Should tanky players use Aspect of the Crab?

I play hardcore sword and board and have 90% estimated physical damage reduction at rest.

It promises 20% physical damage reduction when fully stacked.
Which would result in a total of only 8% damage coming through.

  1. Does Aspect of the Crab stack on top on the physical damage reduction from armor?
  2. Is the Crab still effective above the 90% physical damage reduction cap?
  3. Since as a tank I'm charging and jumping into enemies all the time, the Crab stack is being reset all the time. Is there even a benefit to using the Crab? The Crab might be more useful to ranged evasion builds.

  1. All sources of "physical damage reduction" stack additively with each other. This includes armour, endurance charges, the chaos golem buff, affixes on gear, pantheon bonuses, and the node Soul of Steel. Physical damage reduction is capped at 90%. Crab barriers provide 2% additional physical damage reduction per barrier, which stacks additively with your other sources. Physical damage reduction has hyperbolic scaling in a manner similar to resistances, which is very powerful if you have a lot of it. You would have 8% physical damage taken (from hits) if you had 90% physical damage reduction and then had Fortify, which is 20% reduced damage taken from hits. Damage taken and damage mitigation apply at separate steps, so their modifiers multiply with each other.
  2. No, it is not.
  3. If you are hit often then it is likely not useful. You may want to use the unique Craiceann's Chitin, which prevents you from losing crab barriers if you have lost crab barriers recently (where recently always means 4 seconds). Since you gain barriers at a rate of 1 per 0.5 seconds, this should allow you to keep a large amount even if you are being hit.

By the way, the "estimated physical damage reduction" on the character sheet is not particularly accurate. This is because armour has poor scaling as the physical damage of a hit increases; against a very large hit, such as that from a boss, armour will likely only provide around 15-35% physical damage mitigation at most. As a defensive layer on its own it is not efficient against this, but when combined with other sources of flat physical damage mitigation, it can be quite good.