I understand what you're trying to say with shields and helmets, but it's odd to say that a metal shield becomes a wooden shield and a metal helmet becomes a light cap. I think it would be better just to say that wooden/light can mitigate damage once and metal can do it twice.
you could give each armor "notches" and have them "notched" with each hit, and possibly be able to repair any armor that hasn't gotten to the point of "destroyed"?
So there are ways to damage armor on critical hits and with acid or some other effects but there is legitimately a part of me who would love some armor wear and tear. I may eventually play around with this or give armor a sort of 'shields must be sundered' use, we shall see.
