WHAT ARE THE DIFFERENT TYPES OF DEFECTS IN TIMBER ?

TYPES OF DEFECTS IN TIMBER :

Defects can occur in timber at various stages, principally during the growing period and during the conversion and seasoning process. Defects affect the quality, reduce the quantity of useful wood, reduce the strength, spoil the appearance and favour its decay.

DEFECT DUE TO CONVERSION :

Conversion is the term used to describe the process whereby the felled tree is converted intob marketable sizes of timber. Conversion defects are basically due to unsound practice in milling or attempts to economise during conversion of timber. A wane occurs in timber which contains, on one or more faces, part of the bark or the rounded periphery of the trunk. This reduces the cross sectional area, with consequent reduction in strength in the parts affected. Excessive slope of grains may also be classed as a conversion defect when conversion has not been done parallel to the axis of the trunk.

SEASONING DEFECT :

These defects are directly caused by the movement which occurs in timber due to changes in moisture content. Excessive or uneven drying, exposure to wind and rain, and poor stacking during seasoning can all produce distortions in timber. These defects result in loosening of fixings or disruption of decoration, or both. The common types of seasoning defects are:

a) Checks : longitudinal separation of fibres not extending throughout the cross-section of wood; 



b) Splitting : separation of fibres extending through a piece of timber from one face to another;

c) Warpage : Warpage consists of cupping, twisting and bowing.

DEFECT DUE TO ABNORMAL GROWTH :

Following are some of the important defect   commonly found in wood due to abnormal growth or rupture of tissues due to natural forces.


a) Checks : Checks is a longitudinal crack which is usually normal to the annual rings. These adversly affect the durability of timber because they readily admit moisture and air.

b) Shakes : Shakes are longitudinal separations in the wood between the annual rings. These lengthwise separations reduce the allowable shear strength without much effect on compressive and tensile values. The separations make the wood undesirable when appearance is important. Boths the shakes and checks if present near the neutral plane of a beam they may materially weaken its resistance to horisontal shear.

• Heart Shakes : occurs due to shrinkage of heart wood, when tree is overmatured. Cracks startb from pith and run towards sap wood. These are wider at centre and diminish outwards.

• Cup Shakes : appears as curved split which partly or wholly separates annual rings from one another. It is caused due to excessive frost action on the sap present in the tree, especially when the tree is young.

• Star Shakes : Star shakes are radial splits or cracks wide at circumference and diminishing towards the centre of the tree. This defect may arise from severe frost and fierce heat of sun. Star shakes appear as the wood dries below the fibreb saturation point. It is a senous fault leading to separated log when sawn.

c) Rind gall : Rind gall is characterised by swelling caused by the growth of layers of sapwood over wounds after the branch has been cut off in an irregular manner. The newly developed layers do not unite properly with the old rot, thereby leaving cavities, from where decay starts.

d) knots : Knots are bases of branches buried by cambial activity of the mother branch. The root of the branch is embedded in the stem, with the formation of annual rings at right angles to those of the stem. The knots interrupt the basic grain direction of the wood, resulting in a reduction of its strength. In addition these affect the appearance of the wood. A dead knot can be separated from the body of the wood, whereas live knot cannot be. Knots reduce the strength of the timber and affect workability and cleavability as fibres get curved. Knots are classified on the basis of size, form, quality and occurrence.

Size Pin knot (under 12 mm), 
small knot (12–20 mm), 
medium knot (20–40 mm) and 
large knot (over 40 mm).

1. Form : Round knot and spike knot (a round knot exposed by sawing lengthwise). 
2. Quality : Sound knot—as hard and solid as the surrounding wood, decayed knot—contains advanced decay and is softer than the surrounding wood, encased knot—the annual rings fail to grow into the fibres of the surrounding wood, tight knot—a knot so securely fastened that it holds its position in the finished product.
3. Occurrence : Single Knot—when wood fibres deflect around one knot, Cluster Knot—when wood fibres deflect about two or more knots as a unit and, Branch Knot—two or more knots radiating from a common centre.

    

e) End Splits : End Splits are caused by greater evaporation of sap at the end grains of log and can be reduced by painting the exposed end grains with a water proof paint or capping the exposed end with hoop iron bandage.

f) Twist Fibres : Twist Fibres are caused by wind constantly turning the trunk of young tree in one direction.

g) Upset : Upsets are caused by the crushing of fibres running transversely during the growth of the tree due to strong winds and unskilled felling consequently resulting in discontinuity of fibres.

h) Foxiness : Foxiness is a sign of decay appearing in the form of yellow or red tinge or discolouration of overmatured trees.

i) Rupture : Rupture is caused due to injury or impact.