Influence of mean stress on the multiaxial fatigue behaviour of defective materials

Artificial spherical defects (400 μm) are introduced at the surface of C35 steel fatigue samples. Fatigue tests are conducted under tension and torsion with different mean stress levels in order to determine the fatigue limit. Experimental results show that the effect of mean stress under both torsion and tension is qualitatively the same than for the defect free material. Under torsion, the fatigue limit decreases when the applied mean stress is higher than the yield stress, below this value, it remains constant. Experimental results are analysed using two multiaxial criteria (Dang Van and Crossland) and Murakami’s models: comparison with experimental results shows that the analysis should take into account for the local stress distribution around the defect. Elastic–plastic FE simulations are conducted: isotropic and kinematic hardenings are compared to determine stress distribution around the defect. A nonlocal criterion based on stress gradient is applied and results using the two different hardening laws are compared. It is shown that the isotropic hardening gives good results when the fatigue test is conducted for a load ratio R = −1; kinematic hardening gives good results when R ≠−1.