The wave distortion tool in conjunction with a little noise distortion might work nicely, but they'll both stretch the surface. If you were crushing the hood of a car, for instance, and the hood is 6000 square inches of metal, the crushed version might accordion into 20000 square inches, with a visibly incorrect stretching action in the animation. Then again, if the animation is quick and the damage is severe, it might look great anyway. By the way, the Anything Goos plug-in from Digital Carvers Guild might help to create a distressed metal look. I haven't used it, but for messed-up metal it looks great. http://www.digitalcarversguild.com