Page 179 - Handbook of Adhesives and Sealants

Standard Test Methods 149

Generally, high modulus adhesives possess the highest tensile
strengths. There appears to be an inverse relationship between thick-
ness and tensile bond strength for particular adhesive-adherend com-
binations. Thin bonds are best as long as the bond-line is not starved
of adhesive. Thicker bonds give lower strengths because cleavage
forces more readily occur due to non-axial loading, or internal stresses
in the adhesive.

4.4.2 Lap-shear tests
The lap-shear or tensile-shear test measures the strength of the ad-
hesive in shear. It is the most common adhesive test because the spec-
imens are inexpensive, easy to fabricate, and simple to test. However,
at times it is difficult to minimize or eliminate bending stresses in
common shear joint specimens. Because the standard lap shear tests
introduce some degree of peel into the adhesive joint, values obtained
for the lap shear strength of epoxy adhesives may average 4000–5000
psi; whereas, values for bulk tensile strengths have been reported up
to 12000 psi. 5
The common lap-shear test method is described in ASTM D 1002,
and the standard test specimen is shown in Fig. 4.4. This is the most
commonly used shear test for structural adhesives. However, due to
the nonuniform stress distribution in the adhesive arising from the
joint configuration, the failure strength values are of little use for en-
gineering design purposes.
Testing is carried out by pulling the two ends of the overlap in ten-
sion causing the adhesive to be stressed in shear. Hence, this test is
frequently called the tensile-shear test. Since the test calls for a sam-
ple population of five, specimens can be made and cut from larger test
panels illustrated in Fig. 4.4b.
The width of the lap shear specimen is generally one inch. The rec-
ommended length of overlap, for metal substrates of 0.064 in. thick-
ness, is 0.5 0.05 in. The length should be chosen so that the yield
point of the substrate is not exceeded. In lap shear specimens, an
optimum adhesive thickness exists. For maximum bond strengths, the
optimum thickness varies with adhesives of different moduli (from
about 2 mils for high modulus adhesives to about 6 mils for low mod-
ulus adhesives). 4
The lap shear specimen can be used for determining shear strength
of dissimilar materials in a manner similar to that which was de-
scribed for the laminated button tension specimen. Thin or relatively
weak materials such as plastics, rubber, or fabrics are sandwiched
between stronger adherends and tested.
Two variations are used to avoid the bending forces that occur with
simple ASTM 1002 specimens: the laminated lap shear specimen