I see an ability like this in monster manual:
Rend Armor (Ex): If a bebilith hits with both claw attacks, it pulls apart any armor worn by its foe. This attack deals 4d6+18 points of damage to the opponent's armor. Creatures not wearing armor are unaffected by this special attack. Armor reduced to 0 hit points is destroyed. Damaged armor may be repaired with a successful Craft (armorsmithing) check.
But I can't find the hit points of an armor. All I have found are the hit points of a weapon and the hit points of certain material per inch. I didn't see where has indicated which armor has how many inch of thickness so can't calculate the hit points.
Is there an official rule somewhere indicates the hit points of each type of armor?