Special topic wood

Wood Density (Mg m- j Young s modulus IGPol Strength (MPaj II to grain Fracture toughness (MPa m j  

is a foamed fibrous composite. Both the foam cells and the cellulose fibres in the cell wall are aligned predominantly along the grain of the wood (i.e. parallel to the axis of the trunk). Not surprisingly, wood is mechanically very anisotropic the properties along the grain are quite different from those across it. But if all woods are made of the same stuff, why do the properties range so widely from one sort of wood to another The differences between woods are primarily due to the differences in their relative densities (see Table 26.1). This we now examine more closely. 

All the properties of wood depend to some extent on the amount of water it contains. Green wood can contain up to 50% water. Seasoning (for 2 to 10 years) or kiln drying (for a few days) reduces this to around 14%. The wood shrinks, and its modulus and 

Strength increase (because the cellulose fibrils pack more closely). To prevent movement, wood should be dried to the value which is in equilibrium with the humidity where it will be used. In a centrally heated house (20°C, 65% humidity), for example, the equilibrium moisture content is 12%. Wood shows ordinary thermal expansion, of 

The moduli of wood can be understood in terms of the structure. When loaded along the grain, the cell walls are extended or compressed (Fig. 26.5a). The modulus E ,ll of the wood is that of the cell wall, E, scaled down by the fraction of the section occupied by cell wall. Doubling the density obviously doubles this section, and therefore doubles the modulus. It follows immediately that 

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It may surprise you to learn that liquids and solids don t burn This statement seems at odds with the common experience of paper, wood, or gasoline burning, but in fact, at the molecular level, the solid or liquid has to be vaporized before it will burn. When a match ignites paper or some flammable liquid, the heat from the match first pyrolyzes (decomposes) the material so that some vapor is produced. (See Special Topic 2.1.2.1 Pyrolysis and Fires.) The heat also starts the chemical reaction of the flammable... 

Each chapter is designed to provide the content of a 50-minute lecture. Each block of four or so chapters is backed up by a set of Case Studies, which illustrate and consolidate the material they contain. There are special sections on design, and on such materials as wood, cement and concrete. And there are problems for the student at the end of each chapter for which worked solutions can be obtained separately, from the publisher. In order to ease the teaching of phase diagrams (often a difficult topic for engineering students) we have included a programmed-learning text which has proved helpful for our own students. 

A set of studies reporting the travel behaviour of professional and work groups is implicitly connected to two other frequently measured demographic descriptors previous travel experience and membership of special associations, interest groups or clubs. In particular measures of club membership as well as personal investment in products help define involvement and specialisation in many market segments (Woods, 2003). This specialisation topic will also be explored in the following section on tourist markets pertaining to particular tourist products. 

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