Wood is an example of a natural composite. It has been used by man for thousand of years due to its large abundance and excellent properties. Today, wood is still being used extensively in the structural/building industry, in transportation, and in furniture and appliances.
In structural design, wood is commonly used for supports such as beams, joists and flooring, which are required to take considerable load. The most important properties of wood, therefore, are Young’s modulus (stiffness) and yield strength or compressive strength. Toughness is also important but to a lesser degree.
The macroscopic structure of wood is shown below:
The Bark (1-2): The bark consists of the deceased outer section and the living inner section. The inner bark transports nutrients from the leaves to the rest of the tree.
The Cambium (3): The cambium is where the wood and bark is formed. This layer is usually microscopically small.
The Sapwood and Heartwood (4-5): The sapwood consists of both living and dead cells. Its primary role is in sap transport and food storage. Heartwood typically consists of inactive cells. There is little difference between the strength of sapwood and heartwood.
The Pith (6): The pith is the area where initial wood growth occurs.