Wood itself is fibrous. Cells are long and slender and are aligned with the long axis of the trunk. It is these fibres that give the grain in the wood, not the growth rings. They also make the properties of wood quite anisotropic with much higher stiffness and strength parallel to the grain than across the grain.
The structure of wood can be likened to a bunch of parallel straws (representing the fibres or grain of the wood), which are bonded together using a weak glue. When load is applied parallel to the axis of the straws (a), they are very strong in tension and have reasonably good compressive strength until they start to buckle. However, if the load is applied perpendicular to the axis of the straws (b), they will tend to crush under compression and are weakest in tension, where the “glue” bond fails and the straws literally tear apart.
Creep
Creep is the term used to describe the changes in microscopic structure of wood that causes deflection of timber over a given time under a given load. (It is not the instantaneous deflection that occurs due to the changes in the level of load.
Natural Defects
Planed lumber can have defects that have occurred during the tree's growth. These include:
Decay, Rot or Unsound Wood Decay results from fungal activity. The wood loses its strength and may become soft or “punky”. Decay can be seen in a variety of forms. These forms are known as “white speck”, “honeycomb”, and “peck”.
Knots Knots are created where branches grew from the stem of the tree. Size and number of knots will affect the grade of the lumber. Additionally, the wood of the knot may fall out and leave a hole. This happens when the branch forming the knot dies and the wood is not bonded to the trunkwood.
Burl Burl is a defect caused by an injury in the living tree. This causes distortion of the grain and affects the strength of the lumber.
Shake A shake is a lengthwise separation of the wood. Shakes are most likely caused by wind damage. When the tree tissues are not elastic enough to withstand the stress as the tree is bent by wind, the tissues separate. If the separation is between or along the annual rings, it is known as a cup shake. If the separations radiate from the heartwood, they are known as heart or star shake. Shakes may also be caused in the drying process.
Timber Breaks Timber breaks are tiny cracks that zigzag across the grain of a board or plank. Their cause is unknown but may be due to wind stresses or hard contact with the ground during felling.
Sap Stain Sap stain is a variation in the natural colour of the wood. This is unsuitable for certain grades of lumber because of appearance requirements. The usefulness of stained wood is reduced to uses where natural finishes will be applied. Stained wood is divided into light, medium, and heavy stained categories. Sap stain will not progress when the wood has been kiln dried and planed.
Heart Stain The heartwood is naturally a darker colour than the sapwood. However, if the colour is in irregular patches, it is called heart stain. This is often a dark or reddish colour, but may range from pink to brown. Heart stain is unsuitable for certain grades of lumber because the strength can be affected.
Bark Pockets Pockets of bark may be present in lumber as a result of the stem of the tree growing outwards around a branch that has broken off. Bark pockets can also occur when scar tissue is formed after an injury.
Insect Damage Wood is prone to attack from a fairly wide array of insects.
Drying Defects Planed lumber may have defects that have occurred during the Drying Process:
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