How do you approach multiaxial fatigue analysis when principal stress directions rotate?

Non-proportional multiaxial loading with rotating principal directions is most damaging. Critical plane approaches identify the plane with maximum damage using combinations of normal and shear strain. The Fatemi-Socie parameter uses maximum shear strain with normal stress correction. Brown-Miller criterion combines shear and normal strains. Additional hardening from non-proportional loading reduces fatigue life by factors of 2-10. FEA with critical plane post-processing is standard practice.