Page 134 - Handbook of Adhesives and Sealants

Joint Design 103

cept this displacement differential, the flexibility of the adhesive is,
involved.
The stiffness of the adherend is characterized by the product of the
Young’s modulus, E, and the adherend thickness, t. Then Et of each
adherend becomes an important factor in the shear stress distribution.
As the product Et becomes large, the shear stress distribution becomes
more uniform.
In a shear joint made from thin, relatively flexible adherends, there
is a tendency for the bonded area to distort. This distortion, illustrated
in Fig. 3.7, causes cleavage stress on the ends of the joint, and the
joint strength may be considerably impaired. Thicker adherends are
more rigid, and the distortion is not as much a problem as with
thin-gauge adherends. Figure 3.8 shows the general interrelationship
between failure load, depth of overlap, and adherend thickness for a
specific metallic adhesive joint. As the adherend thickness (i.e., the
relationship Et) increases, the failure load increases for identical over-
lap lengths. For constant adherend thicknesses or constant Et, the
failure load increases with increasing overlap length up to a certain
point. Beyond that overlap distance the failure load remains constant.
In this region the entire load is supported by the edge region of the
overlap. The central section of adhesive is not contributing to the
strength of the joint.

Figure 3.7 Distortion caused by loading can introduce cleavage stresses and must be
considered in the joint design. 6