Fatigue strength of spring steel under axial and torsional loading in the very high cycle regime

The fatigue strength of an oil-tempered Si–Cr steel for valve springs (JIS G3561, SWOSC-V) was investigated. Smooth specimens without residual surface stresses were fatigued with two kinds of ultrasonic fatigue testing machines to clarify the fatigue properties up to very high numbers of cycles (giga cycle regime) under axial and torsional loading. The maximum inclusion size in the critical volume of a specimen predicted by the extremal statistics is 7.9 μm. Although scatter is somewhat large, the S–N data could be approximated by linear lines in double logarithmic plots for both loading conditions up to the giga cycle regime. The ratio of fatigue strength under torsional and axial loading at the same number of stress cycles is about 0.68 and is almost constant even in the giga cycle regime. Cracks were initiated from the specimen surface under tension–compression as well as torsion loading. No specimen showed crack initiation from the interior. Inclusions and granular facet areas could not be observed. Under torsional loading, cracks initiated either perpendicular or parallel to the longitudinal direction of the specimens. After shear crack propagation to a crack length of about 30 μm, crack branching and mode I propagation took place. The size of characteristic defects calculated on the basis of the propagation threshold of long cracks is much larger than the inclusion size calculated by the extremal statistics.