During reactor operation blunt flaws may form in Zr-2.5Nb pressure tubes that pose no immediate threat to the integrity of the tube. However, these flaws are potential fatigue crack initiation sites that must be dispositioned to address fitness-for-service requirements. An understanding of the effect of the reactor coolant environment on fatigue crack initiation is important in this context.
Fatigue tests were conducted on notched transverse tensile Zr-2.5Nb specimens at 300 °C in heavy-water environment. The loads were applied using a saw-tooth wave form, i.e., with a load rise time between 50 s and 3600 s, with a short unloading time. The effects of applied loading frequency and the effect of hydrides in the metal on fatigue crack initiation in both an air and water environment were investigated. Following fatigue testing, select samples were characterized using transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) to elucidate the microscopic mechanisms for fatigue crack initiation. The fatigue test data and characterization results highlight that fatigue crack initiation in water involves complex synergistic interactions between the mechanical stress, environment conditions, microstructure, oxidation, hydrogen ingress and diffusion.