Numerical simulation of tube hydroforming adaptive loading paths

Abstract: The tube hydroforming process is still to be considered a new and advanced technique. The process has been adopted into several industries, e.g. automotive and aero. A tube that has been cut to appropriate length, and by bending or crushing often been preformed, is placed in a die. The tube is filled with a hydraulic liquid and the ends are closed by side cylinders that press against the ends, creating an axial force in the tube. Simultaneously, the liquid is pressurized and the material of the tube yields and flows into the die cavities. The part is formed. In simulations of forming processes, users prescribe the fluid pressure in the work piece and the axial load exerted by the cylinders. Nowadays, many simulations must be performed, trial-and-error, to find appropriate loading paths for the pressure and the axial load. A more effective technique would be that the simulation program itself generates the pressure and the axial load. Depending on the magnitude and the proportionality between the pressure and the axial load, the tube fails either by rupture or wrinkling. In between these two failure boundaries there is a safe area, a process window, where the simulation yields useful results. An adaptive loading procedure would react to the boundaries and change the pressure and axial load accordingly to avoid failure. Today, the preferable virtual verification tool for tube hydroforming processes is the explicit finite element method. The economical cost of simulations by explicit time integration methods is directly proportional to the computational time. It is desirable to prescribe the simulation time to be as short as possible. Till now, program users have set a very high simulation time to avoid the problem with shorter simulation times - unreliable results due to dynamic effects. An easy way of defining the limit of the simulation time when it goes from reliable results to unreliable would be desirable. A part of the process window is established for different simulation times. It is shown that the simulation results changes abruptly at a certain value of the simulation time. Also, adaptive loading algorithms, the process window and the simulation time problem are investigated. A thorough literature survey is carried out in the tube hydroforming area.