Failure criteria of a carbon steel pipe elbow for low-cycle fatigue using the damage index

A seismic isolation system installed in a nuclear power plant will respond to the load caused by an earthquake. As a result, a larger displacement may occur in the presence of the system, compared to its absence. This increased displacement may involve higher seismic risks for some facilities. Probabilistic seismic fragility analysis is conducted to assess the seismic safety of the major elements and systems of a nuclear power plant that may be at increased seismic risk. The installation of piping systems that connect seismic-isolated and general structures may involve increased seismic risks because they are expected to have a large displacement. For the reliable analysis of the seismic fragility of piping systems, the failure modes and criteria that can represent actual failure must be defined. Therefore, in this study, the ultimate state of the elbow, the vulnerable part of piping systems, was defined as leakage, and an in-plane cyclic loading test was conducted. Moreover, an attempt was made to quantify the failure criteria for a 3-inch carbon steel pipe elbow using a damage index, which was based on the dissipated energy that used the moment-deformation angle relationship. •A leakage line and low-cycle fatigue curves for an SCH80 3-inch carbon steel pipe elbow are presented.•The ultimate state, involving leakage, of the SCH80 3-inch carbon steel pipe elbow is quantified using a damage index.•Failure criteria for the 3-inch carbon steel pipe elbow are presented using force-displacement and moment-deformation angle relationships.•Quantitative failure criteria for the 3-inch carbon steel pipe elbows of SCH40 and SCH80 are presented.