On a natural 20 or 1, a non-metal weapon take a cumulative -1 on damage roll (max -5), starting on the next turn. On a short rest, the weapon is consider repaired and return to no penalty.
Option ++: A roll of 20 or 1 with a weapon at -5 destroy the non-weapon metal. It cannot be repair or use after that turn.

Yes, it’s simple, and I do at least like that it’s not a permanant destruction (until really late). Good use of the extended rest mechanic there.
Problem is (and this really does extend back to the 2nd edition rule you’re basing this on) is that it’s based on critting. You’re punishing players for rolling well, and that’s really not a good thing.

Or you could just set the error range for any given material/weapon such that a roll of 1-2 damages some 1-3 for others 1-5 for obsidian, let magic bonus’s lower the error range and or have feats or custom powers that add/subtract from your/enemies error range for various reasons.
Examples: power attack adds 1 just because, no other effect. The never used Carfull strike lowers the error range of any weapon, Thunder Hammer adds 2. Flavor gimmicks that will never really influence the game (because they have to come up 6 times) other than making people carry around a backup weapon or two that isnt quite as awesome, which is exactly how DS should be.

Thanks hendell but i seek something very simple to keep track. I consider Nibenay will use mostly Wood and Urik it will be obsidian, Tyr is bone, so i dont want to make a difference, only Metal or Not-Metal :)
Thanks flip for your answer. Maybe only on a 1? Maybe if they do maximum damage? Help me with some simple alternative please :)

I like the idea of:

on a 1 the weapon breaks, becoming an improvised weapon and dealing 1d4 damage, possibly counting as a light blade when applicable.

after 3 criticals, the same thing happens.