FOREST PRODUCTS LIBRARY
FOREST RESEARCH LABORATORY
OREGON STATE UNIVERSITY
RESISTANCE OF SEVERAL TYPES OF GLUE
IN WOOD JOINTS TO FATIGUE STRESSING
Information Reviewed and Reaffirmed
June 1955
No. 1539
UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST SERVICE
FOREST PRODUCTS LABORATORY
Madison 5, Wisconsin
In Cooperation with the University of Wisconsin
RESISTANCE OF SEVERAL TYPES OF GLUE IN
WOOD JOINTS TO FATIGUE STRESSINGBy
W. Z. OLSON, Technologist,
D. W. BENSEND, Technologist,
and
H. D. BRUCE, Chemist
Forest Products Laboratoryr2– Forest Service
U. S. Department of Agriculture
MI AN. •nn
Introduction
Glued joints in wood aircraft structures are subjected to repeated
stressing. Failures in the glue bonds are usually attributed to poor
gluing technique or to deterioration of the glue by exposure, but so
far as is known have never been traced to fatigue due to repeated
stressing. Nevertheless, it is possible that some failure may be the
result of fatigue. It is, therefore, important to know how glues resist the weakening influence of vibration and how different types of
glues vary in this respect.
Several types of glue have been tested at the Forest Products Laboratory by repeated stressing of the glue joint of a specially designed
specimen. The specimens were made with either thick or thin glue lines.
In addition, matched specimens of the same type, and also lap joints of
plywood, were tested in shear to detect any relation that might exist
between resistance to fatigue stressing , and shear strength.
1
–This is one of a series of progress reports prepared by the Forest
Products Laboratory relating to the use of wood in aircraft. Results
here reported are preliminary and may be revised as additional data
become available.
-Maintained at Madison, Wis., in cooperation with the University of
Wisconsin.
Report No. 1539
-1-
Agriculture-Madison
Preparation of Test Specimens
The eight glues included in this study were of seven types as listed
in table 1. They were all commercial glues currently on the market.
They were mixed and used according to the manufacturers' directions.
Gluing details are given in table 1.
The type of specimen used in the fatigue tests is shown in figures 1
and 4. Assemblies were constructed by gluing thin plywood (1/10-inch)
to thick plywood (1/2-inch), both of high quality yellow birch veneer
bonded with phenolic film glue. The thin plywood consisted of 1/32inch face plies on a 1/24-inch core. The thick plywood was made of
eight plies of 1/16-inch veneer with the grain of the two center plies
in the same direction to provide balanced construction. The three-ply
material was cut into 5- by 12-inch pieces and the eight-ply material
into 2-1/2- by 12-inch pieces, with the grain of the face plies in
both at right angles to the long dimension. After conditioning to
12 percent moisture content the surfaces to be glued were lightly
stAnded.
These pieces of three- and eight-ply plywood were glued together, with
the adhesive being investigated, to make a 1/2-inch lap joint. This
was accomplished as shown in figure 2 using a piece of the eight-ply
plywood, waxed and taped to prevent sticking, as a filler block. After
removal from the press, the filler blocks were discarded and the assemblies were conditioned for at least 1 week at 80° F. and 65 percent
relative . humidity, after which a groove was sawed to the glue line at
a in figure 2 to leave a lap joint of exactly one-half inch. The assembly was then sawed into specimens 1 inch wide, as shown in figure 1,
and a 3/8-inch hole was bored through the thin plywood, seven-eighths
inch from the end to accomodate a fastening in the fatigue machine.
The specimens with thick glue lines were prepared in much the'same
way, except that 1/20-inch veneer shims were used, one under the filler block, the other, 1-7/8 inches wide, between the thin and the thick
plywood, and extending from the left edge of the specimen as shown in
figure 2, to the point where notch a was later cut. This left a trough,
1/20 inch deep, 5/8 inch wide, and 12 inches long, which was filled with
glue and covered by the piece of thin plywood to be glued. As the glue
cured, the glue-line thickness became less than 1/20 inch. The shrinkage amounted to 12 to 6o percent. Average thicknesses of the glue lines
when tested are given in table 2, column 3. Specimens were cut from
this assembly in the same manner as those with the thin glue lines.
Four assemblies, glued as shown in figure 2, were prepared with each
glue, two with thick glue lines and two with thin glue lines, except
for the alkaline hot-press phenol-formaldehyde glue, C, with which
Report No. 1539
-2-
only thin glue lines were prepared. From each assembly, eight specimens
were obtained of which four were tested in fatigue and four in shear,
alternately selected. The shear test specimens were cut to a length
of 3-1/4 inches and tested in the standard plywood shear-testing machine.
For a further comparison of shear and fatigue-test results, specimens
were prepared for shearing by gluing two pieces of three-ply 1/10-inch
birch plywood together and cutting the assembly into lap-joint specimens with, 1/2-square-inch shear area.
Test Procedures
Fatigue-testing machines of the constant-deflection type (fig. 3) operating at 1,790 revolutions per minute were used. The machines were provided with automatic counters and a precision switch which shut the current from the motor whenever a specimen broke.
