A Study of Subsurface Crack Initiation and Propagation Mechanism of High-Strength Steel by Fracture Surface Topographic Analysis

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.