Design Optimisation of Lower-Bound Buckling Capacities for FRP-Laminated Cylindrical Shells

The imperfection sensitive buckling loads of fibre reinforced polymeric (FRP) composite cylindrical shells under axial compression can be optimised with respect to many material and geometric parameters. Current approaches, using mathematical algorithms to optimise the linearised classical critical loads with respect to many design variables, generally ignore the potential reductions in elastic load carrying capacities that result from the severe sensitivities of buckling loads to the effects of initial imperfections. This paper applies a lower-bound design philosophy called the reduced stiffness method (RSM) to the optimisation design of FRP shell buckling. A physical optimisation in terms of parametric studies is carried out for simply supported, 6-ply symmetric, glass-epoxy circular cylindrical shells under uniform axial load. It is shown that under the guidance of RSM, safe lower-bound buckling loads can be enhanced greatly by choosing appropriate combinations of design parameters. It is demonstrated how this approach encourages the delineation of those components of the shell’s membrane and bending stiffness that are important and those that are unimportant within each of the prospective buckling modes. On this basis, it is argued that the RSM provides not only a safe but also a more rational strategy for better design decision making. 1. Introduction Due to their high strength-to-stiffness and strength-to-weight ratios, fibre-reinforced-polymeric- (FRP-) laminated shells are widely used in the weight sensitive industries such as aerospace, automobile, and offshore engineering. For thin FRP-laminated shells, the typically low elastic stiffness-to-strength ratios result in the elastic buckling playing a greater role in the design process compared with equivalent metallic structures. Relative to conventional metallic shells, the buckling capacities of FRP laminated shells will depend upon a much larger number of additional design variables, such as fibre distribution and orientation, lamina stacking sequence and thickness, and material selections. Identifying optimum design choices is consequently a more complex problem than for metallic shells. Current approaches [1–5] to the problem share a methodology based upon mathematical optimisation algorithms seeking the maximum linear classical critical loads of perfect FRP-laminated shells with respect to many design parameters. These approaches have inherent defects. Firstly they heavily rely on immense mathematical and computational efforts. Secondly they usually leave basic understanding of the