Benchmark tests of the strength of adhesive joints were performed by twelve members (institutes), tocusing on the difference in strength between several different types of joints. A series of experiments on four types of joints on (1) butt joints, (2) single-lap joints, (3) double-lap joints and (4) double cantilever beams (DCB). was carried out using two types of epoxy adhesives (adhesive A, ductile ; adhesive B, brittle) and steel adherends. The adhesive joint strengths are strongly affected by the mechanical properties of adhesives and adherends. The adhesive strengths of the butt and double lap joints of adhesive B are found to be higher than those of adhesive A, whereas the single-lap joint and DCB of adhesive A are stronger that those of adhesive B. The difference in adhesive strength between these joints can be explained by considering two different types of fracture criteria : (1) the fracture criterion by crack initiation and (2) the fracture criterlon by crack propagation. The results obtained suggest that we should make an appropriate choice of adhesive test to evaluate the strength of actual adhesive joints.
