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This Report is One of a Series
Issued in Cooperation with
AIR FORCE.NAVY-CIVIL
on
AIRCRAFT !DESIGN CRITERIA
Under the Supervision of the
AIRCRAFT COMMITTEE
MUNITIONi r
BOARD
Madison 5, Wisconsin
In Cooperation with the University of Wisconsin

Sr'. iCT OF ELEVATED TEMPERATURES ON THE
STRENGTHS OF SMALL SPECIMENS OF SANDWICH
CONSTRUCTION OF THE AIRCRAFT TYPE-
Tests Conducted Immediately After the Test Temperature
Was Reached
By EDWARD W.
KUENZI, Engineer
Summary
This study investigated the behavior of sandwich constructions at elevated temperatures up to 600° F. (315.6° C.). Tests of 21 different constructions included column compression, edgewise compression, shear, and tension normal to facings. Tests were conducted immediately after the specimens reached the desired temperature. Apparatus and testing methods are described, and the results of the tests are presented in tabular and in graphical form.
Introduction
Sandwich constructions for aircraft comprised of thin, strong facings bonded to relatively thick lightweight cores have the advantages of being stiff, and therefore stable, as well as being light in weight. The smooth surfaces available make the construction particularly useful for airfoil sections. The use of aircraft at high speeds will cause heating of the surfaces due to skin friction. There may also be other uses in which sandwich constructions will be subjected to loads at elevated temperatures.
In order to be able to design a structure to carry loads under conditions of elevated temperatures, it is necessary to have information as to the behavior of the construction at an elevated temperature. Tests of small specimens at elevated temperatures provide data for judging which constructions may be satisfactory.
This progress report is one of a series prepared and distributed by the
Forest„ Products Laboratory under U. S. Air Force No. AAF(33-038)48-190E and U. S. Navy, Bureau of Aeronautics No. NBA7NAer 00675, Amend. 1, and°
NAer 00730. Results here reported are preliminary and may be revised as additional data become-available. This report was originally puxlished in August 1(2,49.
Report No. 1804

The work of this study included the testing of specimens of sandwich construction in column compression, edgewise compression, shear, and tension normal to facings at temperatures of 75°, 160°, 250°, 350°, 450°, and 600° F.
Twenty-one different constructions were tested as columns, 16 different constructions in edgewise compression, and eight representative constructions in shear and in tension.
The tests were conducted immediately after the specimens reached the desired temperature. No work was done to evaluate creep characteristics or the behavior after long-time exposure to elevated temperatures.
Materials
The materials of which sandwich constructions are made can be divided into three main groups: facings, cores, and adhesives. The facings used in this study were clad 24S-T3 aluminum-alloy sheets in thicknesses of 0.012,
0.020, and 0.032 inch, or five or seven plies of plain-woven glass cloth
112-114 or 181-114. The cores used in this study were as follows:
'End-grain balsa wood having a density of 5 to 8 pounds per cubic foot, cut so that-the grain direction was normal to the facings.
Cellular cellulose acetate of a density of about 7 to 8 pounds per cubic foot (including the skin formed in extruding). The material was oriented so that the loading direction for compression and shear tests was parallel to the extruded direction in all cases except one construction having 0.020-inch clad 24S-T3 facings, in which case the extruded direction was placed at right angles to the direction of loading.
Urapped cellular cellulose acetate consisting of the same material as-described above, but cut to strips 1 inch wide and spirally wrapped with one thickness of glass eloth•112-114 impregnated with wet resin ,1 (a laminating resin of the polyester diallyl phthalate-alkyd type). The strength properties of this particular polyester resin at elevated temperatures are among the best of the currently available polyester laminating'resins.
Glasscloth honeycomb of 3/16-inch hexagonal cells made by impregnating glass cloth 112-114 with resin 1, the same resin that is used in the - glasscloth facings. The apparent density was from
7 to 10 pounds per cubic foot.
Cotton-cloth honeycomb. of. 3/8- to 7/16-inch hexagonal cells made of
4 -ounce cotton sheeting impregnated with a phenolic-type resin. The apparent
Aluminum-alloy honeycomb: of 3/8-inch nominal hexagonal cells made of
0.003-inch plain 2SH-18 alloy foil. The aluminum honeycomb did not have perforations. The corrugations were bonded together with the same adhesive that was used to bond on the facings (adhesive
The apparent density was 4.375 pounds per cubic foot.
Report No. 1804 -2-

