NFL

Yes, you would certainly want a fairly robust material that would weaken at a predictable rate. After thinking about it more, I'm now considering a re-usable system, made of some sort of rigid plastic, where there's a gap between the head and helmet maintained by interlocking teeth. Under a heavy load, the plastic would deform, collapsing the space. Then you could just pop it back out after. Much like a medicine bottle's child safety, the teeth would provide the bulk of their resistance in only one direction.

It's not too different from re-usable breaking boards that martial arts schools sometimes use. Just much smaller, and repeated in a sort of matrix formation.

It's definitely feasible, we possess the technology to make this happen. Might be expensive, I don't know. Might also be inferior to this design:

https://scitechdaily.com/new-stanford-developed-high-tech-helmets-could-protect-football-players-from-debilitating-concussions/?expand_article=1

Something is worth it to reduce concussion rates, I do hope the NFL eventually takes this more seriously, though I know they don't actually give a shit about the players. Current helmets are an absolute joke, though, it should be embarrassing in these modern days. Kinetic energy can be absorbed/converted/dissipated in many different ways.