The effect of the pre-bending operation on ductility can be significant in determining the limit strains of the final product. The strain path experienced in straight tube bulging is significantly different from that of elbow (tube with pre-bending), leading to a reduction in bulge height. The Tube bending introduces strain gradient both along the tube and across the tube. In this work the effect of pre-bending on limit strains during tube bulging process is predicted — and the results are compared to the limit strains of bulged tubes without pre-bending. The Finite Element (FE) model of the bending operation is developed which utilizes an explicit dynamic finite element formulation. The PAMSTAMP 2G code is used to perform the numerical pre-bend (and bulging) simulations.
Tension side of bend tube axial strain is found to be positive and hoop strain as negative and vice versa along the compression side. During the bulging, the neck usually develops perpendicular to major strain direction. During bend tube bulge test with fixed expansion and axial feed expansion of bend tubes, in both cases the crack is found to be in the axial direction.