Postbuckling behaviour of optimized rectangular composite laminates

Abstract

In the literature, the linear buckling load of rectangular plate elements is maximized by optimizing fibre orientation and thickness in a preselected lamination sequence. The paper considers the effects of laminate optimization on the postbuckling behaviour of biaxially compressed, specially orthotropic laminates and suggests modifications of the lamination parameters to improve postbuckling performance. Postbuckling studies, in general, involve the formal derivation of nonlinear equilibrium paths using, say, the methods of Galerkin, finite element, etc. In contrast, a simpler but effective method is proposed whereby the initial postbuckling stiffness, defined by the slope of the postbuckling load-end shortening relation at bifurcation, is adopted as a qualitative index characterizing postcritical behaviour. An explicit solution is described for simply supported plates. It is established that the postbuckling stiffness and linear buckling load are governed by completely different functions: the former depends exclusively on inplane (membrane) stiffness while the latter is a function of bending stiffness only. It has been suggested that unfavourable postbuckling performance usually follows optimization of the buckling load. A detailed parametric study substantiates this proposition by illustrating that laminates, optimal in linear buckling, do exhibit inferior postbuckling characteristics due to reduced stiffness. Improved laminate designs are suggested that exhibit superior performance in both pre- and postbuckling.