TABLE OF CONTENTS Joints for adhesive bonding should be designed particularly for the use of adhesives. The habit of beginning with a design used for another method of fastening and modifying it slightly for adhesive bonding is a poor one, often leading to disastrous results. The aim of joint design is to obtain maximum strength for a given area of a bond. In designing joints specifically for adhesive bonding, the basic characteristics of adhesives must dictate the design of joints. Adhesive bonds act over areas and not a single point. For this reason, the joint should be designed with the objective of minimizing concentration of stress.
The selection of joint design is influenced by limitations in production facilities, production costs, and the desired final appearance of the part. The strength of an adhesive joint is determined primarily by (1) the mechanical properties of the adherend and the adhesive, (2) the residual internal stresses, (3) the degree of true interfacial contact, and (4) the joint geometry. Each of these factors has a strong influence on joint performance. The design engineer must be concerned with the elimination of stress concentrations, which reduce the strength and useful life of the joint. Localized stresses are not always apparent and may occur as a result of differential thermal expansion of the adhesive and adherends. Another cause is shrinkage of adhesive during cure, when volatiles are given off. These volatiles may become entrapped. Internal stresses decrease as adhesive thickness decreases, reducing the tendency to trap volatiles.
