Environmental Effect on Fatigue Crack Initiation and Growth of Stainless Steel for Flaw Tolerance Assessment

Fatigue life can be divided into cycles of crack initiation and those in which the initiated crack grows to macroscopic size. In crack growth analysis, it is possible to consider the effect of the strain or stress gradient in the depth direction on the fatigue life. Therefore, flaw tolerance assessments allow reasonable fatigue life prediction. The fatigue life is reduced in the primary water environment of pressurized water reactor (PWR) nuclear power plants, and the correction factor F_en is used for considering the fatigue life reduction in fatigue damage assessments. To apply the flaw tolerance concept to a PWR water environment, the correction factor must be applied not to the fatigue life but to the number of cycles for crack growth. In this study, the fatigue life reduction in the PWR environment was correlated to the crack growth acceleration for a flaw tolerance assessment. The crack growth rates were obtained from fatigue life tests and crack growth tests performed in the PWR environment using Type 316 stainless steel. Then, the fatigue life was estimated by predicting the crack growth from an initial depth of 20 μm. It was concluded that a reasonable flaw tolerance assessment can be performed by using the strain intensity factor. The fatigue life reduction was successfully replaced with the crack growth acceleration.