The clad 24S-T3 facings were bonded to the end-grain balsa or cellular cellulose acetate cores with adhesive 25 (a high-temperature-setting formulation of thermosetting resin and synthetic rubber) and adhesive 30A (al intermediate-temperature-setting, acid-catalyzed phenol resin). The clad
24S-T3 facings were bonded to cotton-cloth honeycomb or
honeycomb cores with adhesive 35 (a high-temperature-setting formulation of thermoplastic and thermosetting resins). Recent tests
indicated that in aluminum-to-aluminum lap shear specimens, combined adhesives 25 and 30A have
.considerably greater resistance to heat above 160° F. than adhesive 35 when tested immediately after reaching equilibrium4
Glass-cloth facings were wet laminated to cores of glass-cloth honeycomb, cellular cellulose acetate, or wrapped cellular cellulose acetate with resin 1.
The manufacturing techniques, as uell as more detailed descriptions of the materials, are given in Forest Products Laboratory Report No. 1574,
"Fabrication of Lightweight Sandwich Panels of the Aircraft Type."
The specific sandwich constructions tested at elevated temperatures
. were as follows:
Facings Cores Average sandwich weight
Lb. per sq. ft.
0.012-inch clad 24S-T3
.020-inch clad 24S-T3
.032-inch clad 24S-T3
.012-inch clad 246-T3
.012-inch clad 24S-T3
..0247inch clad 24S-T3
1/2-inch end-grain balsa 0.557
1/2-inch end-grain balsa
1/2-inch end-grain balsa
.822
1.024
1/4-inch end-grain balsa .453
1/2-inch cellular cellulose acetate .614
1/2-inch cellular cellulose acetate . .765
.032-inch clad 24$-T3
.020-inch clad 24S-T3
':020-inch clad 24S-T3
1/2-inch cellular cellulose acetate 1.029
1/4-inch cellular cellulose acetate:
.624
1/2-inch aluminum honeycomb `'.698
.032-inch clad 24S-T3 1/2-inch aluminum honeycomb .988
.020-inch clad 24S-T3 1/2-inch cotton-cloth honeycomb .708
.032-inch clad 248-T3 1/2-inch cotton-cloth honeycomb
.937
.020-inch clad 24S-T3 1/4-inch cotton-cloth honeycomb ' '.615
plies of glass cloth 112-114 1/2-inch cellular cellulose acetate .506
7 plies of glass cloth 112-114 1/2-inch cellular cellulose acetate .623
plies of glass . cloth 112-114 1/4-inch cellular cellulose acetate .384
5 plies of glass'cloth 112-114 1/2-inch glass-cloth honeycomb .577
plies of glass cloth 112-114 1/4-inch glass-cloth honeycomb
plies of glass cloth
1/2-:inch wrapped cellular cellulose
.422
acetate .619
5
1/2-inch cellular cellulose acetate 1.211
7 plies of glass cloth
1/2-inch cellular cellulose acetate
Figure 19 shows the behavior in aluminum lap-joint specimens tested at elevated temperatures of adhesives 25 and 30A. when combined and also adhesive
These data were taken from Forest Products Laboratory Report No.
"Strength of Aluminum Lap Joints at Elevated Temperatures."
Report No. 1804 3 -

Testing Apparatus and Testing Procedures
procedure for all types of tests was to surround the specimen general arrangement showing the testing machine, the .
oven, and the controller is . shownin figure 1. The oven was simply a means of heating the specimens.
AllAparatus.neCessary_tO apply loads to ' the specimen was passed through openings in the top and bottom of the oven. Tests were made at, the follow,ing temperatures: 75° F. (23.90'-C,), 160° F.- (71.1° C.), 250° F. (121.1° C.),
30° F..(176.7° C.), 450° F. (232.2° 0.), and 600° F. (315.6° C.).
The testing machine was a hydraulically operated_ machine equipped with pacing mechanisms to enable the operator to control the rate of head.
movement . orof load application. The capacity was adjusted to meet the requirements of the particular tests. Most of the testing was dane.with the machine capacity adjusted to a 24 500-pound load, at which capacity the smallest division'on the'dial corresponded to a load of.2-1/2,poUnds.:
The oven Was of A type usually used ' for ` tempering steels,:
It was capable of continuous operation at 1,200° F. (648.9° C.). The heat was supplied by electrical heating elements located in the two sides of the oven.
vA circulating fan was locatecrin the back of the oven. This fan was in opera-
: *ionwhen the door was closed. Opening the door automatically opened the
'eleetrical'aircuit to the fan and thereby stopped it and prevented a blast of hot air from leaving the oven. The . door of the oven had a heat-resistant glasalvindow at the center for rise in observing the performance Of thespeciben%
•
The temperature ofthe oven was controlled by an indicating-controlling pyrometer (right background, fig.-1). This pyrometer was essentially a wheatstdne bridge arranged to be in continuous balance during operation.
The thOmocOuple of the pyrometer vas placed near the Center of the oven, direCt1S- behind the specimen. The input to the heating elements was switched by a relay controlled by the pyrometer.
A rheostat was provided to permit, reduOing the heater voltage i put so as to obtain closer control of the
• temperature without overheating just after the desired temperature was reached. It greatly reduced the inertia effect, which would be 000iderable if the Oven were operated under full voltage at a low temperature, such as,
160° F.
An additional potentiometer (right foreground, figs 1) having thermo-.
couples'in connection with the oven or 'specimen and an icetwater bath
(reference temperature 32° F.) was used to measure temperatures in the specimen or temperatures at various points in the oven.'
A test of t4OemperatalL 6 distribution in the oven was made by placing thermocouples
A various points.
The locations of the thermocouples are shown in figure The temperatures were read immediately after the pyrometer indicateditqf the desired temperature had been reached. Results of.the test are also given in figure 2. The results show that the temperature
• •
•
Report 1804

