The objective of this thesis is to develop the optimal design of stiffened composite panels subjected to uniaxial compression. To accomplish this objective, a finite element analysis program which calculates the critical buckling load by solving an eigenvalue problem was developed. For the verification of the program, the buckling load and the deformed shapes of stiffened panels with various configurations were compared with those from the nonlinear finite element analysis and the experiment. The genetic algorithm was implemented for the optimization method to manipulate the discrete ply angles as the design variables and its high reliability to find the global optimum.
To verify the possibility of the optimal design by genetic algorithm, the fiber orientations and the stacking sequence of rectangular plates with various sizes was optimized to maximize the critical buckling load. The critical buckling load with the optimized stacking sequence was found to be 1.1~1.6 times higher than that of the quasi-isotropic plates. The optimal design of stiffened composite panels with two stiffeners was performed by genetic algorithm to minimize the weight of a stiffened panel for the given design buckling load limit. The optimal design problem was carried out in two cases. One is the optimal design of a blade-stiffened panel and the other is the optimal design of an I-stiffened panel. The design variables of the skin were the number of plies, the fiber orientations and the stacking sequence. The design variables of the stiffener were the number of plies, the fiber orientations, the stacking sequence, the size, and the location. The optimal designed stiffened composite panels could resist the design buckling load and were lighter by about 23~26% than the stiffened quasi-isotropic panels.