How can I determine the lifetime of a spring?

Question

How can I correlate the lifetime of a spring relative to the percentage of it's elastic limit that the spring is expected to cycle through? Is there a rule of thumb or is can find a function that relates lifetime versus percentage of elastic limit? Thanks.

Answer 1

Generally, as long as you are under the elastic limits for a small number of reps, you are ok. But for mutiple reps, you will be dealing with failure caused by crack propegation and fatigue (which are interelated).

You should look at the 'fatigue' limit for the material which will relate to the elastic limit. However, a spring could fail (rarely) even if under the fatigue limit. You might have some microscopic defects and cracks will tend to propegate under stress.

It's been a long time since I dealt with this type of issue, but there is a good article in Wikipedia that I found. http://en.wikipedia.org/wiki/Fatigue_%28material%29

Good luck

Answer 2

You'll need to review the S-n curves for the spring material. No rule of thumb or function. It's dependent on the material, condition (of the material), stress in the spring, stress ratio (i.e. what is the ratio of minimum stress to maximum stress, or is the stress completely removed every cycle), etc.