My understanding was that damage to shield was the same as to a player with that armor rating, times 1-0.08*(shield skill). And axes just have their base damage doubled before being multiplied by 1-0.08*(shield skill).
I always shoot for at least 20 shield armor so arbalest bolts don't penetrate, but additional armor above that seems to be largely "wasted" in the sense that shields are balanced to trade a lot of HPs for a little armor.
Using the damage calculator, my +3 Hand Pavise is about the same durability against axes as a +3 Plated Covered Round Shield (which is a whopping 25 speed slower!). Plate covered may withstand more hits from a peasant knife, but in a real battle, it's axes that break your shield, not hundreds of hits from weak cut weapons.