1 - Wood microstructure – A cellular composite

Abstract

Wood possesses a cellular, three-dimensional microstructure and is described as a natural composite material with orthotropic elastic properties. The mechanical properties of the wood cell wall, comprised of primary and secondary cell wall layers, are dictated by the orientation of stiff cellulose microfibrils in a matrix of hemicellulose and lignin. The hemicelluloses are closely associated with the cellulose microfibrils and these carbohydrate polymers are encrusted with non-carbohydrate lignin, affording protection from moisture and microbial organisms. The orientation of microfibrils has a direct influence on elastic properties of the wood cell wall and varies as a function of position in the tree and within annual rings. Numerous models of the wood cell wall have explored this relationship. Bound water content in the cell wall also affects mechanical properties together with swelling and shrinkage. The chapter concludes by presenting expanded forms of Hooke’s law for both wood (3D) and balanced veneers (2D), which are directly related to the wood composite microstructure.