چكيده به لاتين
To reduce energy consumption, the use of lightweight metal alloys is one way to decrease the weight of vehicles structure. Meanwhile, the use of magnesium alloys as the lightest metal alloys has always attracted the attention of manufacturers and researchers. In the field of mechanical energy absorption, magnesium alloys limited applications are obvious due to the poor formability, and consequently, there are few pieces of research have been focused on this issue. Hence, in this work, the crash energy absorption behaviors of thin-walled magnesium-lithium ultralight alloy cylindrical crash energy absorbers, are investigated experimentally and numerically. The metallurgical and mechanical properties of the two alloys are studied in order to obtain a suitable alloy composition as well as the conditions for the production of proper sheets required for the fabrication of the crash absorbers. The first compound contains Mg, Li (7 Wt.%), and Zn (1 Wt.%) (LZ71) and the second one contains Mg, Li (9 Wt.%), and Zn (1 Wt.%) (LZ91). Experiments included microstructure observation utilizing optic microscopy and scanning electron microscopy, fuzzy analysis by XRD, microhardness, tensile test, fractography, anisotropy investigation, and fracture toughness test. After selecting the proper alloy and the appropriate heat treatment condition the required mechanical properties are extracted from the related samples to use in the numerical simulations. Moreover, in order to investigate the energy absorption performance of these alloys, tubes are manufactured by rivet joints and subjected to crash testing. Experimental analysis of the produced thin-walled energy absorber is done under quasi-static compressive load to derive force-displacement graphs in order to validate the numerical simulation results. Afterward, two different finite element analyses are performed using ABAQUS/Explicit. In the first analysis, loading and boundary conditions are considered close to the experimental situations of homogeneous tubes. In the second analysis, similar to the experiments performed in this study, simulations were performed for tubes made by rivet joints. In both groups of simulations, the results have shown proper agreement with the outcomes of experiments. Then, the validated finite element models are used to investigate the effect of each of the geometrical parameters including diameter, thickness and rivet space, on the main energy absorption parameters. These parameters include Mean crushing force (FMean), Peak crushing force (FMax), Total Absorbed Energy (TEA), Crushing Force Efficiency (CFE), Specific Energy Absorption (SEA), and Total Efficiency (TE). The results showed that the LZ71 alloy relatively proper energy absorption performance compared to the AA6082 and AA6061 aluminum alloys, due to the fact that the SEA, as well as the CFE of the LZ71 alloy, were significantly more proper than the aluminum alloys. Utilizing the GMDH type artificial neural networks, metamodels of each of the above-mentioned energy absorption parameters have been developed in terms of geometric variables namely diameter, thickness, and rivet space, based on the data obtained from finite element modeling. These metamodels are extracted and presented as mathematical equations to utilize for optimization. Furthermore, Multi-objective optimization of the energy absorber was performed by considering the opposite objective functions namely mass, energy absorption, and peak crushing force, and then Pareto front have been presented. Finally, some compromise points for the magnesium alloy energy absorbers have been introduced, by analyzing the optimization results.
