Good info in this thread, but for the record, ABS is what they make football shoulderpads, soccer shin guards, etc out of. If you can step on it and break it, you're either going too thin with the ABS, or you're waaay overweight

It's not urethane, although that would certainly be within reach of those manufacturers to use instead.
It flexes, but actually breaking it though pressure is reserved for the more brittle non-flex plastics, though it's possible to overstretch thin ABS and run into problems if you step on it or stomp on it.
I've booted one of our helmets as hard as I could a few weeks ago just to see what would happen. Kicked right into the ABS visor, which is the weakest part, and nothing happened. Of course on an assembly like that, you'd probably tear out the rivets if you stood on it and essentially stomped on it.
Course if this is an armor costume to be worn, you shouldn't be getting into those kinds of situations with it anyways.
Maybe I'm wrong here, but I was under the impression that urethane was the expensive way to go.