PRELIMINARY REPORT ON THE 

STRENGTH OF FLAT SANDWICH PLATES 

IN EDGEWISE COMPRESSION 

May 1947 

This Report is One of a Series 

lssued In Cooperation with the 

ARMY-NAVY-CIVIL COMMITTEE 

on 

AIRCRAFT DESIGN CRITERIA 

Under the Supervision of the 

AERONAUTICAL BOARD 

No. 1561 

UNITED STATES DEPARTMENT OF AGRICULTURE 

FOREST SERVICE 

FOREST PRODUCTS LABORATORY 

Madison, Wisconsin 

In Cooperation with the University of Wisconsin 
- - - - 

PRELIMINARY REPORT ON THE STRENGTH OF 

FLAT SANDWICH PLATES IN EDGEWISE COMPRESSION1 

By 

KENNETH H. BOLLER, Engineer 

Summary 

This report presents data on the edgewise compressive strength of 

short columns of flat sandwich plates. It contains the results of a limited 

number of tests that were made on sandwich constructions to determine the 

failing stress of the facing material. This report also presents some 

mechanical properties of the core and facing materials used in the sandwich 

plates, and describes the methods employed to obtain then. 

Introduction 

In the design of sandwich constructions the maximum stress of the 

facings that is obtained in an edgewise compression test of the construction 

is an important criterion in the determination of the suitability of the 

component materials. The data on the strength and related properties of 

sandwich plates are presented in this report to provide assistance in the 

development of theories and formulas for this criterion. 

These data are the results of tests for maximum strength on 169 sandwich constructions. The results were obtained by testing six facing materials in 29 combinations with 10 core materials. In sone of the 29 

combinations the thicknesses of the component materials were varied, resulting in the 169 sandwich constructions. 

Data are presented to show a few properties of the facing and core 

materials that are related to the edgewise compressive strength of the 

sandwich. The methods that were employed for obtaining these properties are 

discussed. 

1

This report is one of a series of progress reports prepared by the Forest 

Products Laboratory. Results here reported are preliminary and may be 

revised as additional data become available. 

Report No. 1561 -1-
Materials 

Facing Materials 

The facing materials that were used and tested in the sandwich constructions are (1) rolled metallic sheets, (2) plastic laminates, and 

(3) veneer laminates. They are described as follows: 

Rolled metallic sheets.--24SH aluminum sheets, 0.005 inch in thickness, and 24 ST alclad aluminum sheets in 0.008, 0.012, 0.018, 0.020, and 

0.032 inch thicknesses were employed. The aluminum sheets in each sandwich 

construction were placed on the two sides of the core with the lengthwise or 

rolled direction parallel to the direction of the compressive stress applied 

in the tests. 

Plastic laminates.--Cross-laminated glass cloth facings were used in 

2-, 4-, 8-, and 16-ply constructions in conjunction with end-grain balsa 

cores. These facings were impregnated to a resin content of about 55 percent 

(based on total weight) with a suitable resin. When glass cloth was used 

with other core materials, 3-, 6-, 10-, and 16-ply constructions were made 

with about 45 percent resin content. (Note: The higher resin content of 

the glass cloth laminate employed with the balsa was found necessary for the 

proper fabrication of the sandwich constructions. In later panels, not 

reported here, means were found by which the resin content was satisfactorily 

reduced to 45 percent.) The glass cloth sheet incorporated in these constructions was 0.003 inch thick, 38 inches wide, and weighed 2.09 ounces per 

square yard. It was of a plain type of weave. 

Cross-laminated papreg was used in 0.009, 0.022, 0.027, 0.042, and 

0.066 inch thicknesses. The base paper was made from an unbleached black 

spruce, Mitscherlich type, sulfite pulp. This paper, 2.5 mils thick, was 

impregnated with 36.5 percent of a thermosetting phenolic resin. The 

percentage is based on the weight of the treated paper.2 

These plastic laminates were placed on the two sides of their respective core materials with the machine direction of the outside sheets parallel 

to the direction of the stress applied in the tests. 

