Improved Flow Curve Modelling for Bending Processes in the Range of Low Plastic Strains

• Applicant: Professor Dr.-Ing. Gerhard Hirt
• Subject Area: Primary Shaping and Reshaping Technology, Additive Manufacturing
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 530544475

The use of high-strength alloys is common practice in the manufacture of sheet metal parts. Due to high ratios of yield strength to Young's modulus, strong springback effects are to be expected, which influence geometric accuracy and thus represent a significant challenge. The extent of springback depends on the material properties of the material used. The now standard use of numerical simulations to model forming processes requires careful determination of the material data in order to ensure realistic material modelling and thus good prediction quality. It has been shown that the use of constitutive models for flow curve approximation can lead to losses in accuracy, particularly in the initial range, i.e. in the transition range from elastic to plastic material behaviour. This can particularly affect processes that only lead to small strains of the material, as is the case for example in levelling or various bending operations. Previous work has also shown the nonlinear character of elastic unloading and reloading, which cannot be fully represented by classical linear-elastic-plastic models. Furthermore, a decrease of the effective Young's modulus can be observed, which leads to an improved springback prediction when taken into account in the FE model and is therefore already widely used in the simulation of sheet metal forming processes. Finally, the results obtained so far show that, in addition to the description of elasticity, the plastic deformation behaviour also exerts an influence on springback; here, both yield point and the flow curve are decisive. Individual influencing factors and their impact on springback prediction have already been extensively investigated. However, a comprehensive investigation of all relevant effects (elastic behaviour, flow curve, yield point) and their mutual interaction has not yet been carried out after reviewing the state of the art. The overall objective of the project is to improve flow curve modelling for forming processes in which low strains occur. The three-point bending test serves as an example process in order to be able to evaluate the effect of occurring accuracy losses in the flow curve approximation in the area of low strains and the influence of the nonlinear transition area including the variable modulus of elasticity on the springback. For this purpose, first a comprehensive characterization of the selected materials will be carried out in order to subsequently investigate the various influencing factors and their mutual interaction by means of FEM. In addition, a new constitutive model combined with a validated and practicable procedure is to be developed which makes it possible to minimize errors that occur and to increase the prediction accuracy.

• DFG Programme: Research Grants