Optimising Buckling Capacities for Composite Shells

Abstract

With their extremely large numbers of independent material and geometric parameters the problem of choosing the appropriate combinations to optimise buckling performance is even greater for advanced composite shells than for equivalent metallic shells. Furthermore, for thin shells constructed from advanced composites it is generally more important to also take account of the reductions in elastic load carrying capacities that result from the severe sensitivities of buckling loads to the effects of initial imperfections. Current approaches of nonlinear numerical analysis combined with optimisation algorithms heavily rely on analytical and computational efforts. Given the immense numbers of independent parameters that might be involved and the complexity of the nonlinear analysis needed to assess the worst effects of initial imperfections, it is unlikely that reliance on just nonlinear numerical solution algorithms will be sufficient for this purpose. This paper will outline an alternative approach based on the so-called “reduced stiffness method” (RSM) [1] that enables an extension of classical buckling analysis to provide safe lower bounds to the imperfection sensitive buckling loads of shells. The RSM has successfully predicted safe lower bounds for a range of isotropic and stiffened shells. Recent carefully controlled numerical analysis shows that RSM can also provide reliable estimates of the lower bounds to composite shells’ buckling loads [2]. Since this method also 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 it provides an intuitive strategy for better design decision making. In this paper by examining the effect of lamina eccentricities to the imperfection sensitive buckling loads of composite shells it is demonstrated how the RSM can be used as a tool for guiding appropriate combinations of parameters to enhance buckling capacities of fibre reinforced laminated composite shells. The simplicity of this analytically based method enables the prediction of the likely consequences of variations of the many material and geometric parameters that govern the safe resistance to buckling. As a tool for guiding appropriate combinations of parameters to affect enhanced, or even “optimum”, buckling capacities this approach will be shown to have considerable advantage over many of the currently available alternative means for improving design performance.