SOME CAUSES OF VARlABlLlTY IN THE 

RESULTS OF PLYWOOD SHEAR TESTS 

Information Reviewed and Reaffirmed 

December 1954 

No. 1615 

INFORMATION REVIEWED AND 

REAFFIRMED JUNE 1959 

DATE OF ORIGINAL REPORT 

JUNE 1946 
SOME CAUSES OF VARIABILITY IN THE RESULTS 

OF PLYWOOD SHEAR TESTS 

By 

D. W. BENSEND, Technologist 

and 

R. J. PRESTON, Technologist 

Forest Products Laboratory,1 Forest Service 

U. S. Department of Agriculture 

Introduction 

The shear test is a generally accepted method for evaluating the quality of 

glue joints in plywood, and the plywood shear testing machine, a modified 

cement-briquet testing machine (fig. 1-A), has been widely adopted for testing 

glues and plywood. Considerable variation is commonly encountered in the 

shear-test values obtained on duplicate specimens of plywood. The causes of 

such variations are usually ascribed to variations in the wood, but they may 

also be due to differences in the moisture content of the specimens, in their 

physical shape and arrangement, or in variations in the operation of the testing machine. 

The following studies were undertaken at the Forest Products Laboratory in 

an attempt to determine the causes of variability in shear-strength measurements made on the plywood shear testing machine and to ascertain the importance 

of the effects of each on the reliability of the results. When these causes 

were determined and evaluated it was the further purpose of this study to 

make recommendations toward their elimination. 

Possible Causes of Variability 

The following 10 factors were considered as possible sources of variation: 

1. Misalignment of jaws. 

2. Equal offset of both upper and lower jaws. 

3. Distance between jaws. 

1 

Maintained at Madison, Wis., in cooperation with the University of Wisconsin. 

Rept. No. 1615 -1- Agriculture-Madison 
4. 

5. 

6. 

7. 

8. 

9. 

10. 

Side play in the grips. 

Jaw pressure. 

Rate of loading. 

Position of saw cuts with respect to direction of slope of grain in 

the face plies. 

Type, depth, and width of saw cuts. 

Moisture content of shear specimens. 

Time of exposure of specimens to relatively dry air. 

Each factor was investigated by separate experiment designed in such a manner 

that the results could be analyzed statistically and the importance of 

individual factors and the interaction between factors could be estimated. 

The 10 experiments are discussed separately in this report. The results of 

all studies were analyzed by analysis of variance, and all conclusions are 

based on those analyses. 

Preparation of Specimens 

Three-ply 3/16-inch plywood made of yellow birch was used in all experiments. 

The standard plywood specimen of the dimensions shown in figure 1 was employed 

in all but experiment 8 where the saw cut was under investigation. 

The 1/16-inch veneer was selected for freedom from defects and for straightness of grain, except in experiment 7 where sloping grain was intentionally 

employed. Unless otherwise indicated the veneer was conditioned to moisture 

equilibrium with air at 80° F. and 65 percent relative humidity before gluing. 

In the preparation of the panels, the sheets of veneer were laid up so that 

any slope of grain in the face plies, however slight, was in the same direction, 

as indicated in figure 2. 

For part (a) of experiment 1, the panels were glued with two sheets of phenol 

film glue at each glue line, cured under pressure of 150 pounds per square 

inch at 320° F. for 8 minutes. Unless otherwise stated, for all other experiments the panels were glued with a melamine-formaldehyde resin glue cured in a 

hot-press at 300° F. for 8 minutes, and the plywood was conditioned after gluing 

for at least a week in an atmosphere of 80° F. and 65 percent relative humidity. 

The panels were cut into shear test specimens in such a way that specimens 

adjacent end-to-end had their saw cuts in reverse positions with regard to 

slope of pain as shown diagrammatically in A and B of figure 2. For each 

variable in each experiment a matched pair of type-A and type-B specimens were 

selected at random from each panel. Enough panels were prepared to provide 

either 12 or 20 specimens for each different condition. 

