The combined high and low cycle fatigue and cracking behavior of 316H stainless steel at 650°C

The effects of the superimposition of high-cycle fatigue (HCF) loadings on the low-cycle fatigue (LCF) behavior of 316H stainless steel were studied. The combined high and low cycle fatigue (CCF) tests with different HCF durations (0s, 120s, 300s, 600s, 1800s) were performed at a constant LCF and HCF strain amplitude of 0.4% and 0.08%. Results showed that the material presented the initially cyclic hardening prior to stress stabilization in both LCF and CCF tests, however, the stress amplitude got decreased with the introduction of HCF loadings. The superimposition of HCF loading significantly reduced the LCF life, and the CCF life was gradually decreased with the increasing HCF durations. The cracking behavior demonstrated the single crack initiation source in CCF tests but the multiple initiation sources in LCF tests. Regarding the crack propagation behavior, the intergranular-transgranular mixed mode was observed in LCF tests. It was thoroughly transgranular mode in CCF tests, corresponding to a much flatter fractography morphologies. The mechanisms of oxidation-assisted crack initiation and propagation behaviors were revealed. Moreover, the crack growth rate was markedly accelerated in CCF tests due to the formation of secondary fatigue striations which bridged the adjacent LCF cracks.