Fatigue crack path and threshold in Mode II and Mode III loadings

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 growth tests were carried out on 0.47% carbon steel specimens. In the torsional fatigue test (SAE52100), the type of inclusion at the torsional fatigue fracture origin was slender MnS inclusions which were elongated in the longitudinal direction. The cracks first propagated by Mode II up to crack length 2a = 100–200 μm (which are almost equal to the length of MnS inclusion) in the longitudinal direction, and then branched by Mode I to the direction (∼ ± 70.5°) perpendicular to the local maximum normal stress (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 direction perpendicular to the remote maximum principal stresses. A fibrous pattern on the Mode II fatigue fracture surface tested in a vacuum was clearer than that in air. The Mode II threshold stress intensity factor ranges, ΔKIIth = 10.2 MPa m (Longitudinal crack) and ΔKIIth = 12.5 MPa m (Transverse crack) in a vacuum were higher than those in air, ΔKIIth = 9.4 MPa m (Longitudinal crack) and ΔKIIth = 10.8 MPa m (Transverse crack). Both in vacuum and in air, the values of ΔKIIth for crack growth perpendicular to the rolling direction were higher than those for crack growth parallel to the rolling direction. The values of KII and KIII at a 3D elliptical crack tip under shear stress were analyzed to investigate the shear crack growth pattern in materials. The 3D crack analysis shows that the most stable aspect ratio b/a of a small planar elliptical crack under cyclic shear stress is b/a = 0.49 in absence of friction on the crack surfaces. The aspect ratio b/a = 0.49 can be explained by the equal resistance against fatigue crack growth both in Mode II and Mode III, i.e., ΔKIIth = ΔKIIIth. However, the aspect ratio b/a for the failure of a real railway wheel did not stay at the stable aspect ratio b/a = 0.49 and continued decreasing. The cause for the decrease in the aspect ratio b/a smaller than 0.49 was revealed to be the friction between crack surfaces.