you mean in seconds a puppy bit through a nylon leash? huh? you must have a puppy that's half shark! LOL!
I guess I have a question for you: when you say tether, you mean the leash is attached to you all the time? If the pup just constantly chews on the leash maybe you need to use a negative marker like "Nope" and when the pup's not chewing it, then you mark that with "Yes!" Watch all the free videos and articles Ed has on using markers in training.
you really can only blame yourself if you are not watching the pup 100% of the time, if you can't really watch the dog then put it in the crate or playpen.
Good luck with little Sharky!