fairly uniform throughout the oven. There were no apparent stratified zones of constant temperature. The temperature of the oven actually was somewhat lower than that indicated by the pyrometer, although in the region where the specimen was tested, this drop was,2.percent , orless, During a test the
'distribution was expected' to have been'better than and:the temperature closer to that indicated by the pyrometer, since a longer period of time elapsed before the specimen was tested after the pyrometer indicated the 'temperature was reached, because 5 to 10 minutes was spent in adjusting the specimen and gages before the test was begun.
:Tests of the temperature distribution along several columns of sandwich construction were made by placing'thermocouplee;at the center of the core and at 1-inch intervals throughout the length of the specimen. The specimens'Were
'pieced under'a load of 25 pounds. All specimens.were 1 inch wide and 10 inches long. The con s tructions tested were 0.012-inch clad 24S-T3 facings on 1/2-inch end-grain balsa, 0.012-inch clad 214S-T3 facings on 1/2-inch cellular cellulose
Acetate, 0.020-inch clad 2LiS-T3 facings on 1/2-inch aluminum honeycomb, and
'five plies of glass-cloth 112-114 facings on 1/2-inch cellular cellulose acetate. The temperature distributions obtained are shown in figures
3,
4,5
6. The ends of the specimens were at a temperature approximately 10 percent and lower than that of the central portion. The central 6-inch portion of the specimens was at a fairly uniform temperature throughout.
Compression tests were made on both long and short sandwich'epecimens.
'The specimens were loaded in a direction parallel to the plane of theificings.
The long specimens were designated as column-compression specimens, the short ones as edgewise-compression specimens. Both types of specimen, were loaded in the same manner. A 1-inch-diameter bar, slotted 3/ ► inch wide.and:to the center, was fitted to each end of the specimen. Shims of brass or aluminum were used to fit the specimen rather snugly in the slot. .The upper loading
`head Was. a stationary flat plate. The lower plate was supported on a knife-
:_ edge and thumbscrew arrangement. The knife edge was placed atJ .ight angles to the plane of the specimen and at the center of the':specimen'to allow selfalinement across-the width of the specimen. The thumbscrew_was arranged to shift the bottom in a direction normal to the facings. , of thespecimen.: means of this thumbscrew, adjustments could be made to equalize the strains in the facings. A Marten/s mirror apparatus for reading Compression over a f-
:" inch-gage length . was placed at the center of theaength of the specimen;.: The
.testing apparatus and arrangement for a column-compression specimen are shown in figure
7, and for an edgewise-compression specimen in figure
8.
The testing procedure was to place the specimen in the heated oven, be-
; tween the heads, and under a small initial load..-A . thermocouple was . placed on one facing. The -Marten's mirrors were adjusted to read on the Scales::: 'Then the oven door was:closed, and, while the:oven was returning to the desired temperature, the - load was applied slowly and several mirror readings were taken. Ifthe'readings on one facing were 20 percent or more different-from the readings on the_ facing, the thumbscrew in the base was adjusted.
This procedure was'follOwed until the strainsin:the facingS,:',as indicated by the mirror readings, were approximately equal,' • This Manipuiation'took-from 5 to 10 minutes. After the temperature of the oven had been reached, the thermocouple on the facing was read and, if agreement was within 5° F., the test was begun.
Rept. No.
180L1

