In this paper, a topology optimization methodology for the minimum weight of the composite stiffened panel with the constraint on the criticial buckling load is presented. An existing finite element solver [1] for the analysis of the stiff- ened tow steered composite panels is extended to perform the topology optimiza- tion for the minimum weight. The panel and the stiffeners are modelled using 3 node traingular Classical Laminate Plate elements (CLPT) and 2 node timoshenko beam elements, respectively. To achieve the independent meshing of the plate and stringers, the Lagrange multiplier based on a weak formulation of the continuity requirements between the plate elements and the beam elements is used. For a specific critical buckling load, the optimum topology of the stiffeners in the stiff- ened composite panel depends not only on the stiffeners but also on the fiber patten of the composite panel. Therefore, design variables corrosponding to both fiber pattern in the skin and stiffeners needs to be considered. Manufacturing mesh ap- proach presented in [1], is used to define the design variables corrosponding to the fiber pattern. The ground structure method is implemented to optimize stringers topology. The cross-sectional area of the stringers in the ground structure are de- fined as the design variables corrosponding to the stiffeners. To perform the robust the optimization, the analytical gradients of the buckling load and the weight of the stiffened panel with respect to design variables are implemented and verified using finite difference. The optimization for the minimum weight is performed for the varied complexities of the ground structures with the constraints on the critical buckling load.

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