Creep and Fatigue

dental guy

What is the difference between dynamic creep and fatigue?
How are they tested?

Mapes

Creep is permanent deformation over time; fatigue is crack propagation over time. Both occur due to applied loads and both can lead to failure. Creep is characterized by looking at the elongation of the sample; fatigue, by elongation of the crack.

FredGarvin

In a nutshell...

Creep is a constant stress situation that is below yield. It is extremely prevelant in higher temperature conditions. Creep is usually tested with a wire of a given size that has a constant load and the elongation is measured over time. Be careful not to mistake creep with stress relaxation.

Fatigue is due to cyclic/alternating loading that can be at any stress level. The loading is repeated until the number of cycles imparted causes failure. This is expressed in S-N curves for particular materials. A material is usually said to have an infinite fatigue endunce limit if it can withstand 10^6 cycles at the particular alternating stress level.

I'll have to look up the particular ASTM specs on each test.

Creep:
http://www.engineersedge.com/material_science/creep.htm
http://www.nuc.berkeley.edu/thyd/ne1...ads/creep.html

Fatigue:
http://www.sv.vt.edu/classes/MSE2094...y/fatigue.html
http://en.wikipedia.org/wiki/Fatigue_(material)

dental guy

Thanks,
If we have to perform experiment on polymers with cyclic loading, then will it be categorised under fatigue and not creep?

FredGarvin

That would be a fatigue type of test. It depends on your outcome and what you are measuring to be able to call it a fatigue test. Are you going to take the samples to failure?

dental guy

Thanks fred,
We do not want to take it to failure. We are interested in change in dimension , if it occurs, say after 12-15 millions cyles.

granpa

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