Fatigue failure of yielded material
Fatigue failure of yielded material
(OP)
We are planning to preload a spring past its yield strength, but before its ultimate tensile strength. The spring will then be used in service for 10^7 cycles, but with a small amplitude. The amplitude stress is about 30% of its endurance strength. Even though the spring has a permanent deformation, it will not break statically, because the stress has not reached its UTS. My question is, how do we analyze if it will fail in fatigue? Thanks





RE: Fatigue failure of yielded material
RE: Fatigue failure of yielded material
RE: Fatigue failure of yielded material
I think I got it though. Let me know if this makes sense. See the attached drawing. So after I preload the spring, it will be at its new yield strength Sy preload. Then after the first cycle, the spring is deformed to a higher stress inelastically to Sy new (along blue line). Then when it is relaxed, it travels down a new elastic line (along green line). The stress will be alternating between the stress at C and B. In reality, the strain difference between A and B would be slightly different than between C and B, because of redistribution of stress due to plastic deformation.
So since the new mean stress is less than its new yield strength, the material could still be safe against fatigue failure provided the stress amplitude is small enough.
It's always reassuring to know when something is already done in industry.
RE: Fatigue failure of yielded material
http://i.imgur.com/qWXz6.png
RE: Fatigue failure of yielded material
RE: Fatigue failure of yielded material
According to Goodman and Soderberg, when calculating fatigue life, one should take into consideration mean stress and stress amplitude. For example, in the extreme case where the mean stress is almost equal to yield and the amplitude is zero, you just have static loading and it will not fail.
You are probably thinking of a cyclic load with zero mean stress (a rotating shaft), in which case the stress amplitude must be below half of the UTS multiplied by Marin factors (an estimate for endurance strength).
RE: Fatigue failure of yielded material