When something deforms it absorbs the incoming energy. Like a cushion or a crumple zone.
When armor dents it eats up some of the incoming energy and keeps that energy from being transferred past itself. So now it has spread out the impact via hardness and then eaten it via deformation.
Like how sparring rapiers work to protect the opponent. If they didn't then both the one hit, and the hitters wrist, would be unhappy right?
Talk to SCA fighters or HMB/ACL guys enough and you'll hear stories about times they got creamed and felt nothing even though their armor was badly dented and potentially destroyed by the incoming blow because the armor worked as designed and in deformation, weight, and spreading of impact ate up the incoming energy. Armor works partly by taking advantage of the physics behind an
Inelastic Collision to reduce the amount of energy transferred to the wearer.
Yes one can use these same physics to hurt someone's brain (see a boxing glove) but the armor, when it dents (aka deformation), is helping prevent harm to the wearer which becomes important when you start talking about brains in helms.
Stainless steel doesn't deform as easily and thus once all the energy that can be eaten via mass & padding has been the rest is transferred to the wearer. Not too bad for gauntlets, rather bad for helms. This is why we see injuries in a gauge of stainless that would not have occurred with mild or spring.
And isn't controlled deformation a key component of longswords, rapiers, armingswords, and zweihanders too?
