Statistical determination of stress threshold and survival analysis on IASCC initiation data of stainless steels in pressurized water reactor conditions

This paper presents the first attempt to perform a rigorous statistical analysis of the compiled IASCC initiation data of 304 and 316 stainless steels available in the existing literature. First, the stress threshold for IASCC initiation was determined statistically as a function of the neutron dose. The 95% lower bound or 2.5% cumulative failure probability line appeared to be a conservative stress threshold, although it could be adjusted according to the required conservatism. Survival analysis was conducted using the filtered data set for 316 stainless steel IASCC initiation times. A proportional hazards model revealed statistically significant covariates in order of greatest impact: stress level as a percentage of irradiated yield strength, neutron dose, and component. Unsurprisingly, the failure rate was found to increase with increasing stress level and neutron dose. It was also found that FTT materials had higher failure rates than BFB materials. The effects of these covariates on the failure rate were constant with time. An accelerated life model based on a Weibull distribution was then used to model the IASCC initiation time. Two different link functions were used for the model: the link function with a power-law form for the stress level and neutron dose fitted the observed data better than a log-linear type link function.