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Shear Testing 

In preparation for the testing, the vertical canter line of the testing grips 

was marked at the points, C, C as shown in figure 1-C, To do this, a plywood 

specimen was placed in the Jaws, put under a 100-pound load, and adjusted so 

that the center line of its core was in line with the center of the hole on 

which the knife edge rests in the upper grip and the center of the screw at 

the bottom of the lower grip, as sighted with a transit. The marks C, C were 

then accurately drawn to coincide with this sighted line. Short millimeter 

scales were mounted on the faces of the grips with the zero graduations 

coincident with the marks C, C. Their location was checked with the transit. 

A traveling telescope was used to line up each specimen tested in all but 

experiment 10, using the scales on the grips as reference for the alignment. 

In experiment 10 the specimens were lined up by eye and the position of the 

upper saw cut alternated between left and right inasmuch as a prior experiment 

had shown that the effect of misalignment of jaws would be eliminated by this 

procedure. In all experiments except 1 (b) the specimens were tested with the 

saw cut to the upper left and lower right as seen by the operator. 

The same machine was used for all experiments, except for experiment 1 (a) in 

which a second machine was also used. 

Unless otherwise indicated, the testing was done in temperature- and humiditycontrolled rooms or the specimens were kept in small tightly capped bottles 

between the time of removal from the constant humidity chambers and the time 

of testing. 

Procedure and Results of 

the Experiments on the 10 Variables Studied 

Experiment 1 Misalignment of Jaws 

(a) In the experiment to determine the effect of the misalignment of the jaws 

of the testing machine the lower jaws were kept centered in the grips but the 

upper jaws were offset from 2 millimeters to the left to 2 millimeters to the 

right. Specimens were tested both dry (conditioned at 80° F. and 65 percent 

relative humidity) and wet (immersed for 3 hours in boiling water) on two 

different machines. Results are presented in table 1 and figure 3. 

The dry shear strength measurements were greatly affected by misalignment of 

the upper Jaws. Displacement of as little as 1/2 millimeter to the right-or 

left of alignment with the lower jaws resulted in a joint strength value about 

50 Pounds per square inch lower or higher than the value obtained when the jaws 

were in perfect alignment. 

Rept. No. 1615 -3-
The shear strength was affected by misalignment much less when the specimens 

Were wet than when they were dry . A displacement of 1 millimeter to the left 

or right made a difference of about 9 pounds per square inch in the average 

wet shear value, which was too low to be shown significant; but a displacement 

of 2 mm. resulted in a significant difference of about 38 pounds per square 

inch. 

The measured value for the shear strength was less when the offset of the 

upper jaws tended to open the saw cuts as in figure 2 (b), but greater when 

it tended to close the saw cuts as in figure 2 (d). 

The effect of misalignment was found with both machines. (b) In experiment 

1 (a) the saw cuts were placed at the upper left and lower right of the operator. In experiment 1 (b), similar tests were made with the saw cuts also at 

the upper right and lower left. The lower jaws were kept accurately centered 

in the grips as before and the upper jaws offset 1 mm. to the left, 1 mm. to 

the right, and centered in line with the lower jaws. The specimens tested 

with the upper saw cut to the right and to the left were matched. They were 

tested dry only after conditioning to moisture equilibrium in air at 80° F. 

and 65 percent relative humidity. The averaged data are given in table 2. 

Analysis of the data indicated that there was no significant difference in 

shear strength of specimens tested with saw cuts to the upper left and to the 

upper right if both jaws were centered in the grips, but, as found in experiment 1 (a), that there was an important effect of position of the saw cuts if 

the jaws were not in alignment. If the upper jaws were offset 1 mm. to the 

side of the upper saw cut (fig. 2 (b)), the average shear strength was 63 

pounds per square inch lower then when both jaws were in alignment, but 56 

pounds per square inch higher when the upper jaws were offset 1 mm. away from 

the side of the upper saw cut (fig. 2 (d)). The average of these two results 

was not significantly different from the average shear strength for specimens 

tested with upper and lower jaws in line. From this it may be concluded that, 

if matched specimens are tested with the saw cuts alternately to the upper left 

and right, the averaged result would not be affected by misalignment of 1 mm. 

or less. If the SR ecimens were not matched, the effect of misalignment would 

at least be minimized. 

