Prediction Concept of Ratchet-Induced Collapse Under Deadweight and Seismic Loading

Abstract

The failure mode estimation of essential components in nuclear facilities under beyond design-basis events (BDBEs) is necessary to mitigate the consequences during severe accident conditions. The plastic collapse induced by ratcheting, defined as the ratchet-induced collapse in this study, is one of the failure modes of piping components under seismic loading. This failure mode was first confirmed in the Pipe-fitting dynamic reliability program (PFDRP) conducted by EPRI in the 1980s. However, the mechanism and occurrence condition of this failure remained unclear for decades because there were no methods to express the continuous transition of failure modes under seismic loading, as ratcheting turned into a plastic collapse behavior in the experiment. In this study, the mechanism of the ratchet-induced collapse is clarified using numerical analyses and experimental tests. The X-Y trajectory representing the transition of the stress levels of deadweight and seismic loading is introduced to express the significant effect of ratcheting. The seismic failure mode map, which has a similar classification of stresses with Bree’s diagram, is also extended to represent the occurrence condition of plastic collapse. The prediction method of the ratchet-induced collapse is proposed by combining the X-Y trajectory and the extended seismic failure mode map.