Metal flow prediction during sheet-metal punching process using the finite element method

In this study, part springback and metal flow caused by punch penetration into a sheet was investigated by punching circular test specimens with concentric circular tools. Strain gauges were bonded on the upper surface of the specimens at radial distances varying from 2 mm to 10 mm from the cutting edge of the punch. The experiments were used to validate a finite element model (FEM) valid for numerical simulation of sheet-metal punching processes. Damage and crack propagation were taken into account by means of an elastoplastic constitutive law. The main difficulty encountered in simulating this operation is describing the behaviour of the sheet continuously from the beginning of the operation up to the total rupture. The choice of a behaviour law is crucial, since each successive step in the whole process has to be described accurately. In this investigation, an elastoplastic behaviour law coupled with damage was retained to describe the progressive damage accumulation in the workpieces. During the analysis, the initiation of a crack is assumed to occur at any point in the structure where the damage reaches its critical value Dc. The crack propagation is simulated by the propagation of a completely damaged area. This is taken into account in the FEM by a decrease in the stiffness of the broken elements. The numerical results obtained by the simulation were compared with the experimental ones in order to verify the validity of the proposed FEM.