Springs

[beenalongtime]

I will text Tom some nudes, but that may not be enough to get him out of retirement.

So we will never hear from him again, after he changes his phone number, after receiving pic's of you.

[pyrotechnic]

It’s been 4-5 years since took my material mechanics and machine design classes, but I’ll try to expand on what fatdog posted.

When a metal is elastically deformed, meaning it will spring back to its original shape after the deforming force is removed, the atoms within its crystalline structure don’t move. The bonds between the atoms are stretched, but none “break”.

When we reach the elastic limit of the metal (yield strength) and then apply a little more force, we plastically deform the part. This is permanent, when we remove the deforming force, the part will still snap back elastically, but not back to its original shape. What has happened is that locations of missing atoms (dislocations) have moved, usually along grain boundaries, and atoms have now re-bonded to other atoms. We have also work hardened the material, as these dislocations pile up, become entangled on multiple planes, and will take more force to move. The electron sharing of metallic bonding is what allows this ductility to occur. When we anneal a part, we increase the kinetic energy of the metal’s atoms, this allows these dislocations to move from areas of high concentration to those of lower concentrations, restoring the ductility and removing the work hardening that was done to the part.

If we keep applying force to our part after we plastically deform it, these dislocations will continue to move and as they pile up on each other they get stuck. The part will continue to deform as we increase the force until it reaches its tensile limit and breaks.

Spring failure when the spring is within its elastic limits is due to fatigue from cyclic loading and unloading. Fatigue is caused by crack propagation within the material as stress risers (imperfections) create localized stresses above the yield strength of the material even though the stress within the bulk of the part may be well within elastic limits of the material. This results in work hardening and embrittlement in those areas. In a typical fatigue case, the initial crack continues to grow as the part cycles through its life, pushing this area of hardening and embrittlement through the part until a critical crack length is reached and it ultimately fails through brittle fracture.
In a magazine spring or recoil spring we rarely see one snap in half, but we do see them become shorter over their lifetime. These localized areas of plastic deformation resulted in permanently altering the shape of the spring just as we permanently changed our part by pulling on it past it’s yield strength. Since we don’t make springs or other parts subject to fatigue out of “perfect” material, imperial testing of fatigue failures is conducted, and statistics used to determine the theoretical life of these parts given the stresses the material will see during normal use.

As far as springs “setting” under a constant deformation. I never studied this in school, it wasn’t mentioned and a properly designed spring used within it’s elastic limits “shouldn’t” exhibit this behavior. However, it sounds a lot like creep and stress relaxation, the examples of which I learned about all occurred at high temperatures or with visco-elastic materials. The mechanism for creep and stress relaxation in metals is also the movement of dislocations through the bulk of the material. Like annealing but with force involved. A quick google search located a presentation “Time Dependent Deformations of Metals at Room Temperature” on the OSTI web site. So I’m going to look into it a bit as I’m interested. I’m also going to load up a couple P-mags after measuring the spring length next time I’m home. I’ll pull one out after 3 months and measure it again. Again, with proper spring design, a magazine is capable of being left loaded without detriment. It will be the cyclic loading and unloading that kills it.

I hope this was somewhat helpful. My only exposure to spring design and fatigue was with undergraduate engineering classes. I hope someone with experience in spring design will chime in, especially if I have errors in the above, and regarding cases of low temperature stress relaxation and creep

[Bart Carter]

...In a magazine spring or recoil spring we rarely see one snap in half, but we do see them become shorter over their lifetime...

Which is why the only recommendations I have seen to check magazine springs is to measure them brand new and then to use that measurement to see if they need replacement. Of course they never tell you how short the difference needs to be from new.

[BWT]

It’s been 4-5 years since took my material mechanics and machine design classes, but I’ll try to expand on what fatdog posted.

When a metal is elastically deformed, meaning it will spring back to its original shape after the deforming force is removed, the atoms within its crystalline structure don’t move. The bonds between the atoms are stretched, but none “break”.

When we reach the elastic limit of the metal (yield strength) and then apply a little more force, we plastically deform the part. This is permanent, when we remove the deforming force, the part will still snap back elastically, but not back to its original shape. What has happened is that locations of missing atoms (dislocations) have moved, usually along grain boundaries, and atoms have now re-bonded to other atoms. We have also work hardened the material, as these dislocations pile up, become entangled on multiple planes, and will take more force to move. The electron sharing of metallic bonding is what allows this ductility to occur. When we anneal a part, we increase the kinetic energy of the metal’s atoms, this allows these dislocations to move from areas of high concentration to those of lower concentrations, restoring the ductility and removing the work hardening that was done to the part.

If we keep applying force to our part after we plastically deform it, these dislocations will continue to move and as they pile up on each other they get stuck. The part will continue to deform as we increase the force until it reaches its tensile limit and breaks.

Spring failure when the spring is within its elastic limits is due to fatigue from cyclic loading and unloading. Fatigue is caused by crack propagation within the material as stress risers (imperfections) create localized stresses above the yield strength of the material even though the stress within the bulk of the part may be well within elastic limits of the material. This results in work hardening and embrittlement in those areas. In a typical fatigue case, the initial crack continues to grow as the part cycles through its life, pushing this area of hardening and embrittlement through the part until a critical crack length is reached and it ultimately fails through brittle fracture.
In a magazine spring or recoil spring we rarely see one snap in half, but we do see them become shorter over their lifetime. These localized areas of plastic deformation resulted in permanently altering the shape of the spring just as we permanently changed our part by pulling on it past it’s yield strength. Since we don’t make springs or other parts subject to fatigue out of “perfect” material, imperial testing of fatigue failures is conducted, and statistics used to determine the theoretical life of these parts given the stresses the material will see during normal use.

As far as springs “setting” under a constant deformation. I never studied this in school, it wasn’t mentioned and a properly designed spring used within it’s elastic limits “shouldn’t” exhibit this behavior. However, it sounds a lot like creep and stress relaxation, the examples of which I learned about all occurred at high temperatures or with visco-elastic materials. The mechanism for creep and stress relaxation in metals is also the movement of dislocations through the bulk of the material. Like annealing but with force involved. A quick google search located a presentation “Time Dependent Deformations of Metals at Room Temperature” on the OSTI web site. So I’m going to look into it a bit as I’m interested. I’m also going to load up a couple P-mags after measuring the spring length next time I’m home. I’ll pull one out after 3 months and measure it again. Again, with proper spring design, a magazine is capable of being left loaded without detriment. It will be the cyclic loading and unloading that kills it.

I hope this was somewhat helpful. My only exposure to spring design and fatigue was with undergraduate engineering classes. I hope someone with experience in spring design will chime in, especially if I have errors in the above, and regarding cases of low temperature stress relaxation and creep

I thanked Fat dog, saw your post, didn’t have time to read all of it but liked it.

Came back and read it now - thanks for your input.