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The influence of mean shear stress on torsion very high cycle fatigue (VHCF) for internal inclusion‐initiated fracture is investigated. Ultrasonic torsion fatigue tests are performed at load ratios R = −1, 0.1, and 0.35 with high‐strength spring steel SWOSC‐V featuring an increased number of nonmetallic inclusions. Based on Murakami and Endo's area parameter model, an equation predicting the torsion fatigue limit in the presence of detrimental defects is rendered. Stress biaxiality is considered using a ratio of torsion to axial fatigue strength of τa/σa = 0.86. Considering the load ratio sensitivity according to ([1 − R]/2)α, the value for α is experimentally determined to be 0.550. A fine granular area (FGA) is generated at an artificial surface defect in a fully reversed cyclic torsion test in vacuum. Measured growth rates in the FGA were 10−14–10−12 m/cycle, and the formation of the FGA consumed 98% of the VHCF lifetime.

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Mean torsional stress is considered to less affect torsional fatigue strength of steels, but several experimental results have been recently reported that mean torsional stress caused significant reduction in torsional fatigue strength in the very high cycle region for shot-peened spring steel. To investigate the effect of mean torsional stress on high strength steel, ultrasonic torsional fatigue tests with mean torsional stress were conducted for spring steel and bearing steel, which are used for mechanical components subjected to cyclic shear stress. Torsional fatigue strength up to 10 9 cycles were obtained for fully reversed torsional loading (□ = −1) to pulsating torsional loading (□ = 0). The results revealed that mean torsional stress caused reduction in fatigue strength in the very high cycle region for both spring steel and bearing steel, and applying higher mean shear stress would result in transition of the fracture origin from surface to an internal inclusion. The reduction in torsional fatigue strength was discussed from the viewpoint of the transition of fatigue origin, and applicability of a √□□□□ parameter model was discussed for predicting the reduction in torsional fatigue strength.