In many engineering applications an optimal design based on continuous variables is usually not practical to produce. A more cost-effective approach is to fabricate the laminate out of standard composite plies with given fibre orientations. Another way of reducing the cost is to opt for a hybrid construction which uses the high-stiffness material in the surface layers and the low-stiffness one in the core layers. The present study is aimed at using the above ideas in the optimal design of symmetric laminates undergoing free vibrations. The frequency of an external excitation can be placed either between zero and the fundamental frequency or in a gap between two consecutive frequencies. In the design optimization problem, the optimal stacking sequence is to be determined such that the frequency separation is maximised (see [1] for the non-hybrid case).
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