No abstract

In order to study the subsurface crack initiation and propagation mechanism of high strength steel under a 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 SKH51) 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. This so-called 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 that multiple microcracks were initiated and dispersed by the decohesion of a spherical carbide from the matrix around a nonmetallic inclusion, and converged into the GBF area during the fatigue process. After the formation of the GBF area, interior cracks grew radially and a fish-eye pattern formed on the fracture surface.

No abstract

In order to study the mechanism of the subsurface crack initiation and propagation of high strength steel in very high-cycle region, computational simulation with fracture surface topographic analysis (FRASTA) method was carried out for specimens of high-carbon-chromium bearing steel, JIS SUJ2, obtained from the rotary-bending fatigue test in air. A remarkable area formed around the nonmetallic inclusion inside the fish-eye zone on the fracture surface, which is a feature on the fracture surface in superlong 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 the FRASTA method that multiple microcracks initiate dispersively 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 formation of the GBF area, interior crack grew radially and the fisheye pattern formed on the fracture surface.