The wood which is suitable for engineering purposes or construction is called timber. The word timber is derived from an old English word ‘timbrian’, which means ‘to build’.
Timber develops many growth characteristics during or after milling. These may be considered defects in timber if they are undesirable visual attributes or structurally affect the timber.
A small mark on timber may not affect it structurally but will render the piece unusable for joinery. A piece of timber may have a defect in it that reduces its structural adequacy but enhances its aesthetic qualities.
A defect in timber is an irregularity or abnormality occurring in or on wood that is responsible for its-
- Strength reduction
- Lowering of durability
- Lowering of utility
- Poor Appearance
Defects In Timber
list the common defects in timber,
- Natural Forces Insects
- Seasoning Defects
- Fungi Defects
- Defects due to insect
Natural Defects In Timber
- Natural defects in Timber are described as features developed in the living tree, or soon after it is felled which may detract from the usefulness of the timber.
- Often it is more economical to remove defects such as dead knots and replace them with plugs.
Today building up long lengths from shorter glued pieces is becoming more common. Building up members by lamination also tends to the more economical use of timber.
1. Defects In Timber Due to Natural Forces
The wood is sometimes discolored by the chemical action caused with it by some external agency. This is known as a chemical stain.
The rind means Bark and Gall indicate abnormal growth. Hence peculiarly curved swelling found on the body of the tree is known as Rind Gall.
They develop at points from where branches are improperly cut off or removed. They are rarely found in a tree and the Timber in this part is very weak and not durable.
If the tree grows rapidly, the annual rings are widened. It is known as coarse grain timber and such timber possesses less strength.
These defects in timber are the bases of branches or limbs which are broken or cut off from the tree. The portion from which the branch is removed receives nourishment from the stem for a pretty long time.
it ultimately results in the formation of dark hard rings which are known as knots. As the continuity of wood fibers is broken by knots, they form a source of weakness.
The timber obtained from dead-standing trees contains dead wood. It is indicated by its lightweight and reddish color.
These defects in timber are indicated by white decayed spots which are concealed by healthy wood they are probably formed by access to fungi.
These are longitudinal separations in wood between the annual rings. These are cracks that partly or completely separate fibers of the wood. The separation makes the wood undesirable when appearance is important.
Types of Shakes
These defects in timber are cracks that extend from bark toward the SAP wood. These are usually confined up to the plane of sapwood.
These are wider on the outside ends and narrower on the inside ends. They are usually formed due to extreme heat or severe frost during the growth of the tree.
It appears as a curved split defect in timber that partly or wholly separates annual Rings from one another. It is caused due to Excessive frost action on SAP present in the tree especially when the tree is young.
These cracks occur in the center of a cross-section of the tree and they extend from pith to SAP wood in direction of the interior part of the tree which is approaching maturity. The heart shake divides the tree cross-section into two or four parts.
When cup shakes cover the Entire, they are known as ring shakes.
These are known as wandering hearts and are caused by the twisting of young trees by the fast-blowing wind. The timbers with twisted fibers are unsuitable for sawing.
These indicate wood fibers that are injured by crushing or compression. The upsets are mainly due to improper felling of trees and exposure of trees at a young age to the fast-blowing wind.
They are particularly formed when a tree receives shock or injury at a young age. Due to its injury, the growth of the tree is completely upset and irregular projections appear on the body of the timber.
A lengthwise ribbon/vein filled with gum that is not breached with wood tissue. It will affect the structural ability of the timber and its appearance.
These are natural grain features but are referred to as defects in timber because they cause working difficulties and affect the overall strength of the timber.
Apart from some growth factors, they are generally caused by the log not being sawn parallel to the bark.
Examples of Grain Defects In Timber
- Diagonal grain- When fibers run at a constant angle or slope to the axis of the timber.
- Interlocking Grain- when fibers of adjacent layers in the growth ring are inclined at different angles to the axis. Boards show ribbon or stripe effects.
- Spiral Grain- Occurs when fibers have taken a simple spiral course in the growing tree as if the tree has twisted.
- Wavy grain-A wavy undulating arrangement of the fibers.
- Wane or waney- The absence of wood on the edge or corner showing the bark
- Cross Grain- Caused when fibers have a varying i or surface of the sapwood, (due to improper conversion)
2. Seasoning Defects In Timber
These are defects in timber caused during the drying of the timber. Extreme care must be taken to control the rate and degree of drying.
More than half the weight of many freshly cut timbers consists of moister or sap. Seasoning is simply the drying-out, out so that timber is conditioned (or pre-shrunk) before use. Seasoning reduces the moisture content of the wood to that of the relative humidity of the atmosphere.
Equilibrium moisture content. Shrinkage is inevitable during drying and efforts are made to control the shrinkage without undue distortion and splitting. Seasoning defects can be further divided into:
Bowing is usually caused by poor stacking during seasoning. It is a serious defect in timber, causing good timber to be suitable only for use in short lengths.
Springing is an edge-way curvature of a board. It is usually caused by the release of internal stresses during seasoning.
Winding or Twist
Winding, also known as a twist, is very serious as it restricts the use of the timber to short lengths. It is caused by poor seasoning and poor stacking.
Cupping is very common in flat-sawn boards. It occurs through shrinkage of the timber when drying.
Shaking is caused by the board being dried too rapidly and is particularly common at the ends of boards, spreading along the grain.
