Project Next Generation Car
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KeywordsTopology Optimisation Shape Optimisation Differential Construction Product Development Process Sheet Metal Part
At the Institute of Vehicle Concepts of the DLR measures are being investigated for reducing mass by adjusting optimisation strategies for the design and the principal component requirements. On the basis of a selected optimisation strategy, a sill structure is being improved both for specific energy absorption and intrusion into the passenger compartment.
The term lightweight has a very high priority in vehicle development today. The objective of lightweight construction is to meet the requirements that legislators and customers have set for vehicles (e.g. vehicle safety, driving comfort, fuel consumption, costs) at minimal mass. Considering the total energy consumption of vehicles it has been shown that up to 2/3 of this is massd ependent . Thus, a low vehicle mass is an essential factor in helping to reduce the energy consumption of a car.
In order to be able to derive concrete research questions, three basic vehicle concepts have been defined and formulated as follows: Urban Modular Vehicle (UMV) is a flexible urban electric vehicle with a modular and multimaterial construction, the Interurban Vehicle (IUA) is distinguished by its long distance comfort, comfortable interior and a body consisting predominantly of fibre-reinforced plastic materials and the Safe Light Regional Vehicle (SLRV) is a light and safe L7e class fuel cell vehicle with an aluminium foam sandwich monocoque construction.
With regard to the vehicle structure, all three vehicle concepts have a common objective of implementing a lightweight, but at the same time safe, body structure.
To achieve the goals of being light and crashproof, the importance of simulation methods and structural optimisation processes continues to increase. It is imperative to systematically incorporate these methods and procedures into the product development process, also in the light of the ever shorter product life cycles which, for example, for the VW Golf model (vehicle generation 1 to 7) has been more than halved .
Optimisation Strategies and Procedures
To achieve an optimal structural design with minimal mass and the required properties, a holistic view of materials, manufacturing technology and construction methods is needed .
This “lightweight construction trialogue” represents a particular challenge that can be supported by using optimisation techniques. By applying this method, it is possible to significantly reduce the number of manual iterations during product development and create a design that already meets the requirements in the concept phase.
▸ choice of construction method
▸ choice of material properties
▸ topology optimisation
▸ shape optimisation
At the beginning of a development process, the design and material selection are determined mainly by using technical feasibility studies or cost analyses. Based on the projected sales, material costs and existing manufacturing plants, amongst other things, the least expensive variant needs to be determined in terms of design and material for each of the respective vehicle components .
In the further course of the development process, the topology optimisation in particular plays a major role in the initial and concept phase. This is used to describe the basic arrangement and number of structural elements. The most common topology optimisation is linearstatic topology optimisation, but there are also approaches that allow a non-linear topology optimisation. These are, for example, the “Hybrid Cellular Automaton” (HCA), which has a uniformly distributed energy density as a target function, or the “Graph and Heuristic based Topology optimisation” (GHT), which was specifically developed for extrusion profiles.
Even at this early stage of development, a manufacturing technology can already be considered in the topology optimisation process. Especially casting, forging and extrusion manufacturing constraints are implemented in the most common software packages. If the production technology is not limited by existing production facilities, a production technology selection also takes place in addition to the joining technology selection.
To bring the concept design optimally in line with the package, the shape optimisation takes place in the concept phase. This can be done either by empirical studies (studies of profile geometries) or with an optimisation process. For series development, dimensioning optimisations are almost exclusively performed. These are carried out on the one hand on the components themselves, but also on the joining technology between adjacent components, on the manufacturing influences on the mechanical behaviour and the production itself, Figure 2.
Due to the variety of possible combinations, an analysis of the optimisation methods and strategies currently in use for lightweight structures is presented in the following section. As one of the first steps in the development of new components is to define the method of construction, the optimisation strategies found in the literature are systematically categorised by the different construction methods (integral, differential, composite and hybrid design).
One concept for the reduction of CO2 emissions from motor vehicles is, apart from alternative propulsion technologies (such as batteryelectric drive systems), to make weight savings, especially in the body. Innovative solutions are required for this, solutions with a high potential for lightweight construction. It is precisely this trend that is visible when the main “body” group is broken down according to the various construction methods.
The opportunity for innovative solutions that lead to mass reduction lies in structural optimisation with differentiated strategies.
The difficulty with this construction method is the implementation of the results from the topology optimisation in a meaningful structure in differential construction. There are various approaches for this. From implementation by experience and knowledge , through the use of construction and regulation catalogues  to a softwarebased solution , which enables automatic derivation.
Shape optimisation and dimensioning follow the CAD implementation, usually taking manufacturing constraints into consideration, particularly for sheet metal parts. In this step, it is important to optimise the joining technology so as to reduce weight and process costs. An exemplary approach for optimising pointshaped joints with sheet metal parts is described in .
