Jay, somewhere I had seen a film of a flight test of a glider whose
wings started a distructive vibration. It was not pretty, the
amplitude of the vibration went from non existant to so great the wing
failed in what looked like 2 seconds. It happened so quickly it looked
doubtful the pilot could have reduced speed enough to stop it before
failure happened. I don't remember the details but the frequency might
have been the order of a couple of hertz -- so the wingtip went up a
little, down a little more, up even more, and after a few such cycles
as I remember it left the airplane.

My memory is not clear, but the suddeness of onset to failure was
something that remains vivid (wonder if it's an accurate memory?).