Reconsideration of crack deflection at planar interfaces in layered systems

Deflection of a crack at a planar bi-material interface in a layered system was investigated using a finite element method from the standpoint of the effects of length scales of two competing putative crack extensions: crack penetration vs. crack deflection. It was predicted that when these length scales are comparable to or larger than the size of K-dominant field and the intact layer is stiffer than the cracked layer, crack deflection would occur under energetically less favourable conditions than those derived from K-dominant field solutions. To the contrary, the crack deflection criteria were predicted to be insensitive to the length scale of the putative penetration and deflection if the cracked layer is stiffer than the intact layer. It was further predicted that when the deflection length is shorter than the penetration length, crack deflection could occur under energetically more favourable condition when the cracked layer is stiffer than the intact layer, and vice versa. Predictions herein better explained the experimental observations on bi-layer systems elsewhere than the existing theories.