Veneer laminates. --Laminatedyellow-poplar veneer was used as a facing 

material on resin-impregnated pulpboard cores. The laminated facings were 

made of from 1 to 10 sheets of 0.01-inch rotary-cut veneer, bonded with 

sheets of resin-impregnated paper. The directions of the grain of the 

individual veneers were parallel to each other. 

2 

Additional information may be found in "Strength and Related Properties of 

Forest Products Laboratory Laminated Paper Plastic (Papreg) at Normal 

Temperature." Forest Products Laboratory Report 1319, revised. 

Report No. 1561 -2-
Aircraft spruce plywood was mads of a three-ply construction having 

1/48-inch faces and 1/32-inch core. 

Both yellow-poplar and spruce facing materials were placed on thle two 

sides of their respective core materials with the grain direction of the 

outside plies parallel to the direction of the stress applied in the tests. 

Core Materials 

The core materials that were used and tested in the sandwich constructions may be placed in four classes, (1) wood and plywood, (2) expanded 

plastics, (3) pulpboard, and (4) honeycomb structures. They are described as 

follows : 

Wood and plywood.--Balsa wood was used in combination with all of the 

types of facing material. The balsa that is referred to as "end grain" 

(E.G.) was placed between the two facings with the longitudinal or grain 

direction perpendicular to the facings. This was accomplished by first surfacing the rough planks, then cutting the planks across the grain to make 

blocks of the required thickness of the core in the sandwich. These blocks 

were glued edge to edge to form the core. In general, a single sandwich 

plate contained blocks from the same plank, but the orientation of their 

radial and tangential directions was not confined to one given direction in 

the plane of the panel. 

The balsa that is referred to as "loaded flat, perpendicular to grain" 

was placid between the two facings with the grain direction parallel to the 

plans of the facings and perpendicular to the direction of the load applied 

in the tests, This construction was made by gluing, edge to edge, planks 

that had been smoothly surfaced to the required thickness of the core material. 

The balsa that is referred to as "loaded parallel to grain" was placed

between the two facings with the grain direction parallel to the plane of the 

facings and parallel to the direction of the load applied in the tests. For 

this construction the planks of the required thickness were glued edge to 

edge to provide sufficient width. 

The spruce plywood that was used as a core material with aluminum 

facings was made in the following constructions: (1) the 3/16-inch thick 

plywood vas made of 1/32-inch faces and core and with 1/20-inch cross bands; 

(2) 3/8-inch thick plywood was made of seven plies of 1/16-inch veneer: 

(3) 7/8-inchthick plywood was made of seven plies of 1/8-inch veneer. The 

grain direction of the outer ply in each construction was placed parallel to 

the direction of the load applied in the tests. 

Expanded plastics.--The expanded plastics consisted of solid materials 

that had been foamed, or expanded, to produce a large number of small voids 

in the mass thereby reducing the over-all specific gravity. Each of the core 

materials of this group had a fairly uniform cell structure and an over-all 

specific gravity of about 0.10. The individual cells varied in size from 

about 0.01 to 0.04 inch in diameter. 

Report No. 1561 -3-
Core materials of calcium alginate, cellular cellulose acetate, 

cellular hard rubber, British hard rubber, and special sponge rubber were 

used. They were in the form of flat plates that ranged in thickness from 

1/2 inch to 1-1/2 Inches, and in width from 2-5/8 inches to 24 inches and 

in length from 1 to 10 feet. The natural skin that covered the manufactured 

product was removed from all faces of these materials in their preparation 

for use as cores. The original direction of the thickness dimension was 

used as the thickness direction of the core in the sandwich. The other two 

directions, lengthwise and crosswise, were placed indiscriminately in the 

direction of the stress applied in the tests. 

Pulpboards3 .--The impregnated pulpboards used consisted of irregularly arranged wood fibers that adhered to each other and were formed into 

sheets of low density, porous core material. The quality of the adhesion 

and strength of the pulpboards were increased by the addition of resin. 

