Development of a new improved closed-form plastic collapse moment solution for structurally distorted and through-wall axially cracked 90° pipe bends subjected to in-plane closing bending moment

Abstract

Plastic collapse loads of 90° pipe bends with structural defects subjected to in-plane closing bending are systematically examined in this study using three dimensional nonlinear finite element (FE) analyses. The analyses consider the large strain formulation option with the pipe bend material assumed to be of the elastic perfectly plastic (EPP) type. Pipe bends are analyzed by changing the ovality at the mid-section which is varied from 0% to 20% by 5% per step. The through-wall axial crack (TAC) parameter at the pipe bend's crown is varied from 0 to 1 by 0.2 per step. Moment rotation curves are drawn for all simulated pipe bend models and plastic collapse moment (PCM) loads are determined from the corresponding moment rotation curves using the Twice-Elastic-Slope (TES) methodology as recommended by clause NB-3213.25 in section III of ASME B&PV code. Both ovality and TAC of pipe bends significantly affects their plastic collapse moments (PCM) and this almost doubles when ovality and TAC act together. The observed effect of ovality and TAC on pipe bends having thin walled and short bend radius does not significantly change with increasing wall thickness and bend radius. The present FE analysis is validated using experimental PCM loads of pipe bends available in open literature. The crack closing effect of pipe bends during closing bending is examined. Based on the analysis, an improved structural integrity assessment equation for shape imperfect pipe bends with TAC is developed for in-plane closing bending moment loads.