Wood is the oldest and one of the most commonly used engineering materials in the world. The earliest evidence for a domestic structure in Britain was that of a tent-like structure made with wooden supports dating to 7000 BC, and wood is the most commonly used building material to this day. Worldwide, 109 tonnes of wood are used per annum, comparable to the consumption of iron and steel. Wood is so widely used because of its low cost, per tonne 1/60th that of steel, and high specific strength (high value of the merit index s/r). Wood also combines high stiffness and high toughness. As wood is a renewable resource, it is a good material from an environmental perspective and its production requires only a low energy input. One mature tree supplies enough O2 gas for 10 people but we are consuming in the UK 12 trees per person per year. We must therefore think carefully about the environmental impact of using large quantities of wood.
Wood is a fibre-composite material (cellulose fibres in a lignin matrix) with complex overall structure. Wood is a cellular material. Cells form the basic unit of life and are immensely complicated. There are roughly 1012 cells of 4 main types in a tree. Cells display a great deal of self-organisation and assembly. Additionally, the constituents of a tree undergo continuous renewal, making a tree a dynamic system.
In this TLP you will learn how the structure of the tree trunk is specially adapted for its functions: to support the leaf canopy, to transport mineral solutions via conduction, and to store food in the form of carbohydrates. Wood has two types: softwoods and hardwoods. There is however little correlation between the type of wood and its properties: some hardwoods are very soft!
This TLP discusses the mechanical properties of wood, and explains wood’s generally high strength under tension. Wood also shows properties of high toughness and stiffness. These values vary greatly depending on the type of wood and the direction in which the wood is tested, as wood shows a high degree of anisotropy. Wood’s properties are also strongly affected by the amount of water present in the wood. Generally, increasing the water content of wood lowers its strength.