Tail's no silent

It was it's primary means of defense, aside from its size.

That's only one of the ideas.
It's not actually known if it was a form of intimidation, communication, or courtship.

Its*

Depending on the taper, you actually wouldn't need a very long boneless area at the end of the tip to reach those speeds, though. At small diameter, skin and tissue would likely hold together reasonably well - that's how leather bullwhips work, after all, and I'd bet dinosaur hide is tougher than cowhide.

Muscle scars on the bones. Some even have healed fractures on the ends of their tails.

I'd actually be very interested in how they distinguish speed injuries as opposed to trauma/predation damage.

They wouldn't have used the tips of their tails for defense, however. Kind of like how humans don't smack people with their fingers but with their palms.

There's also a fossil of an Allosaurus with a shattered lower jaw that was thought to have been broken by a Diplodocus earlier on its life, though it managed to live for many years afterwards.