Elastic-Plastic Correction Factor of Elbow Pipe for Bending Displacement

Abstract For fatigue assessments under seismic loads, elbow pipes tend to be critical components. The elastic-plastic correction factor (Ke) prescribed by design codes has been determined without focusing on plastic deformation of elbow pipes. This study is aimed to investigate the Ke value of elbow pipes under in-plane and out-of-plane bending displacements. First, elastic and elastic-plastic finite element analyses were conducted for given rotation angles. The equivalent strain was shown to reach its maximum near the crown. The maximum equivalent strains were derived by the elastic and elastic-plastic analyses, and their ratio was defined as Ke. The analyses were conducted under various geometrical and analytical conditions to derive a conservative Ke. It was revealed that a conservative Ke could be obtained for a thicker wall thickness, larger pipe radius, and smaller plastic slope without applying internal pressure. Although the conservative Ke was larger than the value prescribed in the design code issued by the Japan Society of Mechanical Engineers (JSME) when the magnitude of the applied displacement was large, the conservatism in the stress intensity evaluation of the design code led to a conservative fatigue damage assessment even when the JSME Ke was used. Second, the Ke was calculated for an elbow pipe subjected to seismic loads. The time series analyses were conducted for two types of input accelerations. Although the obtained Ke depended on the natural period of the model, it was lower than the JSME Ke and the value obtained under constant in-plane bending displacement. The hysteresis damping caused by the cyclic plastic strain reduced the Ke. Finally, the JSME Ke was concluded to be applicable to elbow pipes for both constant displacements and seismic loads in the fatigue damage assessment in accordance with the design code.