Three Common Mistakes in Mechanics of Composites

Abstract

Mechanics of composite materials is a well-established subject after active research for more than fifty decades. The two most fundamental theories in the mechanics of composites are the Rules of Mixtures (ROMs) and the Classical Lamination Theory (CLT). ROMs are the simplest micromechanics theories which are used to compute the effective properties of a composite material in terms of its constituents. CLT is the simplest structural mechanics theory to model composite laminates in terms of lamina constants and layup angles. During the course of researching and teaching the mechanics of composites for over two decades, the author has discovered three common mistakes made in most textbooks and other literature related to the mechanics of composite materials. The first mistake is that most textbooks and other literature claim an incorrect upper bound. The second mistake is the in-compatibility of the plane-stress assumption and the Kirchhoff assumption used in deriving the CLT. The third mistake is the common belief that the extension-bending coupling stiffness B must be symmetric, which is only true for laminated plates. Symmetric B is not guaranteed for curved panels or panels with microstructures featuring arbitrary heterogeneity and anisotropy, such as sandwich panels with corrugated cores or 3D woven composite panels. It is the intention of this paper to disclose these mistakes so that they will not be propagated into future generations.