PostprintThis is the accepted version of a paper published in Structural and multidisciplinary optimization (Print). This paper has been peer-reviewed but does not include the final publisher proof-corrections or journal pagination.Citation for the original published paper (version of record):Kaufmann, M., Zenkert, D., Wennhage, P. (2010) Integrated cost/weight optimization of aircraft structures. July 2007A methodology for a combined cost/weight optimization of aircraft components is proposed. The objective function is formed by a simplified form of direct operating cost, i.e. by a weighted sum of the manufacturing cost and the component weight. Hence, the structural engineer can perform the evaluation of a design solution based on economical values rather than pure cost or weight targets. The parameter that governs the balance between manufacturing cost and weight is called weight penalty and incorporates the effect of fuel burn, environmental impact or contractual penalties due to overweight. Unlike previous work, the analytical cost and structural models are replaced by commercially available software packages. They allow for a more realistic model of the manufacturing cost, and for arbitrary constraints in the structural analysis. By means of parametric studies it is shown that the design solution strongly depends on the magnitude of the weight penalty.

Weight reduction is commonly adopted in vehicle design as a means for energy and emissions savings. However, selection of lightweight materials is often focused on performance characteristics, which may lead to sub optimizations of life cycle environmental impact. Therefore systematic material selection processes are needed that integrate weight optimization and environmental life cycle assessment. This paper presents such an approach and its application to design of an automotive component. Materials from the metal, hybrid and polymer families were assessed, along with a novel self-reinforced composite material that is a potential lightweight alternative to nonrecyclable composites. It was shown that materials offering the highest weight saving potential offer limited life cycle environmental benefit due to energy demanding manufacturing. Selection of the preferable alternative is not a straightforward process since results may be sensitive to critical but uncertain aspects of the life cycle. Such aspects need to be evaluated to determine the actual benefits of lightweight design and to base material selection on more informed choices.

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An optimisation methodology is developed and applied on a FRP sandwich body of an electric vehicle -ZBee, where single-objective and multi-objective optimisation studies are performed stepwise using a commercially available software package. The single-objective optimisation allows the identification of the load paths within the composite body, according to the loading conditions previously defined. Within the multi-objective optimisation, the optimum thickness and distribution for each of the layers that form the composite body are searched within the design space so as to obtain the best performance with respect to weight, material cost, global and local stiffness. Strength requirements are also considered as constraints within the optimisation. A conflict situation appears when several objectives are considered within the optimisation, meaning that an increased performance in one objective may often lead to a decreased performance for the others. Therefore, a trade-off between objectives is needed. The interpretation of results is partially made by using trade-off plots, the so-called Pareto frontiers. A method for the overall selection of the most beneficial solutions is proposed and applied in order to choose between the best obtained solutions according to the importance of the objectives.

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