Characteristics of Fatigue Crack Growth Rates for Ferritic Steels in Water Environment Provided by the ASME Code Section XI

Reference fatigue crack growth rates da/dN for ferritic steels in boiling water reactor environment are provided in Appendix Y of the ASME Code Section XI. Therein, da/dN are expressed as a function of stress intensity factor range ΔK and as a function of the stress ratio parameter R. Different expressions apply for low ΔK and high ΔK. Current expressions result in lower fatigue crack growth rates in the water environment compared to the air environment for certain values of ΔK. Therefore, the ASME Code states that reference fatigue crack growth rates in water environments are given by the expressions which results in higher fatigue crack growth rates. Sample calculations on fatigue crack growth length a versus number of cycles N were conducted for R = 0.25 and = 0.9 in the range from the fatigue threshold ΔKth to approximately ΔK = 80 MPa√m. As the fatigue crack growth rate da/dN in air is faster than that in water for lower values of ΔK, crack growth length a in air is larger than a in water. The crack growth length a in water is significantly lower in the water environment, when the crack starts to grow near the ΔKth. The crack length a for R = 0.9 was calculated as low as 5 mm after N= 1.0E+09 cycles, which means that fatigue crack growth rates in water for R = 0.9 at low ΔK area are not necessary. At around ΔK = 4.5 to 7.5 MPa√m, da/dN in water is faster than that in air. When calculating the crack growth length by combining the air and water expressions whichever results in higher da/dN, the calculated crack is larger than original cracks in air and water, as expected. Hence, it is recommended to keep the current ASME code procedure for the fatigue crack growth rate of ferritic steels in water environment.