[upofadown]

But cracking involves a very small amount of energy absorption. Once the crack starts it tends to propagate with very little required force.

[varjag]

The process once it gets going is pretty quick but the energy buildup required is fairly substantial and is defined by material properties (shear modulus).

[upofadown]

To absorb energy in a useful way you need both force and distance. The idea is to reduce peak G force ... not that just reducing peak G force is all we need to do...