Designing of Smooth Composite Panels Providing Stability and Strength at Postbuckling Behavior

Buckling of skin is admissible for upper load-bearing panels of a wing box and mechanization units of a light aircraft in case of loads close to operating level. Method of determining the optimum parameters of composite skin under action of compressive and shear flows is considered. Peculiarities of the optimum design problem are formulated as follows. Firstly, two loading levels are considered when designing the panels. At the first level, panel buckling is not admissible. At the second level, subject to the admissibility of postbuckling behavior, the panel thickness is calculated from the conditions for reaching the critical stresses determined accounting for the geometrically nonlinear relations. Secondly, the thickness and width of the rectangular panel are taken as variable parameters. Thirdly, in order to determine the optimum parameters of the panel, the conditions of realization of the minimum stability margin at the first loading level and minimum strength margin at the second loading level are taken into account. The stability and strength margins at the corresponding loading levels have to be equal unit. The loading levels above-mentioned, in general case, have to be coincided with the operating and ultimate loads considered when designing the aircraft structures, but can be given by an aircraft designer based on the special design conditions. It was noted that analytical solutions of the geometrically nonlinear problems have to be used in determining the design parameters according to the technique of postbuckling state. The relations presented take into account the membrane stresses arising at buckling of a thin panel. The technique developed can be applied at the early design stages. The relations for designing the hinge-supported orthotropic panels in compression, shear and combined loading are presented.