Optical observations of fatigue crack growth behaviour of a low-alloy pressure vessel steel in high-temperature pressurized water

Abstract

Optical observations of fatigue crack growth behaviour of a low-alloy pressure vessel steel in an autoclave filled with high-temperature pressurized water were conducted. The optical system used consisted of a long focal relay lens and an automatic photographing device. Single-crystal sapphire glasses were employed as solid optical media in high-temperature water, to reduce distortions of the observed images caused by the local thermal fluctuation of water, and as pressure boundary windows of the autoclave. Observations were carried out at temperatures from room termperature to 288°C in boiling water reactor environments. Although the total resolving power of the optical system decreased with increasing temperature, it satisfied the requirements of the ASTM Standard E647 even at 288°C. Fatigue crack growth curves were obtained from successive photographs taken at specific intervals in cycles, and were compared with those obtained by a compliance method using a linear variable differential transformer for the same specimen. Good correlations between the two measurements were observed. The temperature dependence of the fatigue crack growth rate in high-temperature water was analyzed and discussed, and the effect of dynamic strain ageing was examined.