A two-step optimisation approach for the design of composite stiffened panels Part II: optimal design of the laminate

25 pagesThis paper deals with the definition of an optimisation procedure for the design of the wing-box composite stiff- ened panels subject to compression loads. In particular, the objective is to obtain a minimum-weight structure ensuring, at the same time, that the first buckling load should be greater then or equal to a threshold value. The optimisation strategy is subdivided into two phases: in the first one, we perform the global optimisation of the whole structure in terms of geometrical and material design variables, while in the second one the de- sign of the laminate (for both skin and stiffeners laminates) is performed in terms of the stacking sequence. Moreover, no simplifying hypotheses are made on the panel geometry and on the behaviour of the laminates, i.e. types of stacking sequences, which compose the structure and the usual mechanical requirements, such as uncoupling and bending orthotropy, are exactly fulfilled. The results concerning the first optimisation phase (structural optimisation) were discussed in a previous paper. In this work, we show the results of the second optimisation step, i.e. the results of the optimal design of laminates, having some peculiar elastic properties. The problem is stated as the unconstrained minimisation of a positive semi-definite function, whose absolute minimum is known a-priori and it is equal to 0. We show the numerical results for three representative cases: the first one concerns the wing-box section with identical stiffeners, the second one is about the wing-box section with non-identical stiffeners, while the last one is the wing box with non-identical stiffeners with a symmetric distribution of the geometrical and polar parameters