Defects in wood

msc wood anatomy courses

1. WOOD Timber LOG Defect of wood presentation LUMBER

2. DEFECTS IN WOOD A defect in wood may be defined as Any abnormality or irregularity that lowers its commercial value by decreasing its strength, By affecting adversely it’s working and finishing qualities or its appearance. Natural deflects Defects due to seasoning , and Defects due to foreign organisms.

3. Natural deflects Natural Defects 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 to more economical use of timber.

4. Knots Knots - branches that grew on the young tree that eventually got embedded within the growing trunk. The vascular cambium in the branch and the trunk are one in the same the branch grows continuously along with the trunk. Tight knots - during periods of cambial activity, new wood is added as a continuous layer over branch and trunk, tightly fixing the knot in the wood of the trunk. Loose knots - a dead branch that losses its bark and is not firmly fixed; a knot not held tightly in place by growth, shape or position .

5. Loose knots Three types of loosen knot spike knot or a splay knot- In radially sawn board it will thus be cut longitudinally round knot - in tangential board it will be cut more or less transversely, so appearing circular or elliptical in shape, arise knot - Ina piece of sawn timber of which the faces are neither truly radial nor tangential, a knot will lie obliquely through it and may emerge on an edge.

6. Effect of knots Knots affect checking (cracking) and warping, difficulty in working and cleavability of timber. They are defects which weaken timber and depreciate its value for structural purposes where strength is an important consideration. The weakening effect is much more serious where timber is subjected to bending and tension than under compression. The extent to which knots affect the strength of a beam depends upon their position, size, number, direction of fiber, and condition. However, in some cases, knots increases the aesthetic value of board/veneer

7. Reaction Wood It is the specialized type of wood that is produced on the wide side of such eccentric cross sections Reaction wood has different characteristics in softwoods and hardwoods. 1. Compression Reaction In softwoods, it forms mainly toward the under­side of the leaning stem. Because the pull of gravity puts the lower side of the leaning stem in compression 2. Tension reaction Tension wood do not easy to detect visually In hardwood trees, reaction wood forms mainly on the upper side of the leaning stem . fuzzy surfaces produced during sawing or planing 2. Growth stresses in wood steam

8. 3. COMPRESSION FAILURES Compression failures are indicative of permanent deformation of the walls of cells which results from compression of the wood parallel to the grain beyond its proportional (elastic) limit .

9. Is an abnormal condition that causes the wood to break suddenly and completely across the grain at stress levels lower than expected, brought about by a decrease in cell wall material. It is caused by abnormal per cent of soft tissues. (Parenchyma Rays) 4. Brashness

10. Grain Defects – these are natural grain features but are referred to as defects 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. 5. DIRECTION OF THE GRAIN

11. Spiral Grain – Occurs when fibers have taken a simple spiral course in the growing tree as if the tree has twisted. Diagonal grain causes the grain of the wood to intersect the surface at an angle. When fibres run at a constant angle or slope to the axis of the timber Interlocking Grain – When fibres of adjacent layers in the growth ring are inclined at different angles to the axis. Boards show ribbon or stripe effect . TYPES of Grain Defects Spiral Grain Diagonal grain Interlocking Grain

12. Cross Grain – Caused when fibres have varying inclination to the edges of the timber . Wavy Grain – A wavy undulating arrangement of the fibers Wane or Waney – The absence of wood on the edge or corner showing the bark or surface of the sapwood, (due to improper conversion). Types of Grain Defects Wane or Waney Wavy Grain Cross Grain

13. Two types of defects develop in the wood of living trees, supposedly as the result of freezing temperatures. These are known as frost rings and frost cracks. Frost Rings . appear to the naked eye as brownish lines within the boundaries of growth rings which extend in the same direction and simulate false rings. Frost Cracks . develop as radial splits in the bark and wood near the base of a tree 6. Frost injuries

14. Number of defects known as pitch, pitch streaks, and pitch pockets are found in softwoods in which resin canals are normal in the wood, Pitch and Pitch Streaks . These defects develop through the accumulation of resin to excessive amount, in localized tracts in the wood; this fills the cell lumina and permeates the cell walls so that resin-soaked patches or streaks of tissue are present in the wood. Pitch Pockets . when viewed on surfaces at right angle to the grai or in edge-grain stock, plan convex cavities that are usually wholly within the boundary of a growth increment 7. Pitch defects

15. Pitch and Pitch Streaks Pitch Pockets

16. Are small patches of bark imbedded in wood . Bark pockets frequently arise as a result of bird pecks ( work of sapsuckers , etc .) and from injuries inflicted on the tree by insects (bark beetles , etc.). 8. Bark pockets

17. Seasoning Defects . These are defects 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 consist of moister or sap. Seasoning is simply the drying-out, so that timber is conditioned (or pre- shrunk) before use. Seasoning reduces the moister content of the wood to that of the relative humidity of the atmosphere, i.e. equilibrium moister content. Shrinkage is inevitable during drying and efforts are made to control the shrinkage without undue distortion and splitting.

18. Checks are ruptures along the grain that develop during seasoning either because of a difference in radial and tangential shrinkage or because of uneven shrinkage of the tissue in adjacent portions of the wood . Types of Seasoning Defects Warp – Any variation of as piece of timber from a flat surface. This tenancy is caused by the way a piece of timber is cut, the presence of other defects, and natural stresses in the tree from which the timber is cut.

19. Types of Warping Bow or Camber – a curvature along the wide surface in the direction of the length of the board. In floor joists and bearers this can be eliminated by using blocking Cup – a concave curvature across the face of the board Spring – a curvature along the edge of the board, not affecting the face. Twist – also known as ‘wind’ is a spiral distortion along the length of the board caused by twisting of the fibres .

20. Case hardening The exposed surface of timber dries very rapidly. It, therefore, shrinks and is under compression. The interior surface which has not completely dried is under tension. This defect is known as case-hardening and is usually occurs in timbers which are placed at the bottom during seasoning Collapse – The flattening or distortion of the cells showing excessive or uneven shrinkage. It can sometimes seriously affect the structural soundness of the timber . Collapse Case hardening Types of Seasoning Defects

21. Honey combing is the internal splitting in wood that develops in drying; caused by internal stresses or by closing of surface checks . Splits and Checks Honey combing Split – A longitudinal crack extending through the log or timber from one surface to another, usually at the end of the piece. Types of Seasoning Defects

22. Defects due to Conversion During the process of converting loges to lumber, the following defects may occur Chip mark marks or signs placed by chips on the finished surface bf timber. They may also be formed by the parts of a planning machine. Tom grain caused when a small depression is formed on the finished surface of timber by falling of a tool or so Waney Edge Occurs when the bark is left on during conversion

23. Tom grain Wane edge Chip mark

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