Plastic deformation is when a material deforms inelastically. Basically, if you stretch it and it stays stretched it's been plastially deformed. If you stretch it and it elastically recoils back to its original dimensions then it hasn't been plastically deformed.
Doug Huffman wrote:
Contrary to popular belief, neither cycling a spring nor not cycling a spring will wear it out. Stressing it into plastic deformation will.
Theoretically that would be correct, but even under the yield strength some plastic deformation occurs in a spring through cycling. Perthe book
Material Science for Engineers Sixth Edition by James Shackelford pg 280-281, a direct quotation, but[] indicates minor edit :
Many structural applications involve cyclic rather than static loading, and a special problem arises. Fatigue is the general phenomenon of material failure after several cycles of loading to a stress level below the ultimate tensile stress. [...] The data indicate that while [a specific material]can withstand a stress of 800 MPa (T.S.) in a single loading (N=1), it fractures after 10,000 applications (N=10[sup]4[/sup]) of astress less than 600 MPa. The reason for this decay in strength is a subtle one [...] repeated stress applications can create localized plastic deformation at the metal surface, eventually manifesting as sharp discontinuities (extrusions and intrusions). These intrusions, once formed, continue to grow into cracks, reducing the load-carrying ability of the material and serving as stress concentrators.
So theoretically, if you neveroverstretched/compressed the spring you would never see plastic deformation, but due tothe formation of cracks (often times at grain boundaries andflaws in the material,)and crackpropogation, repeated cycles within the range will lead to some plastic defomation. It will be very slow, and may takemanycycles before any noticeable difference, wheras if youexceeded its strength by overtstretching ityou would instantly compromise it.
Springs used in magazines are designed to withstand a lot of cycles, so I wouldn't worry about it in your regular use of a magazine.
Also see:
http://en.wikipedia.org/wiki/Fatigue_limit --> Depending on what the spring is made out of (and I actually have very little understanding of the properties of magazine springs in particular), the cycles it may withstand may be much more than what you are likely to encounter.
Leaving the magazine loaded will not cause any wear.
