Characteristic Properties of Wood
As natural products of biological origin wood is characterized by a high degree of diversity and variability in its properties. No one knows exactly how many species of trees there are in the world, but estimates run from 30000 upwards. A large proportion of these are to be found in the tropical regions, which frequently have an incompletely explored flora. The number of commercial timbers is only a small fraction of the total number of tree species. Since most wood properties are related to wood density, the fact that balsa, the lightest commercial species, has d density of about 160kg/m3 for ligament, one of the heaviest illustrates the diversity that can arise from species differences. In addition, considerable variability is found within species due to genetic and environmental factors that influence tree growth. Such variability is found not only from tree to the next, but also within trees, in part due to natural patterns of growth that make wood properties dependent on radial position and height within a stem. This variability must be taken into consideration if wood is to be used properly.
Wood is a highly anisotropic material
that is, its properties depend significantly on the direction in which they are measured. The principal directions in wood are the longitudinal direction is parallel to the cylindrical axis of the tree stem, and is also referred to as the direction parallel to grain since the majority of the constituent cells of wood are aligned parallel to it. If a tree stem is cut at right angles to its long axis, a series of concentric rings can often be seen on the cross section. These rings are markings produced by annual growth increments. The radial and tangential directions are those which are normal and tangential, respectively, to the growth rings on the cross section. The radial and tangential directions are also referred to collectively as directions perpendicular to grain. The tensile strength and stiffness of wood is highest parallel to grain and very low perpendicular to grain. Conversely, the shrinkage of wood that accompanies loss of moisture is very small parallel to grain and very much higher perpendicular to grain. In each of the properties of tensile strength, young’s modulus and shrinkage, the largest and smallest values are in the ratio of about 25:1.The degree of anisotropy is thus extremely high.
Wood can be very durable materials
The key to durability is proper use and understanding of the factors that destroy wood. Wood is biodegradable, which is essential in mature and can be an advantage when wood in use becomes unserviceable and must be disposed of. The same natural process becomes a severe disadvantage when it occurs as biodeterioration of wood in use. The key to preservation is to create conditions unfavorable to the organisms that cause biodeterioration, principally decay fungi and termites. In the case of fungi this means keeping wood dry.
Wood is susceptible to damage by fire
In match—stick sizes wood burns readily but it is much more resistant in larger sizes. With suitable construction methods wood can provide a high degree of fire safely in the early stages of a building five giving time for occupants to egress and the fire to be brought under control. This is possible because wood is poor conductor of heat.
As a hygroscopic material
wood will take up or give off moisture depending on the temperature and relative humidity of the surrounding atmosphere. Changes in moisture content below the fiber saturation point, which is the state where the cell walls are saturated with adsorbed water and no free water in present in the cell cavities, have an effect on virtually all properties of wood. Wood also shrinks as it dries, and conversely swells when it is netted again. The total shrinkage in volume of wood from a fresh felled tree dried in an oven will range from about 6% to 20%, depending on species. About two-thirds of the shrinkage will be in the tangential direction and one-third in the radial direction, the longitudinal shrinkage being almost negligible.