Subsurface Crack Initiation and Propagation Mechanism in the Super-Long Fatigue Regime for High Speed Tool Steel, JIS SKH51, by Fracture Surface Topographic Analysis

Abstract

In order to study the subsurface crack initiation and propagation mechanism of high strength steel in very high cycle fatigue regime, computational simulation with fracture surface topographic analysis (FRASTA) was carried out for subsurface fatigue crack initiated specimens of high speed tool steel, JIS SKH 51, obtained from the rotating bending fatigue test in air. A remarkable area formed around the nonmetallic inclusion inside the fish-eye region on the fracture surface, which is a feature on the fracture surface in super long fatigue and named as GBF (granular-bright-facet), was observed in detail by a scanning probe microscope and a three-dimensional SEM. The GBF area, in which a rich carbide distribution was detected by EPMA, revealed a very rough and granular morphology in comparison with the area inside the fish-eye. It was clearly simulated by FRASTA method that multiple microcracks initiated depressively by the decohesion of spherical carbide from the matrix around a nonmetallic inclusion and coalesce each other into the GBF area during fatigue process. After the formation of GBF area, interior crack grew radially and the fish-eye pattern formed on the fracture surface.