Fatigue stress ratio effect on fretting-fatigue crack nucleation: comparison between multi-axial and uni-axial predictions

This research focuses on high cycle fretting fatigue crack nucleation prediction. A plastic steel on steel cylinder/plane contact was investigated keeping constant the normal force and the maximum fatigue stress but varying the fatigue stress ratio (R-F=0.6 to 1). The evolution of the crack length as function of the applied fretting tangential force amplitude at 10(6) cycles allows us to formalize the crack nucleation condition. It shows that the threshold tangential force marking the crack nucleation (i.e. b(p_th)=-0 mu m) is not affected by the fatigue stress ratio. But an increase of the fatigue stress amplitude sharply increases the crack extension. To model the experiments, a 2D plastic plain strain FEM modeling is performed. As expected, the computed stress field description is mesh-dependent. However we demonstrate that a representative stress description is obtained at the 3rd node (i.e. 2nd node below the surface). By coupling this mesh condition and non-local critical distance approach, reliable prediction of the crack nucleation risk can be achieved either considering a Crossland multi-axial fatigue analysis or using a basic uni-axial Haigh's description.