13 diseases

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?