Session: SE-09-01 Advanced Seismic Evaluation and Code (Joint session with C&S)

Paper Number: 152806

152806 - Simplified Elastic-Plastic Modeling for Fatigue Analysis of Piping Systems in Seismic Pra 

Abstract: 

In Japan, seismic probabilistic risk assessment of nuclear power plants is required from the perspective of voluntary safety improvement. Seismic probabilistic risk assessment necessitates the fragility evaluation of individual equipment and structures. Especially, piping systems are important equipment from a safety perspective. Because piping systems function as pressure boundary systems.

In fragility evaluation, it is necessary to consider realistic failure modes, and shaking table tests have confirmed that the realistic failure mode for piping systems is fatigue. Fragility evaluation that takes into account fatigue as failure mode requires elastic-plastic strain-time histories. To obtain an elastic-plastic strain-time history, finite element analysis (FEA) is needed. However, since FEA is time-consuming, applying FEA to numerous piping systems is impractical. Therefore, there is a need to develop a method to evaluate the fatigue of piping systems without using FEA.

The authors have developed a method for fragility evaluation in which the piping system is modeled with a single degree of freedom system composed of an elastic spring, a damper, and a mass, to evaluate fatigue. In this method, the displacement amplitude-time history of the mass qualitatively matched the strain amplitude-time history obtained from elastic FEM. By converting displacement amplitude to strain referring to the peak stress method described in JEAC4601, which is the seismic safety design standard in Japan, the converted strain amplitude-time history accurately matched the strain amplitude-time history obtained from elastic FEA. However, since this method uses an elastic spring, it did not correspond to elastic-plastic FEA results. In this study, the authors developed a method to predict elastic-plastic strain, modeling the piping system with a single degree of freedom system composed of an elastic-plastic spring, a damper, and a mass.

In this method, the load-displacement relationship of the elastic-plastic spring was approximated using a bilinear model, and the mass of the single degree of freedom system is set equal to that of the piping system. The initial stiffness of the load-displacement relationship is set so that the natural frequency of the single degree of freedom system matches the first natural frequency of the piping system. This is because the first natural frequency which causes large strain amplitudes is the dominant natural frequency for fatigue.

Using this method, the displacement amplitude-time history was evaluated for a piping system used in past shaking table test, and the results were qualitatively similar to the strain amplitude evaluated by elastic-plastic FEA. Future work will require the development of a method to convert displacement amplitude to strain amplitude. The peak stress method described in JEAC4601 may be applied to convert displacement amplitude to strain amplitude. However, this design method contains conservatism, so it is considered necessary to develop a method that more realistically predicts the strain amplitude.

Presenting Author: Yohei Ono Central Research Institute of Electric Power Industry

Presenting Author Biography: Academic degree:
2012.03 Master of mechanical engineering (The University of Waseda, Japan)

Work Experience:
2012.04- Toshiba corporation. Division of Nuclear plant material development
2018.04- Central Research Institute of Electric Power Industry. Earthquake engineering sector
2021.07- Central Research Institute of Electric Power Industry. Structures and Earthquake Engineering Division

Authors:

Yohei Ono Central Research Institute of Electric Power Industry
Masato Nakajima Central Research Institute of Electric Power Industry
Michiya Sakai Central Research Institute of Electric Power Industry
Ryuya Shimazu Central Research Institute of Electric Power Industry