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Grid stiffened composite shells are one of the most important structures in aerospace industries. In this study critical buckling load of these structures with diamond shape stiffeners under axial loading based on smeared method is presented. The effect of shell thickness, angle of fibers in shell and the direction of stiffeners into the buckling load is determined. First-order shear deformation theory based upon the Ritz method is used to calculation of critical buckling load of these structures. In the use of FSDT theory, transverse shear forces in stiffeners have been considered. The results are compared with FEM solution with clamped boundary condition.

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Nowadays, grid stiffened composite shells have many applications in aerospace. These structures include an external shell that some helical and circumferential ribs, are placed in the inner surface of shell, are being used to reinforce it. Conical shells are one of the types of these structures that are used in the construction of space projectile body. In this study, buckling behavior of grid stiffened composite conical shells under axial loading, have been investigated. For this purpose, both analytical and finite element methods have been used and effects of external shell winding, helical ribs angle, ribs number and vertex angle of cone parameters on the buckling load of these structures were investigated. In analytical method, stringers by a shell, that have equivalent stiffness, were smeared. Based on this analysis, the extensional, coupling and bending matrices associated with the stiffeners were determined. Then by use of Ritz method, buckling load was calculated. Also, in the finite element method, conical shells by use of ANSYS software was modeled and analyzed. In finite element analysis, two kinds of mode shape for these structures were observed.