Stacking sequence optimization and modeling of laminated composite plates for free vibration

Abstract

Composite materials, especially fiber reinforced composites, have been extensively used in various engineering fields such as automotive, aerospace, aircrafts, defense, marine and so on due to having their high specific strength to weight and stiffness to weight ratios. In these last years, vibration problem has become more and more important in the structures where thin plates are used. Therefore, free vibration characteristics of composite structures under the influence of dynamic forces should be determined in the design process. Accordingly, in this thesis, optimum designs, which maximize the natural frequencies of laminated composite plate, are investigated by using hybrid algorithm combining the genetic algorithm (GA) and generalized pattern search algorihm (GPSA). Composite plates made of graphite/epoxy have been considered and assumed to be symmetric with continuous fiber angles in the laminate sequences. The natural frequency of plates is obtained bu using the Rayleigh Ritz method analytically. Free vibration equation is taken as objective function and fiber orientation angles are chosen as design variables. The natural frequency is maximized for various boundary conditions, aspect ratios, number of ply and material properties. The optimum designs obtained are verified by finite element method, and mode shapes of laminated composite plates are presented. A comparison between continuous and conventional (laminate in which the orientation angles are limited to the conventional orientations) designs is performed in order to show the reliability of continuous plates. As a results, it is observed that material properties, boundary conditions and dimensions of composite plates play important role on vibration behavior of composite plates. On the other hand, the natural frequencies and the optimum fiber oriantation angles are not affected from the change of number of plies.