Typically if an attack roll equals the AC of a target then the attack does normal damage. I quite like the idea of making it a glancing blow, dealing half damage instead, this would help clue the PCs into what the AC of the target roughly is.

My only worry would be that this might create balance issues. I will make sure that the rule is the same for when monsters attack, so it is a general rule "ties do half damage." What would the consequences of this house-rule be?