Microscopic and Macroscopic Analysis of the anelastic behavior of steel with regard to springback prediction

• Applicants: Dr. Michael Hofmann; Professor Dr.-Ing. Wolfram Volk
• Subject Area: Primary Shaping and Reshaping Technology, Additive Manufacturing
• Term: from 2020 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 429432653

In terms of lightweight design, for example, the use of high and ultra-high strength steels leads to challenges in elastic springback and thus dimensional accuracy. Although a large number of studies have already dealt with this phenomenon, there is no holistic view on the macroscopic and microscopic mechanisms and analysis of their correlation. The anelastic, so called plastically reversible, portion of the elastic unloading is differently explained in the microscopic level. Phase transformations, experimental setup errors, Poission's ratio, and movement of dislocations are current assumptions for the observed nonlinear behavior. However, the arguments are based on macroscopic experiments and cannot prove of the microscopic assumptions. Microscopic investigations have shown that at the atomic level effects occur which differ from the macroscopic behavior. A coupling of microscopic parameters, such as local dislocation densities, phase transformations and microscopic stresses with macroscopic stress-strain behavior considering the elastic behavior of metallic materials, is lacking so far and should therefore be carried out in this research.The aim of this project is to analyze the elastic and anelastic behavior of the materials IF220 and DP1000 during loading and unloading, taking into account the rolling directions as well as the pre-strain, in order to develop, validate and verify a comprehensive and physically consistent model to predict springback of formed sheet metal components. Normal and cyclic tensile tests are carried out for this purpose and evaluated on the one hand microscopically by means of in situ neutron diffraction and on the other hand macroscopically by optical and tactile strain measurement. As part of the microscopic investigations in addition to the lattice strains, the change in the dislocation density and the texture development is analyzed using pole figures. These microscopic parameters are the basis of a physically consistent description of the processes. The microscopic results can be correlated with the macroscopic material behavior by means of parallel, synchronous measurement of the macroscopic deformation and the global load condition. Based on the interactions including pre-strain and rolling directions, a loading and unloading model is derived. Finally, the model will be implemented into a simulation environment as well as validated and verified using real experiments.

• DFG Programme: Research Grants