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/ne161/jlrhoads/creep.html

Fatigue:
http://www.sv.vt.edu/classes/MSE2094_NoteBook/97ClassProj/anal/kelly/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.