Failure fronts in brittle materials and their morphological instabilities

There are various observations and experiments showing that in addition to standard shock-wave fronts, which propagate with trans-sonic velocities, other much slower wave-fronts can propagate within glass or ceramic substances undergoing intensive damage. These moving fronts propagate into intact substance leaving intensively damaged substance behind them. They have been called failure fronts or waves. Failure fronts can be modeled either as sharp interfaces separating two states - the intact and comminuted states - or, alternatively, as continuous traveling waves with large spatial gradients of a damage parameter. Our approach is motivated by the analogy between failure fronts and fronts of slow combustion. In this paper we present two main theoretical results that require experimental verification. One of them concerns the speed of a failure wave generated by oblique impact of a brittle target. The other establishes a criterion for morphological instability of failure fronts.