30 Accuracy, unmodified Dragonbone Bow, Dragonbone Arrows.
101 Damage dealt to thrall with 0 Armor.
111 Armor = 30% damage reduction.
111 Armor -9% = 101.01
101 Armor = 28% Damage Reduction.
So 9% armor pen should deal 72.72 damage.
111 Armor -18% = 91.02.
91 Armor = 26% damage reduction.
So 18% armor pen should deal 74.74 damage
Now for what I see…
73 Damage dealt, that's a 27.72% damage reduction.
A second test…
40 Accuracy, unmodified Dragonbone Bow, Dragonbone Arrows. Armor pen should now total 27% with Crevice in the Armor.
113 Damage dealt to thrall with 0 Armor.
111 Armor -27% = 81.03
81 Armor = 24% damage reduction.
So 27% armor pen should deal 85.88 damage.
83 damage dealt, that's a 26.54% damage reduction.
A third and final test.
40 Accuracy, Spiked Dragonbone Bow, Dragonbone Arrows. Armor pen should now total 49.5% with Crevice in the Armor.
113 Damage dealt to thrall with 0 Armor.
111 Armor -49.5% = 56.05
56 Armor = 18% damage reduction.
So 49.5% armor pen should deal 92.66 damage.
83 damage dealt, that's a 26.54% damage reduction.
Conclusion: It looks like arrow and bow armor pen stacks and it appears that Crevice in the Armor does not stack with the spiked attachment on bows, but it definitely does stack with it on melee weapons as can be seen in my Crevice in the Armor test results.
Please let me know if any of my math is off.