Page 102 - Handbook of Adhesives and Sealants

70 Chapter Two

2. Stress due to differences in thermal expansion coefficients of the
adhesive and adherend (mainly associated with adhesives that cure
at temperatures different than their normal service temperatures)
3. Stress due to shrinkage of the adhesive or sealant as it cures

2.4.2.1 Localized stress. Losses of theoretical adhesive strength arise
from the action of internal stress concentrations caused by the trapped
gas and voids. Griffith 16 showed that adhesive joints may fail at rel-
atively low stress if cracks, air bubbles, voids, inclusions, or other sur-
face defects are present. If the gas pockets or voids in the surface
depressions of the adherend are all nearly in the same plane and not
far apart (as is shown in Fig. 2.10 upper), cracks can rapidly propagate
from one void to the next. However, a variable degree of roughness,
such as shown in Fig. 2.10 lower, provides barriers to spontaneous
crack propagation. Therefore, not only is surface roughening impor-
tant, but the degree and type of roughness may be important as well.
It has also been shown 17 that a concentration of stress can occur at
the point on the free meniscus surface of the adhesive (edge of the
bond-line). This stress concentration increases in value as the contact
angle, , increases. At the same time, the region in which the maxi-
mum stress concentration occurs will move toward the adhesive-
adherend interface. The stress concentration factors for a lap joint in
shear with a contact angle ranging from 0 to 90 degrees are summa-
rized in Fig. 2.11. For contact angles less than about 30 degrees (i.e.,
for good wetting) the maximum stress occurs in the free surface of the
adhesive away from the edges, and the stress concentration is not
much greater than unity. For larger contact angles, the maximum
stress occurs at the edges A, A (i.e., at the actual interface between
adhesive and adherend). The stress concentration factor increases
until, for a value of 90 degrees (non-wetting), it is greater than
2.5. Thus, poor wetting will be associated with a weak-spot at the

Smooth Adherend

Adhesive
Gas Bubbles

Rough Adherend
Figure 2.10 Effect of surface roughness on coplanarity of gas bubbles: upper ad-
herend is smooth and gas bubbles are in the same plane, lower adherend has
roughness and gas bubbles are in several planes. 13