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In order to investigate the fatigue crack propagation under Mode III,torsional fatigue tests which use shallow micro notches on a SAE 5135 gear steel werecarried out. The initial small cracks were introduced by a preliminary Mode I fatiguetest pre-cracking, in order to promote co-planar crack propagation using the specimenswith the artificial defect. Torsion fatigue tests for a shallow micro defects showed thatstarting from the √area of micro defects equal to the 221.2 μm it was possible to obtainstable co-planar fatigue crack growth in Mode III, at stress levels slightly higher thanthe Mode I threshold ΔKth,I. Moreover, in order to investigate the crack path undermixed mode loading, some in-phase and out-of- phase axial torsion fatigue tests werecarried out. The results of the multiaxial fatigue tests with mode mixity similar to rolingcontact fatigue condition show a stable co-planar growth.