DEFECTS IN WOOD

DEFECTS IN WOODVarious abnormal conditions and features of wood which permanently reduce the economic value of wood are termed as defects. However, defect caused by fungal attack and decay in wood is known as Unsoundness. The term is generally applied to the discontinuity of tissues and abnormal fibre development in wood, and unsoundness to some form or stage of decay in wood. These defects and or unsoundness in wood may either just reduce its utility or render it entirely valueless.

Defects in wood can be broadly classified into two categories which are as follows:

(A) NATURAL DEFECTS (Knots, Shakes, Cross grain, Compression)

(B) OTHER THAN NATURAL DEFECTS: These defects include the

(1) Defect caused during treatment of felled timber (Seasoning defects and Conversion defects); the seasoning defects include:- (Warping, Split, Shake, Collapse, Case hardening) and the defects due to conversion includes:-( Box heart, Machine burnt, Machine notches, Miscut, Imperfect grain)

(2) Defect resulting from activity of external agent (animals, insects and fungus)

(A) NATURAL DEFECTS:

1. Knots: Knots are common types of natural defects. As the tree increases in diameter it covers the bases of the lateral branches. The portions of the branches enclosed within the wood are called knots. If the branches are alive at the time of inclusion, their tissues are continuous with main stem of the trees are called live knots. But when a branch dies and a part of it is gradually covered by the live tissues of the wood is called dead knots.

Knots vary in its size from Pin head several centimeters in diameters.
Classification of Knots:
(i) Pin knots: less than 6.5 mm in diameter.
(ii) Small knots: 6.5mm to 20mm in diameter
(iii) Medium knots: 20mm to 40mm in diameter
(iv) Large knots: above 40mm in diameter

Knots spoil the appearance and reduce the strength properties of wood. It also raises the seasoning defects and makes difficulties during wood working.

2. Shakes: A separation of fibre along the grain of standing or freshly felled timber is called shakes. This forms crack or fissures that is generally confined to the interior part of the timber but sometimes extends to one surface.
For example: If a tree is growing in high wind areas, different stresses are set up inside it. A tension occurs on the windward side whereas compression occurs on the leeward side. If the tree is not sufficiently elastic, then separation of tissues takes place inside the trunk.

3. Cross grain: This is general term depending the deviation of the wood fibres from a direction parallel to the longitudinal axis of the tree. This can be diagonal, spiral or interlocked types in nature.

4. Reaction wood: A distinctive anatomical characteristics form typically in parts of leaning trees and its branches is called reaction wood. In branches of trees, the under parts have compression wood denser and darker than upper part.

5. Compression failures: Minor fractures are running across the grain and the fibres show crinkles structures due to compression.

6. Resin pocket: Due to excessive accumulation of resin, resin patches are found in wood is called resin pocket.

7. Constriction due to climber: This defect occurs due to climbing plants. These climbing plants do considerable damage to the tree by binding round the stem.

(B) SEASONING DEFECTS: This is caused by faulty techniques of seasoning. The different types of permanent distortion of timber and ruptures of tissues constitute separately or together, they are referred as seasoning defects.

Types of seasoning defects:

(a) Warping: The distortion in converted timber caused departure from its original plane usually during seasoning period is called warping. Warping can be culping, bowing, twist, string.

(b) Check, Split and Shakes: These are the examples of separation or ruptures of the wood along the grain. These three forms differ in whether the crack is confined to the interior of the wood or extends to the surface.

Check: In check, there is a separation of fibres, which crack or fissures do not extend through the piece from one face to face of wood. This term is applicable for the converted timber.

Split: Crack extends from face to face of the wood. An end split is one that occurs at the end of log or a piece of timber.

Shakes: Separation of wood fibre along the grain and occurs in different shapes such as star, ring, etc. Shakes may be Heart and Star shake, Radial shakes and Cup and ring shake.

(c) Case hardening: During the wood seasoning, surface layer of wood usually dry before the interior layers and tend to shrink but they are prevented from doing so by the wetness of the wood. This is situation is called Case hardening. The case harder timber is liable to cuping, warping and other forms of distortion during planning and resawing.

(d) Collapse: This is abnormal and irregular shrinkage of wood. This defect is seen when very wet heartwood is dried.

(C) DEFECTS DUE TO CONVERSION: Timbers may sometimes contain defects due to faulty conversion.
Some of them are:

(a) Boxed heart: This term is applied to the timber, which is sawn in a way that the pith or the centre heart falls entirely within the surface throughout its length.
(b) Machine burnt: Defect due to overheating.
(c) Machine notches: Due to bad holding and pulling.
(d) Miscut: Careless during sawing of wood.
(e) Imperfect grain: Not matching with grain alignment.

(D) FUNGAL DEFECTS:

(a) Stain: Fungi causing stain in wood, when it feeds only on food materials stored in the sapwood. In this case, fungi do not attack the heart wood which normally does not contain food material within the cell. Stain defect does not affect strength properties of wood. For example: Ceratocystis.

(b) Decay: This is observed due to wood destroying or wood rotting fungus of wood. These fungi nourish cell wall material and break down the cell structure and enzymatic activities. Decay fungi attack both sapwood and heartwood. This defect reduces the strength properties of wood.

(E) DEFECTS DUE TO INSECTS AND OTHER ANIMALS:

(a) Insects: Insects borers and termites together constitute one of the most destructive biological agencies causing defects in timber. Some insects infest standing trees others infest felled logs before conversion or converted timber. The damage is visible in the form of tunnels and wood dust packed galleries in timber.

(b) Other animals:

Marine borer: Mostly these insects are found in costal regions and affects wood. For example: Crustaceans, Molluscs, etc

Birds and mammals: In birds, only wood peakers do some damage to individual trees by excavating holes in stem. Similarly, some of the animals such as deer, boar, etc cause serious wounds by peeling of the bark and damaging the cambium part of the tree.

So, defects in the wood is necessary to identify so as to prevent the timber from being damaged by insect, fungi and other animals so as to obtain the desired quality of timber.











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