What causes springs to wear out?

[RestlessMind]

Hi guys!

I was curious if anyone here knows what causes springs to wear out. Obviously, repeated stretching and releasing springs causes wear on the spring, no?

However, I was also curious as to whether keeping springs compressed for extended periods of time (say, up to a year or two) will cause the spring to lose its regular function.

Thanks a bunch in advance!

 

[hotvette]

As long as the stresses are below the yield limit, I would guess creep and fatigue would be the predominant causes.

https://www.physicsforums.com/showthread.php?t=259098

 

[granpa]

I thought steel didnt fatuguehttp://www.ent.ohiou.edu/~bayless/seniorlab/samplereport.pdf

The endurance limit is defined for ferrous (steel and iron) material as the stress level below
which the material can be cycled infinitely without failure. (Shigley et al., 2003) This is very
important, because the result of exceeding this point most likely will be fatigue failure. However,
for non-ferrous materials, such as aluminum, there is no true endurance limit. Given enough
cycles, the material will fail in fatigue. (Stinson, 2003)

http://sfbay.craigslist.org/forums/?ID=145262849

 

[RestlessMind]

Interesting, but that thread seems to imply that creep is caused when a spring is stretched, not compressed. Or, does it happen in both cases?

 

[Ntstanch]

Ever sleep on a spring mattress? Which parts become less "springy" over time when force is applied? Seems like the same elements would apply if you stretched it... in both cases you're taking its natural/original state and changing it with unnatural force, which it adapts to on a structural level which changes it fundamentally from it's original and optimal form.

It doesn't really wear out in my opinion... it just changes. The change would be infinitesimal if the spring were meant to constantly bear a load however.

Edit: Infinitesimal if the structure of the spring didn't give out or fail for design reasons or external reasons. What I was getting at is that the spring and the forces impacting it would reach a balance. Though with the load constantly being applied it's not unlikely that something could go wrong suddenly -- or minutely with a sort of exponential implication. Like a small chip in glass becoming exponentially worse over time versus a structural flaw that causes immediate shattering.

 

[OmCheeto]

It's been decades since I've studied such things, but my best guess would be "work hardening".

Similar to bending a paperclip over and over again, until it breaks.

Each time a spring is compressed or stretched, atoms realign themselves into a slightly more crystalline structure. Crystal's make very poor springs from what I've heard.

That's the way I remember it anyways.

 

[Lsos]

I thought steel didnt fatugue

It is possible to design a steel part so that it supposedly has an "infinite" fatigue life, but this doesn't mean that everybody does it for every design. There are different grades of springs depending on how severe their service is. Valve springs, for example, suffer extremely severe service and yet you hardly ever hear of them failing even after many years and hundreds of thousands of miles.

As for what causes a spring to fail (when it does), I would say mostly fatigue unless the temperature it operates in is high, where creep might become an issue. As for normal temperatures, I doubt a permanately deformed steel spring (not too extreme) would creep even after many years.

Interesting, but that thread seems to imply that creep is caused when a spring is stretched, not compressed. Or, does it happen in both cases?

Keep in mind that when you compress or stretch a helical spring, for example, the actual stresses on the spring are torsional...so it doesn't really know the difference. Same goes with many other types of springs that I can think of.

 

[mrspeedybob]

It is possible to design a steel part so that it supposedly has an "infinite" fatigue life, but this doesn't mean that everybody does it for every design. There are different grades of springs depending on how severe their service is. Valve springs, for example, suffer extremely severe service and yet you hardly ever hear of them failing even after many years and hundreds of thousands of miles.

Valve springs do fail. If you take a spring with 1,000,000 miles on it and compare it to a new spring, the new spring will require significantly more pressure to compress. The only reason valve spring failure is not more common is that the rest of the car wears out first.

A previous post mentioned gradual hardening of a spring over many cycles but this cannot be the cause of valve spring failure because they get softer with time.

The same thing applies to suspension springs. They oscillate every few inches while driving, but seldom break except due to rust. However, if a large person drives a small car, after several years the drivers side of the car will sit lower due to greater spring fatigue on that side.

I've wondered about these phenomena, but never found satisfactory answerers.

 

[RestlessMind]

Oh, I definitely see what you guys are saying about springs wearing out by repeated stretching/compression, but I'm just talking about (steel) springs being compressed and staying in the same position for a long period of time.

 

[Ntstanch]

Oh, I definitely see what you guys are saying about springs wearing out by repeated stretching/compression, but I'm just talking about (steel) springs being compressed and staying in the same position for a long period of time.

Imagine being on another planet with higher gravity than ours for a year. Or being in zero gravity for a year. Your body adapts and changes... though for metals in general, being less complex than the human body, those changes are basically permanent. The stresses change it fundamentally in structure to adapt to the new environment (in the springs case bearing weight or being stretched for years or however long you want). And unless something goes "horribly wrong" they will stay in that new state, under a constant load, until time and the elements corrode them to the point of failure.

To answer your original question, yes... it will lose it's regular function. Have you never slept on a steel spring mattress? :P

 

[Chronos]

The laws of thermodynamics demand structural decoherence of any solid over time - loads merely accelerate the process. The properties of all materials are altered with the passage of tine. Some are more resilient than others, but, none are immune.

 

[RestlessMind]

Alright, thanks very much for the info everyone!