Morphological instabilities of dynamic fractures in brittle solids

We present a study of the stress fields in the neighborhood of a moving crack tip in the framework of linear elastic fracture mechanics. This approach is found to be physically relevant for a large range of the crack speeds. We show that the stability analyses based on conditions of attainment of a critical tensile stress on some plane are inadequate to describe the instabilities of the crack path. A study of the largest principal stress in the neighborhood of the crack surface is reported. We show that at ``low'' crack velocities the path of the crack extension is an opening mode. However, this property disappears when the crack speed exceeds a critical velocity Vc and reappears again beyond a faster speed VB, but at a different orientation from that of pure opening mode. These variations have been interpreted as the onset of roughening and branching instabilities. {} 1996 The American Physical Society.