I've been told 2 armor reduces dmg by 1, which I'm fairly sure is true.
So take my Plate Covered Round Shield for example. Its got 61 armor, which reduces dmg it takes by 30.
Any weapon or hit that doesnt do 30 dmg is basically negated and the shield is unaffected. Swords that hit for 35 dmg~ are only doing like 5 dmg a swing so the shield will last a very long time with 175 HP.
an axe however, does double dmg and completely bypasses shield armor, the shield will die in about 3-4 hits