Loss of constraint on fracture in thin film structures due to creep Original Research Article

Fracture in thin films is normally constrained by the substrates. If the substrate creeps, however, the constraint will be lost over time. This paper presents a two-dimensional model for channel cracks in an elastic film on a viscous layer, and implements an extended finite element method to evolve the displacement field and the stress intensity factor with relatively coarse meshes. Solutions are obtained for stress intensity factors of channel cracks with several in-plane geometries. The stress intensity factor increases with time, indicating the loss of constraint. Several scaling laws are obtained. Extensions of the present model are outlined for nonlinear creeping and viscoelastic layers, as well as a thick viscous substrate. Fracture in thin film structures subjected to ratcheting deformation under cyclic temperatures using the analogy between creeping and ratcheting is also discussed.