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

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.