The resin-impregnated pulpboards used had resin contents ranging from 0 to 

70 percent. The percentage is based on the total weight of the impregnated 

board when dry. The specific gravities as well as the strengths of the 

core materials increase with increases in resin content. These boards were 

not stripped of their outer skin, prior to their use as cores, as were the 

expanded plastics. This skin prevented excessive penetration into the core 

material of the glue to affix the facings. The boards were manufactured in 

thicknesses of 3/8 and 3/4 inch and were used unaltered. 

Honeycomb structures.--Thehoneycomb material that was used in this 

study was made of either resin-impregnated glass cloth or paper sheets. The 

glass cloth sheets were the same as those used in making the facings. The 

paper sheets were made of 4-mil kraft paper, corrugated, and pretreated with 

10 percent of phenolic resin. Both kinds of sheeting were impregnated with 

a contact type of resin and assembled in large separate blocks. The crest 

of one corrugated sheet was placed on the crest of another forming tubes 

about 0.18 inch in diameter. The cores for the sandwich were sliced from 

the blocks so that the thickness dimension of the sandwich was parallel to 

the axes of the tubes. This core material was oriented in the sandwich so 

that planes of the corrugated sheets were perpendicular to the direction of 

1oading. 

Fabrication of Sandwich Plates 

Facing and core materials were combined to form 169 different sandwich constructions. Each construction consisted of sheets of facing material 

glued to a core to form flat sandwich plates, In some cases more than one 

layer of core material was required to form the core so that several layers 

were glued together. The thicknesses of the core and facing materials in 

these plates are listed in tables 1 through 5. 

3

Addititional information may be found in "Resin Treated Pulpboard Core 

Material for Sandwich Construction." Forest Products Laboratory Report 

R1623. 

Report No. 1561 -4-
Three sizes of sandwich plates were made: (1) square, 12 inches on 

a side, (2) rectangular, 12 inches long and 4 times the thickness plus 

1 inch in width, (3) rectangular, 2 inches wide and 4 times the thickness 

plus 1 inch in length. In the fabrication of the rectangular plates, 12 

inches long, strips of wood were placed between the facings in conjunction 

with the core material. These strips, 1/2 inch wide, 12 inches long, and 

the same thickness as the core material, were located at both 12-inch 

edges as shown in figure 1. 

Methods of Test 

Tests of Sandwich Constructions 

Compression and tension tests of sandwich constructions were made, 

except for a few modifications, according to the tentative methods described 

in Forest Products Laboratory Report 1556.4 

Compression edgewise.--The compression tests in the edgewise direction were made according to the procedure given in section 6 of report 1556.4 

Although this method was used and found satisfactory for a majority of the 

specimens, sone of them failed adjacent to one of the loaded edges. These 

failures were subsequently prevented in two different ways, (1) by the 

addition of wooden strips, shown in figure 1, to provide internal support of 

the facings at the loaded edges in conjunction with the steel clamps 

described in report 1556,4 and (2) by the adoption of plaster disks, as 

shown in figure 2, The specimens equipped with the plaster disks were prepared by grinding the bearing edges of the facings smooth ana parallel and 

removing 1/4 inch of the core material at each bearing end. The protruding 

edges of the faxing material were cast in plaster disks so that the bearing 

surfaces of the facings were flush with the surfaces of the disks. 

The particular specimens that were modified in each of these ways are 

indicated by footnotes in tables 1 through 5. 

Tension flatwise.--The tension tests in the flatwise direction were 

made according to the procedure given in section 8 of report 1556.4 The 

sandwich material for the specimens used in these tests was obtained from 

the 12-inch square plates as shown by the cutting diagram (fig. 3). 

Compression flatwise.--The compression tests in the flatwise direction 

were made according to the procedure given in section 7 of report 1556.

4 The 

sandwich specimens used for these tests were also obtained from the 12-inch 

square plates. 

4

Tentative Methods for Conducting Mechanical Tests of Sandwich Constructions. 

Forest Products Laboratory Report 1556. 

Report No. 1561 -5-
Tests of Core Material 

Compression and shear tests of the core materials were made according 

to the tentative methods described in Forest Products Laboratory report 

15555 with an additional method for determining the modulus of rigidity. 

