Looking into the documentation around bicycle helmet design, it appears that the outer surface is actually less for energy absorption, and more for preventing any "catching" on the road surface which would add torque to your head in a crash.

Cracking, in this case, is the expected failure model, since the material has to be hard to provide a sliding surface.

Helmets should not be re-used after a crash.

http://www.bhsi.org/ideal.htm point 5 - outer shell

hmmm ... interesting point