Buckling Analysis and Optimum Design of Multidirectionally Stiffened Composite Curved Panel

Continuous filament grid-stiffened structure is a stiffening concept that combines structural efficiency and damage tolerance. However, buckle resistant design optimization of such structures using a finite element method is expensive and time consuming due to the number of design parameters that can be varied. An analytical optimization procedure which is simple, efficient and supports the preliminary design of grid-stiffened structures for application to combined loading cases is needed. An analytical model for a general grid-stiffened curved panel is developed using an improved smeared theory with a first-order, shear-deformation theory to account for transverse shear flexibilities and local skin-stiffener interaction effects. The local skin-stiffener interaction effects are accounted for by computing the stiffness due to the stiffener and the skin in the skin-stiffener region using the neutral surface profile of the skin-stiffener semi-infinite plate model. The neutral surface profile for the skin-stiffener semi-infinite plate model is obtained analytically using a stress function approach, minimum potential energy principle, and statics conditions. Analysis methods for buckling of general parallelogram-shaped and general triangular-shaped curved panels are developed. These analyses are required in order to assess the local buckling of grid-stiffened curved skin segments. The buckling analysis makes use of circulation functions as Ritz functions which account for material anisotropy and different boundary conditions. The local buckling of stiffener segments between stiffener interaction points are also assessed. Using these analyses and a genetic algorithm as optimizer, an optimization tool is developed for minimum weight design of composite grid-stiffened panel subjected to combined in-plane loads with a global buckling design constraint. Design variables are the axial and transverse stiffener spacings, the stiffener height and thickness, and the stiffener pattern. Results are presented for buckling loads of composite grid-stiffened panels which are obtained using the improved smeared theory and are compared with detailed finite element analysis. Buckling loads for anisotropic skewed and triangular plates, and curved panels are presented and compared with results from finite element analysis. Finally, designs for grid-stiffened panels obtained using the design optimization process are presented