(c) In view of the great difference found in experiment 1 (a) in the effects 

of misalignment on dry and wet plywood specimens, it seemed desirable to study 

the effect of misalignment on dry specimens that varied only moderately in 

moisture content. For this purpose matched specimens were conditioned to three 

different moisture content values and tested, as in experiment 1 (b), with upper 

and lower jaws in alignment and with the upper jaws offset 1 mm. to the right 

and 1 mm. to the left of the lower jaws. The averaged data are given in table 

3. 

As in previous parts of this experiment there was a striking difference in 

shear strengths for the different alignments at a particular moisture content 

and also for different moisture contents at a particular alignment, but the 

Rept. No. 1615 -4 -
-- 

difference in the moisture content of the specimens from 5.3 to 10.2 percent 

did not significantly influence the effect of the misalignment on the shear 

strength. 

Experiment 2 Equal Offset of Both Upper and Lower Jaws 

Twenty specimens were tested with both the upper and the lower jaws centered 

in the grips and symmetrical with the center line of pull, and 20 matched 

specimens were tested with both the upper and the lower jaws offset 1 mm. to 

the right of the center line of pull. The average dry shear strengths were 

637 and 633 pounds per square inch, respectively, with 100 percent wood failure 

in all specimens. The small difference between these values indicates that, 

if the upper and lower jaws are in vertical alignment, equal displacement of 

both jaws in the same direction up to 1 mm from the center line of pull is 

unimportant. 

Experiment 3 Distance Between Jaws 

The jaws in the plywood test grips are normally 1 inch wide and 1 inch long. 

If the full area of each jaw is in contact with the specimen, the distance 

between the jaws is 1-1/4 inches. Usually the specimen is inserted so that 

the full area of the lower jaws is in contact with the specimen, but it frequently happens that only a portion of the upper jaws is in contact due to the 

failure of the operator to adjust the machine after testing the previous specimen. In practice the distance between the jaws may vary from 1-1/4 to about 

1-1/2 inches. It is important to know the effect of this variation on the 

shear strength results. In Great Britain plywood testing jaws 2-1/4 inches 

apart are specified, and it was deemed desirable to know how the results would 

be affected by jaws at this spacing. Accordingly, experiment 3 was conducted 

to obtain data (a) with jaws 1-1/4 inches apart, (b) with jaws 1-3/4 inches 

apart, the lower jaws in full contact and the upper jaws in half contact and 

(c) with jaws 2-1/4 inches apart, using jaws 1 inch wide and 1/2 inch deep. 

These three conditions are shown in figure 4. 

Matched specimens were tested with jaw conditions A, B, and C (fig. 4), not 

only with jaws centered and in alignment, but also with the upper jaws offset 

1 mm. to left and right, to detect any interrelation between effect of 

misalignment on the effect of distance between the jaws. The averaged results 

of the tests (each value based on 20 tests) are given in table 4. 

The shear strength values were importantly influenced by the position of the 

jaws on the plywood specimen. The magnitude of the effect was increased or 

decreased if the misalignment was such as to increase or decrease, respectively, 

the shear strength value. Higher strengths were obtained with the jaws 1-1/4 

inches apart than with the Jaws further apart. With the small jaws in the 

position shown as C figure 4, values were obtained intermediate between the 

values obtained with the larger jaws in the positions shown as A and B. 

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-- 

-- 

-- 

-- 

Experiment 4 Side Play in the Grips. 

