Two of the major factors influencing the design of lap joints are the magnitude and direction of the load that the joint must bear. Most adhesives used for bonding flat surfaces are relatively rigid, strong in shear, and not so strong in peel or cleavage. Thus by designing the joint so that the adhesive is in shear, the effect of peel or cleavage stress is minimized.
Lap shear joints can be affected by shear concentration when the adherends yield. A common lap shear joint “A” in Figure 1-2 tends to deflect (yield) under stress and aligns itself to a shape resembling “B”. Thus, instead of a simple shear stress, the tension effect at edges 1 and 2 creates a peel stress because a high proportion of the load is carried at the edges of the lap. Figure 1-3 illustrates several joint designs. Some show how the problem of substrate yield can be minimized, and others show the strengths and weaknesses in various bonded joints.