Compression flatwise.--Compression tests of the special sponge rubber 

were made in the flatwise direction according to the procedure described by 

paragraphs 5 through 10 and figure 2 of report 1555.5 

Compression tests of honeycomb materials were made on specimens 2 

inches square by 6 inches long (length parallel to axes of cells). These 

specimens were tested between the heads of a testing machine by applying the 

load in a direction parallel to the axes of the cells. A 2-inch Marten’s 

mirror arrangement was used for measuring the deformations. 

Other core materials were tested in compression flatwise as sandwich 

constructions as previously described. 

Conpression edgewise. --Compression tests in the edgewise direction 

were made according to the procedure described in paragraphs 12 througn 17 of 

Forest Products Laboratory report 1555.5 Materials for these tests were 

matched with those used as cores of the sandwich constructions tested in 

edgewise compression. Core materials from the square plates, which were 3/4 

of an inch in thickness or thicker, were prepared by removing the facings 

from that portion of the plate marked "core, edgewise compression," as 

indicated in figure 3. Plates that were thinner than 3/4 of an inch did not 

provide material of sufficient thickness for testing. 

Shear.--The modulus of rigidity vas determined by one of three methods, 

(1) plate shear, (2) torsion pendulum, or (3) frame shear. Balsa, cellular 

cellulose acetate, and cellular hard rubber were tested by the plate shear 

method, which is described in paragraphs 33 through 37 of Forest Products 

Laboratory report 1555.5 Calcium alginate, pulpboards, British hard rubber, 

and sponge rubber were tested by the torsion pendulum method; paragraphs 39 

through 45 of report 1555. 

The frame shear method of test was used to determine the modulus of 

rigidity for honeycomb structures because the other methods were considered 

not to be applicable. Figure 4 shows the. dimensions of the assembly of 

specimen and frame used for this test. Figure 5 shows the assembly with 

dial located between the heads of a testing machine. The axes of the cells 

in the honeycomb are perpendicular to the plates and the planes of the 

corrugated sheets are parallel to the 6-inch dimension. Loads were applied 

through either a spherical head (fig. 5) or shimmed bearing blocks and thus 

distributed uniformly across the width of the specimen. 

5 

Tentative Methods of Test for Determining Strength Properties of Core 

Material for Sandwich Construction. Forest Products Laboratory Report 

1555. 

Report No. 1561 -6-
During the application of the load, data were taken for use in ;lotting the load-deformation curves. Deformations or shear strains were 

obtained at equal increments of load to the nearest 0.0002 radian by means 

of a dial indicator that was mounted on one of the steel plates. This 

indicator measured the movement of one plate with respect to the other. 

The modulus of rigidity was determined by calculating the slope of the loaddeformation curve according to the following formula: 

G = Pc 

awr 

where G = the modulus of rigidity associated with shearing strains in the 

plane parallel to the load and perpendicular to the plane of the steel 

plates, in pounds per square inch. 

P = load, in pounds 

c = thickness of the core material between steel plates, in inches 

a = length of core material, in inches 

w = width of core material, in inches 

r = displacement of one plate with respect to the other at load E, 

in inches. 

There is some question as to the accuracy of this method of test. 

Values that were obtained by this method for some weak materials are 50 percent higher than those obtained by other methods. On the other hand, values 

for dense core materials obtained by this method agreed with those obtained 

by other methods. It is believed that the values obtained by the frame 

shear test of honeycomb core materials are reasonable. 

Tests of Facing Material 

Compression edgewise.--Load-deformation curves for the 0.005-inch 24 

SH aluminum sheet were obtained by testing 1/4- by 1- by 4-inch rectangular 

specimens that consisted of sheets of aluminum laminated with an adhesive. 

Stress-strain curves of the aluminum itself were calculated from the stressstrain curves of the laminated specimens by taking into account the stressstrain curve of the bonding agent. 

