Morphological instabilities of dynamic fractures in brittle solids

We present a study oi the stress fields ill 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 V-c and reappears again beyond a faster speed V-B, but at a different orientation from that of pure opening made. These variations have been interpreted as the onset of roughening and branching instabilities.