Some experiments suggest the presence of a sharp interface between comminuted and uncomminuted regimes in a ceramic subject to impact by a penetrator. This in turn suggests that one might model the associated dynamical process with the help of recently developed continuum models of the macroscopic response of solids undergoing phase transitions. A highly idealized phase transition model of such a com- minuted process is analyzed here. The model accounts for the kinetics of the phase transition. If the shear wave speed in the comminuted material is small compared to that in the uncomminuted portion, it is found that the energy reaching the uncomminuted portion of the target is greatly reduced in comparison to its value in the absence of the phase transition.