Page 72 - Handbook of Adhesives and Sealants

40 Chapter One

failure occurs when the failure is such that a layer of adhesive or
sealant remains on the adherend. When the adherend fails before the
adhesive, it is known as a cohesive failure of the adherend. The var-
ious modes of possible bond failures were shown in Fig. 1.1.
Cohesive failure within the adhesive or one of the adherends is
sometime considered a preferred type of failure, because the maximum
strength of the material in the joint has been reached. However, fail-
ure mode should not be used as a sole criterion for a useful joint. A
cohesive type of failure does not necessarily insure a practical or ec-
onomic assembly or one that will even survive the expected life. The
function of the joint is a more important criterion than the mode of
joint failure. However, an analysis of failure mode can be an extremely
useful guide to determine if the failure was due to a weak boundary
layer or improper surface preparation (see Table 1.11).
An inference to the nature of the failure can be made by examining
the failure mode. For example, if the adhesive failure is interfacial,
the bulk cohesive strength of the adhesive material can be assumed
to be greater than the intermolecular strength of adhesion. If the over-
all joint strength is not sufficient, the user needs to address the ‘‘weak-
est link’’—probably the surface condition of the substrate. Improving
cohesive strength of the adhesive or sealant would not add to the qual-
ity of the joint in this case.
If on the other hand, the failure exhibits a cohesive mode (either
the adhesive or the adherend), then the bond strength is stronger than
the forces holding the bulk together. To improve the characteristics of
this joint, one must look at the bulk adhesive or sealant material. In
practice, however, one usually cannot improve the adhesive and keep
the interphase area untouched. These interrelated effects make it ap-
pear as if the science of adhesion is primarily one of trial and error.
The exact cause of a premature adhesive failure is very hard to
determine, because so many factors in adhesive bonding are interre-
lated. However, there are certain common factors at work when an
adhesive bond is made that contribute to the weakening of all bonds.
The influences of these factors are qualitatively summarized in Fig.
1.11. This is a very useful illustration showing why one can never

TABLE 1.11 Failure Mode as an Inference to Bond Quality
Failure mode Inference
Adhesive failure (interfacial) Cohesive strength interfacial strength
Cohesive failure (bulk) Interfacial strength cohesive strength
Adhesives/cohesive (mixed failure mode) Interfacial strength cohesive strength