The procedure described is represented in an abstract way in the first step and must inevitably be adjusted and detailed. An important role is played here by the requirements or the target function according to which the body component should be optimised. Depending on these component-specific target functions, the appropriate procedure, for example, for topology optimisation can be chosen. The same is true when selecting the method in the subsequent shape optimisation (empirical, geometrybased shape optimisation, netbased shape optimisation).
▸ Structures of the passenger compartment require high strength to guarantee the survival space of the occupants.
▸ Stiffness structures are mainly cross beams in the roof and rear area, which are characterised by a high moment of inertia to give the body the necessary rigidity.
▸ The structure nodes that are located primarily in the area of the A- and C-pillars mainly have the task of connecting as many adjacent structures as possible.
▸ Energy absorbing structures are used primarily to reduce the energy in the event of a crash. They are found in both the front and rear of the chassis but also in the sill area.
▸ Shear panels mainly increase the torsional rigidity of the structure and are therefore relevant to the driving dynamics of the vehicle.
▸ The outer skin structures and mounting parts are designed for specific concerns such as dents or a class A surface finish.
▸ Doors and closures both have ergonomic requirements, but are also relevant components for passive safety.
NGC UMV Side Crash Concept
For the structures in the crash relevant and energy absorbing region, a structure optimisation strategy for an energy absorbing structure in the form of a sill, including the underlying crash absorber, is shown as an example.
By applying the optimisation strategy, Figure 7, the specific energy absorption could be improved by 24 % and the intrusion by ~30 % compared with the initial design from crash test (compare also ).
Conclusion and Outlook
The opportunity for innovative solutions for body construction that lead to mass reduction lies in structural optimisation with differentiated strategies. These must be specifically adapted for the design and objective function of the components. The strategy described for the sill structure investigated improves both the specific energy absorption and the intrusion into the passenger compartment.
More possibilities are seen in particular in the application of optimisation strategies in the area of shear panels, such as the end wall. Here even more potential is found, since the use of specific optimisation possibilities, for example, for composite sandwich structures is not yet fully exploited and applied. The challenge with optimising lies in particular in the large number of optimisation parameters, the combined load types and the location of the component within the body. The location in particular poses major challenges since, especially for the optimisation of crash load cases, the calculation time required during the optimisation must also be considered.
- Friedrich, H. E.; Hülsebusch, D.: Elektro-Fahrzeugkonzepte und Leichtbau: Anforderungen für neue Werkstoffe. 1. Internationaler eCarTec Kongress für individuelle Elektromobilität, München, 2009Google Scholar
- Münster, M.; Schäffer, M.; Sturm, R.; Friedrich, H.E.: Methodological development from vehicle concept to modular body structure for the DLR NGC-Urban Modular Vehicle. 16th Stuttgart International Symposium, Stuttgart, Germany, 2016Google Scholar
- Klaiber, M.: Einsatz innovativer 3D-Druck-Technologien zur flexiblen Prozessverkettung. 2. Technologietag Hybrider Leichtbau, Stuttgart, 2015Google Scholar
- Schmit, L.A.; Mallet, R.H.: Structural Synthesis and Design Parameters. In: Hierarchy Journal of Structural Division, ASCE, Vol. 89, No. 4 (1963), pp. 269–299Google Scholar
- Volz, K. H.: Physikalisch begründete Ersatzmodelle für die Crashoptimierung von Karosseriestrukturen in frühen Projektphasen. München: Shaker-Verlag, 2011Google Scholar
- N.N.: Gewicht sparen mit Stahl — Massiver Leichtbau. In: emobility tec, No. 03/2015, pp. 50-53Google Scholar
- Lesemann, M.; Bröckerhoff, M.; Urban, P.: Leichtbaukonzepte für die Zukunft. In: Lightweight Design, No. 1 (2008), pp. 32–36Google Scholar
- Müller-Bechtel, M.; Jankowski, U.; Nösner, R.: Topologieoptimierung in der Rohkarosserie-Entwicklung. 3. Landshuter Leichtbau-Colloquium, Landshut, 2004Google Scholar
- Chapple, A.: eDICT — Evolutionary Design in Chassis Technology. Altair Technology Conference, Ludwigsburg, 2009Google Scholar
- Han, Y. H.; Witowski, K.; Lazarov, N.; Anakiev, K.: Topometry and Shape Optimization of a Hood. 10th European LS-DYNA Conference, Würzburg, 2015Google Scholar
- Wilde, H.-D.; Urban, T.; Strating, A.: Body Structures based on Audi's Modular Longitudual Architecture. Strategien des Karosseriebaus, Bad Nauheim, 2016Google Scholar
- Schäffer, M.; Münster, M.; Sturm, R.; Friedrich, H.E.: Development of an optimised side crash concept for the battery-electric vehicle concept Urban Modular Vehicle. 14. LS-DYNA Forum, Bamberg, 2016Google Scholar