Defects in timber | Factors Affecting Strength of Timber

 

Defects in timber

 

Defect occurring in timber are grouped into the following five categories:

· Defects due to conversion

· Defects due to fungi

· Detects due to insects.

· Defects due to natural forces.

· Defects due to seasoning.

 

1. Defects due to Conversion -

During the process of converting Timber to the commercial form, the following defects may occur:

a. Chip Mark: This defect is indicated by the marks or signs placed by chips on the finished surface of timber.

b. Diagonal Grain: Formed due to improper sawing of Timber. It is indicated by diagonal mark on straight grained surface of Timber.

c. Torn Grain: In the conversion, many tools are used. If any of the tools or any other heavy things are dropped accidentally on the finished surface of timber it will cause small depression, which is called torn grain.

OR   -  Small depression formed due to falling of a tree.

d. Wane: This defect is denoted by the presence of original rounded surface on manufactured piece of Timber. (OR) The edge part of the timber log contains a rounded edge on one side because of its original rounded surface. This rounded edge is called wane.

2. Defects due to Fungi -

Fungi attacks the timber when moisture content is greater than 20% or there is presence of air and warmth in timber. Some of the important defects due to fungi are :

a. Dry Rot: Certain types of fungi attack on wood and convert it into dry powder form.

 

b) Wet Rot: Certain types of fungi cause chemical decomposition of wood and convert timber into grayish brown powder.

 

3. Defects due to Insects -

The insects responsible for decay of timber are beetles, marine bores, termites.

4. Defects due to Natural Forces -

Following defects are caused by natural forces:

·       Burls

·       Callus

·       Chemical stain

·       Coarse Grain

·       Deadwood

·       Druxiness

·       Foxiness

·       Knots

·       Rind galls

·       Shakes

·       Twisted Fibers

·       Upset

·       Water stain

Burls -

These formed when a tree has received shock or injury, the growth of the tree is completely upset and irregular projections appear on the body of timber.

Callus - 

It indicates soft tissue or skin which covers the wound of a tree.

Chemical strain -

The wood is sometimes discolored by the chemical action caused by some external agency. This is known as the chemical strain.

Coarse grain -

If a tree grows rapidly, the annual rings are widened. It is known as coarse-grained timber and such timber possesses less strength.

Dead wood -

The wood obtained from the cutting of the dead tree is light in weight and is actually defected. It is reddish in color and its strength is very less.

Druxiness -

Druxiness is a defect of timber in which the top surface of timber indicates white spots. These spots will give the access to fungi.

Foxiness -

 This defect is indicated by red or yellow tinge in wood or reddish-brown stains or spots round the pith of tree discoloring the timber. It is caused due to poor ventilation.

Knots -

These 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 long time and it ultimately results in the formation of dark hard rings, which are known as the knots.

 

Rind galls - 

Rind galls are curved swellings of trees which are formed at a point where a branch of the tress is improperly removed or fell down.

Shakes in Timber -

Shakes are nothing but cracks which separate the wood fibers partly or completely. Different shakes are formed in different conditions as follows:

· Cup shakes are formed due to the non-uniform growth of a tree or excessive bending by cyclones or winds. In this case, the shakes develop between annual rings and separate them partly.

· Heart shakes, the other type of shakes which develop in maturity approaching trees whose inner part is under shrinkage. The shake spread from pith to sapwood following the directions of medullary rays.

· Ring shakes are similar to cup shakes, but they completely separate the annual rings.

· Star shakes are formed due to extreme heat or severe frost action. They develop wider cracks on the outside of timber from bark to the sapwood.

· Radial shakes are developed radially from pith to the bark.

Twisted Fibers -

When the tree in its younger age is exposed to high-speed winds, the fibers of wood gets twisted. This type of wood is not suitable for sawing. So, this can be used for making poles, posts, etc.

Upset -

These are also known as the ruptures and they indicate the wood fibers, which are injured by crushing or compression.

Water stain -

The wood is sometimes discolored when it comes into contact with water. This defect is usually found in converted timber.

5. Defects due to seasoning -

Following defects occur in the seasoning process of wood.

Bow -

When the converted timber is stored for a longer time, some timber planks may have a curve along its length, which is known as Bow. The defect is indicated by the curvature formed in the direction of length of timber.

Cup -

If the timber planks curve along its width, then it is called Cupping of timber.

Check -

Check is the formation of a crack in the wood, which will separate the wood fibers. They form due to over seasoning of timber.

Split -

Split forms when a check extends from one end to the other end, which will split the wood into a number of pieces.

Twist -

Twist forms when the timber piece is distorted spirally along its length. It looks like a propeller blade after twisting.

 

Honeycombing -

Honey combing occurs in the inner part of the timber, which cannot be identified by just seeing. It is mainly due to stresses developed during the drying of timber.

Case Hardening -

Case is nothing but the top surface of wood, which dries rapidly during seasoning, but the inner part didn’t. Then this defect is called as case hardening.

 

Collapse -

During drying, some parts of the wood may dry rapidly while some may not. Because of this, improper drying shrinkage of wood occurs, that results in the defect called collapse.

Warp -

Warping is the loss of shape of wood due to stresses developed during drying. Cupping bowing, twisting of wood come under warping.

FACTORS AFFECTING STRENGTH OF TIMBER -

·       Density

·       Moisture Content

·       Temperature

·       Position in tree

·       Condition of growth

·       Presence of knots, shakes, etc.

 

1. DENSITY -

Density of wood is defined as the mass or weight per unit volume. Moisture in wood has a very large effect on the specific gravity as well as the density. Timbers of young tree has a very low density, therefore reduced stresses used for such material. Weight of timber reduced by drying while most strength properties are increased.

 

2. MOISTURE CONTENT -

Moisture content in a living tree varies with the species. Even in the same species, variation in moisture content depends on the age & size of the tree and its location. Mechanical properties of wood influenced by moisture content, but modulus elasticity is less affected by changes in moisture. Strength of wood increase as the moisture content decreases. Moisture content determined by oven-dry method or by electric moisture-meter method. Drying of timber from the green condition as cut to constructional usable content of say, 18% moisture content will cause shrinkage.

3. TEMPERATURE -

Strength of timber decreases, together with increasing temperature. Permanent loss of strength may happen if wood is held at high temperature for along period.

4. POSITION IN TREE -

In the early tree’s life, wood often tends to become stronger with increasing distance from the pith.

The cross section of a tree shows concentric rings and the number of rings per unit width is an indication of the growth of the tree. For Structural Timber 5 rings per 20 mm width may be specified.

 

5. CONDITION OF GROWTH -

Environmental factor such as height above the sea level, temperature, type of soil, rainfall, spacing between the trees have effect on the strength of the properties of the timber.