it depends on the construction of the carbon. some are made to be extremely stiff and shatter on impact to dissapate energy. others, including the AC-12, are designed to be very strong but still extremely flexible. the most it will do is crack, just like any other composite based helmet shell will (arai, suomy, shoei, hjc, kbc, vemar, etc etc etc). all modern helmets have at least a little carbon fiber in them.