| dc.description.abstract | The global effort of decreasing energy consumption and CO2 emissions has resulted in an important increase in the use of aluminium in the automotive sector. Aluminium (in the form of aluminium alloys) possesses many attractive properties to this industry. Automotive structures in aluminium mostly consist of extrusion-based products, which are able to provide a very good compromise between weight, stiffness, strength and ductility. All of these features make these components optimal solutions to be utilized in energy-dissipating structures. In a car collision, the energy absorption for these components is obtained by different mechanisms, such as crushing and bending of the side and inner walls composing the extruded profile. Small variations in the geometry or material properties can lead to variations in the behaviour of these structures. Thus, if the energy dissipates in an inappropriate way because of a non-robust design, this can produce unexpected accelerations and intrusions inside the vehicle. In order to get a good understanding of the structural capacity and prediction of the energy-dissipating response of these components, the automotive industry uses large-scale finite element models during the process development of the final product. As a result, the robustness and accuracy of the finite element models of these structures are imperative to make proper, faster and cost-effective decisions, as well as to improve the behaviour of these metallic components in terms of weight and crash performance.
This thesis is divided in three parts. In the first part the differences observed in the structural response by the type of element used in the discretization of single and double-chamber extruded aluminium profiles under axial crushing and three-point bending was investigated, paying special attention to the deformation patterns and force levels. In addition, the effects of the shell element’s drilling degree of freedom were analysed. The previous was studied first numerically in a hollow short tube with square-cross section (simple model) and then in a longer hollow profile under axial crushing. Secondly, these numerical models were validated against axial crushing and three-point bending tests on a component built from a double-chamber profile in a 6xxx aluminium alloy in temper T6.
In the second part, a window of chemical compositions for five different alloys was considered (i.e., variations in mechanical properties through virtual material models with calibrated parameters) that resulted in a corresponding window of mechanical response (i.e., variations in the structural capacity of aluminium components). These variations in the structural response between the different alloys were analysed through large-scale analyses of double-chamber extruded aluminium profiles under axial crushing and three-point bending that were carried out with five NaMo-based isotropic user-defined material models. These material models were previously calibrated by use of virtual tensions tests on flat specimens and localization analyses. These five alloys were compared against the experimental and numerical results of an alloy of reference.
In the third part, the influence of variations in plastic and failure anisotropy on the structural behaviour of double-chamber extruded aluminium profiles under axial crushing, three-point bending and lateral crushing was studied. These large-scale analyses were carried out with both isotropic and anisotropic material models. The isotropic materials were used as the baseline for comparison against the anisotropic materials. Prior to the large-scale analyses, the effects of variations in plastic anisotropy on the ductile failure was investigated with non-linear finite element simulations of tensile tests on flat specimens carried out in one direction for two isotropic alloys and in seven in-plane directions for four anisotropic alloys. After the virtual tension tests, localization analyses were run to predict the onset of ductile failure in the tensile specimens for the different materials considered. By doing so, it was possible to obtain the fracture parameters for the isotropic and anisotropic regularized damage model (TTR), as well as the anisotropic scaling factors under pure bending and membrane loading, which later were introduced in the user-defined material models to use in the large-scale analyses.
Finally, itwas concluded that activating the drilling stiffness up to a certain level had positive consequences on the structural behaviour of extruded aluminium profiles subjected to axial crushing and three-point bending, giving a better estimation of both the force levels and deformed shapes when compared to the experiments. It was also seen how the variations in the structural response of extruded aluminium profiles under axial crushing, three-point bending and lateral crushing were significantly influenced by the variations in the chemical composition, plastic and failure anisotropy. | en_US |
