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.

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

“…The experimental results obtained from the high speed tool steel, JIS SKH51, were almost same as a highcarbon-chromium bearing steel, JIS SUJ2, which contains 1.01%w/w carbon as reported previously (19), (20) . Based on the experimental results, a mechanism for the formation of the GBF area during the high-cycle fatigue process is proposed as a 'dispersive decohesion of spherical carbide' model, which was proposed for a high-carbon-chromium bearing steel in previous papers (19), (20) .…”

“…Based on the experimental results, a mechanism for the formation of the GBF area during the high-cycle fatigue process is proposed as a 'dispersive decohesion of spherical carbide' model, which was proposed for a high-carbon-chromium bearing steel in previous papers (19), (20) . The model is illustrated in Fig.…”

“…The formation mechanism of the GBF area has yet to be elucidated, because it is difficult to directly observe the behavior of subsurface crack initiation and propagation. The authors have proposed the mechanism of GBF formation in a very high cycle fatigue regime as a 'dispersive decohesion of spherical carbide' model, based on the experimental results of detailed observations of fracture surface using specimens of high-carbon-chromium bearing steel, JIS SUJ2, of which the carbon content is 1.01%w/w (19), (20) . The aim of this study is to clarify the mechanism of the formation of a GBF area in the vicinity of an inclusion, and to verify the adequacy for the model of dispersive decohesion of spherical carbide.…”

Very high cycle fatigue behaviour of 2000‐MPa ultra‐high‐strength spring steel with bainite–martensite duplex microstructure

“…This rough region was first named ODA (optically dark area) by Murakami et al when observed by optical microscopy 13 . It was also named GBF (Granular bright facet), by Shiozawa et al; 14 FGA (fine granular area), by Sakai et al; 15 FCT (facet), by Tanaka et al; 16 and RSA (rough surface area), by Ochi et al 17 when observed by SEM. It is called GBF in the present paper.…”

The influence of surface layer microstructure evolution of M2 steel cold-ring rolling mandrel roller on fatigue crack initiation

“…Therefore the major part of fatigue life is consumed in the period of fatigue crack initiation. Thus the fatigue life of high strength steel largely depends on the length of crack initiation period [1][2][3].…”