Long term reliability of nuclear power plants (NPPs) are especially important for lifetime extension. There are issues that should be concerned to renew the license of a nuclear power plant, especially, environmentally-assisted fatigue (EAF). The American Society of Mechanical Engineers fatigue design curve of nuclear power plants was established by numerous fatigue data obtained in ambient air. The environmental effect was not fully addressed in the fatigue design curve. However, in the laboratory test, the fatigue life of austenitic stainless steel, which is one of the core component materials, was significantly decreased in simulated pressurized water reactor (PWR) primary water. There were many investigations regarding environmental effect depending on various factors, including strain rate, strain amplitude, temperature, hold time, and surface finish, etc. In particular, the strain rate significantly influences the fatigue life of austenitic stainless steels in a simulated PWR primary water environment. The environmental effect increases with decreasing strain rate. However, it is regarded that the strain rate effect is saturated at a specific strain rate although the data is not enough to accurately understand the trend of environmental effect depending on the strain rate. It is desirable to evaluate fatigue behavior in the low strain region because the loading mode of EAF gradually approaches that of stress corrosion cracking (SCC) when the strain rate becomes very low. Thereby, SCC as well as EAF must be carefully taken into consideration for the lifetime extension of the NPPs to be realized by precisely predicting the remaining life of the components because of higher environmental effect at lower strain rate.

The objectives of this research are to achieve the time domain analysis (TDA) to evaluate the crack growth rate (CGR) in the conventional saturation region, to clarify the environmental acceleration in the low strain rate region. The material used in this study was type 316 stainless steel. The EAF tests for the crack initiation stage and the crack propagation stage were conducted using a tubular smooth bar-type and compact tension specimens in simulated PWR primary water at 325ºC. The TDA was used to clarify the presence of saturation of environmental acceleration at very low strain rate region with a special emphasis on crack size. The behavior of short and long crack growth in lower loading rate region was evaluated to clarify the continuity of environmental acceleration on the crack length. The average CGR was well-interpreted on the TDA and showed similar behavior of those indicated in the short crack concerning the strain rate effect. All CGR data sets can be well-interpreted on the TDA and simply compared to each other to understand the most suitable evaluation method of structural integrity by using the CGR on the TDA. The environmental acceleration obtained from the longest crack observed in the cross-section of the tubular smooth specimen was much larger than that obtained from the averaged value. This result clearly indicates that the effect of the environmental acceleration of CGR increases larger in the shorter crack. Therefore, it is very important to measures the environmental acceleration at the very low strain rate precisely by using a very short crack.


Effect of Crack Length on the Environmental Acceleration in Fatigue Crack Growth of 316 Stainless Steel in a Simulated Pwr Primary Water

Division / Sponsors: 
Power Division & Nuclear Engineering Division (NED)

Power Conference
Paper Proceedings: https://asmedigitalcollection.asme.org/POWER/POWER2020/volume/83747

International Conference on Nuclear Engineering
Paper Proceedings: https://asmedigitalcollection.asme.org/ICONE/ICONE2020/volume/83761

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