Study on Evaluation Method for Safety Margin of Degraded Piping Against Seismic Loading with Multi-Fracture Mode

管道系统是核电站的主要设备之一。由于假定管道的断裂会对核电厂的整个系统造成严重损害，因此必须确保足够的安全性，以防止管道的缺陷和引起的劣化。此外，由于地震频繁，日本的地震安全裕度很大。然而，老化管道对抗震安全性的影响尚未明确，核电厂老化管道可能存在应力腐蚀开裂（SCC）裂纹和局部减薄等缺陷。为了评估退化管道的公差极限，作为国家地球科学和灾害预防研究所资助的“老化管道委员会”项目的一部分，进行了一系列大型直管四点弯曲试验和三维管道系统模型振动台试验，以及退化管道的低周疲劳试验。2002年在日本国立地球科学与灾害预防研究所进行了局部减薄弯管弯曲试验。本文旨在建立一种分析方法，可以模拟退化管道在地震载荷作用下的多断裂模式的低周疲劳强度和失效行为。对该模型进行了一系列的有限元分析，包括动态分析和静态分析。对局部薄壁直管在循环四点弯曲载荷作用下和局部薄壁弯管在循环面内或面外弯曲载荷作用下以及三维管系在动载荷作用下进行了一系列弹塑性分析。已经发现，在实验中观察到的棘轮或椭圆化行为可以很好地模拟有限元分析。

The piping system is one of the major equipment in the nuclear power plant. Because it is assumed that fracture of the piping cause serious damage to whole system of the nuclear power plant, sufficient safety must be ensured against the defect and caused degradation of the piping. In addition, large margin of seismic safety has been taken in Japan because of a lot of earthquakes. However, effect of aged piping on seismic safety has not been clarified.The aged piping of a nuclear power plants has possibility to have some defects such as SCC (Stress Corrosion Cracking) cracks and local thinning. In order to evaluate tolerance limit of the degraded piping, a series of large scale four-point bending tests on straight pipe and shaking table tests using 3-D piping system models, low cycle fatigue tests for the degraded pipes were carried out as a part of the "Aged Piping Committee" project sponsored by National Research Institute for Earth Science and Disaster Prevention. And local thinning bent pipes bending tests were done in National Research Institute for Earth Science and Disaster Prevention in 2002.The purpose of this study in this paper is to establish analytical approaches by which the low cycle fatigue strength and failure behavior of degraded piping can be simulated against seismic loading with multi-fracture mode. A series of finite element analyses, dynamic analysis and static analysis, have been performed. A series of elasto-plastic analyses have been carried out for the locally thin-walled straight piping loaded cyclic four-point bending and locally thin-walled elbow piping loaded cyclic in-plane or out-of-plane bending and then 3D piping system loaded dynamic loading. It has been found that the ratcheting or ovaling behavior observed in the experiment can be simulated well by finite element analysis.