[varjag]

> People will so often put up photos on social media of obliterated helmets and say, “Holy crap, look at my helmet! It saved my life!” But helmets are not supposed to shatter. When a helmet protects your head from a serious injury, the styrofoam inside will be compressed and stay that way. Most of the pictures I’ve seen are of helmets that have broken apart.

Just.. what?

Any sort of mechanical deformation is essentially an energy release. Be it compression, cracking or what else. The energy that went to cracking the polycarbonate shell is the energy that didn't reach your skull.

[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...

[collyw]

Exactly. What would there head have looked like if the helmet didn't save it.