Page 176 - Handbook of Adhesives and Sealants

146 Chapter Four

cate all of the pertinent conditions that are required to assure repro-
ducibility in subsequent testing. It is also very useful to describe the
failure mode of the tested specimens. An analysis of the type (or mode)
of failure is a valuable tool to determine the cause of adhesive or seal-
ant failure. The failed joint should be visually examined to determine
where and to what extent failure occurred. The percent of the failure
in the adhesion and cohesion mode should be provided. A description
of the failure mode itself (location, percent coverage, uniformity, etc.)
is also useful. The purpose of this exercise is to help identify the weak
link in the joint to better understand the mechanism of failure.
Numerous standard test methods have been developed by various
government, industrial, and university investigators. Many of these
have been prepared or adopted under the auspices of the ASTM Com-
mittee D 14 on Adhesives, D 24 on Sealants, or other professional
societies. Reference to the appropriate standards will adequately
equip one with the background necessary to conduct the test or a ver-
sion of it. Several of the more common standard tests will be described
in this section. Numerous variations exist for specific applications or
materials. In these descriptions, the emphasis will be on understand-
ing the reasons for the test, its relationship to a specific adhesive prop-
erty, advantages and limitations of the test, and possible variations or
extrapolations of the test method. The detailed description of the test
mechanics will be kept to a minimum, since they are adequately cov-
ered in the existing standards and specifications.

4.4.1 Tensile tests
The tensile strength of an adhesive joint is seldom reported in the
adhesive supplier’s literature because pure tensile stress is not often
encountered in actual production. An exception to this is the tensile
test of the bonds between the skin and core of a honeycomb or com-
posite sandwich. However, the tensile test is not only useful as a qual-
ity control test for metal and sandwich adhesives; it can also be em-
ployed to yield fundamental and uncomplicated tensile strain,
modulus, and strength data for the adhesive.
The ASTM D 897 tensile button test is widely used to measure ten-
sile strength of a butt joint made with cylindrical specimens (Fig. 4.2).
The tensile strength of this bond is defined as the maximum tensile
load per unit area required to break the bond (psi). The cross-sectional
bond area is specified to be equal to one square inch. The specimen is
loaded by means of two grips that are designed to keep the loads ax-
ially in-line. The tensile test specimen requires considerable machin-
ing to ensure parallel surfaces.
A similar specimen design uses a sandwich construction with a dis-
similar material bonded between the two cylindrical halves of the but-