Dominant factors for very-high-cycle fatigue of high-strength steels and a new design method for components

Dominant factors affecting fatigue failure from non-metallic inclusions in the very-high-cycle fatigue (VHCF) regime are reviewed, and the mechanism for the disappearance of the conventional fatigue limit is discussed. Specifically, this paper focuses on the following: (i) the crucial role of internal hydrogen trapped by non-metallic inclusions for the growth of the optically dark area (around the non-metallic inclusion at fracture origin), (ii) the behaviour of the crack growth from a non-metallic inclusion as a small crack and (iii) the statistical aspects of the VHCF strength, in consideration of the maximum inclusion size, using statistics of extremes. In addition, on the basis of the aforementioned findings, a new fatigue design method is proposed for the VHCF regime. The design method gives the allowable stress, sigma(allowable), for a determined design life, N-fD, as the lower bound of scatter of fatigue strength, which depends on the amount of components produced.