Do belts on belt-drive bicycles stretch under load and spring back? Reputedly they don't stretch over their lifetime Does a belt on a belt-drive bicycle stretch under load and then elastically return?
The only current manufacturer I know (Gates Carbon Drive) claims the belt does not stretch over its useful lifetime, but my question is does it stretch under load and then return?
I have read some forum posts (can't find them now), that say they stretch under load, so are no good for power applications such as single-speed racing. This argument would not seem to hold up since Gates now have a single speed racing team and they've won some races.

Does anyone have any factual information or practical experience that can answer whether the belt stretches under load and elastically returns?

A related question is if there is some slight elasticity would that help prevent the damage that can be caused to internal hubs from changing gears under heavy load?
Everything stretches under load.  How much is the question.

The belt manufacturers should have technical data, including stress/strain graphs, modulus numbers, or other "hard" numbers.

But, generally speaking, carbon fiber and steel both have a modulus of around 200.  It's not clear now much carbon fiber is in the belts (and the cross-section area times the modulus effectively tells you how much stretch will occur), but when you consider the thinnest parts of a steel chain, and the fact that the pins will tend to flex in tension, it's probably comparable.

In any event, stretching should not be a concern so long as it's not sufficient to cause the belt to skip on the cog, or some other mechanical issue.