Timber—a review of the structure‐mechanical property relationship

SUMMARY Following a brief introduction concerned with present consumption and future prospects of timber and timber products the principal structure‐property relationships of this low density, cellular, polymeric composite are reviewed and discussed. Structure is examined at four levels of magnitude—macroscopic, microscopic, sub‐microscopic and chemical—and the various models used to interpret its composite nature are described. The dimensional instability of timber and loss of strength on wetting are discussed in terms of its fine structure. At low levels of stressing, and for short periods of time, timber can be treated as an elastic material, but at higher stresses and prolonged periods, especially with alternating humidity, timber behaves as a linear orthotropic viscoelastic material; the various factors influencing the elastic constants and the relationship of creep to fine structure are discussed. The anisotropy of wood is related to cell arrangement and microfibrillar orientation: strength and its variability are discussed in terms of structure at all four levels. Comparison of the strength of timber with that of other constructional materials especially on a weight basis shows timber in a very good light; the combination of high stiffness and high toughness is unique. Recent models using three‐dimensional anisotropic elastic analysis to understand strength and deformation of timber are described. The morphology of fracture under different forms of stressing is illustrated.