From what armors have a durability? How does it works?
No, nothing about durability. Material strengths in DF work on a linearized model of an elastic stress-strain curve. As you apply stress (force per area) the material deforms linearly up to a point and then fails catastrophically. The parameters are (1) the slope: how much it bends for a given stress. And (2) the breaking point: how much strain it takes before it breaks. The deformation can allow a blow that doesn't break the armor to still cause blunt trauma.
So much like real life you can have materials which are strong and brittle, strong and ductile, weak and brittle, weak and ductile, or anywhere in between. Ashland-glass and mithril are both great armor materials but they are different: mithril is less likely to fail against an attack from a strong steel/ashglass/mithril blade, but ashland-glass will protect you more fully from an iron/bronze/copper strike that isn't going to penetrate anyway.
There are also different parameters for impact (blunt) vs. shear (edged) strength, so for example, orichalcum has a particularly excellent impact modulus, so you might consider layering a mail or cloak of it with your deep-bronze armor which has superior shear modulus.
Volcanic is very strong and quite low deformation, so if I remember correctly, it trumps the other high end materials in every category. But it still is partially elastic, so it will deform under heavy blows.
The very best mythical materials adamantine and bifrost, are both very strong and perfectly brittle, they don't deform at all.
edit. To be clear, when I use the term "breaks", i mean: the calculation determines that enough force was applied by the attack that the blade penetrates the armor and the wearer is vulnerable to the weapon attack. The armor itself is never permanently damaged in combat. Although I wouldn't be surprised if that gets implemented by Toady in the future.
