von Mises or principle stress for fatigue analysis? 3

[EricZhao]

When the fatigue life is considered. Mean Stress and stress amplitude governs according to Goldman’s theory. Does the stress refer to von Mises or principle stress?

Thanks,

 

[rb1957]

umm, so will max. principal

 

[cbrn]

zcp, rb1957,
I think you're both right. as I said before, VonMises, MaxPrincipal, Stress Intensity, etc, are all based upon ASSUMPTIONS (as every analytical theory does, anyway), and neither can completely capture the behaviour of "real" materials over all the possible stress fields: there are cases where MP becomes totally unrealistic, others where VM will under-estimate the equivalent stress. Because the crucial point is this: we always want to reconduct a stress field in a point to a single "sigma" value in this point...
Finally, I think it's important to refer to the Norms that are valid for the country you work in (or that are requested by the customer, it depends): for ex., if you refer to ASME then you will be obliged to carry all the calculations with Stress Intensity, no matter if you disagree or not in some points...

Regards