A symmetric, cross-ply laminate of sandwich hybrid construction is optimally designed with the objective of maximizing the fundamental frequency and the separation between two adjacent frequencies. The laminate, which is subject to an in-plane compressive load, is constructed with inner layers made of a low-stiffness material and outer layers made of a high-stiffness material. Optimal hybridization is achieved by determining the optimal ratio of the thickness of the low-stiffness layers to the overall laminate thickness. Moreover, the fibre orientations of high and low stiffness materials (0° or 90°) are determined optimally noting that the orientations of outer and inner layers affect the maximum frequency values in the case of hybrid laminates although they do not have any effect in the case of non-hybrid laminates. Comparative numerical results are given for graphite/glass hybrid and non-hybrid laminates. The optimal value of the hybridization parameter is found to depend on the level of prestress as well as on the aspect ratio. The optimal thicknesses of layers are shown to be non-unique for certain aspect ratios in the case of non-hybrid laminates of a given staking sequence.

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