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.