A mechanistic approach to the life prediction of 316L stainless steel under combined creep/fatigue cycling

In this work, the relaxation stress behaviour of a 316L stainless steel cycled at high temperature has been analysed and a critical strain rate, which marks the transition from a transgranular to an intergranular dominated damage regime, was obtained. This strain rate has been used to partition the relaxation strain into its transgranular and intergranular components. Coffin-Manson type relationships for both damage regimes have been derived and the corresponding intrinsic parameters, ductility and irreversibility damage coefficient, determined. A cumulative damage rule, resulting from equalling the sum of the transgranular and intergranular damages to unity, allows the calculation of the material life under creep/fatigue conditions. These predicted values have been tested against previously reported data obtained from the same material. It is shown that, irrespective of the imposed strain range, the duration of the hold period and the ratio between the transgranular and intergranular components of the damage, predicted and experimental lives are in close agreement. The points determined by the calculated transgranular and intergranular components of the overall damage fall closely along the failure locus as defined by the cumulative damage rule.