Extension of the forming limits during deep drawing by additional force transmission

The material-specific process limits in deep drawing depend on the stress distribution in the workpiece. If the stress limit of the material is exceeded in a certain area, material failure occurs. By locally applying an additional force, a pressure superimposed stress state can be induced, which relieves the sheet material in critical areas for crack initiation and extends the process limits. Here, the finite element method is an indispensable tool for a numerical prediction of the process window regarding the material failure. The aim of this project is to determine the deformation capacity of the steels HCT600X and HX340LAD in a broader spectrum of strain states (pure shear to equibiaxial pressure) compared to the forming limit curve and to integrate it into the FE simulation of the conventional deep drawing process as well as deep drawing with additional force transmission in the form of a suitable stress-based fracture model. The influence of the process-related stress state on the deformation capacity is to be precisely identified by experimental tests. Within the material characterisation, the deformation capacity of HCT600X and HX340LAD with a sheet thickness of 1.0 mm is determined considering different stress states. With the shear tensile tests developed at IFUM, it is possible for the plastic deformation to localise independently of the specimen's orientation and the stress state in the centre of the load region of a butterfly specimen. Subsequently, the stress-based fracture criterions (Johnson-Cook, Modified Mohr-Coulomb and DF2016) are calibrated. The application of the criterions are tested and validated with regard to the numerical prediction of the material fracture during deep drawing with activation of the additional force transmission. Furthermore, the surface pressure that is imposed on the material from a certain stage of the deep drawing process is also taken into account in the fracture modelling. The project results should contribute to increase the accuracy of the simulation-based design of conventional sheet metal forming processes as well as the processes with additional force introduction with regard to the use of high-strength steels.