Turtles are fighting, biting each other?

I have 2 red eared sliders, about 3 years old. Today for the first time, I noticed the male chasing the female around the tank and cornering her, and biting her tail and legs. She has a chunk missing from her tail, and is huddled in her shell on land. I've seen them mating before, and it really doesn't look like that at all. I have no idea how to stop him from killing her :(