Mean stress sensitivity of spring steel in the very high cycle fatigue regime

The influence of load ratio on the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) properties of shot-peened VDSiCr spring steel is investigated. S–N curves are measured with ultrasonic fatigue testing equipment at load ratio R = −1, R = 0.1 and R = 0.5 up to 1010 cycles. Failures either occur below 5 × 106 or above 5 × 108 cycles. In the HCF regime cracks start exclusively at the surface. In the VHCF regime crack initiation occurs solely in the interior, at grain boundaries, inclusions or (rarely) in the matrix. The S–N curves continue to decrease beyond 109 cycles, most pronounced for R = −1 where the mean cyclic strength at 1010 cycles is 5 % lower than at 109 cycles. VHCF strength presented in a Haigh diagram can be very well approximated with a straight line. The mean stress sensitivity factor is M = 0.47 for load ratios between R = −1 and R = 0.5.