Diversity of damage evolution during cyclic loading at very high numbers of cycles

Abstract

Over the last decade, it has been shown for a number of metals that failure occurs even beyond the classical fatigue limit. High frequency testing techniques make it possible to conduct fatigue tests up to 10⁹ or even 10¹¹ cycles for various alloys, ranging from aluminium to high strength steels. As a consequence, the characterisation of fatigue life and damage mechanisms at N>10⁷ cycles [very high cycle fatigue (VHCF)] has become a major research issue. Fatigue life in this regime is dominated by crack initiation. With the overall strain being in the purely elastic range, microstructural features acting as stress raisers lead to localised and inhomogeneously distributed irreversible deformation. Hence, microstructural discontinuities become the leading features controlling fatigue life at very high numbers of cycles. The present survey will focus on dislocation arrangement, grain orientation, grain size and surface roughening and their implications on the VHCF behaviour for selected virtually defect free metals, thus providing a sound basis for a detailed understanding of the relevant deformation and damage evolution mechanisms. It will also focus on the VHCF behaviour of materials representing a ‘transition’ between non-defect related damage evolution and defect based crack initiation, thus pointing out the complexity of damage evolution in the VHCF range. In this context, the term defect is limited to hard non-metallic inclusions, which can be found, among others, in high strength steels as well as pores in casting materials, both dominating the VHCF behaviour of these material types. In contrast, second phases, precipitates or intermetallic particles are considered as irregularities of the microstructure and will not be classified as defects. The current review will show that a true understanding of the VHCF behaviour requires a careful differential analysis of the possible microstructural features leading to localised plastic deformation and that not only crack initiation, but also crack growth behaviour analysis is essential to gain a sound basis for a reliable fatigue life prediction.