If you're talking about the Zoo Med Floating Turtle Dock, I had the same prob. After replacing it a few times, I found the trick. The turts were looking for calcium. It sounds funny, but I really do believe that they know what they're missing as far as nutrients in their diet and they do their best to try and get it and that's why they were chewing the dock. Now, I make sure that there is always a piece of cuttlebone in their tank and they chew on that instead. It took them a few days to figure out that it was edible, but they haven't tried to eat the dock since. Try it
Just remember to take off the hard backing first because they can't digest that. Also, I use about 1/3 of the bone and just throw it in. Some turts prefer small, pea sized pieces. See what yours prefers and lemme know if it works
The things that come to those who wait may be the things that were left by those who got there first - Steven Tyler