FRACTURE LOAD PREDICTIONS FOR ADHESIVE JOINTS UNDER MIXED-MODE I-II-III LOADING

Application of a method for predicting the fracture strength of adhesi ve joints under mixed mode I-II-III conditions is presented. The expre ssions used for the average energy release rate in Mode I, Mode II and Mode III along the idealized straight crack front of a cracked, gener alized adhesive sandwich are based on a beam-on-elastic foundation mod el. Fracture experiments were performed with obliquely loaded unequal- adherend split-cantilever-beam (SCB) specimens of different widths and loading angles, using two different epoxy adhesives. The fracture str engths agreed reasonably well with predictions based on the previously measured fracture envelopes (G(c) vs. mode ratio) obtained for these epoxies using equal-adherend obliquely loaded SCB specimens. It was se en 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 thick ness of the residual adhesive half-way across the specimen width. As i n the case of equal-adherend specimens, the addition of Mode I greatly reduced G(c), and caused the effect on G(c) of Mode II to be similar to that of Mode III.