Buckling optimization for composite panels with elastic tailoring

Elastic stiffness tailoring of composite stiffened panels and tow placed plates is presented. The stiffened panels made from straight fiber laminates are designed using a previously developed bi-level optimization approach, including panel level optimization of the cross sectional panel dimensions and laminate level design of the stacking sequence using a genetic algorithm. Variable ply percentages for the skin and stiffeners are selected within practical manufacturing constraints, and the effect on panel weight is presented. It is shown that there is 10% weight variation when the ply percentages are within a specific range, whereas out of the range, the weight penalty could be 50%. The optimum designs maximize the proportion of ±45 degree (0 degree) plies in the skin (stiffener), indicating that a soft skin is better for stiffened panel design. For the particular example considered, an I-stiffened panel offers about 6% weight saving compared with a blade-stiffened panel when the load case is compression combined with out-of-plane pressure. Tow placed plates made from curved fiber laminates are modeled efficiently using the exact finite strip program, VICONOPT, for buckling analysis, and results are validated using the literature. A parametric study is preformed in terms of plate aspect ratio, and different reference fiber paths. It is shown that the buckling loads of tow placed plates with soft stiffness in the plate center are up to 30% higher than straight fiber plates, without any increase in weight, suggesting that tow placed plates can be used in stiffened panels to further reduce weight.