I am working on a ballistics model, and have been trying to find some material on impact forces. Does anyone have any reference material for creating an approximation of the brittle behaviour of materials over short time scales? At sufficient impact velocity a material will appear to be more brittle than it is, even ductile materials. There is no time for energy to be dissipated into the target material, and most of the kinetic energy of the bullet goes into fracturing the material. Is there any simple way of approximating this behaviour? Also, does crack formation and propagation have any effect on penetration? I.e. does a crack propagating ahead of a bullet along its axis weaken the material, and would it have any noticable effect relative to yield/drag forces?