Effects of mixed strain rates on low cycle fatigue behaviors of austenitic stainless steels in a simulated PWR environment

• Effects of mixed strain rate on low cycle fatigue in PWR water were studied.
• Measured by Fen value was comparable to those estimated by MRA.
• Cyclic stress hardening is dominated by the slower of the mixed strain rate.
• Increase in striation spacing is consistent with increase in dislocation density.

Though strain rates are hardly fixed during the actual transients in nuclear power plants (NPP), most of the existing environmental fatigue ε–N data were obtained under simple loading histories with constant strain rates. In an effort to incorporate the actual loading conditions, strain rate was changed during the low cycle fatigue (LCF) test. The LCF tests were performed in a strain control mode in both simulated pressurized water reactor (PWR) environment and 310 °C air to evaluate the effects of mixed strain rates on fatigue life. The test results indicated that the modified strain rate approach and average strain rate method could be applied to the fatigue evaluation considering environmental effects. The cyclic stress response showed that the trend of hardening with mixed strain rate was consistent with a negative strain rate sensitivity, which was observed in the constant strain rate test. In addition, the increase in striation spacing on the fracture surface is in good agreement with the increase of bulk dislocation density and reduction of fatigue life.