抄録
The fatigue crack growth mechanism under Mode II and Mode III loadings and the threshold stress intensity factor ranges, ΔK_<IIth> and ΔK_<IIIth>, were investigated on an annealed 0.47% carbon steel (JIS S45C). The dependency of ΔK_<IIIth> on crack size was also investigated. The layers of microstructural change were observed in the vicinity of Mode II and Mode III fatigue cracks. The Vickers hardness of the layers was much higher than that of the original matrix (Mode II: HV=196→629, Mode III: HV=175→438). The layers consisted of very fine grains (Mode II: 102nm, Mode III: 64nm). Thus, the mechanism of Mode II fatigue crack growth is presumed to be essentially identical to that of Mode III fatigue crack growth. The Mode III fatigue crack growth threshold ΔK_<IIIth> had a dependency on crack size. For the large crack size, √<area>_p ≈ 4900μm, ΔK_<IIIth> (9.6 MPa√<m>) was higher than ΔK_<Ith> (7.8 MPa√<m>) and the factory roof morphology was formed after ΔK_<III> reached ΔK_<IIIth>. For small cracks of √<area>_p < 1000μm (√<area>_p ≈ 450 and 140μm), ΔK_<IIIth> (8.2 and 5.2 MPa√<m>) were approximately equal to ΔK_<Ith> (7.3 and 5.0 MPa√<m>, respectively), and the factory roof morphology was formed after ΔK_<III> reached ΔK_<IIIth>.
