Abstract
Understanding the mechanisms leading to very high cycle fatigue is necessary to make predictions about the behavior under various conditions and to ensure safe design over the whole lifetime of high-performance components. It is further vital for the development of possible measures to increase the very high cycle fatigue strength. This review therefore intends to give an overview of the properties of the fine granular area that have been observed so far. Furthermore, the existing models to describe the early crack initiation and crack growth within the very high cycle fatigue regime are outlined and the models are evaluated on the basis of the identified fine granular area properties. The aim is to provide an overview of the models that can already be considered refuted and to specify the respective open questions regarding the other individual models.
Highlights
The fatigue failure beyond the traditional fatigue limit above 107 cycles is called very high cycle fatigue (VHCF) [1], ultra-high cycle fatigue [2], ultralong life fatigue [3], as well as very high fatigue life regime or gigacycle regime [4]
Concerning the model first postulated by Murakami et al [9], which is based on hydrogen-induced short crack growth, resulting in the granular area, the following questions arise in consideration of the present state of knowledge; while trapped hydrogen is observed around non-metallic inclusions, influencing the crack propagation of short cracks, only the rough surface can be explained by hydrogen-induced short crack propagation
Because of the fact that FGA corresponds to a three-dimensional fine-grained layer, early models that are based on hydrogen-induced short crack propagation or the decohesion of carbides, that neither take grain refinement into account nor explain it can be regarded as disproven
Summary
The fatigue failure beyond the traditional fatigue limit above 107 cycles is called very high cycle fatigue (VHCF) [1], ultra-high cycle fatigue [2], ultralong life fatigue [3], as well as very high fatigue life regime or gigacycle regime [4] This fatigue regime is constantly gaining attention because of the efforts in lightweight construction and the resulting use of highly stressed components, as well as the higher traveling speed combined with the demand of high service life. Sakai et al [8] can be observed in the HCF-regime a unique change in microstructure in the direct vicinity of the crack-initiating defect is observed in VHCF-regime. This characteristic microstructural feature in the VHCF-regime was designated “optically dark area” (ODA) [9], “granular bright facet”
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