Preparation for beyond design basis events (BDBE) becomes an important issue as the lessons learned from the Fukushima nuclear accident. IAEA proposed the best estimation approach for strength evaluation under BDBE. It is required to identify dominant failure modes for best estimation approach. Ratcheting, collapse and fatigue are the probable failure modes which can occur due to dynamic loadings like the seismic load. The current studies describe an attempt to clarify occurrence conditions of such failure mode as ratcheting and collapse for pipes. Elbow pipe components have been analyzed by using the inelastic finite element method. Gravity load was the primary loads, while the base acceleration of sinusoidal waveform of different frequencies was considered as pseudo secondary load. The results have been put in a nondimensional stress parameter plot similar to the Bree diagram for thermal ratcheting, paying attention to the similarity between thermal load and dynamic load. From above results, authors have proposed the failure mode map which can evaluate the occurrence conditions of ratcheting and collapse failure mode for pipes under dynamic loadings.

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