Mixed mode I, II, and III fracture characterization of adhesive joints

This paper examines mixed-Mode I-II-III fracture of adhesive joints using a new experimental method. The expressions used for the average energy release rate in Mode I, Mode II, and Mode III along the idealized straight crack front of a cracked, generalized adhesive sandwich are based on beam theory, the J-integral and a beam-on-elastic foundation model. Fracture experiments were performed with adhesively bonded, obliquely-loaded, equal adherend, split-cantilever-beam (SCB) specimens of different widths and loading angles, using two different adhesives. It was seen that the fracture surface had an arrowhead shape for small loading angles (similar to that of SCB-type specimens), and a thumbnail shape for larger loading angles (similar to that of double-cantilever-beam (DCB)-type specimens). Moreover, a sharp step was observed in the thickness of the residual adhesive half-way across the specimen width. When no Mode I was present, the effect of a Mode II loading component on the critical energy release rate G(C) was different from the effect of Mode III for one epoxy, but was the same for the other. The addition of Mode I caused the effect on G(C) of Mode II to be similar to that of Mode III for both adhesives.