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Due to high strength and stiffness-to-weight ratio of composite cylindrical shells, they are increasingly being used in different industries. Applying different types of stiffeners is one of the ways to improve the buckling resistance of these structures. In this paper new analytical method based on smear method is developed to analyze the stiffened composite shells. The main difference of this method and previous methods is on technique of combination of shell`s and stiffeners` stiffness parameters, and calculating the equivalent stiffness parameters. In the suggested method a three layered shell is designed in such a way that this shell and stiffeners have the same volume and stiffness. Putting these layers under the main layers of shell, the equivalent stiffness parameters could be calculated easily. Using the Ritz energy method the critical load of axial buckling of shell is calculated. The method is verified using finite element ABAQUS package. The results show that the proposed method has less difference from finite element`s results compare to previous methods. In addition, the effects of different parameters on buckling load and special buckling load of stiffened shell is investigated. The results show that in order to have efficient stiffened structure; there must be an adequate number of ribs and unit cells. It also shows that, although adding stiffening ribs increase the buckling load of the shell, the special buckling load does not increase necessarily. The optimum angle for helical ribs is 30 to 40 degrees respect to axis of the cylindrical shell.

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Although many researchers investigated the effect of geometrical imperfection on the buckling load of unstiffened shells, the stiffened shells were not studied yet. In this paper, the effects of geometrical imperfection the buckling load of unstiffened and stiffened composite shell with and without cutout are investigated. For this goal, several specimens are manufactured and tested. The mechanical properties of fibers and resin matrix and volume fraction of fibers in the shell and the stiffeners are determined based on the standard tests. Finally, the mechanical properties of each component are calculated by micromechanical relations. These properties are used for finite element modeling by ABAQUS package. Linear eigen value analysis and nonlinear RIKS method -which can consider the geometrical imperfection- are used. FE results are validated in comparison with experimental tests. Using FE model, the effects of imperfection amplitude on the buckling behavior of unstiffened and stiffened shell with and without cutout are studied. The results show that geometrical imperfections have more effect on the buckling load of unstiffened shells in comparison with stiffened ones. Nevertheless, ignoring these imperfections and using eigen value analysis overestimate the buckling load. This fact is more evidence for shells without an opening. In perforated shells, the cutout itself represents an imperfection that is much more significant than geometric imperfections.

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In this paper, axial buckling of a composite cylindrical shell with and without a rectangular cutout is studied based on the first-order shear deformation theory. The equations are derived in a general form and can be converted to Donnell`s, Love`s, and Sanders` theories. To investigate the perforated shell, a physical domain is decomposed into several elements with uniform boundary and loading conditions in each element edges. In each element, the governing equations are discretized in both longitudinal and circumferential directions by the use of generalized differential quadrature method (GDQM). By assembling these discretized relations, a system of algebraic equations is generated. The boundary conditions at the shell and cutout edges, and the compatibility conditions at the interface boundaries of adjacent elements are also discretized by GDQM. Finally, the buckling load is calculated by an eigenvalue solution. To validate the presented method, the results of GDQM are compared with the available ones in the literature and also with ABAQUS finite element model. Then a parametric analysis is performed to investigate the effects of different parameters on the buckling behavior of the shells with and without cutouts. This study illustrates that the shell layup has a great effect on the buckling load of a shell. In addition, the influence of increasing the cutout size is not identical for different layups. However, the buckling behavior is independent of the shell material. Moreover, it was concluded that the shell with a square cutout has higher critical load than the one with a rectangular opening.