Fatigue experiments were carried out to investigate the effects of mean-stress and phase-difference on the tension-torsion fatigue life of 30CrMnSiA high strength steel

while the experimental data and plane stress characteristics of different mean stress and phase difference were analyzed. The test results showed that the fatigue life increased with the growing phase difference without mean stress

but decreased with the increase of phase difference when mean stress existed

whether mean normal stress or mean shear stress. Criteria based on the linear combination of the shear stress amplitude and normal stress on the maximum shear stress amplitude plane cannot reflect the test results correctly to some degree. Moreover

the initial crack angle was measured showing that its direction was close to the maximum shear stress amplitude plane. Finally

the defects of the criteria based on the linear combination shear stress amplitude and maximum normal stress on the maximum shear stress amplitude stress plane were explained by stress analysis.