Thanks to FO4Edit!
This is a fairly straightforward resource- it adds a Damage Threshold actor value. When an NPC is hit, their DT is removed from the damage done before Resistance is applied. If the damage is below the Damage Threshold, then no damage is done at all. DT is modified by Armour Piercing just like DR.
Example 1: If you have 20 DT and take a 30 damage hit, 10 damage will go through and be reduced by your DR
Example 2: If you have 30 DT and take a 30 damage hit, you’ll be completely unharmed
Example 3: If you have 20 DT and take a hit with 30% AP, you’ll subtract 14 damage from the attack before DR
Thanks to some neat work by Bethesda, NPCs will recognise if they can’t damage a target and won’t attack it. If they have no targets they can hurt, they’ll simply flee.
This resource does not add DT to anything, it simply provides a framework for adding it. Mods that implement it include:
Impervious Power Armour: Adds DT to power armours
Tougher Vertibirds: Adds DT to vertibirds
HOW TO USE
It’s easy. Copy either the “Template Fortify DT” Enchantment or Ability from Damage Threshold.esm into your file as a new record. Tweak the magnitude to whatever you want. Either add the Ability to an NPC or perk, or add the Enchantment to some armour or an object modifier. Don’t edit anything in Damage Threshold.esm.
Then just list DT as a requirement on your file and voila!