Fatigue-life prediction for loading conditions with very small strain amplitudes, i.e., high-cycle fatigue regime (HCF), is mostly based on S/N diagrams in the range of the fatigue limit. This solely phenomenological approach is reasonable and valuable from the point of view of applicability to design purposes, however a sound mechanistic understanding of the physical reasons for the cyclic life behaviour in the HCF and in particular in the VHCF (very-high-cycle fatigue) range is still lacking. In general, it is assumed that crack initiation mechanisms and short fatigue crack propagation processes govern fatigue life in these regimes. Moreover, it is now becoming accepted that the conventional fatigue limit does not imply complete reversibility of plastic strain. Local slip irreversibility causes crack initiation far below the fatigue limit. However, interaction of the crack tip with microstructural barriers, such as grain boundaries or second phase particles or grains, leads to a decrease and eventually to a stop in the crack-propagation rate.
In the present contribution examples for propagating and non-propagating conditions of short fatigue cracks are given from numerous Ph.D. works carried out in the author`s laboratory on various structural alloys, such as beta and alpha+beta titanium alloys, duplex steels, metastable austenitic stainless steels, and different ferritic and martensitic steels. To classify the results within the scope of predicting the service life for HCF- and VHCF-loading conditions, a numerical model based on the boundary-element method has been developed, where crack propagation is described by means of partially irreversible dislocation glide on crystallographic slip planes in a polycrystalline model microstructure (Voronoi cells). It is demonstrated that this approach is capable to account for the strong scatter in fatigue life for very small strain amplitudes, to model the effect of hydrogen and to contribute to the concept of tailored microstructures for improved cyclic-loading behaviour.