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In order to investigate the crack path under Mode II or Mode III loadings,reversed torsion tests were carried out on SAE52100 and Mode II fatigue crack growthtests were carried out on 0.47 % carbon steel specimens. In the torsional fatigue test(SAE52100), the type of inclusion in the torsional fatigue fracture origin was slenderMnS inclusions which are elongated in the longitudinal direction. The cracks firstpropagated by Mode II up to crack length 2a = 100 ~ 200 μm (which are almost equalto the length of MnS inclusion) in the longitudinal direction, and then branched byMode I to the direction (~ ± 70.5 deg.) perpendicular to the local maximum normalstress (σθmax ) at the crack tip.In the Mode II fatigue crack growth test (0.47 % carbon steel) in air and in a vacuum,the cracks first propagated by Mode II. After the Mode II fatigue crack growth stopped,the crack branched to the direction perpendicular to the local maximum normal stress(σθmax) at the crack tip, and finally branched to the angle close to the directionperpendicular to the remote maximum principal stresses.A fibrous pattern on the Mode II fatigue fracture surface tested in a vacuum was clearerthan that in air. The Mode II threshold stress intensity factor ranges, ΔKIIth = 10.2 MPam (Longitudinal crack) and ΔKIIth = 12.5 MPa m (Transverse crack) in a vacuumwere higher than those in air, ΔKIIth = 9.4 MPa m (Longitudinal crack) and ΔKIIth =10.8 MPa m (Transverse crack). Both in a vacuum and in air, the values of ΔKIIth forcrack growth perpendicular to the rolling direction were higher than those for crackgrowth parallel to the rolling direction.The values of KII and KIII at a 3D elliptical crack tip under shear stress were analyzedto investigate the shear crack growth pattern in materials. The 3D crack analysis showsthat the most stable aspect ratio b/a of a small planar elliptical crack under cyclic shearstress is b/a = 0.49 in absence of friction at crack surfaces. The aspect ratio b/a = 0.49can be explained by the equal resistance against fatigue crack growth both in Mode IIand Mode III, i.e. ΔKIIth = ΔKIIIth. However, the aspect ratio b/a for the failure of a realrailway wheel did not stay at the stable aspect ratio b/a = 0.49 and continueddecreasing. The cause for the decrease in the aspect ratio b/a smaller than 0.49 wasrevealed to be the friction between crack surfaces.