In the sketch of the grips in figure 1 it may be seen that the upper half is 

free to slide up and down between two side plates rigidly screwed to the 

lower half. Since the freedom between the upper half of the grips and the 

side plates might have an effect on the test results, it was chosen as one of 

the variables for study. The side play was regulated by shims under the side 

plates used to hold the grips in line. Without shims the grips were closely 

fitted, with just enough play to avoid friction. Three different thicknesses 

of shims were used and 20 specimens tested dry with each thickness. The average results are given in table 3. 

Analysis of the results showed that side play in the grips of as much as 1 mm. 

had no significant effect on the shear strength measurements. 

Experiment 5 Jaw Pressure 

In tightening the jaws the pressure applied to the shear test specimen will 

vary with the operator, the length of the handle on the screw, and whether the 

screw is tightened by hand or by wrench. To determine the effect of jaw 

pressure on the shear strength measurements, 20 specimens were tested when the 

jaws were tightened by hand, and 20 matched specimens when the jaws were 

tightened with a hand wrench, The average shear strengths were 625 pounds per 

square inch for specimens tightened by hand and 650 pounds per square inch for 

specimens tightened as tight as possible with the wrench. This difference was 

highly significant indicating the desirability of uniform tightening of the 

jaws for all specimens. 

Experiment 6 Rate of Loading 

The plywood shear testing machines are loaded by lead shot running from an 

orifice. The normal rate of loading is approximately 800 pounds applied to 

the shear test specimens per minute. In this experiment the size of the 

orifice was altered so that the rate of flow of shot was both one-half, and 

about twice the normal rate. At each rate of loading 20 matched specimens were 

tested dry. The averaged results are given in table 6. 

The results indicate little or no difference in the shear strength measurements made by loading at 400, 800, or 1,600 pounds per minute. 

Experiment 7 Position of Saw Cuts With Respect to 

Direction of Slope of Grain in Face Plies 

To determine the effect of the position of the saw cuts with respect to the 

direction of slope of grain in face plies the plywood was made with straightgrained face plies and also with the grain of the face plies sloping 1:10 and 

Rept. No. 1615 -6-
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1:5 from the glue lines, the direction of slope being parallel in both faces. 

The cores of the plywood were in all cases straight grained. Matched specimens 

were cut with saw cuts A and B as shown in figure 2. The specimens were tested 

dry after conditioning to about 12 percent moisture content; those with a slope 

of 1:10 and those without slope were also tested wet after boiling in water for 

3 hours. In the tests, the upper saw cut was always placed to the operator's 

left. The test was repeated under three conditions: with the upper jaws (1) 

centered and in line with the lower jaws; (2) offset 1 mm. to the left; and 

(3) offset 1 mm. to the right. The results are presented in table 7. 

From the data of table 7 it may be seen that there was little difference between the results for saw cuts A and B when there was little or no slope of 

grain. This result was to be expected, since in this case there was little 

or no distinction between A and B. When there was slope of grain from the 

glue line in the face plies of as much as 1:10, however, the location of the 

saw cuts had a pronounced effect. In the dry tests, type-B specimens averaged 

210 pounds per square inch higher than type-A specimens for the 1:10 slope, 

and 92 pounds per square inch higher for the 1:5 slope. In the tests on wet 

specimens of 1:10 slope, type-B specimens averaged 60 pounds per square inch 

higher than those of type A. Of the specimens tested dry all type-A specimens 

failed 100 percent in the wood, but the percentage of wood failure for the 

type-B specimens averaged only about 62 percent. Of the specimens tested wet 

both A and B failed 100 percent in the wood, but the failure of type-A specimens 

was in a face ply and of type-B specimens in the core. 

