The issue with wooden arrows passing through adamantine armor and still doing damage has to do with a couple of outstanding "features" with arms and armor in game. First, wear is never considered. The armor has invisible holes punched through it for the arrows that never affect it again, and the arrows, upon successfully passing through, are still the same rail gun ammo that left the craft table pencil sharpener.
The differences in blunt force damage for different materials, however DOES correlate fairly well to RL physics. The Iron breastplate has more mass than the Adamantine breastplate. This is because the density of Iron is much higher, but the volume of material used is the same. DF does not consider plastic deformation vs. elastic deformations. That would still make Iron a good choice vs. a perfectly rigid material as it could "bounce back" from a hit, to a certain extent. But we'll disregard that, as it is not modeled. What IS considered, however, is calculations involving the mass of the armor layer. The combat mechanic does a sort of energy moment calculation using this. Since the mass of Iron is high, its moment is high, more of the energy of the striking blunt instrument is used to overcome the moment of the armor piece resulting is less energy being absorbed by the soft, blood and vomit filled bag of beard hair within.
Extreme Example:
Setup:
1 Regulation Softball, placed atop a T-ball post
1 Ceramic Ball, same VOLUME as the Softball, placed atop a T-ball post
1 Steel Ball, same VOLUME as the Softball, placed atop a T-ball post
3 Ash Softball Bats
1 Force measurement guage affixed to a vertical plate, 2 ft. distant, horizontally, from target objects above
Process:
Strike each of the target objects with one of the bats with equal force. Observe the force applied at the force measuring guage by the target object when it collides.
HypothesisTheory:
The force imparted at the force measurement guage by Ceramic Ball will be greater than that of the Regulation Softball which will in turn be greater than that applied by the Steel Ball. This assumes that all test objects survive the test process.
Edit: Corrected terminology