Durability loss of equipment should equal Material Hardness of attacking weapon vs Material Hardness of defending weapon. Also, make it so dental, horn and crepite have a negative modifier to these totals due to a brittleness created when under tension/pressure of the bent bow giving woods more use.
Heavy metal weapons will utterly destroy the bow pretty quick if used to block. The damage of the bow will drop fast (if not break it) if the archer decides to use his bow to defend, but sometimes it may feel worth the decision to give up your bow damage vs your health. May even need to carry multiple.
Would also potentially give reason to use other materials in the bow makeup, rather than just pure cookie cutter bows that currently exist.