Mechanical and microstructural investigations of the biaxial Bauschinger effect in sheet metals

• Applicant: Professor Dr.-Ing. Martin Franz-Xaver Wagner
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: from 2016 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 318682861

The well-known Bauschinger effect refers to changes of the yield stress associated with load reversal. Numerous studies have been reported on the Bauschinger effect under uniaxial loading and on the corresponding microstructural mechanisms. However, similar effects can be expected to occur under multi-axial loading conditions, particularly during deformation of sheet metals, where such effects may well be highly relevant for forming technology. In this research project, biaxial in-plane deformation of sheet metals with a load reversal from biaxial tension to biaxial compression is studied for the first time with a focus on biaxial Bauschinger effects. Special new experimental setups were developed at Technische Universität Chemnitz, allowing for uniaxial tension and compression testing, on the one hand, and for biaxial compression-compression testing, of sheet materials, on the other hand. These new mechanical testing methods will now be used to tackle the interesting materials science challenge of documenting biaxial Bauschinger effects, and to identify their microstructural origins. As model sheet materials, technologically pure aluminum (EN-AW 1050) and DC06 steel will be used. In detailed mechanical investigations, cruciform samples will be deformed (in collaboration with partners in Erlangen) systematically to different biaxial tensile strain levels, and then smaller samples will be cut from these samples and subjected to biaxial compressive loading. Complementary Finite Element simulations will be used to analyze the corresponding stress and strain states, with a special focus on the effect of textures in the sheet materials on anisotropic material behavior. Moreover, the simulations will allow for a separate consideration of strength differential effects. Special attention in this project will be paid to the identification and quantification of the microstructural mechanisms (most importantly, the formation and annihilation of dislocation cells) that strongly affect the biaxial Bauschinger effects during the various deformation stages, by extensively using electron microscopy (SEM, EBSD, STEM, TEM, transmission Kikuchi diffraction) and X-ray diffraction methods. Furthermore, the generation and evolution of residual stresses during the loading history will be considered in detail and related to microstructural observations using X-ray diffraction. Both ex situ measurements of samples after different amounts of deformation (prior to and after the load reversal), and in situ measurements during biaxial deformation will be performed, the latter in collaboration with colleagues from Villigen, Switzerland, at the dedicated POLDI setup for neutron diffraction.

• DFG Programme: Research Grants
• International Connection: Switzerland
• Cooperation Partners: Professorin Dr. Helena van Swygenhoven; Professor Dr.-Ing. Kai Willner