With the Adamantium Skeleton perk, your limbs only receive 50% of the damage they normally would.”— In-game description, Fallout 3
Fallout 3 and Fallout: New Vegas
Adamantium Skeleton simply takes any sustained limb damage and halves it, making it twice as difficult for you limbs to be crippled. This perk does not affect limb damage from falling, nor does it prevent any scripted situations that cripple limbs, such as falling in the Mothership Zeta trash compactor.
With the Adamantium Skeleton perk, your bones are infused with unbreakable metal, which reduces - and may even completely prevent - limb damage.”— Fallout 4 loading screen hints
With the first rank of this perk, damage done to limbs is reduced by 30%. The second rank raises this reduction to 60%, and the third rank negates all limb damage entirely.
Behind the scenes