Well, it's not supposed to crack. The liner is supposed to crush. If a helmet cracks, it cannot absorb the energy it was designed to do.

from

http://www.magma.ca/~ocbc/hfaq.html#A4

The helmet's liner is made of foam sufficiently stiff that the head inevitably will absorb some of the impact. The stiffer the liner, the more shock the head will absorb. Theoretically, the liner is supposed to limit the deceleration for a typical fall on to a flat hard surface to a sub-lethal level, ie. less 300g's, by absorbing energy.