Stress-strain curves for the 24 ST alclad aluminum sheet and glass 

cloth laminate were obtained by testing dumbbell-shaped specimens (fig. 6), 

which had an over-all width of 2 inches necked down to 15/16 inch and a 

length of about 4 inches. Their bearing surfaces were ground smooth and 

parallel. The specimens were tested between ground surfaces of steel 

I-shaped plates that provided lateral support for these thin sheets of metal 

(fig. 7). The distance between the plates was adjusted to provide 0.002-

inch clearance between the specimen and the plates. The upper and lower 

platens were alined by means of a cast-iron frame. Deformation measurements 

were made by means of a Marten's mirror arrangement with a 1-inchgage length. 

Tests similar to these in which rectangular brass plates were used for lateral 

Report No. 1561 -7-
support have been conducted at Langley Memorial Aeronautical Laboratory6 

and found to give satisfactory results within certain limits. 

Edgewise compressive data for the papreg and wood facings were obtained by testing rectangular-shaped specimens that were laterally supported 

at equal intervals along the entire length. The type of apparatus and the 

testing procedure used are described in ASTM Tentative Standard D805-44T.

7 

Yellow-poplar facings that were 0.010 and 0.021 inch thick were too thin for 

this test and therefore data for these thicknesses were obtained from tests 

of 0.032-inch facings which matched them. 

Shear.- Thenodulus of rigidity of aluminum in the plane of the sheet 

was determined by the torsion pendulum method of test that is described in 

sections 38 to 43 of Forest Products Laboratory report $555.5 The moduli of 

glass cloth laminate and papreg was determined by the plate shear method of 

test described in sections 33 to 38 of report 1555. 

Presentation of Data 

The results of tests on the sandwich plates, facing materials, and 

core materials are shown in tables 1 through 7. The first five tables 

present the results of tests of sandwich plates. Table 1 presents the 

results of tests on sandwich constructions having facings of 24 SH aluminum 

sheet; table 2 having facings of 24 ST alclad aluminum sheet; table 3, glass 

cloth laminate; table 4, papreg; and table 5, wood. Table 6 presents the 

properties of the facing materials and table 7 those of the core materials. 

Typical stress-strain curves for some of these materials are presented in 

figures 8 and 9. 

Tables 1-5, Sandwich Plates 

Each table from 1 through 5 presents the results obtained from sandwich constructions having facings of different materials and tested in 

edgewise compression and flatwise tension. The core materials used in combination with the individual facing materials are listed in each table. The 

results obtained in flatwise compression tests of sandwich plates are 

presented in table 7 with other results relating to core materials, The 

columns and the symbols denoting the properties and the types of failure are 

common to tables 1 through 5. Each value that is presented is the average 

of five test specimens. 

6NACA Wartime Report L-189 "Investigation of Methods of Supporting SingleThickness Specimens in a Fixture for Determination of Compressive StressStrain Curves." Joseph N. Kotacnchik, Waltser Woods , Robert A. Weinberger. 

7 Report of Committee D-7 on Timber, ASTM Tentative Standard D805-44T, 1944 

"Proposed Methods of Testing Veneer, Plywood, and Wood-base Laminated 

Materials." 

Report No. 1561 -8-
Columns 1-3.--These columns list the core materials and the thicknesses of the cores and facings. In column 1 each of the core materials is 

indicated by name. Columns 2 and 3 show the nominal thicknesses of the 

facings and core materials, respectively. 

Columns 4-5.--Column 4 shows the average total thickness of the 

individual specimens, and column 5 shows the weight of each sandwich construction in pounds per square foot. These values include the thickness and 

weight of the bonding agent, respectively. 

Columns 6-8.--Results of edgewise compression tests of the sandwich 

constructions are presented in columns 6-7. The loads, Pm, column 6, have 

been converted to pounds per inch of loaded edge and the strains, column 7, 

have been converted to unit measure. The values of maximum stress in the 

facing; material listed in column 8 were computed according to the formula: 

