Crack penetration and deflection at a bimaterial interface in a four-point bend test

The four-point bend (4PB) test is an industry standard in assessment of adhesion in thin films. Obtaining crack propagation along the desired interface plays a cardinal role in the success of this test. Crack penetration and deflection at a bimaterial interface has been extensively studied in the past. Most general results are based on asymptotic analyses involving the interface between two semi-infinite media. The results of these analyses cannot be directly used in case of the 4PB test since the boundary conditions and finite size effects in actual test specimen geometry are not accounted for. The purpose of this paper is twofold: first to develop a simple methodology to understand the preferred failure mode in a 4PB test and gain insight into the factors governing its success; second to examine the extent to which the results can be reconciled with those reported previously based on analysis of a crack in two semi-infinite media bonded together at the interface.