Fatigue, Cyclic Deformation and Microstructure. Subsurface Crack Generation in High-cycle Fatigue for High Strength Alloys.

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Abstract
In order to progress in the understanding of fatigue fracture process for high strength alloys, the previous studies of the subsurface fatigue crack generation are reviewed. A change in crack initiation mechanism from specimen surface to specimen interior often introduces a plateau (knee) followed by a rather sharp drop in the shape of S-N curve. Various subsurface crack origins are related with microstructural crackings and pre-existing defects. The subsurface initiation site is formed as a Stage I crack. A new explanation of microcrack growth is proposed for the subsurface crack generation process. The subsurface crack size is the most important parameter to determine how the crack becomes a fatal crack. The size highly depends on the maximum cyclic stress range, which implies a ΔKth threshold controlling mechanism. The dislocation structures in high-cycle fatigue are fairly planar for both austenitic steel and titanium alloy

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