Novel in-plane torsion specimen for the characterization of damage and hardening - curved sheets and components

• Applicant: Professor Dr.-Ing. A. Erman Tekkaya
• Subject Area: Primary Shaping and Reshaping Technology, Additive Manufacturing
• Term: since 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 327544970

For an accurate numerical analysis of forming processes on sheets, tubes and components exact material parameters are a basic requirement. Tests for material characterization have to meet the following requirements: (1) The exact determination of stress-strain-values of the pure measured values such as force and displacement, and (2) the determination of the characteristic values up to high true strains (in sheet metal forming up to 1). Since optical strain measurement is possible nowadays, the requirement reduces to the correct determination of the stresses based on the force measurements. Obtaining characteristic stress-strain data and fracture strains requires a homogeneous stress state. The necessity of a homogeneous stress state represents an implied process limit, for example in tension, compression, and classical shear experiments. The reason is that high strains cannot be achieved without inhomogeneity. Thus, most experiments are not suitable for determining the critical fracture strain. The in-plane torsion test has outstanding properties for the material characterization of sheet metals: The shear stress is homogeneous over an annular area and can be clearly calculated on the basis of the measured torque. This means that the in-plane torsion test is the only test having a "pure" shear load. Due to this and the lack of influences by friction, instabilities are controllable and high strains high strains can be achieved. The strain, which is not directly measurable through the rotation-angle, can now be determined very accurately using optical strain measurement and, thus, provides for new opportunities with this test. Therefore, the in-plane torsion test is very well suited for the determination of kinematic hardening. The in-plane torsion test on curved sheets and components was successfully applied in the first funding phase. For curved specimen, however, there are additional challenges with regard to suitable clampings, the inhomogeneity of the stress state and the evaluation of stresses and strains in the test.The aim is to significantly increase the range of specimens for the in-plane torsion test. Based on new results of the first funding period, the application of the in-plane torsion test on curved sheets and components is investigated. The expected benefits are:1. the exact local determination of the strength and flow curve on curved sheets, components and tubes,2. the exact determination of the fracture strain under simple shear,3. the ability to determine the kinematic hardening for curved specimens.The research activities include the understanding of the effects of the specimen curvature and the specimen material properties on the measurement results, the development of a suitable evaluation method and the implementation of the new test setup for flexible specimen shapes.

• DFG Programme: Research Grants
• Co-Investigators: Professor Dr.-Ing. Till Clausmeyer; Dr.-Ing. Rickmer Meya; Dr.-Ing. Heinrich Traphöner