Keywords: optimisation, damage resistance, weight, costs, genetic algorithms, bit-masking method, FEM.

Composite materials in aerospace design are being more and more considered as a valid alternative to metals due to the relevant influence of the weight on the overall aircraft performance and the need for robustness, effectiveness and reliability [1,2,3]. However composite materials, even if characterised by high strength-to-weight and stiffness-to-weight ratios, are extremely expensive in terms of manufacturing and maintenance. For these reasons, the adopted structural solutions should take into account not only the weight and the performance requirements but should also minimise the costs.

According to these considerations, in this paper, a multi-objective optimisation procedure based on the adoption of genetic algorithms is presented. The optimal configuration with minimum weight and cost of a damage resistant stiffened composite panel with buckling constraints has been determined. The numerical procedure is based on an in-house optimisation code [4,5] in conjunction with the ANSYS finite element (FE) code [6].

The presence of both continuous and high sensitivity discrete design variables, suggest the adoption of a genetic algorithm (GA) with a special bit-masking data structure able to increase the computational efficiency by switching specialized crossover operators depending by the nature (discrete or continuous) of the variables.

In order to determine the optimal configuration of stiffened panels resistant to buckling and to the impact damage, a parametric FE model has been implemented in the FE code ANSYS. The parametric procedure is able to handle up to twenty design variables governing geometry, topology and material properties (hence the manufacturing process) of the stiffened panels.

The introduced parametric FE model adopts the linearised buckling approach, based on eigenvalue calculations, for the buckling load determination and the impact threshold force empirical approach for the impact damage resistance estimation [7].

The life cycle costs are evaluated by considering the contribution of material, labour and repair costs [8]. The repair costs are determined taking into account the probability of impact and are considered dependent on the impact location.

Several optimal configurations of the stiffened panel, are finally analysed and critically discussed focusing on the influence of the damage resistance constraint on the overall costs.

1

Budiansky B., "On the Minimum Weight of Compression Structures", International Journal of solids and Structures, Vol. 36, 1999, pp. 3677-3708. doi:10.1016/S0020-7683(98)00169-3

2

Fukunaga, H and Vanderplaats G.N., "Stiffness Optimization of Orthotropic Laminated Composites Lamination Parameters", AIAA Journal, Vol. 29, 1991, pp. 641-646. doi:10.2514/3.59931

3

Nagendra, S., Haftka, R.T. and Gurdal, Z, "Stacking Sequence Optimization of Simply Supported Laminates with Stability and Strain Constraints", AIAA Journal, Vol. 30, 1992, pp.2132-2137. doi:10.2514/3.11191

4

Iuspa L. and Scaramuzzino F., "A Bit-Masking Oriented Data Structure for Evolutionary Operators Implementation in Genetic Algorithms", Soft Computing, Vol. 5, 2001, pp 58-68. doi:10.1007/s005000000066

5

Iuspa L., Scaramuzzino F. and Petrenga P., "Optimal Design of an Aircraft Engine Mount via Bit-masking Oriented Genetic Algorithms", Advances in Engineering Software, Vol. 34, 2003, pp.707-720. doi:10.1016/S0965-9978(03)00100-5

6

ANSYS®, Reference Manual, Version 8.0

7

Davies G.A.O., Zhang X., "Impact damage prediction in carbon composite structures", International Journal of Impact Engineering, Vol.16, 1995, pp. 149-170 doi:10.1016/0734-743X(94)00039-Y

8

Creemers R.J.C., "DAMOCLES 2 Deliverable for Work Package 1.11", NLR-CR-2004-054, NLR, Amsterdam, 2004

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