2054. The Ten-Percent Rule for Preliminary Sizing of Fibrous Composite Structures

Paper

Abstract

Simple, reliable methods are presented for calculating the fiber-dominated in-plane strengths of well designed fibrous composite laminates. This is accomplished by a simple rule of mixtures, the Ten Percent Rule. The primary fibers for each uniaxial load condition are considered to develop 100 percent of the reference strength of the composite material for each environment, while the secondary (transverse) fibers are credited with only 10 percent of this strength and stiffness, whether they be inclined at 90′ to the primary fibers or at ?45 deg. The procedure is applied to uniaxial loads, to biaxial loads of the same sign, and to biaxial loads of opposite signs (which is equivalent to in-plane shear with respect to rotated axes). A worked example for a wing skin is included to show how very rapidly this method converges on the most suitable design to withstand a set of loads, whether applied simultaneously or separately. Since neither this nor any other method less complicated than micromechanics is capable of identifying structurally inferior (matrix-dominated) fiber patterns that should be avoided, the preferred fiber patterns within the O deg, ?45 deg, and 90 deg family are established by other analyses and empirically acquired wisdom. Various other simple formulae suitable for calculating the laminate strengths and elastic constants are included. Differences between the predictions of this failure model and those of better publicized models are attributed to the failure of other authors to recognize that homogenizing the fibers and resin matrices into a single ”equivalent” orthotropic material is permissible only for calculating elastic properties and is scientifically incorrect for predicting the strength of distinctly heterogeneous conventional fiber-polymer composites.