Growth from initial to self-similar shape of an interface crack front

Initiation and growth towards a self-similar/final shape of an interface crack is analysed. Asymmetric double cantilever beam experiments are performed under prescribed displacement loading conditions. The onset of crack growth from an initially straight front is followed using digital microscope. It is observed that at first the crack grows from the longitudinal axis of the specimen symmetry. Subsequently, the crack spreads in both the longitudinal and the transverse directions finally forming a curvilinear front. From this stage, the crack grows in a self-similar shape and propagates along the interface. Such crack formation may result in a nonlinear force vs. displacement relationship which is otherwise related to constituent or interface constitutive nonlinearities. To study the consequences of the initial crack growth process a cohesive zone model is established. The numerical analysis starts with the assumption of a straight crack front configuration and continues up to the self-similar stage of propagation. A maximum cohesive zone stress is used as criterion to determine the position and the shape of the process zone front while critical crack opening is used to follow the crack front shape. It is concluded that the non-linear part of the force vs. displacement curve observed during loading can be at least partially interpreted as a crack front reformation from the straight to a steady-state quasi-parabolic shape.