Collapse is a rare defect caused by too rapid drying in the early stages of seasoning. The moisture is drawn out too rapidly causing dehydrated cells to collapse.
Case hardening is caused by too rapid drying, resulting in the outside cells of the timber drying and hardening, sealing off the moisture in the central part of the board.
3. Defects Due to Insects
Defects Caused by Termites
- Termites are the most dangerous to structural timbers as far as insect attack is concerned. They attack the sapwood and the heartwood of most timbers. But prefer the sapwood and the pith.
- Lives in the colony and is very fast in eating away the wood from the core of the cross-section.
- Makes tunnels in different directions and usually does not disturb the outer shell or cover.
- The timber piece attacked by termites may look sound until it completely fails.
- Few good timbers like Teak, Sal, etc. can resist the action of termites.
- Some Australian timbers because of the cellular structure, are resistant to termite attack for example.
- Cypress pine
- Grey Ironbark
- Red Gum
Termites are of two wide species
- Subterranean species, which
- live in the ground
- The dry wood species live in timber above the ground.
- Subterranean or ground-dwelling termites can often be found in pieces of timber that are left on the ground.
- Ground dwelling termites can often be found in pieces of timber that are left on the ground. These termites cause the most damage to buildings by preceding from their colony through formed subterranean galleries to the timber. The colony can be enlarged to encase a timber member.
- Smaller tunnels may radiate out from one member to another e.g. bearer to joist. This enables the termites to find more cellulose material
- (which is contained in the sapwood).
Subterranean species and Dry Wood species continue
- The dry wood species are wood dwelling and able to live in seasoned timber with low moisture content. Colonies are situated in and between timber members such as in a wall frame or tree branch.
- Subterranean termites travel inside tunnels, as they avoid light and prefer darkness. Metal and caps used on top of piers and walls will not prevent termite infestation but they can make it obvious as they build out over the angled metal. Creosote is a method of deterring termites but one must ensure that the timber is coated on the ends.
- Treatment can be carried out after completion but it is of course easier to do it during construction. Common methods today are mechanical prevention such as Granit Gard, Termtimes, and TERMISHIELS. Spraying is no longer carried out.
Defects in Timber caused by beetles:
- They form pin holes of size about 2MM dia in wood
- Tunnel formation is done in sapwood by larvae of the beetle
- Conversion of timber into a flour-like powder
- They do not disturb the outer shell or cover
Defects Caused By Beetles
- They are found in salty water
- They form tunnels or bores to take shelters
- The diameter and length of holes are as high as 25MM and 60MM respectively
- Affected wood loses its color and strength
- No timber is completely immune from the attack of marine borers
- There are several vanities of insects that attack wood. One that does serious damage to furniture is the common furniture beetle.
- The beetle lays its eggs in cracks and crevices of the wood. These hatch into small worms which burrow into the wood. They continue to eat their way through the wood for up to two years. The worms then change into beetles and come out leaving a woodworm hole.
- It is thus possible for wood to be affected by woodworm for a long time before the woodworm holes are seen. The beetles are very small about the size of a pin head and light brown in color in color. They are active in the summer months and during this time they fly about and lay their eggs on furniture. They are very fond of the plywood backs of cupboards. Although little can be done about the worms in the wood, several preparations are on the market that poison the beetles as they come out and destroy any eggs laid” on the surface.
4. Defects Due to Fungi
- The sap of wood is stained to bluish color by the action of a certain type of fungi.
- Brown Rot:
Fungi of certain types remove cellulose compounds from wood and hence the wood assumes a brown color.
It is just the opposite of brown rot. In this certain type of fungi attack, lignin of wood assumes the appearance of a white mass consisting of cellulose compounds.
Some kind of fungi caused the chemical decomposition of wood timber and in doing so converted timber into a greyish-brown powder. It is known as wet rot. Some important points to be remembered about wet rot are.
The alternate wet and dry conditions favor the development of wet rot. If unseasoned or improperly seasoned timber is exposed to rain and wind, they become easily liable for the attack of wet rot. To prevent wet rot, the well-seasoned timber should be covered by tar or paint for protection against moisture.
Some types of fungi feed on wood and convert it into dry powder form. This is known as Dry rot. The following facts are to be noted. Dry rot occurs in places where there is no free circulation of air such as improperly ventilated basements, rooms, etc., and in a damped situation like kitchen toilets, etc.
The unseasoned sapwood is easily attacked by dry rot. The favorable for the growth of fungus causing dry rot is the absence of sunlight, dampness, presence of sap, stagnant air, and warmth.
It is also caused by charring, painting, and tarring the unseasoned timber. The dry rot may be prevented by using well-seasoned timber free from sap.
When a part of the tree is seriously affected by dry rot, the damaged portion may be completely removed and the remaining unaffected portion should be painted with a solution of copper sulfate.
Heartwood is exposed to the attack of atmospheric agents; the tree becomes weak and gives an out hollow sound when struck with a hammer.
5. Defects In Timber Due to Conversion
Chip Mark Indicated
Chip Mark is Indicated by marks or signs placed by chips on the finished surface of the timber.
The Diagonal Grain
These defects in timber are formed due to improper sawing of timber. It is indicated by diagonal marks on the straight-grained surface of the timber.
The Torn Grain
These defects in timber are caused when a small depression is formed on a finished surface of timber by falling timber.
Denoted by the presence of the original rounded surface on the manufactured piece of timber.
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