniques, even though carefui1 Y
controlled, may result in wide differences in the quality of the bond :
. , t, It-- has been mentioned that the, panel made for these tests wa s
assembled according to the recommendations of the manufacturer, and that the
shear ,strength of the core, matertea- itself could not be developed beceuse : of
. w akn :'ss i i the core-toegacing-glu • . bond. A limited number of' flatwis e e
e
t . ns on .tes :tf were ., de»at the Forest Products-Laboratory from a similarly
fabricated sandwich panel . Result s. of these testa- showed a 16W tensil e
strength due to weakness in the core-to-facing glue bond . In an effort t o
improve the tensile properties of the bonded area, two small panels wer e
made by a modified fabrication method . The core was not dipped in a thi n
solution of rosin 2, but instead was roller coated with resin 2 to provid e
a spread of about 10 grams of resin per square foot . The panels were other wisc,aassempled :according to the manufacturer .' s r redtmmendationE . Th e
flatwi e .tensile strength of the latter -panels-was' about-thre'e or four time s
F greatcr - ti e.n,,thgt of - tie original panel . • -Thus " it a ppears that - other -'
assembly technaqu •; nig} tbe_deyelope .d'%hich would:improuee :the ,c•ore-tofacing bond, and such improvement would be expected to increase substantiall y
e Rhear ;tstreBg Yi : of, the sandwichs-•materdal ..
.. .
A greater number of fatigue tests may have been desirable to establis h
the S-N curve for this particular panel more definitely, but, in view of th e
preceding discussion, additional tests did not appear warranted . The result s
of these limited tests indicate that, for this particular sandwich panel, th e
endurance limit is about 64+ percent of the static strength for the conditio n
of eloadix g us-era . , . .
r
,' . .r
Report No . 1559-I
..4-
Appendix I '
Description of Resins, Adhesives, and Material
Used in These Tests-.
1.
Core 3 1
A waffle-type core material . Material used in these test s
obtained from manufacturer in 1949 .
2.
Glass clot h
A glass fabric designated as 112-114, which has been heat cleane d
and chemically treated withia chrome complex bath .
3.
Resin 2
A high-temperture-setting, low-viscosity, laminating resin o f
the polyester (styrene-alkyd) type .
4.
Adhesive 25
A high-temperature-setting formulation of thermosetting resi n
and synthetic rubber .
5.
Adhesive 29C
An intermediate-temperature-setting, acid-catalyzed, pheno l
resin (with 24 percent of catalyst) .
Report No . 1559- I
-5-
Fatigue tests
: Control test s
Specimen : Maximum : Ratio of
No .
repeated maximum
shear
: repeated
:
. stress :shear stress :
: to contro l
strength .
(2 )
(1)
P .s .i .
F7-1-1
3
4
6
7
9
to
12
13
:
;
:
:
:
:
:
15
16
:
19
:
18
21
22
24
:
:
:
91 .1
56 .2
71 .1
88 .1
85 .2
68 .2
77 .4
96 .6
79 .6
62 .6
74 .o
96 .6
77 .0
71 .1
82 .4
88 .6
Report No . 1559- I
:
Cycles
to
failure
(3)
(4)
Percent
Number
80 .0
200
100,000
31,900
77 .3
77 .8
:
:
: F7-1-2
:
:
32,966,800+ :
32,346,000+ :
72,300
35,000
:
: Av
. 31,057,800+ .
31,026,400+ .
.
.
42,200
.
9,785,800
.
15,200
1,478,100
11,700
(6 )
(5 )
34,134,000+ :
31,260,000+ :
49 .3
62 .4
74 .8
59 .9
68 .o
84 .8
69 .9
55 .o
65 .o
84 .8
67 .6
62 .4
72 .3
: Specimen : Shear
No .
: strength
.
.
5
8
11
14
17 .
20
23
:
:
:
:
:
:
:
:
P .s .i .
85 . 9
108 . 1
120 . 6
132 ., 8
107 . 8
107 . 3
106 . 8
142 . 1
113 . 9
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_
.
.
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44 T
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•
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4.
- -,••-
-
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1
Figure 1 .--Waffle-type core material of resin-impregnate d
glass-fiber mat
(ZM 81907 F)
.:
:,
■.
Report No . 1559-1
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Report No .
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