Calculation of B2′ Stress Indices for Elbows Using Finite Element Analysis

Abstract

Nuclear power plant components are designed to withstand reversed dynamic loading like earthquake loading. Such reversed dynamic loads may induce plastic deformation in nuclear power plant components like pipe elbows. Plastic deformation in nuclear power plant components is limited by equation (9) of ASME Boiler &Pressure Vessel Code, Section III, NB-3652. ASME B&PV Code was revised in the year 2000 to accommodate plastic ratcheting as a mode of failure instead of plastic collapse under reversed dynamic load. The modified Code contains B2′ index, which is given as 2/3 rd of B2 index for butt-welded elbows. In the earlier work [1] B2′ indices were determined for several elbows using quasi-static nonlinear finite element analysis. In the present work an attempt is made to determine the ratio B2/B2′ for elbows using plastic nonlinear dynamic finite element analysis. Elbows of different sizes were considered in the present study. For each elbow linear static, linear dynamic, plastic nonlinear static and plastic nonlinear transient dynamic analyses are carried out to determine B2′ index in terms of B2 index. Elastic-perfectly plastic material model is used for the elbows. Collapse loads are obtained under static and dynamic conditions. Load vs. deflection curves are obtained for elbows under linear static and nonlinear quasi-static analyses. Deflection vs. time-curves are obtained from linear dynamic and plastic nonlinear dynamic analyses. The ratio B2/B2′ is computed for elbows of different sizes. The computed stress indices are compared with the Code values.