Types of Stress

(Courtesy 3M)

Adhesive strength can be readily matched to the substrate and stresses (figure 1) to which the bond will be subjected. Most adhesives perform better when the primary stress is tensile or shear.

Figure 1 - Stresses on a Joint
Fig 7 July 

In most industrial applications, however, a combination of stresses are involved that may include cleavage and peel. Tensile is pull exerted equally over the entire joint. Pull direction is normal to the adhesive bond. Shear is pull directed across the adhesive, parallel to the adhesive bond, forcing the substrates to slide over each other. Cleavage is pull concentrated at one edge of the joint, exerting a prying force on the bond. The other edge of the joint is theoretically under zero stress. Peel is concentrated along a thin line at the edge of the bond where one substrate is flexible. The line is the exact point where an adhesive would separate if the flexible surface were peeled away from its mating surface. Once peeling has begun, the stress line stays out in front of the advancing bond separation.

The leverage effect of cleavage and peel forces concentrate stress at smaller areas of the bond causing failure at lower force levels than those observed in tension and shear. Flexural yield of the substrate (figure 2) can lead to cleavage and peel stresses in the bond.

Figure 2 - Substrate Flexural Yield Can Lead to Cleavage