The dynamics of rapidly moving tensile cracks in brittle amorphous material

The dynamics of fast fracture in brittle amorphous materials are reviewed. We first present a picture of fracture in which numerous effects commonly observed in dynamic fracture may be understood as resulting from an intrinsic (micro-branching) instability of a rapidly moving crack. The instability, when a single crack state undergoes frustrated microscopic crack branching, occurs at a critical propagation velocity. This micro-branching instability gives rise to large velocity oscillations, the formation of non-trivial fracture surface structure, a large increase in the overall fracture surface area, and a corresponding sharp increase of the fracture energy with the mean crack velocity. We present experimental evidence, obtained in a variety of different materials, in support of this picture. The dynamics of crack-front interactions with localized material inhomogeneities are then described. We demonstrate that the loss of translational invariance resulting from this interaction gives rise to both localized waves that propagate along the crack front and the acquisition of an effective inertia by the crack. Crack-front inertia, when coupled with the micro-branching instability, leads to an understanding of the chain-like form of the micro-branch induced patterns observed both on and beneath the fracture surface.