Structure of Wood

Wood is a cellular material consisting of tracheid or fiber cells. Essentially wood is made up from cellulose and lignin, amongst other things. A 10mm³ cube of wood can contain approximately 1.5 milllion cells. Cells are structured such that they are multilayered closed end tubes, this gives increased voids and an excellent strength to weight ratio. Some cells are responsible for carrying food and water and others provide the strength to the material. Because of the structure of wood, it can be said that it is a composite material, such that it relies on several components within its structure for it to perform.

There are several components or rather regions of a tree that are useful to be aware of, these being (working from the centre outwards);

Pith – This is the tissue at the centre of the tree.
Heartwood – This surrounds the pith, it forms spontaneously and once formation has completed it is essentially dead. That is to say that the tissue has ceased functioning and is now blocked by resins, tannins and oils.
Sapwood – The newly formed outer wood comprising of living tissue.
Cambium – The cambium layer acts as a sort of membrane between the bark of the tree and the sapwood. The function of the layer is to control growth of the tree.
Inner and Outer Bark – Acting as the skin of the tree, the bark forms a durable protective layer around the more delicate wood within.
During the growth of wood growth rings form. These rings are formed on a yearly basis and can be used to age the tree. In addition growth rings can also be used to provide information on the growing climate over the years, with wider growth rings suggesting faster growth rates and better growing conditions and narrower rings indicating the opposite.

It is possible to categorise wood in to two types, softwood and hardwood. This refers to the reproductive habits of the tree and the response to seasonal changes. In the first instance, hardwood trees produce seeds with some sort of covering (angiosperms), i.e. an acorn, whilst softwoods produce seeds that do not have this covering (gymnosperms). Furthermore, during winter months, hardwood trees shed their leaves whilst softwood trees maintain theirs. Hardwood is therefore said to be deciduous and softwood evergreen. It is a common misconception that hardwood is actually harder than softwood, this is not the case. Take for instance Balsa wood, this is actually a hardwood but it is by its nature very soft.

Source: http://www.timberfreak.co.uk/wood-timber/

Introduction of Wood

The Mechanical properties and availability of wood have made it a natural material for building structures, furniture, tools, vehicles, and decorative objects. Worldwide it is used more than metal or plastic.

Wood is a natural product and when used responsibly is a sustainable resource which need not result in damage to the environment. Forests can be protected by recycling and reusing the wood, using less wood and by supporting sustainable forest management

All wood is composed of cellulose,lignin, hemicelluloses, and minor amounts (5% to 10%) of extraneous materials contained in a cellular structure.

Wood comprises about 50% of cellulose which responsible for most of its mechanical properties.

Natural wood is generally composed of bundles of long fibres which are effectively water carrying tubes.   These fibres are laid in the direction of the tree trunk or branch from which the wood is removed.

The strength of wood is highly dependent on the loading direction. Wood is strongest in tension along the fibres and is weakest in the radial and tangential direction.  When loaded in its strongest direction (longitudinal along the grain - see figure below) wood can have a strength to weight ratio advantage relative to steel of 2:1.  However when wood is loaded in other directions (radial and tangential to the grain- see figure below) this advantage disappears

To use wood to its best advantage and most effectively in engineering applications, specific characteristics or physical properties must be considered.

Source: http://www.roymech.co.uk/Useful_Tables/Matter/Wood.html