Failure initiation in an epoxy joint between two steel plates

An Irwin-like criterion, initially developed to predict crack initiation in monolithic v-notched specimens, is used to examine the failure of two steel plates bonded together by an epoxy joint. It is based on an estimate of the critical value of the intensity factor of the elastic singular fields appearing in some regions of the structure, due to the geometry and to the elastic mismatch of the components. The analysis is worked out at two scales: a macroscopic one ignoring the thin epoxy layer and considering the two plates as perfectly bonded along an idealized interface and a microscopic one taking into account the elastic fields within the joint. It is shown that these two approaches are equivalent and that a macroscopic failure criterion can be developed. The characteristic failure parameter is a macroscopic critical intensity factor. It can be derived from the knowledge of microscopic data such as the strength and the toughness of the epoxy.