Fatigue Crack Path and Threshold in Mode II and Mode III Loadings

ABSTRACT. 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 in the torsional fatigue fracture origin was slender MnS inclusions which are 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 deg.) 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 cleare