The specimen was so adjusted in the machine that the motion was the repeated-stress cycle illustrated by figure 4A, that is, the deflection
waS always to the tension side of the neutral point of zero stress on
the glue line and never to the side at which the glue line would have
been under compression.
Previously, exploratory trials had been made of the reversing-stress
cycle, represented by figure 4B, with which it was found that the threeply birch plywood invariably broke through the face plies at the point
corresponding to a in figure 4A before there were any signs of failure
in the glue line. Hence, for a test of the glue joint rather than of
the plywood, the repeated-stress cycle was preferable.
As a first step, exploratory experiments were made to find a load that
would result in failure within a reasonable length of time. An average static load of 6.7 pounds, applied 3 inches from the point a in
figure 4A so as to put the glue in tension, caused failure when fatigue
specimens glued with urea-formaldehyde glue were tested as cantilever
beams, but a 4.0-pound initial load induced failure at about 300,000
cycles of repeated stress. Because the thin plywood varied in stiffness, the deflections resulting from a particular load varied somewhat
between specimens,but the initial deflection produced by a load of 4.0
pounds was arbitrarily chosen for all the fatigue tests subsequently
made in this study.
Whenever a specimen withstood 10,000,000 cycles without breaking, it
was removed from the machine.
Report No. 1539
-3-
Results
The results of the fatigue tests are given in table 2. The data on
the thin glue lines fell into three groups as follows:
1. Glues A, C, D, E, and F withstood means of 4,431,100+ to
6,627,500+ cycles before failure, with the means calculated on the
assumption that unfailed specimens broke at 10,000,000 cycles.
2. Glues B and G withstood means of 306,900 to 397,300 cycles
before failure.
specimens failed.
al
3. Glue H withstood a mean of 31,000 cycles before failure.
All specimens failed.
The data of the fatigue tests on the thick glue lines fell into five
groups as follows:
1.
Glues E and D were not broken by 10,000,000 cycles.
2. Glues A and F withstood means of 2,273,500+ to 3,152,400+
cycles before failure, with the means calculated on the assumption
that unfailed specimens broke at 10,000,000 cycles.
3. Glue B withstood a mean of 628,400 cycles before failure.
All specimens failed.
4. Glue H withstood a mean of 75,100 cycles before failure.
All specimens failed.
5. Glue G withstood a mean of 20,200 cycles before failure.
All specimens failed.
The differences between the groups is highly significant, but there is
no statistical significance between the means for the glues within the
groups.
These results indicate considerable differences between types of adhesives in their ability to withstand repeated stressing. The resorcinol,
melamine, and alkaline phenol resins were superior to the urea resins or
to the casein.
The failure of the urea- and casein-glued joints in the fatigue tests
was largely in the glue, whereas in most others, the failure was largely
in the wood. The wood adjacent to the acid phenol glue, B, was clearly
Report No. 1539
-4-
injured by the acid of the glue and failed with shallow wood failure.
There was no indication that the wood was injured by the less concentrated acid of the urea and melamine glues, F and G, or by the strong
alkali of glues C and H.
With both
stressing
true also
Likewise,
glue. The
the thick
resorcinol glues, the thick glue lines withstood repeated
better than did the thin glue lines. This appeared to be
for the casein glue but the difference was not important.
the difference may not be significant for the acid phenol
thin glue lines with urea glue were definitely superior to
glue lines.
Table 3 gives the results of shear tests on both fatigue-type specimens and the birch plywood lap-joint specimens. In a comparison of the
results of the fatigue and shear tests, disregarding the data of the
acid-damaged specimens of glue B, there is some indication that the
lower the wood failure in the sheared specimens, the lower the number
of repeated stress cycles to cause failure. The evidence, however, is
not definite but there appears to be no relation between the shear
strength values and the number of repeated stress cycles to cause failure. The conclusion is that a shear test of a glued joint is not a
satisfactory criterion as to the behavior of the joint in fatigue
stressing.
Report No. 1539
-5-
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Table 3:--Averaged results of shear teats on glued srciméns
Glue : Glue-line :
Fatigue-type specimens
symbol : thickness : : Shear strength : Wood failure
:
A
B
.:
P.s.i.
: thin
: thick
:
:
421
thin
thick
:
:
577
:
701
:
699
: Lap joints of three-ply
: 3/32-inch birch plywood
:
:Shear strength:Wood failure
Percent
:
P.s.i.
:
Percent
716
:
92
68
:
36
:
97
44
:
:
590
:
.
100
:
C
: thin
95
639
:
96
D
:
thin.94
: thick
612
:
94
680
:
88
E
:
F
G
H
thin
thick
:
;
751
:
87
47
685
:
92
:
:
:
79
647
:
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:
:
:
91
6o
685
413
672
340
:
:
78
1
617
95
: thick
: thin
: thick
689
424
:
:
19
3
748
62
: thin
: thick
:
thin
i f-port No. 1539
758
538
Figure 1.--Fatigue test specimens with thin and thick glue lines.
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