I would chalk this one up to a lesson learned. My husband and I have been landlords for 10 years and we find this 'gray area' of wear and tear damage to be somewhat of a thorn in our side. As someone else pointed out, if your tenants are good tenants otherwise and currently have no intentions of moving out, I would let this go, but would fix it and supply some sort of runner to prevent further damage. Also, if you have not specified in your lease or house rules that you require mats or carpeting under wheeled chairs, then your are pretty much SOL. Chances are good that if they cracked that easily, the wood would have cracked sooner than later anyway. It sucks, but such is the life of a FAIR landlord.