What stress to use for fatigue analysis

[scootypuffsnr]

Hi,

I was wondering which stress values from a finite element program to use for fatigue analysis, (von mises, principal, tresca). I was leaning towards principal stresses but then would my stress range be σ11 from load case 1 (LC1) to σ11 of LC2, or would i need to do σ11-σ33 first and then would my stress range be (σ11-σ33) LC1 to (σ11-σ33) for LC2. Sorry I am a bit confused. Thank you

 

[Mech_Engineer]

It will largely depend on your material for analysis and loading conditions. Ductile materials with combined loading conditions (e.g. shear + tension) will tend to use Von Mises compared to the material's S-N curve. Brittle materials or composites will have very different analysis methods though...