Five specimens were tested of each construction given in table 1.
The column-compression specimens were 1 inch wide and 10 inches long. They were loaded under continuous motion of the movabis head of the testing machine at a rate of 0.020 inch per minute until failure occurred. The time from the beginning of loading until failure was measured with a stop watch. The edgewise-compression specimens were 2 inches wide and 3 inches long and were loaded under continuous motion of the movable head of'the testing machine at a rate of 0.009 inch per minute until failure occurred.
Shear tests were made on specimens 1 inch wide and 6 inches.long. All cores in these specimens were 1/2 inch thick. The particular constructions tested are given in table 1. Five specimens were tested of each construction.
Bars of stainless steel 1 inch wide and 3/8 inch thick were glued to each facing with adhesive 35. These bars, had hales at one end for receiving the loading bolts. The, bars were positioned, depending on the facing thickness, so that the direction of loading force coincided with the , diagonal connecting one end of a facing with the other end of the opposite facing. Deformations were observed by means of two mirrors arranged similarly to the Marten'smirror apparatus. One mirror was fastened between two projecting ears on the facings, and the other mirror was fastened to the loading bar of the specimen.
Thus any motion of the mirror between the ears, relative to the mirror on the bar, indicated shearing deformation between the two facings.. Therefore the moduli of rigidity computed from the data are effective moduli and include deformations in the facing-to-core adhesive as well as in the core but not deformations in the facings. Thus, the effective shear modulus is given by the formula
G where P = load, c = core thickness, b = width of core, ble
1 = length of core, e = deformation (slip between insides of facings). A photograph of the apparatus arranged for testing is shorn in figure 9.
The testing procedure was to place a. cold specimen in position in the heated oven and,to adjust the mirrors to read on the scales. Approximately
5 minutes after the oven had reached the desired temperature, the test was begun. Tests made of core temperatures of similar or failed specimens placed near the test specimens showed that about 5 minutes were 'required to ensure that the specimen temperature was correct. Thus the total time the specimen was being heated prior to testing was 5 minutes plus the time necessary to adjust the apparatus and the time necessary for the oven to reach proper temperature. This total time was from 10 to 20 minutes. The load was applied by continuous motion of the movable head of the testing machine at a rate of 0.01
inch per minute until failure occurred. The time from beginning of loading until failure was measured with a stop watch.
Tension tests normal to facings were made on specimens 1 by 1 inch in cross-sectional dimension. Cubes of aluminum were bonded to the facings with adhesive 35.
These cubes were positioned so that the holes for the loading pins were at right angles and thus formed a universal joint with the specimen for a center link. Figure 10 shows the testing apparatus and arrangement.
The testing procedure was the same as for the shear tests, except that the speed of testing was
0.035 inch per minute.
Report No. 1804

Results of Tests
The numerical results, such as facing stresses or core moduli, are given in table 1 and, also in.graphical form
: in figures 11 through 18. The time involve&in testing each specimen, from the beginning of loading until failure is , given in table 2.
The results of the column-compression tests are shown in figures 11 and
12 for constructions having cores 1/2 inch , thiek and in figure 13 for those having cores V4 inch thick. The facing stresses at-failure are plotted against the,temperatures at the time of test. The type of failure was by Euler buckling, which resulted permanently bending the specimen with or;vithoutshear failures4n the core, or. by , severe H orimping-of the construction near one end.
Compression failures were observed in some of the i glasscloth facings.
The results of the edgewise-compression tests are shown in figures 14 and 15. The facing stresses at. failure are plotted against the temperatures at the time of test. Failures were by bending of the specimen with or without shear failures in the core or by_severe_crimping near one end.
, Facing wrinkles were observed in many of the specimens at failure.. Some compression failures were observed in glass-cloth facings.'
The results of the shear tests are shown in figures 16and 17. - Figure 16 shows the effective moduli of rigidity plotted against the temperatures at the time of test. These moduli are the results of measurements of the relative slip between the inner surfaces of the facings and, therefore, they include deformations in the facing-to-core adhesives as well as deformations in the core, but do not include deformations in the facings. Figure 17 shows the shear stresses in the core at the failing load of-the specimen plotted against the temperatures at the time of test.- Failures occurred in the cores, in the adhesive between the facing and the cores or in the adhesive used to bond the loading plates to the facings. As indicated above, failure in the bond between cube and facing might have been reduced by using a more heat-resistant adhesive.- Failures in the continuous types of cores appeared as jagged breaks due to tension set up by the applied shear. Failures in the honeycomb types of cores occurred by buckling of the cell walls and subsequent crushing of the core.
_The results of the tension tests are shown in figure 18. -The tensile stresses in the'cores at the failing loads of the specimens are plotted-against the temperatures at the time of test. Failures occurred in the cores, in the adhesive between the facings and the cores, or in the adhesive.'used to bond the loading cubes to the facings. Failure of the bond between'Ioading plates and facings could probably-have been materially reduced by use of a more heat-resistant adhesive that 35. At the time of 'these tests, little information on heat resistance of adhesives was available. Adhesives 25 . and
30A when combined had a considerably higher short-time strength than did adhesive 35• Lap-joint tests also show this to be true. Tests on . these adhesives'and others are reported in
Forest Products Laboratory Report
No. 1808. The joints were made by lapping two
1-inch . strips-of 0.04-inch clad 24S-T3 alloy for'one-half inch. Figure 19, gives the strength of the-combination of adhesives"25 and 30A, and also the strength of adhesive 35 when plotted against temperature., less
The behavior of the materials as the - temperature was increased, regardof the type of test, was of a general degrading or disintegrating nature.
The glass finally cloth impregnated with resin 1 discolored slightly at 350° F. and became a golden brown in color at 600° F. The cotton-cloth honeycomb
Report No. 1804
_7_

