Since the accident at the Fukushima Daiichi power plant, in addition to “design to prevent accidents”, “mitigating the severe accident” has come to be emphasized. Thus, it is necessary to evaluate the actual failure mode under beyond design basis events (BDBEs). In this study, authors focus on the failure mode of piping in nuclear power plants under excessive earthquakes. The piping design of nuclear power plants has been conservative assuming that seismic load acts as load-controlled and the collapse happens by maximum acceleration. However, the test conducted by Electric Power Research Institute (EPRI) confirmed that when excessive vibration load was applied to the piping with the elbow, ratchet deformation occurred with time and eventually collapsed. Unfortunately, this failure mechanism is not clear, so it is highly important to consider the actual failure mode, namely ratchet deformation leading to collapse. Authors tried to clarify the mechanism of ratchet deformation by experiments and analyses of inputting acceleration to a beam simulating piping. According to these results, it is identified that ratchet deformation is likely to occur when the vibration load whose frequency is lower than resonance frequency is applied, and is difficult to occur on the higher frequency area. Hereafter, the ratio of the frequency of vibration load to the natural frequency of beams is referred as “frequency ratio”. In this study, half-cycle vibration load was applied to the beam, and the frequency dependence of the collapse phenomenon was investigated.
Ratcheting is a progressive incremental inelastic deformation or strain which can occur in a component that is subjected to variations of mechanical stress, thermal stress, or both. This study concentrated on the ratcheting occurrence of the piping model under the combined effect of constant external force and dynamic cyclic vibrations. Bent solid bars represented piping models, and sinusoidal acceleration waves were loaded. Characteristics of seismic loads between load-controlled and displacement-controlled properties were studied from the viewpoint of the frequency ratio of the forcing frequency to the natural frequency of the piping model. Besides, the ratcheting occurrence conditions of the beam and the piping model were compared in one normalized diagram to display the general mechanism of ratcheting with the consideration of the effect from the difference of shape and material. Results show that ratcheting occurs easily with a lower frequency ratio in both beam and piping models. In addition, it is meaningful to use beam models to understand the ratcheting mechanism of piping models. Describing the occurrence of ratcheting using the normalized ratcheting diagram for different components is feasible.
Ratcheting is one of the dominant failure modes under excessive earthquakes and may cause extreme failures of structures (e.g., collapse). We focused on clarifying the ratcheting mechanism of piping under sinusoidal excitations. Both finite element analyses and experiments were conducted on bent solid bars, which represented piping in this study. Seismic ratcheting occurred due to the combined effect of constant external compressive force and cyclic vibrations. The external compressive force acted as a load-controlled load. Vibrations were applied to provide the source of the dynamic load. Characteristics of vibrations between load-controlled and displacement-controlled properties were studied from the viewpoint of the frequency ratio of the forcing frequency to the natural frequency of the piping model. In addition, the influence of supports on the occurrence of ratcheting was also considered. The results showed that the resonance effect was evident in the piping model compared with the beam model due to the limited plastic area in the piping model. The vibration with a lower frequency had load-controlled characteristics. In contrast, the vibration with a higher frequency presented displacement-controlled properties. In terms of the occurrence of ratcheting, providing more supports sometimes increased the possibility of the occurrence of ratcheting under relatively higher forcing frequencies because more supports increased the natural frequency and decreased the frequency ratio.
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