Fracture initiation at three-dimensional bimaterial interface corners

Abstract

We propose an approach to correlate fracture initiation at three-dimensional bimaterial interface corners based on the existence of a universal singular stress field in the context of linear elasticity. The idea is to correlate fracture initiation with critical values of the stress intensities of the singular fields calculated from a linear elastic stress analysis. The approach is in the spirit of interface fracture mechanics but applicable to a different class of problems, specifically, situations without a preexisting crack and situations where subsequent crack propagation does not necessarily occur along the interface. In order to validate the proposed approach, we designed and fabricated a series of aluminum/epoxy bimaterial specimens that possess well-defined three-dimensional interface corners. They consist of two perfectly bonded square prisms with varying interface dimensions and surface finishes. The specimens were loaded in four-point flexure until fracture initiated at the three-dimensional interface corner. The nominal stress at failure varied significantly with interface dimensions, thus invalidating its use as a fracture criterion. From a rigorous asymptotic analysis of the three-dimensional interface corner stress state, we determined the order of the singularity and the angular variation of the stress and displacement fields. We determined the corresponding stress intensities via full-field finite element analyses of the aluminum/epoxy specimens. Although the measured failure stress varied significantly with interface dimensions, the corresponding critical stress intensity did not, although as expected it varied with interface surface finish. These findings support the use of critical stress intensities to correlate fracture initiation at three-dimensional bimaterial interface corners.