nad a dark-brown color at 450° F. and . was black and charredat
600° F. The end-grain balsa. showed light-brown charring throughout at :•45O° F. and was black and charred at 600° F.; several specimens actually ignited during the test at
600° F. Several specimens of cellular cellulose acetate showed bulges at
250° F. and at 350° F. Most of the specimens were severaly bulged. This bulging was more Prominent on the specimens 2 inches:wide. than on specimens'
1 inch wide. At 450° F., the cellular cellulose acetate melted to, a rather shapeless mass. The degree of disintegration, as described above, depended upon the length of time the specimen was exposed to the elevated temperature.
No definite data concerning this relation was obtained because Most specimens received the same period of exposure; however, it was observed in some:cases, such as those in which the balsa ignited, that longer exposures would decidedly cause further disintegration.
Discussion of Test Results
Results of a study of this nature are difficult to evaluate unless the end use of a construction is known. Even•the construction showing the poorest performance when considering absolute-strength values may be used in a structure because of its light weight, low cost, or, perhaps, because of other inherent advantages peculiarly' adaptable to the structure. Thus a designer will have to consider the economies, weight, ease of fabrication, availability, etc., for a particular ` structure before judgment can be passed concerning the most desirable sandwich construction.
Based on absolute strength, in column- or edgewise-compression tests, the results showed that up to and, including a temperature of 450° F. combinations of Clad 24S T3 facings on end-grain balsa cores performed the best. At
600° F., or probably at some temperature between 450° F. and 600° F., the combination of clad 24S T3 facings on aluminum honeycomb cores was the best.
The strengths might have been greater if a more heat-resistant adhesive was used both in the fabrication of the core and for bonding facings to the core.
The effective modulus of rtgidity.ef the clad 248-T3 facings on aluminum honeycomb coresas grOater than that of clad 24S-T3-balsa construction up to temperatures of k50° F. Above this temperature, the ,clad
24S-T3-balsa construction was better.
Combinations of clad 24S T3 facings on end-grain balsa cores tended to maintain strengths until higher temperatures were reached, above which a rapid drop occurred.. The other sandwich constructions tested showed more abrupt decreases in strength at smaller increases in temperature, but better maintenance of this decreased strength at 'h i gher temperatures. The decrease in strength may have been due to a softening-'of the adhesive.
Results of compression tests on constructions having glass-cloth facings were less erratic than those on constructions having clad 24S T3 facings, probably because the compressive strength was more often the failing mode as compared to the instability typek , of failures observed for specimens having clad 24S T3 facings and because-' a a difference in heat resistance of the resins and adhesives used in the two types of constructions.
Report No. 1804 -
8
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Figure 1.--General view of apparatus for testing at elevated temperatures.
ZM79837F

Figure 7.--Apparatus for column-compression tests of sandwich constructions at elevated temperatures.
ZM79839F

Figure 8.--Apparatus for edgewise-compression tests of sandwich constructions at elevated temperatures.
ZM79838F

Figure 9.--Apparatus for shear tests of sandwich constructions at elevated temperatures.
ZM79840F

Figure 10.--Apparatus for tension tests of sandwich constructions at elevated temperatures.
a179841F