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Fatigue strength in presence of inhomogeneities is controlled by the threshold condition of small cracks which nucleate at the defect tip. In biaxial in-phase fatigue strength is controlled by the threshold condition of small cracks at Mode I branches. This paper presents the results of set of torsional fatigue tests in presence of shallow micronotches for a gear steel. The results show that the fatigue strength mechanism under torsion is controlled by the competition between Mode I and Mode III propagation.