On Jun 23, 3:51*pm, Derek C wrote:

Most modern gliders are stressed to take at least +5.3/-2 g without
damage...

It is true that operation within the limit load should not cause any
damage in the sense of bent, stretched, or torn structure that
precludes continued operation. However, at the same time every single
load cycle causes the accumulation of fatigue that, given a long
enough service life, will eventually cause failure.

Aluminum structures require careful consideration of fatigue and
service life, since there is no level of stress below which fatigue
does not accumulate. If you take an aluminum wing designed to a 5.3g
limit load and load cycle it from 0g to 1g and back a relatively large
number of times, it will eventually break without ever having been
stressed over 1g. That is one of the reasons that there is some margin
(usually 50%) between limit load and ultimate load.

The key, question, of course, is how many cycles does it take? The
extreme example I cite above will probably take many, many times the
number of cycles equivalent to the flight hours in the planned service
life. Given greater loads, the number of cycles to failure is reduced.
And of course, given lower loads, the number of cycles is increased.
But the important thing is that, for aluminum at least, there is no
level of loading at which the cycle count goes to infinity. An
infinitesimal loading, repeated enough times, will result in eventual
failure.

Thanks, Bob K.