A Crack Driving Force Criterion for the Prediction of Interface Crack Kinking in Thin-Film Composites

Interfacial fracture mechanics is a relatively new field with many issues that have not yet been resolved. One such issue is the ability to accurately predict whether or not a bimaterial interface crack will propagate along the interface, kink into the film or substrate, or not propagate at all. In the present work, a crack driving force criterion is proposed in order to predict the level of applied load required to propagate a pre-existing interface crack in thin-film composites subjected to thermal loading. A primary objective is to predict the critical length of an interface crack at which it kinks into the substrate. The phenomenon of interface cracks kinking into the substrate is frequently observed when the film is under tensile loading and the substrate is brittle. An interface crack that advances into the substrate eventually propagates parallel to the interface at a certain steady-state depth. Ultimately, the portion of the structure that remains above the crack may spall, resulting in catastrophic failure. The crack driving force criterion is applied to a two-dimensional, plane strain interface crack problem, and the results compare favorably with available experimental results.