Environmental Effects on Fatigue Crack Initiation in Piping and Pressure Vessel Steels (NUREG/CR-6717, ANL-00/27)
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Manuscript Completed: October 2000
Date Published: May 2001
O.K. Chopra, W.J. Shack
Argonne National Laboratory
9700 South Cass Avenue
Argonne, Illinois 60439
J. Muscara, NRC Task Manager
Division of Engineering Technology
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
The ASME Boiler and Pressure Vessel Code provides rules for the construction of nuclear power plant components. Appendix I to Section III of the Code specifies fatigue design curves for structural materials. However, the effects of light water reactor (LWR) coolant environments are not explicitly addressed by the Code design curves. Test data illustrate potentially significant effects of LWR environments on the fatigue resistance of carbon and low-alloy steels and austenitic stainless steels. This report summarizes the work performed at Argonne National Laboratory on the fatigue of piping and pressure vessel steels in LWR coolant environments. The existing fatigue S-N data have been evaluated to establish the effects of various material and loading variables, such as steel type, strain range, strain rate, temperature, and dissolved-oxygen level in water, on the fatigue lives of these steels. Statistical models are presented for estimating the fatigue S-N curves for carbon and low-alloy steels and austenitic stainless steels as a function of material, loading, and environmental variables. The influence of reactor environments on the mechanism of fatigue crack initiation are discussed. Decreased fatigue lives of carbon and low-alloy steels and austenitic stainless steels in water are caused primarily by the effects of environment on the growth of short cracks. The results suggest that for carbon and low-alloy steels, the growth of these small cracks in high-purity oxygenated water occurs by a slip oxidation/dissolution process. A fracture mechanics approach has been used to evaluate the effects of environment on fatigue crack initiation in carbon and low-alloy steels. Environmentally assisted reduction in fatigue life of austenitic stainless steels is most likely caused by other mechanisms such as hydrogen-enhanced crack growth. Two methods for incorporating environmental effects into the ASME Code fatigue evaluations are discussed. Differences between the methods and their impact on the design fatigue curves are also discussed.