Assessment of surface fatigue crack behavior on NPP piping under seismic loading

Abstract

Nuclear power plants (NPPs) are composed of various types of pipes, and these piping systems are designed, manufactured, and monitored according to strict standards because various types of loads can affect the effective functioning of an NPP. Under the existing design criteria for seismic loading conditions, equipment and facilities of NPPs are designed to exhibit an elastic behavior, and the structural integrity is evaluated by applying linear elastic analysis and stress-based acceptance criteria. However, when an earthquake exceeds the design criteria, a large-amplitude load is repeatedly applied

consequently, the nonlinear behavior of the equipment should be considered in flaw evaluations. In the current procedure for flaw evaluation, the stress intensity factor range is generally used as the main fatigue parameter. However, as this evaluation is an analysis based on linear elastic fracture mechanics, simulating an elastic–plastic behavior is difficult. Therefore, this study aims to develop a fatigue crack growth model that can be used under conditions of large and variable loads such as seismic loads and to analyze the characteristics of crack growth in pipes. In this study, as a factor of crack growth, the concept of change in net-section strain energy, which can be applied to both elastic and plastic regions and has the advantage of simplifying the relationship, was applied. First, the results of finite element analysis on plate specimens were compared with the experimental results of other studies, and we confirmed that these results can be applied regardless of plastic deformation. Moreover, a crack growth model is proposed to predict the crack growth rate. The performance of this model was verified through comparison with experimental results and existing crack equations under variable loads. The validated methodology was extended to semi-elliptical cracks in the piping system. Unlike cracks in plate specimens, a geometrical correction factor was considered owing to the complex shape of cracks in pipes. The crack growth rate could be predicted through a neural net fitting, and the accuracy was confirmed by comparison with actual experiments results. In contrast to the crack evaluation method based on the American Society of Mechanical Engineers standard, the proposed crack growth prediction methodology obtained the results by considering plastic deformation. Thus, the results showed that the proposed method can be used to evaluate the behavior of cracks under various seismic loading conditions.