Fatigue Behavior of Pressure Tube Material Zr-2.5Nb in Air and in Simulated CANDU-Reactor Water Environments

Abstract

An experimental program was initiated to develop fatigue data in strain-controlled conditions for an assessment of the fatigue behavior of Zr-2.5Nb pressure tube material in support of CANDU reactors for Atomic Energy of Canada Limited (AECL)/Chalk River Laboratories (CRL). Fatigue tests were conducted on specimens fabricated from a Zr-2.5Nb pressure tube to determine the fatigue performance in air and in simulated CANDUreactor water environments. This report summarizes the fatigue data of Zr-2.5Nb material obtained in both environments. Fatigue tests in air were performed under constant total strain amplitudes in the fully reversed (R=-1) strain control. Fatigue behavior of Zr-2.5Nb pressure tube material was examined over a range of total strain amplitudes from 0.4 to 0.75% at 300°C with a balanced triangular wave form (equal strain rates in tension and in compression) at a strain rate of 0.4%/s. The temperature dependence of the fatigue response was examined at 20 and 100°C at total strain amplitude of 0.6%. The strain rate dependence of the fatigue performance was investigated by applying an unbalanced triangular waveform with a slower strain rate of 0.004%/s in tension and a faster strain rate of 0.4%/s in compression. Fatigue tests in water environments were performed under the stroke control mode. The axial stroke response was correlated with the axial strain control from the air tests, and was used to provide control for the fatigue tests in the autoclave system. Fatigue behavior of Zr-2.5Nb pressure tube material was examined over a range of total strain amplitudes from 0.45 to 0.75% at 300°C with an unbalanced triangular waveform with a slower strain rate of 0.004%/s in tension and a faster strain rate of 0.4%/s in compression in lowdissolved oxygen (DO) LiOH-containing water. Fatigue data were analyzed to obtain the strain-life relation, the cyclic stress response during fatigue cycling, half-life hysteresis loops, and half-life cyclic stress-strain curves. The cross-sectional fracture surfaces and longitudinal surfaces of the gage section of tested specimens were examined by scanning electron microscopy (SEM) to understand the fatigue crack initiation and propagation behavior in this material. The fatigue life of Zr-2.5Nb was significantly reduced in simulated CANDU-reactor water environments at 300°C, compared to the fatigue data obtained in air. The synergistic effect of the aqueous environment and the strain rate is most detrimental to the fatigue resistance of Zr-2.5Nb. The fatigue life of Zr-2.5Nb in water under the slow-fast cyclic loading was reduced in both high-strain and low-strain amplitudes when compared with the fatigue data in air under the fast-fast cycling. The influence of the aqueous environment on the fatigue resistance is more pronounced in the high strain amplitudes than in the low strain amplitudes when compared with the fatigue data in air under slow-fast cyclic loading. Significant reduction in fatigue life was observed at 300°C in air when a lower strain rate (0.004%/s) was ap...