The thickness and strength of the adherend are equally important, especially in situations where the elastic modulus of the adhesive is related to the joint design. Flexible materials such as rubber, thin metal sheets, and plastic films are prone to bending during use, and hard and brittle adhesives should not be used for bonding. This is because such adhesives may crack, leading to a decrease in bonding strength. Differences in the flexibility or thermal expansivity of the adherends can generate internal stresses within the adhesive layer. Such stresses may cause premature failure of the joint before it bears any external load.
This kind of joint is particularly dangerous when used at sub-zero temperatures. To a certain extent, stresses can be minimized through joint design, but stresses still exist in the cured adhesive layer. The smallest stresses occur between identical materials. By adjusting the composition of the adhesive and selecting appropriate adhesives, it is expected to obtain an adhesive whose rheology, thermal expansivity, and chemical stability are similar to those of the adherend materials. When one of the joint materials swells in a chemical medium (gas or liquid), stresses will be induced near the interface between the adhesive and the adherend.