It may be concluded from these observations that the glue line would be put to 

a more severe test if the face veneer was not of straight grain, but was 

selected to have an appreciable slope, such as 1:10, and if the saw cuts into 

the core were of type B (fig. 2-C), whereupon the measured strength would be 

higher and, in the dry tests, the percentages of wood failure lower. With a 

slope of grain of as much as 1:5, the test on the glue line would be less 

severe, since such specimens often fail at relatively low load by splitting 

along the grain between the saw cut and the end of the specimen. Since it is 

not easy to select grain of a particular slope, the practical procedure would 

be to employ only straight-grained veneer for the faces of the plywood used 

for glue-joint tests. 

Experiment 8 Types, Depth, and Width of Saw Cuts 

Matched specimens of the plywood were tested with three different depths of 

saw cuts made with a swage-set saw. The averaged shear strength results, based 

on 20 specimens each, are given in table 8. 

The differences between the shear strength averages are highly significant and 

indicate the importance of control of the depth of saw cut. 

Specimens cut with a spring-set saw cutting through the face plies and core 

and slightly nicking the opposite face ply gave an average strength of 540 

pounds per square inch for 20 specimens. Although this value is less than the 

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560 pounds per square inch average value for cuts through the face and core 

by the swage-set saw, the variation among individual specimens was sufficiently large that the difference between these two average values was not statistically significant. 

To investigate the effect of width of saw cut, matched specimens from birch 

plywood panels glued with a room-temperature-setting urea-resin glue and with 

a thermoplastic resin glue at 300° F. were cut with a swage-set saw two-thirds 

through the 1/16-inch core, half with a saw cut 3/32-inch wide and half 5/32­

inch wide. These specimens were tested both dry and wet after soaking 48 

hours. The results of the shear tests are given in table 9. 

There was no significant difference between the results for the narrower and 

wider widths, wherefore it may be concluded that a variation of ± 1/32-inch 

from the 1/8-inch standard dimension for-the width of the saw cuts as shown in 

Figure 1 would have a negligible effect on the test results. 

Experiment 9 Moisture Content of Shear-Test Specimens 

(a) Birch sapwood and heartwood veneer was conditioned in an atmosphere of 

80° F. and 65 percent relative humidity, then glued with melamine glue into 

three-ply plywood panels. Groups of 20 sapwood and 20 heartwood matched sheartest specimens cut from these panels were conditioned for 8 to 10 days in 

atmospheres of 5 different relative humidities, soaked for 48 hours in water 

at room temperature, and boiled in water for 3 hours, after which they were 

tested for joint strength. The moisture content of the dry specimens at the 

time of testing was determined by weighing, drying, and reweighing the specimens immediately after testing. 

Averaged results are given in table 10 and plotted in figure 5. 

The moisture content of the specimens at the time of testing greatly influenced 

the shear strengths (table 10 and fig. 5). The shear strengths were highest 

for specimens conditioned and tested at 80° F. and 80 percent relative humidity 

and having an average moisture content of 11.8 percent. At lower or higher 

moisture contents there was a sharp reduction in shear strengths. Soaked and 

boiled specimens were likewise lower in strength. 

(b) To ascertain the way in which the moisture content of the veneer at the 

time of gluing might affect the relation between the joint strength and the 

moisture content of the plywood at the time of testing, birch veneer was conditioned to moisture equilibrium with air at 80° F. and 30, 65, and 80 percent 

relative humidity, then glued with melamine and urea resin glue into plywood 

panels. Shear test specimens cut from these panels were evenly divided into 

three groups, conditioned to equilibrium in air at 80° F. and 30, 65, and 80 

percent relative humidity, and tested. The results are given in table 11. 

Rept. No. 1615 -8-
-- 

No significant differences in shear strengths were noted for the various 

moisture contents at the time of gluing within the limits required for glue 

joints of high quality. The conclusion is that, within the limits of this 

experiment, moisture content at time of gluing had little or no influence on 

the relation between the shear strength and the moisture content at the time 

of testing, providing the moisture content was within the range required for 

good glue bonds. 

