SHIFT OF S–N CURVES OF FERRITIC NODULAR CAST IRON DUE TO LOADING CYCLE ASYMMETRY

Increasing the mean stress of loading cycle causes a shift of fatigue (Wöhler’s or S–N) curves towards higher values of fatigue strength if the curves are represented by the upper stress of loading cycle in dependence on the number of cycles to fracture. A successful quantitative description of the shift in high-cycle region was published by Walker. The aim of the present paper consists in deriving relations that describe the shift of fatigue curves in the whole cycle region from UTS to permanent fatigue limit. Their applicability is demonstrated by successful fit of the family of fatigue curves representing the fatigue behavior of ferritic nodular cast iron loaded at different loading cycle asymmetries when the curves are described by the Stromeyer equation as well as by the Kohout and Vechet equation. Also the limitation of Walker’s approach in the case of high mean stresses is discussed and a certain solution is proposed.