Rotating Bending Fatigue Stress Range

[corus]

For steel material properties the fatigue limit, in terms of the stress range, is often given in rotating bending. If the mean stress is zero how can the compressive stress contribute to the fatigue damage when there are no welds?
Also, in the British Standard on fatigue it is said that, for parent metal, when part of the cycle is compressive then the contribution to the stress range is 60% of the compressive stress. Where does this figure come from and why add any proportion of the compressive stress part of the cycle when the damage must come from tensile stresses?

corus

http://www.corusresearch.com

 

[ishvaaag]

The traditional understanding has been being that only stresses in the tensile range were required to properly protrait fatigue phenomena. This was an election, it was thought that considering only such stress ranges and their magnitudes was enough to portrait to what the practicing engineer was relevant.

More perfect understanding of the fatigue phenomena must have lead some to the convenience of using as well the compressive stress range as a factor contributing if not to fatigue itself, to a proper practical portrait of the same for the practicing party, to whom the only question that matters is if the necessary requirements are met.

For example at some opening crack microdefoliation along the tensile stress can start. Sucessive iterated compressive cycles won't place the then buckled microparts of failed material where theyt initially were. Then it can act as the stone for the lever in the compression cycles -or as included rust would do- and so the compression cycles can be thought as well contributors to a quicker opening of the crack.