Wood Flashcards
advantages of wood
- natural beauty
- availability
- cost
- ease of production
- ease of use
- low density
- biodegradable
- sustainable
- energy efficient
- durability
endogenous
intertwined growth - very strong and lightweight, not generally used for engineering applications
exogenous
outward growth - fibers grow from centre outwards by adding concentric layers, gives more predictable engineering properties
hardwood
broad leaf tree - does not necessarily indicate hardness
softwood
coniferous trees with needle like foliage and cones
deciduous
typically a broad leaf tree which looses its leaves in autumn
evergreen
tree which replaces its foliage gradually
Anatomy of a Tree
roots-anchor tree and allow uptake of moisture and mineral
crown - leaves and supporting branches which produce food for trunk and seed
trunk-provides strength and rigidity
Macro Structure of Wood
- outerbark
- inner bark
- cambium
- sapwood
- heartwood
- pith
outer bark
dense rough layer that protects interior of tree
inner bark
transports sap from leaves to growing parts of tree
cambium
layer of material between bark and wood-forms both new wood and bark
sapwood
near outside of log - takes moisture up from roots and stores food for future growth
heartwood
inner core - nonliving cells, more resistant to decay
pith
located at centre of trunk - small cylinder of primary tissue, originally formed as new shoot of growing tree
earlywood or springwood
formed during periods of rapid growth - cells with large diameters and thin cell walls, light in colour
latewood or summerwood
forms later in growing season - cells grow more slowly with smaller diameter but thicker cell walls, darker in colour
Reasons for variability of wood
- over 30000 species
- composite material, made up of components of very different properties
- many flaws and imperfections, different sizes and degrees of severity
- wood is an anisotropic material
- properties highly sensitive to moisture condition
Tracheids
longitudinally aligned cells, responsible for the mechanical support and transport of water and sap
cellulose
approx. 50% by weight - polymer formed from glucose, forms high density ordered strands (fibrils)
lignin
25-33% softwood, 16-25% hardwood
- built up of phenylpropane units and act as glue to hold cells together
- longtudinal shear strength determined by strength of lignin bonds
hemi-cellulose
15-20% softwood, 20-30% hardwood
-polymeric unit composed of various sugars
extractives
5-30%
- includes tannins, oils, resins, wax, gums
- some extractives toxic to fungi, provide natural durability
ash
- 1-3.0%
- calcium, phosphate, potassium and silica
parallel compression
very strong, 30-70 MPa
perpendicular compression
strength increases with deformation, max when wood compressed to about 1/3 original thickness
2 - 12 MPa
parallel tension
also strong, about twice compressive stress, 70 - 150MPa
perpendicular tension
relatively weak, 2-9 MPa
bending
relatively strong, failure begins as compressive failure (stronger in tension)
flexural strength = 40-100MPa
longitudinal shear
often a controlling a factor in beam design, 5 - 15 MPa
hygroscopic material
evaporates or absorbs water until moisture content in equilibrium with surrounding air, function of temperature and relative humidity
fibre saturation point (FSP)
cell cavity is empty (no free water) but cell walls are fully saturated (M.C. at FSP = 25-30%)
free water
liquid filling the wood cell cavities
bound water
liquid or vapour chemically bound by hydrogen bonding to the cellulose of the wood cell walls
water content greater than FSP
no general change on dimensions of wood
water content decreasing below FSP
reduction in volume, shrinkage (process is reversible)
shrinkage
- greatest in tangential direction
- slightly less in radial direction (about 2/3 tangential)
- very little in logitudinal
Lumber Process
- Harvesting
- Sawing
- Seasoning (Drying)
- Surfacing (Planing)
Harvesting
harvesting when trunk reaches optimal size for processing
Sawing
- live (plain) sawing: most rapid and economic
- quarter sawing: maximum amount of prime (vertical) cuts
- combination: most typical
Seasoning (Drying)
methods of seasoning
- air drying (cheap and slow)
- kiln drying (fast and expensive)
- usually a combination
Surfacing (Planing)
- takes approx 2-5mm from each side
- nominal sizes refer to rough-sawn dimensions of lumber
Visible features in sawn timber caused by:
- natural wood growth
- seasoning too fast
- wood diseases
- animal parasites
- faulty processing
knots
result of wood grain flowing into branches of a living tree
- loss of load carrying cross section
- fibres in area of knots are distorted
- checking or slitting often occurs around knots
- greatest impact on tensile strength
sloping grain
reduce strength and stiffness but largest effect on tensile strength, not always easy to detect
- grain disturbed locally in growing tree due to branch
- board sawn parallel to pith but log had a significant taper
- log had fibres growing in a spiral direction about the trunk
corewood (juvenile wood)
wood within the first 5-10 growth rings from the centre - less dense than rest of wood and may contain many small knots (significant problem in radiata pine)
reaction wood
can lead to significant shrinkage and warping after sawing - density 30-40% greater than normal wood
checks
cracks - an almost unavoidable consequence of differential drying in larger pieces of timber
shake
separation occuring between annual growth rings
wane
lack of wood at corner of a board
Two types of grading
- Pre-sorted (typically visual grading)
- Strength class (machine grading)
visual grading
segregating timber for different uses, limit size of knots and other visible defects according to the reduction they make in properties of clear, defect free timber
machine grading
- constant load plank grader
- constant deflection plank grader
- joist grader
- acoustic grader
major causes of deterioration
- fire
- decay
- termites
- insects and marine borers
- mechanical abrasion
- chemicals
- UV light
- moisture fluctuations
Prevention of decay
- moisture control (good construction practices)
- exclusion of air
- lower temperatures
- wood preservatives
How fire retardant treatment works
- reduce amount of flammable gas released
- reduce amount of heat released in initial stages of fire
methods of fire resistance
- pressure impregnation with water soluble salts
- fire-retardant chemicals painted on surface
Composite benefits
- resource utilisation (can used insect and fire damaged timber, juvenile, small diameter, recycle plastics)
- dimensional stability
- potentially greater durability
- less variability
- improvement in some mechanical properties
Composite drawbacks
- emissions (panels)
- mechanical properties
- moisture/temp. effects
- manufacturing energy
- resource competition
Laminated Veneer Lumber (LVL) advantage
spread any defects over length of material
- less localised stresses
- reduce uncertainty
Four requirements for fungal growth
- air
- moisture (fungi inhibited below 20%)
- favourable temperature
- food
Fungi Types
- Visual Health
- moulds
- stainers - Strength
- soft rot fungi
- wood-rotting basidiomycetes
Microstructure of Wood
- 50% cellulose
- 23-33% (softwood) or 16-25% (hardwood) lignin
- 15-20% (softwood) or 20-30% (hardwood) hemicellulose
- 5-30% extractives
- 0.1-3% ash
