Polikristal metallerin farklı ölçeklerde deformasyon ve kırılma öngörüsü: Darbe ve lokalizasyon üzerine vaka çalışması

The widespread use of polycrystalline materials in advanced and infrastructural technologies demands a proper understanding of the mechanical response at multiple length scales. Today's high demand to foster higher speeds in public transportation media has introduced the need for designing new materials that can be safely employed under severe loading conditions, in addition to being impact-tolerant. Furthermore, ductile fracture of polycrystalline metals commonly ensue plastic flow localization through the mechanisms involving void nucleation, growth and coalescence. As a precursor of failure and a limit that mark uniform straining of ductile metallic materials, detection of the localization is highly crucial. In order to investigate multiscale impact and failure response of the polycrystalline metallic materials two case studies were considered here. In the first part a microstructural-sensitive simulation of the high-manganese austenitic steel under impact loading was performed. Next, using in-situ SEM tests of AISI 316L stainless steel macroscopic plastic localization was investigated. In part (I) microstructurally-informed macroscopic impact response of a high-manganese austenitic steel was modeled through incorporation of the visco-plastic self-consistent (VPSC) crystal plasticity model into the ANSYS LS-DYNA non-linear explicit finite element (FE) code. Voce hardening flow rule was utilized in the VPSC crystal plasticity model to predict the micro-mechanical response of the material, which was calibrated based on experimentally measured quasi-static uniaxial tensile deformation response and initially measured textures. Specifically, hiring calibrated Voce parameters in VPSC, a modified material response was predicted employing local velocity gradient tensors obtained from the initial FE analyses as a new boundary condition for loading state. The updated micro-mechanical response of the material was then integrated into the macro-scale material model by calibrating the Johnson-Cook (JC) constitutive relationship and the corresponding damage parameters. Consequently, we demonstrate the role of geometrically necessary multi-axial stress state for proper modeling of the impact response of polycrystalline metals, and validate the presented approach by experimentally and numerically analyzing the deformation response of the Hadfield steel under impact loading. In part (II) of this study, plastic localization was investigated by tracking the captured incremental images during the micro scale in-situ scanning electron microscope (SEM) tensile experiment of samples made from AISI 316L stainless steel. A novel hybrid-numerical experimental (HNE) approach for small scale tensile testing of materials is developed. An unconventional miniaturized test set-up was designed to reach smaller scales in testing and also to eliminate displacement errors due to vertical spatial distortion during the in-situ SEM measurements. By arranging each pair of sequentially taken surface images – over deformation history – in alternate order for several repetition, plastic localization regions became manifest on surface. Results clearly prove feasibility of the presented method in recording primary plastic localization events within an in-situ SEM tensile testing framework.