Inclusion initiated fracture in spring steel under axial and torsion very high cycle fatigue loading at different load ratios

Inclusion initiated fracture under axial and torsion very high cycle fatigue loading at load ratios R = -1, 0.1 and 0.35 is compared. High strength spring steel with intentionally increased density of aluminate inclusions was tested at ultrasonic frequency. Failures under axial loading started at internal aluminate inclusions at all load ratios, and at aluminate and MnS inclusions in cyclic torsion at positive load ratios. Considering crack initiation at same size aluminate inclusions, a cyclic torsion stress amplitude at the inclusion that is a factor 0.86 +/- 0.05 smaller than the cyclic tension stress amplitude has an equally detrimental effect on cyclic strength.