Transition of crack path at bi-material interfaces

•Interface incident crack propagation transitions between penetration and deflection.•Near transition, competing process zones elevate the applied load for propagation.•Far from transition, single process zones lower applied load to LEFM predictions.•An LEFM expression is derived to bound transition conditions.•A material strength ratio which guarantees penetration does not appear to exist.

The manner in which a crack incident on an interface transitions between penetration across the interface and deflection along it has not been widely studied. This paper uses a stress-and-energy-based (cohesive-zone) method to show that at and near transition, competing process zones exist causing both applied load for propagation and transition toughness ratios to differ from energy-based-method predictions. This finding is supported through comparison to an LEFM solution for propagation of branching cracks. However, far from transition only a single significant process zone exists and the two methods are in agreement. Results are also provided for cases of modulus mismatch.