Experiment 10 Time of Exposure of Specimens to 

Relatively Dry Air 

Matched specimens were tested after they were conditioned at 80° F. and 80 

percent relative humidity and then after they were exposed for several different periods of time to air at 80° F. and 30 percent relative humidity. The 

specimens had been glued with a hot-setting melamine-formaldehyde and a roomtemperature-setting urea-formaldehyde glue. Wet tests after the specimens were 

boiled for 3 hours were made on the melamine-bonded specimens after the same 

periods of exposure to 80° F. and 30 percent relative humidity. The results 

are presented in table 12. 

No important change was noted in the dry strengths up to 10 minutes exposure 

in the dry air, but between 10 and 20 minutes the strengths dropped significantly. Longer exposure up to 240 minutes effected little or no further change 

in the shear strength. The wet strength of the specimens appeared not to be 

affected by any of the prior exposures used. 

Summary of Conclusions 

A study was made to determine some of the causes of variability in plywood 

shear-strength measurements. Standard shear-test specimens of three-ply 3/16­

inch yellow birch (fig. 1) were used, The plywood was made with melamine glue 

and in a few instances with phenol thermoplastic or urea-resin glues, The 

independent variables studied included: alignment of jaws., distance between 

jaws, side play in the grips, Jaw pressure, rate of loading, position of saw 

cuts with respect to direction of slope in the face plies, type, depth, and 

width of saw cut, moisture content at time of gluing and testing, and time of 

exposure of specimens to relatively dry air. Each experiment was so designed 

that the data could be analyzed statistically. 

1. Misalignment of the upper and lower jaws of as much as 1/2 mm. resulted 

in large differences in dry shear strengths, The difference was higher or 

lower than the shear strength with perfect alignment depending on whether the 

saw cuts were narrowed or widened by the misalignment. It would be advisable 

to bevel the face of the grips and mark the center line of pull at the points 

C, C in figure 1 so that the operator could align the two sets of jaws more 

accurately by inserting the specimens with the center of the core in line with 

the marks C, C. The effect of moderate misalignment of jaws, however, could 

Rept. No. 1615 -9-
be eliminated or minimized by alternating the upper saw cut to the left and 

right in testing a given set of specimens. 

2. Offsetting the specimen an equal distance up to 1 mm. from the center 

line of force in the grips did not appreciably affect the shear values. 

3. Dry shear strengths were sufficiently reduced by an increase in the span 

between the two sets of jaws that care should be taken at each test to adjust 

the machine so that the full length of the upper jaws is in contact with each 

specimen. 

4. Side-play in the grips of as much as 1 mm. had little or no effect on the 

shear-strength values. 

5. The force with which the jaws were tightened had only a slight effect on 

the shear-strength results but enough to justify the attempt to tighten the 

jaws always to about the same extent. 

6. Variation in the rate of loading from 400 to 1,600 pounds per minute were 

not important. 

7. The practical conclusion from the results of tests on specimens made with 

and without sloping grain in the face plies is that only straight-grained 

veneer should be employed for the face plies of shear test specimens used for 

testing the quality of glues in dry joints. Sloping grain in the face plies 

puts the glued joint to a less or more severe test depending on the slope of 

grain and the location of the saw cuts with respect to the direction of slope. 

8. It is desirable that the depth of the saw cuts in the shear-test specimens 

be controlled, since variations in the depth of the saw cuts resulted in important variations in shear-strength values. Small variations in width of 

saw cut were not important. 

9. Variations in moisture content of the veneer at the time of gluing had no 

important effect on the shear strength providing the moisture content was within the range required for good glue bonds. 

10. Because the shear strengths were greatly affected by the moisture condition of the specimens at time of testing, the necessity for careful control 

of moisture content of the specimens at the time of testing is indicated. 

11. The average shear strength of the specimens from some panels were importantly and significantly different than those from other panels similarly 

glued and constructed for the same set. This indicates the desirability of 

carefully matching face plies of comparative panels and of testing an equal 

number of specimens from each panel for every testing variable being investigated. 

