New experimental approach for yield loci determination using a modified Nakajima setup

• Applicant: Professor Dr.-Ing. Wolfram Volk
• Subject Area: Primary Shaping and Reshaping Technology, Additive Manufacturing
• Term: from 2017 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 363839128

In the state of the art the Nakajima test is used to determine die forming limit of sheet metal. In the common test setup, consisting of a spherical punch, a die and a blank holder, only the strains in the sheet are measured. The aim of the research project is the development of a method for the experimental determination of stress states by use of a modification of the Nakajima test. For this purpose, different measurement concepts are developed for the detection of the normal pressure between the apex area of the punch and different tailored specimens by means of a simulation and a calibrated test rig. The high strength steel DP600 is used as a test material. On the basis of the experiments as well as the virtual experiments with specimens, which have different widths, a mathematical method is designed for differentiation of the measured normal pressure into the membrane stresses. This approach takes into account the complete punch stroke and thus the entire forming history of the test sheet. By means of the resulting knowledge, it is possible to determine flow curves or yield loci for sheet metal, at different stress states in the first quadrant of the stress space, as well as for each strain state of the deformation history up to the determination of the local necking. The methodology is validated by using of prescale foil, which can measure the pressure, and the bulge test. Finally, experimental yield loci are determined exemplary. The Nakajima tests, which are required in the research project, are determined on the BUP1000 universal sheet metal testing machine procured within the scope of the DFG large instrument application. The new approach to determine the yield locus offers the possibility to understand the plastic material behavior better, to compare numerical material models with actual material parameter, and to develop new yield function approaches. Performing all the necessary tests on one test machine and one experimental setup, which already finds a wide application for the determination of the forming limit, minimizes the extensive effort required for the determination of yield loci.The main objectives of the research project include:1.Construction of measuring concepts and a calibrated test rig2.Derivation of calibration function depending on the different specimen widths3.Development of a mathematical method for the decomposition of the normal pressures in membrane stresses4.Determination of flow curves and yield loci by means of the membrane equation

• DFG Programme: Research Grants
• Co-Investigator: Dr.-Ing. Roland Golle