Page 125 - Handbook of Adhesives and Sealants

94 Chapter Three

bution analysis will lead to methods of maximizing the efficiency of
the joint.

3.2 Types of Stress

A uniform stress pattern in an adhesive or sealant joint is seldom
produced by application of an external force. Rather, non-uniform
stress distributions are the norm. Since fracture initiates when and
where local stress exceeds local strength, stress concentrations have
a large influence on the breaking strength of a joint. Residual internal
stresses and their tendency to form stress concentration regions were
discussed in the previous chapter. This chapter considers the appli-
cation of external loads on the joint.
External loads produce local stresses that may be many times the
average stress. These stress concentrations are often unexpected, and
they may determine the actual force that the joint can sustain. It is
the responsibility of the joint designer to compensate or to minimize
these effects, but first they must be understood.
Four basic types of loading stress are common to adhesive or sealant
joints: tensile, shear, cleavage, and peel. Any combination or variation
of these stresses, illustrated in Fig. 3.1, may be encountered in an
application.

3.2.1 Tensile and compressive stress
Tensile stress develops when forces acting perpendicular to the plane
of the joint are distributed uniformly over the entire bonded area. In
tension, the adhesive develops high stress regions at the outer edge
(Fig. 3.2), and those edges then support a disproportionate amount of
the load. The first small crack that occurs at the weakest area of one
of the highly stressed edges will propagate swiftly and lead to failure
of the joint. However, if the joint is properly designed, it will show
good resistance to tensile loading because the loading is more easily
distributed.
Proper design requires that the joint has parallel substrate surfaces
and axial loads. Unfortunately in practical applications, bond thick-

Figure 3.1 The four basic types of stress common to adhesives and
sealants.