Pm pf = (Ey ) c 2f + c (Ey ) f 

where Pm = load at failure, pound per linear inch 

(Ey)c = the modulus of elasticity in edgewise compression of the core 

material in a direction parallel to the load on the sandwich 

(Ey)f = the modulus of elasticity in edgewise compression of the 

facing material in a direction parallel to the load on the 

sandwich 

c = core thickness in inches 

Column 9.--Designations of the types of failure observed are listed in 

column 9. Six types of failure ware observed and are designated and 

described as follows: (1) Failure of the facings in compression. Glass 

cloth, papreg, and wood facings usually failed in this manner. (2) Face 

wrinkle, apparently good bond. When failure occurred the facing material 

buckled, wrinkled, formed waves, or popped off the core but the rest of the 

specimen remained straight and. the bond appeared good. This, type of failure 

was sometimes difficult to recognize because the amplitude of the waves was 

very minute and the plane of the failure was often so close to the bond that 

good and bad bonds were hard to distinguish, (3) Offset failure. This 

failure consisted of a short crimp near the center of the specimen, the 

remaining parts of the specimen remaining straight and parallel but not 

co-linear. The offset consisted of a bending failure in the facings and a 

shear failure in the core. (4) Face wrinkle, apparently a faulty bond. 

This failure consisted of a sudden separation of one of the facings from the 

core and was attributed to inadequate adhesion between them. (5) Failure at 

the bearing ends. This type of failure occurred at the portion of the 

facings that were within the clamps, The ends of the facings failed by 

Report No. 1561 - 9 -
bending toward the core at the point of contact between the loaded end of 

the specimen and the bearing block. The two modifications of the test 

specimens previously described eliminated this type of failure. (6) Failure 

of cores in compression. When the core supports a substantial portion of 

the total load its failure is immediately followed by failure of the facings 

because they are not able to support the total load alone. 

Column 10.--Column 10 presents the ratio of the average maximum 

stress in the facings to a standard stress value. This ratio is equal to 

pf

/py for aluminm and papreg facings, and pf /pm for glass cloth laminate 

and wood facings, where py is the yield stress of the facing material at 0.2 

percent strain offset and pm is the maximum stress obtained by edgewise compression tests. The values of pf are shown in column 8 of tables 1 through 

5, and the values of py and pm are shown in columns 6 and 8, respectively, 

of table 6. 

Columns 11 and 12.--Theresults of tests of the sandwich plates in 

flatwise tension are tabulated in columns 11 and 12. Column 11 presents the 

maximum tensile strength of the construction in a direction normal to the 

surface. These values are the strength of the weakest part of the sandwich. 

Column 12 indicates the predominant type of failure. The types of failure 

are symbolized by the letters B, C, F, and O. The letter B indicates a 

failure in the bond; C indicates a failure in the core; F indicates a failure 

in the facing material, which was in most cases a delamination of the 

facings; and O indicates a failure that occurred outside the sandwich construction either in the loading block or in the bond between the loading 

block and the facing of the sandwich. 

Table 6. Facing Materials 

Table 6 presents data for sone of the properties of the facing materials that were used in the sandwich constructions listed in tables 1 through 

5. Each value presented is the average of the results of tests of five 

specimens. These values were obtained by testing specimens that were 

representative samples of each thickness of facing material, except those 

shown for papreg, which were obtained from Forest Products Laboratory report 

1319.2 

Columns 1 and 2.--Thefacing materials and their respective thicknesses are listed in columns 1 and 2. 

Column 3.--Column 3 tabulates the specific gravity of the facing 

materials. 

Columns 4 through 7.--Theresults of the tests in edgewise compression 

are shown in columns 4-7. The Young's modulus of elasticity, (Ey)f , was computed according to the formula: 

Pg (Ey

) f = bfY 

Report No. 1561 -10-
where P is the load in pounds on the specimen, b and f are the width and 

thickness, respectively, at the net section, and Y is the deformation at 

load P, measured over a gage length, g. Young's modulus of elasticity and 

the stress at proportional limit (cols. 4 and 5, respectively), were obtained for all the facing materials. The values of yield stress (col. 6), 

which is defined as the stress at 0.2 percent strain offset, were obtained 

for the alminum sheets and the values of maximum stress (col. 7) were 

obtained for the glass cloth laminate, papreg, and wood facings. In conjunction with these values, typical stress-strain curves for the facing 

materials are shown in figure 8. 

Column 8.--Column 8 presents the values for the modulus of rigidity 

in shear. 

