Strain controlled LCF testing extended to 10 million cycles revealed an abrupt endurance limit enforced by secondary hardening. In elevated temperatures the ε-N curve is rotated and endurance limit is lowered, but not vanished. When very low strain rates are applied at 325°C in simulated PWR environment, fatigue life is reduced, but far less than predicted according to NUREG/CR-6909. It is possible, but not probable that the difference is due to different stainless grades studied. We assume that the test method plays a more important role.
We have repeatedly demonstrated in different tests campaigns that interruptions of straining with holds aiming to simulate steady state normal operation between fatigue relevant cycles can notably extend the fatigue endurance. Further proof is again presented in this paper. The suspected explanation is prevention of strain localization within the material microstructure and also in geometric strain concentrations. This actually suggests, that hold effects should be even more pronounced in real components.
Cyclic behavior of austenitic steels is very complex. Transferability of laboratory data to NPP operational conditions depends on test environment, temperature, strain rate and holds in many ways not considered in current fatigue assessment procedures. In addition to penalty factors, also bonus factors are needed to improve transferability. Furthermore, it seems that the load carrying capacity of fatigued stainless steel is not compromised before the crack growth phase. Tensile tests performed after fatigue tests interrupted shortly before end-of-life condition in 325°C (N ≈ 0.85 × N25) showed strength and ductility almost identical to virgin material.
This paper provides new experimental results and discusses previous observations aiming to sum up a state of the art in fatigue performance of German NPP primary loop materials.
