1【 匂 匹 vords ： Shea← type 飴tigue crack growth ， Biaxia1 飴tigue test ； Bearing steel， Cmck 甲 face friction ；Crack branching

Shear-type fatigue crack behavior in a bearing steel, SAE52100, was investigated in a biaxial fatigue testing machine using cylindrical specimens. Either of the following two types of artificial defect with the total length of 400 ~ 440 %m and the depth of 200 ~ 300 %m was introduced into the specimen surfaces: (a) a semi-elliptical pre-crack emanating from 2 adjacent holes by a tension-compression loading, (b) 3 adjacent holes oriented in the axial direction which had slits made by the focused ion beam technique at the both ends. Fully reversed torsion with a shear stress amplitude at specimen surface, τa = ~ 600 MPa, was applied to the specimens under the static axial compressive stress σm = −1000 ~ −1200 MPa. In case of the defect (a), a shear-type crack propagated from the pre-crack in direction perpendicular to the specimen axis. At the specimen surface, the shear-type crack periodically branched from the crack tip and propagated 5~10 %m in Mode I directions. The shear-type crack growth was decelerated with an increase in the crack length and finally stopped at N ≅ 7.5×106 cycles and 2a ≅ 600 %m. On the other hand, in case of the defect (b), the shear-type crack propagated in axial direction, and the crack growth was accelerated with an increase in the crack length. In addition, the threshold stress in case without the compressive stress on crack-face was determined by a τa-decreasing test. The results revealed that the shear-type crack growth was strongly influenced by the crack-face friction and the existence of the Mode I blanched cracks.

Growth behaviour of shear-type fatigue crack in a bearing steel, JIS SUJ2, was investigated by a biaxial fatigue testing machine using round specimens. Either following two kinds of defect with total length 400〜440μm and depth 200-300μm was introduced into specimen surface ; (1) a semi-elliptical pre-crack emanating from a 2-hole defect by a tension-compression loading, (2) a 3-hole defect oriented in the axial direction which has FIB slits at the edge of the defect. Fully reversed torsion corresponding to shearing stress amplitude at specimen surface, τ_a=600MPa, was applied to each specimen under static compressive stress σ_m=-1000〜-1200MPa. In case of defect (1), a shear-type crack propagated from the pre-crack in direction perpendicular to the specimen axis. At the specimen surface, the shear-type crack periodically branched from the crack tip and propagated 5〜10μm in Mode I directions. The shear-type crack growth was decelerated with the increase in the crack length and finally stopped at N≅7.5×10^6 cycles and 2a≅600μm. On the other hand, in case of defect (2), the shear-type crack propagated in axial direction, and the crack growth was accelerated with the increase in the crack length. These results revealed that friction between crack faces is a strong resistance in the shear-type crack growth.