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.