Buckling of laminated-composite cylindrical shells under axial compression

A deep investigation upon stability of laminated-composite cylindrical shells under axial compression is presented and analytical methods for shell design criteria are demonstrated and discussed. In the first part the classical Donnell-type solution of the buckling problem of geometrically perfect cylindrical shells is presented; a comparison between the results found out with this solution and those obtained with Finite Elements Analysis in NASTRAN highlights the need of different approaches for orthotropic shells. New algorithms SOLBUC-1 and SOLBUC-2 are proposed and appear in good accordance with FEM results. In the second part, an analysis of experimental results on a selected group of laminated-composite cylindrical shells puts in evidence the high sensibility of these structures to geometrical imperfections. NASA approach currently used in industry is based on the introduction of "knock-down factors" that, infact, yield over-conservative Critical Buckling Loads; new design approaches are proposed leading to a good mark-up in comparison to NASA results. In particular, an analytical solution of non-linear fourth-order partial differential stability equations considering axis-symmetrical imperfection is presented in new algorithm SOLDIF and leads to new Buckling Load "knock-down factors". These new "knockdown factors" appear substantially improved and hence less conservative than the corresponding "knockdown factors" presently used in the industry. Furthermore, the results show that the improved analytical-based knockdown factor presented always yields a safe estimate of the buckling load of the shells examined in this work