The purpose of using drawbead in the sheet metal forming process is to control material flow into the die during the binder wrap process and the stamping process. The dimension of drawbead is relatively small in comparison with other die dimensions. It is very difficult to include drawbead in finite element analysis of the sheet metal forming process since computation time is drastically increased. Many researchers have focused on equivalent boundary conditions to include drawbead effects during the sheet metal forming process.
Two dimensional simulations are carried out to obtain the equivalent boundary conditions in the binder wrap process and the stamping process. Elasto-plastic finite element formulation is derived from the equilibrium equation and related boundary conditions considering the proper contact conditions. The developed finite element code is applied to the bead forming process and the drawing process. The simulation of the bead forming process are carried out to obtain the equivalent displacement boundary condition in the binder wrap process. In this simulation, the distribution of stress in the cavity region and elongation which is used for the equivalent displacement boundary condition, varies with respect to various drawbead dimensions. The simulation of the drawing process are carried out to obtain equivalent force boundary condition in the stamping process. In this simulation, the restraining force in the blank resulted from friction and bending-unbending process in the drawbead region varies with respect to various drawbead dimensions. In order to inspect the effects of various die geometries, parameter studies are performed with the variation of parameters such as the blank length, the drawbead depth, the drawbead radius, the inclination of die and the friction coefficient.
In order to demonstrate the validity of equivalent boundary conditions, obtained boundary conditions are applied to the front door panel forming process, NUMISHEET’99 benchmark example. The elongation in the bead forming process is applied to the equivalent displacement boundary condition at the binder wrap process and the restraining force in the drawing process is applied to the equivalent force boundary condition at stamping process. The calculated result with equivalent boundary conditions is more similar to the experimental result than the simulation without considering drawbead effects. Consequently, the numerical results fully demonstrate that drawbead simulations for the calculation of boundary conditions are successfully carried out.
