Influence of Mean Stress on the Fatigue Behavior of 304L SS in Air and PWR Water

Load-controlled experiments on 304L stainless steel were run in Air and PWR water, at 150°C and 300°C, with and without a mean stress of 100MPa. These experiments were run to determine the influence of temperature, environment, and mean stress on the 10⁷ Cycle Fatigue Limit stress amplitude. A 100MPa mean stress was found to have different effects at the different temperatures and environments. In contrast to all the conventional models used to describe the effects of mean stress, when the testing was done at 300°C (for both air and PWR water), a 100MPa mean stress was found to raise the 10⁷ Cycle Fatigue Limit relative to that observed without a mean stress. This was ascribed to the effect of the hardening due to the initial straining and to secondary hardening, both of which are more pronounced at 300°C than at 150°C. The increased initial and secondary hardening resulted in the development of less non-elastic strain, thereby improving the fatigue behavior. In PWR water at 150°C, a 100MPa mean stress reduced the 10⁷ Cycle Fatigue Limit by more than that predicted by conventional mean stress models, but in air at 150°C, the decrease in the endurance limit was more in keeping with the predictions of these models. This difference was ascribed to the effect of the PWR water, in the absence of significant initial straining and secondary hardening.