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WOOD MATERIALS TECHNOLOGY
4th Edition
Chapter 13Diseases and Defects
Defects Defects in timber can occur1. naturally (during the growth of a tree) or 2. artificially (as a result of incorrect stacking or
seasoning)Some defects can be decorative and very
pleasing to the eye, so these will be keptOthers may cause problems during the
manufacture of a project, such defects must be overcome
Natural DefectsOccur during the growth of a tree
– Knots– Resin pockets– Shakes– Waney Edge
Knots:These are formed from small
branches which are cut off the tree or fall off.
They reduce timber strength and make it hard to work
Resin pocket:• Small cavities in the wood
which are full of resin. Exist deep in the wood, especially in softwoods.
• Cracks caused by high wind or extremes in temperature
• Reduce the strength of the timber− Live knot
− Dead knot(loose)
− Radial shakes
− Tangential shakes
Shakes
ShakesShakes are splits in the end grain of wood
Occur along either: Ray linesAnnual rings
Caused by tension forces which build up as the tree is growing. When it is felled, or during seasoning weaker points break and the wood splits causing shakes
Waney Edge
Occurs during the conversion processOccurs when the bark is left on the edge
of the plank
Waney edge
Artificial DefectsOccur as a result of incorrect stacking or
inappropriate drying schedule, stresses will be created in the wood that will cause– Cupping– Bowing– Twisting / Warping– End Splitting– Case Hardening– Honeycomb Checks
Cupping Caused by
differential shrinkage
Planks will cup away from the pith
Annual rings appear to try to straighten out
Bowing
Takes the form of a bend along the length of the piece
Can be as a result of improper stacking – stickers too far apart
Twisting
Occurs when opposite corners move in a similar direction and the plank loses its flatness as a result
End Splitting
Caused if the ends of the boards dry out too quickly due to exposure to the sun or heat
Occurs during Air Seasoning
Can be prevented by sealing or covering the end grain of the planks of wood
Artificial defects – seasoning
− Boards bend when tension is released
− Moisture is trapped in the centre if surfaces dry too quickly
Case hardening
Fungal life cycle
Types of rotDry rot
− Under wood floors− Musty smell − Poor ventilation− Charred wood appearance
Wet rot− Occurs outside − Doors and window frames− White residue
Dry rot
Treatment− All infected wood removed
and 500mm beyond− All blockwork and wood
treated with fungicide− The cause of infection must
be found and fixed− All new timber must be
treated with preservative
Most common wood boring insect
The furniture beetle (woodworm)
Insect life cycleEggs are laid in woodThe larva (worm) hatches
and begins eating the woodWhen grown, the larva
makes a cocoon (pupa) and changes into the adult beetle
Adult emerges at the surface
Insect attack
Quiz
Name the defects shown in the diagrams
− Cupping− Heart shake
− Warping/twisting
The life cycle of a wood boring insect is shown. At which stage does the most damage occur?
Most damage occurs while the insect is at the larva stage
The larva bores tunnels under the surface of the wood
List the conditions necessary for a fungal attack to occur in wood
Conditions– A moisture content
above 20%– A supply of food (wood)– Oxygen
Questions1 What is the difference between natural and
artificial defects in timber.2 Give an example using a Sketch of both a
radial shake and a tangential shake.3 Describe using sketches what is meant by
bowing, cupping and twisting in planks.4 Why does case hardening occur?