Alternating Stresses for Fatigue Calculation: Same Stress Component or Stress with higher magnitude?

[struclearner]

To calculate Fatigue using alternating stresses, from two loading conditions, if Max. Principal Stresses from one load is selected, for the second load, same Max. Principal Stresses would be selected or the stress component with higher magnitude will be selected, if Min. Principal Stress value is higher than Max. Principal Stress value at the same location in the case estimating Fatigue Life from FEA or from Test Data from a Rosette Strain gauge.
Thank You Very Much for Your Help and Clarification!

 

[rb1957]

I think most would use max principal for both load levels and just accept the difference in direction. If max principal for load2 is higher than load1 then the cycle (presumably) 0 -load1 -load2 -0 would be effectively 0 -load2 -0 (as the load increases through load1 and crests at load2.

"truth" would be to use rainflow to combine the spectrum stresses.

even more "truth" would be to calculate fatigue at a bunch of different directions (at least the two principal directions from load1 and load2) ... but that's a lot of work for little gain.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[SWComposites]

The only way a min principal stress is higher magnitude than the max principal stress is if the value is compressive. And compressive stresses are usually not critical for fatigue analysis of metallic structure.

 

[rb1957]

but Min1 could be higher than max2.

another thought, if you're looking at a web, would be to use von Mises to combine the two principals.
Not saying it's Right, just another approach.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.