Transition of fatigue failure mode of Ni–Cr–Mo low-alloy steel in very high cycle regime

Transition of the fatigue failure mode in very high cycle regime was investigated based on the experimental results obtained under a fully reversed tension–compression test in air at room temperature using a hourglass shaped specimen of a Ni–Cr–Mo low-alloy steel, JIS SNCM439. A clear duplex S–N curve existed in an emery-polished specimen as a result of the change from surface-induced failure to internal-induced failure with GBF area. On the other hand, a smooth and continuous S–N curve appeared in a specimen treated by a shot-peening, even though the failure mode changed depending on applied stress amplitude level. An internal-induced failure without GBF area was observed on the fracture surface of the shot-peened specimen in addition to the above two failure modes. Stress amplitude for the transition of the failure mode was explained by considering an effect of compressive residual stress in the surface layer, and competing subsurface crack growth rate with surface one. A map for transition of failure mode was proposed through the experimental and theoretical investigation.