Implications of dynamic strain aging under LCF-HCF interactions in a type 316LN stainless steel

Influence of dynamic strain aging (DSA) under sequential low cycle fatigue (LCF) and high cycle fatigue (HCF) loading was investigated by conducting HCF tests on specimens subjected to prior LCF cycling over a wide range of temperature from 573 to 973 K. DSA was found to be pronounced at 823-873 K depending on the magnitude of the stress employed under HCF cycling. DSA was seen to have contrasting implications under LCF and HCF deformation resulting in an anomalous fatigue behavior in terms of remnant HCF life under LCF-HCF interaction. LCF-HCF interaction was found to be pronounced at intermediate levels of prior LCF exposure, where the remnant HCF life is dictated by competitive damage mechanism resulting from the influence of DSA under LCF as well as HCF. Detailed fracture surface examination revealed that extensive hardening associated with DSA leads to an extended zone of faceted appearance with river markings (Stage-I crack) under HCF cycling (with or without LCF exposure). This reduces the crack growth rate, delaying the transition of crack from Stage-I to Stage II, thereby leading to an extension of life in such cases. On the other hand, a highly striated fracture surface indicating a quick transition in crack from Stage-I to Stage-II, was observed for loading conditions with minimal or no influence of DSA, thus leading to lower life compared to the previous case.