Defects in timber may be broadly classified into three major categories
- Due to abnormal growth of trees
- Due to conversion or seasoning and
- Due to fungi and insects.
The defects in each category are described as under.
Defects Due to Abnormal Growth of Trees
Defects due to abnormal growth of trees are natural in nature and they are knots, stakes, twisted fiber and rind galls.
Knots are too much frequent in many trees. They break the continuity of fiber and weaken the timber. These defects may of dead or live kind. Fig. bellow shows a knot defect. A dead knot will come out of the wooden piece leaving a knot hole. It is produced when a branch is broken off before the tree has finished growing. A live knot occurs when a branch separates from the tree after felling of the tree. Live knot will not become loose and fall out of its position but may tend to crack. This provides inlets for any fungi to attack the wood. If these knots are not too large and not too near the edge of the plank, they will not present a great problem however dead knots in timber makes the wood unsuitable for structural use.
Shakes in timber are splits in the grain and often arise as a result of uneven seasoning. They may be of radial, heart, star, cup and upset kind. An upset or rupture is a shake in which the fracture occurs across the grain. This is thought to be caused by violet jarring of the timber during felling and is often found in mahogany.
Sometimes, the fibers of a tree may get twisted due to wind action in branches. This defect is known as twisted fibers defect in timber. The fibers of wood have different inclinations with its axis. They are no more parallel to the axis of the wood. Twisted fibers in timber offer difficulty in working and a smooth surface cannot be obtained.
Rind gall is the defects due to abnormal growth of trees. It is the wound created on the tree while a branch breaks and parts off or gets cut. Decay may occur at that point.
Defects Due to Conversion and Seasoning
Defects due to conversion and seasoning of timber involve shakes, warping, bowing, twist, diamonding, casehardening and honey combing. Some of such important defects are discussed as under.
Warping is a kind of variation from a true or plain surface and may include a one or combination of cup, bow, crook and twist. Warping board which is tangentially sawn may invariably warp. This takes the form of a hollowing or cupping across the face of the board and when wide flat boards are required this will act as a serious drawback. Wind or twist defect occurs when thin boards are cut from a log having curved longitudinal grain. This tendency is for the board distort spirally.
Diamonding in timber is the tendency of square cut pieces to become diamond shaped when cut from certain areas of the log. This happens when the piece has been cut with growth rings running diagonally, causing the unequal shrinkage between summer and spring growth to pull it out of shape