Dear Sara,
todays helmets generally do not crack. The "styrofoam" core usually compresses a bit. If you notice any deformation on the "styrofoam", please replace your helmet. My serious crashes resulted in "styrofoam" compressing about an 1/8 inch or so but it didn't crack. The plastic shell are generally made out of some sort of film like mylar or some kind of polycarbonate and they tend to flex rather than crack. They do get scratched but they don't rip nor do they crack.
If you hit your head pretty hard, I would highly recommend that you replace your helmet.
be safe,
smilingcat




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