Colums 9 and 10.--Columns 9 and 10 contain cross references from 

table 6 to tables 1 through 5. Column 9 contains the number of the reference 

table. Column 10 designates the core material of the particular sandwich 

construction referred to. 

Table 7, Core Materials 

Table 7 presents the results obtained from the core materials tested 

in flatwise and edgewise compression and transverse shear. It should be 

noted that the individual values in this table for the nodulus of elasticity 

in flatwise compression were obtained either from the sandwich specimens or 

the core specimens. The core materials and their respective thicknesses are 

listed and cross referenced in the same sequence as they were listed in 

tables 1 through 5. Each value is the average obtained from five specimens. 

Column 1.--Column 1 lists the core materials that were used in the 

construction of the sandwich plates. The orientation of the core materials 

in the sandwich plates is indicated. 

Column 2.--Incolumn 2 the average specific gravity is listed for 

each core material. 

Columns 3 and 4.--The Young's moduli of elasticity that were determined by the compression tests in the flatwise direction are listed in 

columns 3 and 4. These values were computed according to the formula 

(Ez ) c = 

Pg 

abY 

in which P = the load on the specimen in pounds at deformation Y in inches, 

g = the gage length in inches, and ab = the cross-sectional dimensions in 

inches. In column 3, the values were determined from the deformation data 

that were obtained by means of strain gages attached to the sides of the 

specimens; in column 4 the values were determined from deformation data 

obtained by measuring the deformations between heads. The values obtained 

by the latter method are lower but more consistent than those obtained by 

the strain-gage method. 

Report No. 1561 -11-
Columns 5 through 9.--The data that were obtained from tests of 

specimens in edgewise compression are tabulated in columns 5-8. Column 5 

presents values of Young's modulus. Columns 6 and 7 present the values of 

the proportional limit stress and the maximum stress, respectively. Sone 

core materials, like balsa, loaded in the radial or tangential direction 

or a honeycomb structure loaded in a direction-perpendicular to the length 

of the cells may not reach a maximum load until they have been compressed 

to a solid mass. The maximum stress for these materials was defined as the 

stress at which there was very small increase in stress for a very large 

increase in deformation. Column 8 presents the values of the strains that 

were recorded at the maximum stress. In conjunction with the data in these 

columns, typical stress-strain curves for these materials are shown in 

figure 9. 

Column 9.--The values that were obtained for the modulus of rigidity 

by tho methods previously described are tabulated in column 9. 

Columns 10 and 11.--Columns 10 and 11 contain cross references from 

table 7 to tables 1 through 5. Column 10 contains the number of the table 

referred to and the thickness of the face of the particular sandwich construction in that table. Column 11 contains the thicknesses of the cores 

of the particular sandwich construction referred to. 

Report No. 1561 -12-
Z M 72682 F 
Z M 72683 F 
Z M 72684 F 
Z M 72688 F 
Z M 72685 F 
Z M 72687 F sheet 1 of 3 
Z M 72686 F 
Z M 72689 F 
Figure 1. --Sandwich plate, compression-edgewise specimens with wood 

inserts at the bearing ends. 

ZM 72644 F 
Figure 2.--Cross section of compression-edgewise 

specimen of sandwich plate with plaster disks at 

the bearing ends. 

ZM 72645 F 
Figure 3.--Layout diagram for edgewise compression tests of 

flat sandwich plates. 

ZM 72646 F 
Figure 4.--Frame-shear test for sandwich core material. 

Z M 68071 F 
Figure 5.--Apparatus for frame-shear test showing

steel plates, honeycomb core, and dial arrangeZM 73512 F ment for measuring deformations between plates. 
Figure 6.--Compressionspecimen for edgewise 

tests of thin single-sheet material. 

ZM 71321 F 
Figure 7.--Compression-edgewise test of thin single-sheet 

material with continuous support: A, Specimen; B, steel 

plates; C, Marten's mirror strain gage; D, supporting jig. 

Z M 71923 F 
Figure 8.--Typical stress-strain curves of facing materials 

in edgewise compression. 

Z M 72647 F 
Figure 9.--Typical stress-strain curves of core materials 

in edgewise compression. 

Z M 72648 F