THE GLUING OF WOOD 71 
The results of long use have shown the limitations of this block-shear test. 
It is necessary, in order to get the most uniform and the most reliable results, 
to standardize the gluing operation so as to avoid the effect of variable factors 
as much as possible and to include all practical gluing conditions. Kind, 
quality, density, and moisture content of the wood, care used in preparing the 
wood for the joints, technic used in applying the glue and pressing, and accu- 
racy with which the specimens are cut and tested all have a bearing on the 
results. This test if properly conducted, however, will show whether or not the 
glue is capable of making a joint as strong as the wood. It will not show how 
much surplus strength the glue may have and therefore it will not permit a 
close comparison of several glues which are all capable of making joints as 
strong or stronger than the wood. 
The block-shear test is very useful only as a measure of the success of the 
gluing operation, and as such it has been extensively used at the Forest Prod- 
ucts Laboratory in studies on the effect of assembly time, pressure, amount of 
glue spread, and other factors which affect the strength of the joint. It is also 
a good test of the strength of the joint in samples of commercially glued wood, 
since test specimens can be cut from glued blocks of almost any shape as long 
as they are of sufficient size and have laminations thick enough. Furthermore, 
the appearance of the glued joint after the specimen is broken often enables the 
observer to determine whether proper conditions were used in the gluing 
operation. 
PLYWOOD-SHEAR TEST 
The plywood-shear test is usually made on glues used for veneer work. The 
regular test at the Forest Products Laboratory is made on three plies of birch, 
each one-sixteenth of an inch thick, glued with the grain of the core at right 
angles to the faces. The plywood is seasoned after it is glued, under uniform 
atmospheric conditions, and then cut into specimens of the standard form shown 
in Plate 13. The specimens are tested in a cement-briquette testing machine 
which is provided with special grips, shown also in Plate 13. 43 The specimen 
is subjected, to tension and the failure is principally in shear although some 
other stresses may occur owing to the slight bending of the specimen. The 
breaking load and the character of failure are recorded. The specimen is easily 
prepared and quickly tested. The shape and construction of the specimen make 
it impossible to develop loads as high as those obtained in the shear-block test, 
but under proper control this test gives comparable results. For direct com- 
parison of glues it is important that the plywood be of uniform construction as 
to species and ply thickness and that the specimens be prepared and tested in 
an identical manner. 
Where the plies are thinner than one-sixteenth of an inch the joint failure 
often occurs in this test by the specimen breaking across one of the faces. In 
such a specimen the strength of the joint can be tested more accurately by reduc- 
ing the shearing area of the test specimen without lessening the width of the 
specimen ; that is, by making the saw cuts across the faces one-half of an inch 
apart instead of 1 inch. Specimens with thin faces made in this way give unit- 
strength values somewhat higher than where the 1 square inch testing area is 
used. When the plies are one-sixteenth of an inch and thicker the specimen 
with 1 square inch of shearing area is used. When possible, however, tests on 
glues are confined to the same wood, ply thicknesses, and form of specimen. 
WATER RESISTANCE 
The plywood-shear test may be made on either dry or wet specimens or on 
both. The wet test is used to measure the water resistance of glues. Speci- 
mens are soaked in water at room temperature for 48 hours and then tested 
while wet. Glues of low-water resistance fail in the soaking vat. Highly 
water-resistant glues when tested wet may show from 50 to 75 per cent of their 
dry strength. The Army and Navy have adopted specifications for water- 
resistant plywood based on the plywood-strength test. In the specifications in 
force in 1926 two grades were provided for by the Navy, known as grades A and 
B. For grade A strengths of 300 pounds per square inch tested dry and 180 
pounds tested wet were required. The requirements for grade B were 225 and 
43 Other types of machines, involving the same general method of test, have been devel- 
oped and used to some extent in testing plywood joint strength. 
