keeping mags loaded.....for how long?

If I can sum up many other threads on the matter. Springs weaken with movement. That is compression and release as an ongoing process.
It's been more than 20 years since my metallurgical engineering course so I'm admittedly a little rusty but if I remember right there are three failure modes for springs:

  • Fatigue Failure: This is when there is a complete breakage of the spring, Usually starts a a flaw in the metal and then and the spring works the crack progresses enough that it can't withstand the stress abd the spring breaks. I've never heard of a mag spring breaking in this way
  • Failure from exceeding ultimate stress: This would be is a spring is stressed (or compressed I suppose in some cases) beyond its designed capacity--like if you grab a spring and stretch the bejesus out of it--it doesn't return to its previous size. But cycling a spring in a magazine does not subject it to stresses that exceed its design stress so this isn't the cause either.
which leaves...
  • Creep: This is a phenomenon where some materials (plastics, especially, but also some metals) deform when their subjected to loads at less than their maximum but still slowly deform (or they take a "set") over time. This is what causes some springs to lose their strength. But, it's static, not dynamic, force (I think) that causes creep (like a car sitting on its springs)--loading and unloading them wouldn't causes this, but leaving them loaded would. Obviously, though, most magazine springs are designed so that creep is not a problem.
It's certainly possible that I'm misremembering something but my hypothesis is that if a spring is going to take a set it's leaving it compressed that'll cause it, not cycling it. Any thoughts?
 
About 25ish years ago I found an Enfield inside a wall in a house being torn down in Rossland, BC. The rifle had been drilled and tapped at one time but no scope. There was a mag inserted, no ammo. There was no rust at all which was surprising as the house had been vacant/unheated for several years. Don't know how long it would have been there, but it was in perfect working condition, mag and all.

I gave it to a needy deer hunter in Manitoba several years later, don't know if he still has it.
 
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