Fatigue Crack Growth Threshold of Austenitic Stainless Steels in Simulated PWR Primary Water

Many studies have revealed that fatigue crack growth (FCG) rate of austenitic stainless steels is accelerated in light water reactor environment compared to that in air at room temperature. Major driving factors in the acceleration of FCG rate are stress ratio, temperature and stress rise time. Based on this knowledge, FCG curves have been developed considering these factors as parameters. Hewever, there are few data of FCG threshold ΔKth in light water reactor environment. Hence it is necessary to clarify FCG rate under near-threshold condition for more accurate evaluation of fatigue crack growth behavior under cyclic stress with relatively low ΔK. In the present study, therefore, ΔKth was determined for austenitic stainless steels in simulated PWR primary water, and FCG behavior under near-threshold condition was revealed by collecting fatigue crack propagation data. The results are summarized as follows : No propagation of fatigue crack was found in high temperature water, and there was a definite ΔKth. Average ΔKeff, th was 4.3MPa·m0.5 at 325°C, 3.3MPa·m0.5 at 100°C, and there was no considerable reduction compared to currently known ΔKeff, th in air. Thus, it was revealed tha ambient conditions had minimal effect, on ΔKeff, th, ΔKth increases with increasing temperature and decreasing frequency. As a result of fracture surface observation, oxide-induced-crack-closure was considered to be a cause of the dependency described above. In addition, it was suggested that changes in material properties also had influence on ΔKth, since ΔKeff, th itself increased at elevated temperature.