Always disturbing to hear of catastrophic structural failure. It frequently makes the headlines, as we all know...
I am frequently quick to put blame on the integrity of those manufacturers, yet that is not entirely appropriate, considering the nature of the beast.
We expect a people-powered contraption to be lite-weight, yet afford the greatest utility, and of course, style !
They, have delivered.
Here, at motorbicyclingDOTcom, have gone the extra mile and modified the engineering marvel of the cantilever frame, with Power Assist ! And this allows us to go farther, faster, and perhaps carry more.
All beyond design specifications.
There is really no fix other than purchase of a proper frame, (perhaps a Worksman?), up to the task.
Dip the frame in JB Weld ! Well, that actually might help dampen vibration, if it were applied uniformly and Very thickly... yet not for long.
Some time back I reviewed bamboo bicycles, built by young fellows, probably engineering students, looking for DIY projects. They wrapped just about everything with fiberglass cloth and resin, and it apparently worked well enough.
The same application over one of these frames might actually lend a bit more longevity, yet there would be an end to it, also.
Mild steel, of heavy wall construction will be the best option, in the end.
Never-the-less, I have seen mc frame failure on products from almost every continent, including the heavy-weight American twins.
IMO, detailing a cycle, on a weekly basis, brings personal attention to all elements of the machine, and also affords an element of safety.
Best, rc