Investigating the effects of cyclic thermo-mechanical loading on cyclic plastic behaviour of a ninety-degree back-to-back pipe bend system

Pipe bends are generally employed for routing piping systems by connecting to straight pipes but back-to-back pipe bends are often necessary for confined space applications. In order to achieve safe operation under complex loading, it requires a thorough pipeline integrity assessment to be commenced. This paper investigates the effects of cyclic thermo-mechanical loading on cyclic plastic behaviour of 90° back-to-back pipe bend system, including temperature-dependent yield stress effects. Structural response interaction boundaries are determined for various different combinations of cyclic and steady loading. Constructed structural responses are verified by full cyclic incremental, step-by-step, Finite Element Analysis. The numerical studies provide a comprehensive description of the cyclic plastic behaviour of the pipe bends, and semi-empirical equations for predicting the elastic shakedown limit boundary are developed to aid pipeline designers in the effective assessment of the integrity of the pipe bends without a requirement for complex Finite Element Analysis.