With the front end concept, the challenge was to reduce the weight of a 76 kg module using a variety of lightweight materials in the optimum places. To proceed methodically, the front end area of the mostly-steel benchmark structure was analysed with regard to the materials used, production methods and component costs and their requirements when the work was begun. Preliminary conceptual considerations for lighter front end structures were made while taking into account aluminium alloy casting, the assembly of parts and the integration of functions.
Using the parts list of the reference structure, Fig. 3, components suitable for integration into a single cast part due to their arrangement, function and specific costs were identified.
In taking these elements into account, the conceptual idea of combining the area of the top longitudinal rail, the strut tower and the engine and transmission mount into a single large aluminium alloy cast component was developed. This area has stringent requirements regarding crash safety of the vehicle, e.g. in case of a head-on collision, and the torsion and flexural stiffness of the entire vehicle structure.
In previous vehicles, these requirements were fulfilled using sheet metal or cast metal solutions made of steel or aluminum. As a result of the goals for weight reduction, the engineers aimed at a solution using magnesium casting despite the great challenges involved. This would involve unique new technologies for the use of magnesium in load-bearing structures. Sheet aluminium was identified as an ideal solution for the adjoining areas and the rest of the front end, Fig. 4.
Detail development took place over several development loops between construction (CATIA V5) and simulation (LS-Dyna). The optimisation of crash behaviour of the multi-material structure, in particular, required more than 80 overall vehicle crash calculations.
First, the crash behaviour of the magnesium component was optimised for the OBDFootnote 1 crash load case (Figs. 5 and 6). Through precise analysis of the calculated component failure, the component was improved geometrically and with regard to wall thickness distribution. To reproduce the material properties of the selected magnesium alloy AM50 as realistically as possible, material failures with a breakage elongation of 8% were computed in the crash simulation. In addition, great importance was attached from the very beginning to the manufacturing capability using the die-cast process. In the second stage of development, the surrounding steel structure was replaced with a lighter aluminium structure. This structure was then also optimised for advantageous crash behaviour in the overall vehicle structure. The force-displacement diagrams, intrusions, compare Fig. 7 and the simulation films of every design variant were analysed. The geometry, wall thickness or material changes of every new variant was examined to determine what benefits they could offer. This allowed the longitudinal rail structure to be better utilised with regard to specific energy absorption and thus lightened in comparison to the respective reference structure.
Despite a reduction in weight of 32% in the front end, excellent crash behaviour was achieved. In several areas, the crash behaviour was actually improved in comparison to the reference structure. For example, the footwell intrusion measurement of 51 mm of the DLR concept is clearly an improvement over the 100 mm of the reference structure.
By reducing the weight of the front end by 24 kg, the set project goal of more than 30% was reached. The highly-integrated magnesium cast component described above, which combines some 12 steel components into a single cast component and reduces the weight by more than 60%, was responsible for a considerable portion of this.