A Finite Element Analysis of the Residual Stresses Incurred During Bending of Pipes

This work illustrates the potential for finite element methods to be used in support of metal fabrication processes. The focus is an analysis of the residual stresses incurred during cold bending of small diameter pipes. The pipe was modeled using 3D constant strain elements. The mandrels used to support the pipe and apply the necessary bend forces were modeled using 2D rigid surfaces. Contact surfaces were defined on the outside of the pipe and the inside of the mandrels. The fabrication process was simulated by programming the nodes of one of the mandrels with prescribed velocities. The finite element analysis was performed using H3DMAP, proprietary software that includes a hybrid explicit/dynamic relaxation module. The technique is a quasi-static approach that discounts inertial effects. The finite element analyses are used to predict the residual stresses and plastic strain in the pipe. The studies involve a constant pipe size. Two stress/strain curves are used. The effect of using isotropic or kinematic material hardening models, compressive pre-stressing and differing bending procedures are considered, and results compared. The details of each simulation are shown to influence the calculated residual stress field.