Helmets are absolutely not meant to crumple. It's not only blunt impact a helmet protects you from. Its also the penetration of your skull by pointy hard objects.
"Helmets are made with an inner EPS (expanded polystyrene foam) shell and an outer shell to protect the EPS. The density and the thickness of the EPS is designed to cushion or crush on impact to help prevent head injuries. Some manufacturers even offer different densities to offer better protection. The outer shell can be made of plastics or fiber materials. Some of the plastics offer very good protection from penetration as in Lexan (bullet-resistant glass) but will not crush on impact, so the outer shell will look undamaged but the inner EPS will be crushed."
