Fatigue failure accompanied by ratchet deformation is well known as one of the failure modes of pressurized pipes under high-level cyclic load. In this research, the process of failure of such pipes was investigated based on the experimental result in which an elbow pipe failed by cyclic input of displacement. The strain behavior, moment-deflection relationship, and observed damage were compared with the stress level used in the seismic design of the piping system. As a result, no damage was detected and the moment-deflection relationship remained almost linear within the primary stress intensity limit and the primary plus secondary stress intensity range limit of 3Sm. In the experiment, visible damage was detected at the applied load levels of approximately 5∼6.6Sm of the primary stress intensity. The test results showed that there is a certain time margin before actual failure occurs to piping systems, compared with its designed stress limitation.

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