Page 129 - Handbook of Adhesives and Sealants

98 Chapter Three

Units of peel or cleavage strength are usually expressed as pounds
per inch of bond width (piw). SI units for peel are kg/m where 1 piw
is equivalent to 17.858 kg/m.
Peel or cleavage stress should be avoided where possible, since the
stress is confined to a very thin line at the edge of the bond (Fig. 3.3
right). Brittle adhesives are particularly weak in peel because the
stress is localized at only a very thin line at the edge of the bond as
shown in Fig. 3.4. The stiffness of the adhesive does not allow distri-
bution of stress over an area much larger than the thickness of the
bond-line. On the other hand tough, flexible adhesives distribute the
peeling stress over a wider bond area and show greater resistance to
peeling forces.
Common rigid epoxy adhesives can generally provide greater than
2000 psi shear strength, but they may only provide on the order of 2
piw of peel strength. Tough, flexible adhesives that are specifically
formulated for high peel resistance could provide peel strength values
in the range of 25–50 piw.

3.3 Maximizing Joint Efficiency
For maximum joint efficiency, non-uniform stress distribution should
be reduced though proper joint design and selection of certain design
variables that are of importance to stress distribution. The number of
variables affecting the stress distribution, even in the most common
joint designs, is large. The following variables are most important.

Figure 3.4 Tough, flexible adhesives distribute peel stress over a larger area. 3