I created a survivor backpack, and I didn’t notice that it lasted any longer than a regular backpack.
I regularly find myself walking away from a battle with a bunch of zeds largley unsacthed, but with all my inventory on the ground, due to my backpack getting shredded in the fight.
If remaining damage is below armor value (ie. if initial damage was at most 200% of armor value), there is a 0.5% chance of damaging the item. Sturdy items and power armors are immune.
Otherwise (initial damage was more than 200% of armor value) there is one_in(damage) chance of avoiding damage. Then again one_in(2) chance of avoiding damage.
For non-physical damage (electric, acid), there is no armor damage. Fire is calculated differently - it burns and doesn’t “chip”.
tl;dr version: if attacks are above 200% of your item’s armor value, they will shred your armor very fast. If they’re below 200%, sturdy items will not take any damage, while other items will slowly get chipped down to nothing.