Effects of inclusions on very high cycle fatigue properties of high strength steels

The effect of inclusion size on fatigue behaviour of high strength steels in the very high cycle fatigue (VHCF) regime (>10(7) - 10(9) cycles) is reviewed. Internal fatigue fractures of high strength steels in the VHCF regime initiate mostly at non-metallic inclusions. The critical inclusion size below which it is hard to initiate fatigue cracking of high strength steels in the VHCF regime is found to be about half the critical value characteristic of the high cycle fatigue (HCF) regime (about 10(5) - 10(7) cycles). A stepwise or duplex S-N curve is observed in the VHCF regime. The shape and form of the S-N curves are affected by inclusion size and other factors including surface condition, residual stress, environment and loading modes. Fatigue strength and fatigue life for high strength steels have been found to obey inverse power laws with respect to inclusion size D of the form sigma(w)proportional to D-n1 and N-f proportional to D-n2 respectively. For fatigue strength, the exponent n(1) has been reported to be similar to 0.33 in the literature for the HCF regime and, more recently, to fall in the range 0.17-0.19 for the VHCF regime. For fatigue life, the exponent n(2) is reported to be similar to 3 in the HCF regime, and in the range 4.29-8.42 in the VHCF regime. A special area was often observed inside a 'fish eye' mark in the vicinity of a non-metallic inclusion acting as the fracture origin for specimens having a long fatigue life. The major mechanisms of formation for this special area are discussed. To estimate the fatigue strength and fatigue life, it is necessary to know the size of the maximum inclusion in a tested specimen, and to be able to infer this value using data from a small volume of steel. The statistics of extreme value (SEV) method and the generalised Pareto distribution (GPD) method are introduced and compared. Finally, unresolved problems and future work required in studying the VHCF of high strength steels are briefly presented.