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.

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

Very high cycle fatigue properties of bainitic high carbon–chromium steel

Notch effect on fatigue strength reduction of bearing steel in the very high cycle regime

Very high cycle fatigue properties of bearing steel under axial loading condition

The nature and the mechanism of crack initiation and early growth for very-high-cycle fatigue of metallic materials – An overview

In order to investigate the fatigue properties of high-strength steels in the very long-life regime up to over 10 9 cycles, cantilever-type rotating bending fatigue tests were carried out for two kinds of high-strength steels, SUJ2 and SNCM439, which were machined by grinding and finished by electropolishing after grinding. And also, the residual stress on the specimen surface of the ground specimen was examined by X-ray diffractometer in order to investigate effects of the residual stress on the fatigue properties. From the investigations, the S-N curves clearly have a tendency to decrease again in the longer-life range over 10 7 cycles for both types of specimen and for both steels. From observations of fracture surfaces, it was found that fatigue crack origins could be grouped into two types: (i) 'surface crack origin type' in the shorter-life regime and (ii) 'internal crack origin type' in the longer-life regime.

S. Yoshida

Papers

High-cycle rotating bending fatigue property in very long-life regime of high-strength steels

Development of estimation method for material property under high strain rate condition utilizing experiment and analysis