Page 128 - Handbook of Adhesives and Sealants

Joint Design 97

Figure 3.3 Stress distribution on an adhesive when stressed in (left) shear and (right)
peel or cleavage. 2

provide the same bond strength as when the entire overlap area is
bonded with adhesive. The reason for this is provided in Section 3.3.4.

3.2.3 Cleavage and peel stress
Cleavage and peel stresses are undesirable for adhesives and sealants.
Cleavage is defined as the stress occurring when forces at one end of
a rigid bonded assembly act to pry the adherends apart. Peel stress is
similar to cleavage, but it applies to a joint where one or both of the
adherends are flexible. Thus, the angle of separation (or the angle
made by the separating substrates) can be much greater for peel than
for cleavage.
Joints loaded in peel or cleavage offer lower strength than joints
loaded in shear because the stress is concentrated at only a very small
area of the total bond. The stress distribution of an adhesive in cleav-
age is shown in Fig. 3.3. All of the stress is localized at the end of the
bond that is bearing the load. The adhesive at the other end of the
bond is providing little to the ultimate strength of the joint.
Cleavage and peel forces are measured as force per linear length of
bond. Consider an application where a 1 in. wide strip of pressure
sensitive adhesive tape is placed on an aluminum substrate and then
peeled off the substrate to measure adhesive strength. The peel
strength would be measured as pounds of force required to strip the
tape off the substrate divided by the width of the bond being peeled,
i.e., 1 in.