Compression Buckling Response of Tailored Rectangular Composite Plates

Abstract

Buckling resistance is often a controlling criterion in the design of structures composed of plate elements. Design concepts that lead to increased buckling loads (or strains) of these plate elements can directly lower the structural cost and/or weight by a number of means. This study quantifies the improvements that can be achieved in compression buckling loads of rectangular composite plates by using a simple stiffness-tailoring concept. The approach is to position the unidirectional lamina through the thickness and over the planform of the plate so that the buckling load is increased with no loss in in-plane stiffness or increase in weight. Finite element analyses have been used to determine the effects of tailoring on the buckling loads of plates with various boundary conditions, aspect ratios, thicknesses, and membrane stiffnesses. Increases in buckling loads (or strains) of 200% or more compared to the uniform plate-buckling loads are shown possible with this tailoring concept. HE design of aircraft primary structural elements is often strongly influenced by a plate-buckling criterion. For ex- ample, the wing cover surface is normally restricted from buckling at loads up to maximum operating or even ultimate levels due to aerodynamic smoothness or global stiffness re- quirements. The fuselage skin is often restricted from buckling up to a relatively low load level to control out-of-plane defor- mations at normal service conditions and to limit these defor- mations and the loss of effective in-plane stiffnesses at higher postbuckling load levels. Similar requirements exist for struc- tural elements in many other types of applications. Design concepts that lead to increased plate-buckling loads (or strains) can directly lower the structural cost and/or weight by 1) allowing supporting substructure to be more widely spaced, thus reducing substructure, fastener, and attachment part count, 2) allowing the use of simpler substructure offering little rotational restraint to the surface structural elements, or 3) reducing the total structural weight by permitting design to a higher strain level governed by the material strength or damage tolerance rather than to lower values governed by buckling. This study quantifies the improvements that can be achieved in compression buckling loads of rectangular composite plates by using a simple stiffness-tailoring concept. It provides guide- lines that designers can use to achieve the improvement. Here stiffness tailoring is defined as the precise placement of lamina with various orientations through the thickness and across the planform of the laminated plate. The local membrane and bending stiffnesses of the tailored plate become nonuniform over the plate. When the loading is applied to the plate in the form of uniform imposed end displacements, stiffness tailor- ing brings about a redistribution of the in-plane loads that can directly benefit buckling response. At the same time, the dis- tribution and magnitudes of the bending and twisting stiff- nesses can also be modified to further improve the buckling load. The properly tailored design will be a function of the type of loading, the plate geometry, the boundary conditions, the relative material properties, and the basic configuration of the tailoring concept. Bulson1 shows that the type of tailoring