Three-dimensional effects in thin film fracture mechanics

This paper presents the results of a three-dimensional finite element analysis of the mechanics of crack growth and decohesion in a highly compliant thin film bonded to a rigid substrate. Essential features of the model are a surface-breaking initial crack which has penetrated to the film-substrate interface, and a remote biaxial residual stress field in the film. Computational analyses show that, in the absence of decohesion, the stress intensity factor along the leading edge of the crack reaches a steady state value when the crack is about twice the film thickness. This steady state condition gives rise to a nearly parabolic crack front along the leading edge. The computed results also confirm the existence of high interface stresses that are the driving force for film decohesion. When decohesion is permitted, the stress intensity factor is observed to increase with debond opening angle. The equilibrium state between crack propagation and decohesion may be determined from the respective stress intensity factors and fracture toughness of the film and the interface.