Double-sided high-pressure tubular hydroforming

It is generally understood that tensile stress loads lead to the shearing and/or expansion of micro voids in a metal specimen. Such deformation leads to stress concentrations that ultimately form the basis of the limiting strain capacity in a metal. Accordingly, a number of formability theories suggest that stress (not strain) is the key mechanism for ductile fracture. Motivated by such theories, an introduction to a special form of tubular hydroforming (THF) is put forth, where hydraulic pressure is introduced on both sides of the metal. Six different formability models are compared in light of their sensitivity to thickness normal stress (σ3) and other process variables associated with THF. It is shown that the models which capture the σ3 effect are most suitable for formability assessment for the double-sided high-pressure (DSHP) process. The stress space forming limit model is then employed in the context of a solid finite element model of a plane-strain THF expansion. The presence of the DSHP boundary condition is shown to lead to increased formability relative to that observed for the traditional single-sided high-pressure (SSHP) process. Research of the DSHP process may lead to discoveries of various avenues towards greater formability. Consequently, the design space currently available to those employing the SSHP process may be significantly increased through the DSHP process. Ultimately, such design space increase may result in lower product cost, greater customer enthusiasm and increased market share for those manufacturers who invest in the DSHP process. This work represents a preliminary numerical investigation into the DSHP process only. Issues of cost, time and fixture design are left untouched.