Is it possible to put damaged item variants on armor?
Solution 1:
Here (archive) Minecraft forum user "Momlob" said:
As far as I know you could do the Items from the Armor, there is no possible Way known to me to do it with the Armor shown on the Player. This is because the Model of the Armor unlike the Items and Blocks is not included in the model folder of a Resource Pack.
I have verified this with the wiki (archive): There is a resource pack folder for item models and one for block models, but none for armor. And since armor items uses textures that are not automatically generated from the worn armor, the texture and model is separate.
So I'm afraid, but currently (1.12) it seems to be impossible to change armor models. And since different textures for all the damage values are part of the model, that's also not possible for armor.