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I'm a structural engineer. The failure mechanism for springs would be fatigue. Springs are made and designed to be compressed, that's what they're good for. In order to induce fatigue you must compress and decompress springs many times.
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You can also induce fatigue in a spring by pressing/pushing that spring FARTHER than it's designer meant it to be compressed, or by keeping it in that compressed state at it's design limits (or beyond). Problems generally arise only at or near that elastic limit.
Other engineers participating here in earlier versions of this discussion, including one who spent some time investigating spring design and behavior, citing in the process a number of technical links (including a few shown below), have addressed this aspect of metal fatigue. It's a problem that occurs near what Wolff Springs call the "elastic limit" and a lot of springs never get NEAR that limit, so they work and work and work... just like Tappet Springs in a car's engine.
Here are some of those links:
Ex:
http://www.spring-makers-resource.ne...les/fig_37.pdf
Also:
http://www.spring-makers-resource.ne...ng-design.html
http://www.spring-makers-resource.ne...g-designs.html
And
http://en.wikipedia.org/wiki/Elasticity_(physics))
http://en.wikipedia.org/wiki/Plasticity_(physics))
http://en.wikipedia.org/wiki/Viscoelasticity
http://en.wikipedia.org/wiki/Creep_(deformation))
Just a snippet:
Taken from http://en.wikipedia.org/wiki/Viscoelasticity
"All materials exhibit some viscoelastic response. In common metals such as steel or aluminum, as well as in quartz, at room temperature and at small strain, the behavior does not deviate much from linear elasticity. Synthetic polymers, wood, and human tissue as well as metals at high temperature display significant viscoelastic effects. In some applications, even a small viscoelastic response can be significant. To be complete, an analysis or design involving such materials must incorporate their viscoelastic behavior. Knowledge of the viscoelastic response of a material is based on measurement"
Many springs in magazines aren't pushed near that elastic limit -- and it's most often only in the high-cap or compact (sub-compact, etc.) mags that designers force the springs to do things that aren't normally asked of springs.
As others have noted, letting springs REST won't restore lost power. Rotating mags won't prolong their collective lifetimes except from NOT being worked. Springs lose power because the internal structure of the spring metal has changed. Coil springs are more inclined to weaken than break, because the work is distributed more uniformly over the entire spring material than with leaf springs.
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