The natural frequencies and mode shapes of composite stiffened plates with stiffener are presented by using the assumed natural strain 9-node shell element. To compare with previous research, the stiffened plates are composed of carbon-epoxy composite laminate with a symmetric stacking sequence. Also, the result of the present shell model for the stiffener made of composite materials is compared with that of the beam model. In the case of torsionally weak stiffener, a local buckling occurs in the stiffener. In this case, the stiffener should be idealized by using the shell elements. The current investigation concentrates upon the vibration analysis of rectangular stiffened and un-stiffened composite plates when subjected to the combination of in-plane compressive and shear loads. The in-plane compressive and shear loads affect the natural frequencies and mode shapes of the stiffened laminated composite plates. As a result of the increase in the in-plane compressive load and the variation of fiber angle of skin plate, the sequence of some of the mode shapes are interchanged. This implies that the present shell model for the stiffened plate produces more accurate results. Therefore, to obtain the correct vibration mode, the stiffeners are modeled as a shell element. To solve the eigenvalue problems, the Lanczos method is employed.The analytical solutions for free vibration of laminated composite plates were discussed in the works of Reddy 121 and Kant et al. 111. Aydogdu and Timarci /4/ presented vibration analysis of cross-ply laminated plates with twelve different combinations of edge boundary conditions using the Ritz method. Han and Choi 151 have published static and vibration analysis of laminated composite plates and shells using the lumped mass matrix. Park et al. 16/ studied linear static and dynamic analysis of laminated composite plates and shells using a 4-node quasi-conforming shell element. A 3-D analysis using a resultant 8-node solid element was presented by Kim et al. 111.
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