Hardwood and Softwood
Trees are divided into two classes: hardwoods and softwoods. The hardwoods such as Teak, birch, maple have broad leaves. The terms “hardwoods” and “softwoods” are not directly associated with the hardness or softness of the wood although in most cases hardwoods are actually harder and tougher than softwoods. In general softwoods originate from conebearing trees and hardwoods from trees that have their seeds contained in a seedcase.
Sapwood plays and important part in a tree’s living process. In general only the last few outside layers of sapwood are alive. The rest of the sapwood carries moisture from the roots to the leaves and store food for the tree. It usually have a higher moisture content than the heartwood.
During the life of the tree, sapwood gradually changes into heartwood. As it does so, it becomes less permeable. Since moisture movement is retarded considerably, heartwood dries more slowly than sapwood. Heartwood usually surface checks and honeycombs more readily than sapwood, hence requires milder drying conditions. It is usually darker in color and also more resistant to decay than sapwood.
Wood and Moisture
All wood in growing trees contains considerable quantities of water. It is present in two forms, i.e. (a) Freewater in the cell cavities and (b) Boundwater bounded to the cell walls. Most of this water should be removed in order to obtain the satisfactory service from the wood in use. The removal of freewater is much easer than that of boundwater from the wood and there is no change in dimension or in strength properties by removing freewater. The wood is said to be in a state of Fiber Saturation Point (FSP) when no freewater is present and the cell wall is saturated with boundwater.In this state, moisture content in wood is between 25% and 30% and removal of water below fiber saturation point (FSP) results dimensional change (shrinkage) and increase in many of the strength properties.
Because of its hygroscopic nature wood is always seeking to equilibrate its vapour pressure with that of the environment. That is wood always try to maintain its Equilibrium Moisture Content (EMC) which correspond to the environment. It is the unique characteristic of wood that expends when it absorb moisture if its EMC is higher than its existing moisture content, whereas contracts or shrinks when it loses moisture if its EMC is lower than its existing moisture content.
Anisotropy of Wood
As wood possesses a complex fiber composite structure, it varies in its most properties with the directions, called anisotropy. Moreover its difference in structure occurs not only between different species but also between trees of the same species grown in different environments or different parts of a single tree. This results undesirable variation in its properties as a material.
The figure below shows three principal axes of wood with respect to grain direction and growth rings.