Timbers and Woods

Abstract

Timber can be considered as a typical natural composite material with a highly anisotropic structure. Indeed, this structure has two chief directions, radial and longitudinal, corresponding to its botanical organization. Furthermore, superimposed to these two degrees of variability are local effects such as growing conditions. For classifications, the terms hardwood and softwood have no relation to the actual mechanical hardness of the wood. It is only a broad botanical distinction. Hardwoods are generally broad-leaved deciduous trees which carry their seeds in seedcases (i.e., Angiosperms), such as ash, balsa, beech, greenheart, oak, obeche, and maple, while softwoods are generally coniferous trees (i.e., Gymnosperms) such as douglas fir, yellow pine, larch, spruce, hemlock, red cedar, and yew. From a structural-botanical point of view, wood contains many cells. These cells have different functions depending on their location in the tree. Inner cells, located in the heartwood, are mostly dead and provide mechanical support for the tree and in which the reverse materials, e.g., starch, have been removed or converted into resinous substances, Heartwood is generally darker than sapwood, although the two are not always clearly differentiated. Cells located in the sapwood store nutrients and act a conduits for water. Only the cambium,one-cell-thick layer, located just beneath the bark, contains new growing cells allowing the tree to grow, and subdivides the new wood from bark cells. This creates the rings each year. Hence, wood is considered from a strict mechanical point of view, as a complex fiber-reinforced composite composed of long, unidirectionally aligned tubular cellulosic polymer cells in a polymer matrix made of lignine. Cellulose is a naturally occurring carbohydrate and a thermoplastic polymer; it is arranged in long chains to form a framework. A bundle of these long chains is enclosed by both hemicellulose, a short polymer, and lignine, an organic cement that bonds these bundles, or microfibrils, together. Many of these unidirectionally aligned microfibrils compose the inner cell structure. Wrapped around the core is the cell wall consisting of more microfibrils, except that they are randomly oriented.