Rept. No. 1615 -10 -
12. In 9 different experiments in this study the averages of the shear 

strengths of the control specimens (each average based on 20 specimens, 2 

from each of 10 panels) fell with ± 6 pounds of 629 pounds per square inch. 

From this it may be concluded that excellent precision can be obtained in this 

plywood shear test by carefully controlling all influencing variables. 

Rept. No. 1615 -11- .2-27 
Table 1.--Average shear test results obtained on three-ply 3/16-inch birch 

plywood specimens tested with the upper and lower jaws in and 

out of vertical alignment 

Rept. No. 1615 
Table 2.--Average results of shear tests on matched three-ply 3/16-inch birch 

plywood specimens tested dry with the upper and lower jaws in and 

out of vertical alignment and the upper saw cuts on the left and 

right of the operator 

Table 3.--Average results of shear tests on thee-ply 3/16-inch birch plywood specimens at three moisture contents tested with the upper 

and lower jaws of the testing machine in and out of alignment 

Rept. No. 1615 
Table 4.--Averaged results of shear tests of three-ply 3/16-inch birch plywood specimens tested with jaws of the testing machine different 

distances apart and with jaws in and out of alignment 

Table 5.--Averaged shear test results on three-ply 3/16­

inch birch plywood specimens tested dry with 

different amounts of side play between the 

upper grips and the side plates 

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Table 6.--Average shear test results on three-ply 3/16­

inch birch plywood specimens teated in a 

plywood shear testing machine at three 

different rates of loading 

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Table 7. --Average shear test results obtained on three-ply 3/16-inch birch 

plywood specimens prepared with main of face plies without slope 

and with slopes of 1:10 and 1:5 

Rept. No. 1615 
Table 8. --Average shear test results on three-ply 3/16-inch 

birch plywood specimens cut with a swage-set 

saw to three different depths 

Table 9.--Average shear-test results on three-ply 3/16-inch 

birch plywood specimens with saw cuts of two 

different widths 

Rept. No. 1615 
Table 10. --Average shear strengths obtained on three-ply 3/16-inch birch plywood specimens after conditioning at various humidities, soaking, 

and boiling in water 

Rept. No. 1615 
Table 11.--Average shear-test results obtained in specimens of three-ply 

3/16-inch birch plywood, prepared from veneer conditioned to 

equilibrium with air at 80° F. and 30, 65, and 80 percent

relative humidity. The specimens were tested after conditioning in air at 80° F and 30, 65, and 80 percent relative humidity 

Rept. No. 1615 
Table 12.--Average shear-test results obtained on three-ply 3/16-inch birch 

plywood specimens, glued with melmine and with urea glues, conditioned to equilibrium with air at 80° F. and 80 percent relative humidity, but tested after various periods of exposure to 

air at 80° F. and 30 percent relative humidity 

Rept. No. 1615 
Figure 1.--Plywood joint test: (A) Plywood shear testing machine; (B) diagram of the standard shear test specimen; 

(C) shear test grips. 

Z M 68796 F 
Figure 2.--Two combinations of positions of sawcuts and grain slope 

(indicated by sloping lines) in face plies of three-ply plywood 

specimens. In (b) and (d) is illustrated the distortion of the 

Z M 68797 F sawcuts caused by misalinement of the upper and lower jaws. 
Figure 3.--Shear strength values obtained on wet and dry three-ply 

3/16-inch birch plywood specimens tested with the upper jaws in 

and out of alinement with the lower jaws. The specimens were 

placed in the machine with the sawcuts to the upper left and 

Z M 68798 F lower right as seen by the operator (fig. 28). 
Figure 4.--Diagrammatic sketch of the three conditions of the grips 

Z M 68799 F in Experiment 3. 
Figure 5.--Average shear strength values obtained on threeply 3/16-inch plywood made of birch veneer conditioned to 

equilibrium at 80° F. and 65 percent relative humidity, 

tested at various moisture contents and after soaking and 

Z M 68800 F boiling in water. 
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