B2' Stress Indices for Elbows Using Non-Linear Finite Element Analysis

Abstract

Nuclear power plants are designed to withstand earthquake loads without severe damage under service level D conditions. Under earthquake induced reversing dynamic load, nuclear power plant components may undergo plastic deformation. Plastic deformation in class I nuclear power plant piping systems is limited by Equation (9) of ASME Boiler & Pressure Vessel Code [14], Section III, NB-3652. In the year 2000, the ASME B&PV Code was revised to accommodate reversing dynamic loading in which the failure mode is fatigue ratcheting, instead of plastic collapse. This modified equation [16] contains B2′ index, which is given as a fraction of B2 index where, B2 is defined for monotonic loading [17]. In this study a new definition is proposed for calculating B2′ stress index which is given by B2′ = MCLcyclicRange,straightpipe/MCLcyclicRange,component, where MClcyclicRange is the range of collapse moment. Incremental elastic-plastic nonlinear finite element analyses are performed considering both material and geometric nonlinearities. Kinematic hardening, isotropic hardening and elastic-perfectly plastic material models have been used to model the material behavior during plastic deformation. Load deflection curves are obtained and from these curves collapse loads for monotonic and cyclic loading are determined. B2 and B2′ stress indices are computed for elbows using the proposed equation. The computed stress indices are compared with ASME Code values.