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ENCN221 > Wood > Flashcards

Wood Flashcards

(63 cards)

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1
Q

advantages of wood

A
  • natural beauty
  • availability
  • cost
  • ease of production
  • ease of use
  • low density
  • biodegradable
  • sustainable
  • energy efficient
  • durability
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2
Q

endogenous

A

intertwined growth - very strong and lightweight, not generally used for engineering applications

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3
Q

exogenous

A

outward growth - fibers grow from centre outwards by adding concentric layers, gives more predictable engineering properties

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4
Q

hardwood

A

broad leaf tree - does not necessarily indicate hardness

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5
Q

softwood

A

coniferous trees with needle like foliage and cones

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6
Q

deciduous

A

typically a broad leaf tree which looses its leaves in autumn

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7
Q

evergreen

A

tree which replaces its foliage gradually

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8
Q

Anatomy of a Tree

A

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

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9
Q

Macro Structure of Wood

A
  • outerbark
  • inner bark
  • cambium
  • sapwood
  • heartwood
  • pith
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10
Q

outer bark

A

dense rough layer that protects interior of tree

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11
Q

inner bark

A

transports sap from leaves to growing parts of tree

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12
Q

cambium

A

layer of material between bark and wood-forms both new wood and bark

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13
Q

sapwood

A

near outside of log - takes moisture up from roots and stores food for future growth

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14
Q

heartwood

A

inner core - nonliving cells, more resistant to decay

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15
Q

pith

A

located at centre of trunk - small cylinder of primary tissue, originally formed as new shoot of growing tree

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16
Q

earlywood or springwood

A

formed during periods of rapid growth - cells with large diameters and thin cell walls, light in colour

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17
Q

latewood or summerwood

A

forms later in growing season - cells grow more slowly with smaller diameter but thicker cell walls, darker in colour

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18
Q

Reasons for variability of wood

A
  • 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
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19
Q

Tracheids

A

longitudinally aligned cells, responsible for the mechanical support and transport of water and sap

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20
Q

cellulose

A

approx. 50% by weight - polymer formed from glucose, forms high density ordered strands (fibrils)

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21
Q

lignin

A

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
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22
Q

hemi-cellulose

A

15-20% softwood, 20-30% hardwood

-polymeric unit composed of various sugars

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23
Q

extractives

A

5-30%

  • includes tannins, oils, resins, wax, gums
  • some extractives toxic to fungi, provide natural durability
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24
Q

ash

A
  1. 1-3.0%

- calcium, phosphate, potassium and silica

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25
parallel compression
very strong, 30-70 MPa
26
perpendicular compression
strength increases with deformation, max when wood compressed to about 1/3 original thickness 2 - 12 MPa
27
parallel tension
also strong, about twice compressive stress, 70 - 150MPa
28
perpendicular tension
relatively weak, 2-9 MPa
29
bending
relatively strong, failure begins as compressive failure (stronger in tension) flexural strength = 40-100MPa
30
longitudinal shear
often a controlling a factor in beam design, 5 - 15 MPa
31
hygroscopic material
evaporates or absorbs water until moisture content in equilibrium with surrounding air, function of temperature and relative humidity
32
fibre saturation point (FSP)
cell cavity is empty (no free water) but cell walls are fully saturated (M.C. at FSP = 25-30%)
33
free water
liquid filling the wood cell cavities
34
bound water
liquid or vapour chemically bound by hydrogen bonding to the cellulose of the wood cell walls
35
water content greater than FSP
no general change on dimensions of wood
36
water content decreasing below FSP
reduction in volume, shrinkage (process is reversible)
37
shrinkage
- greatest in tangential direction - slightly less in radial direction (about 2/3 tangential) - very little in logitudinal
38
Lumber Process
1. Harvesting 2. Sawing 3. Seasoning (Drying) 4. Surfacing (Planing)
39
Harvesting
harvesting when trunk reaches optimal size for processing
40
Sawing
- live (plain) sawing: most rapid and economic - quarter sawing: maximum amount of prime (vertical) cuts - combination: most typical
41
Seasoning (Drying)
methods of seasoning - air drying (cheap and slow) - kiln drying (fast and expensive) - usually a combination
42
Surfacing (Planing)
- takes approx 2-5mm from each side | - nominal sizes refer to rough-sawn dimensions of lumber
43
Visible features in sawn timber caused by:
- natural wood growth - seasoning too fast - wood diseases - animal parasites - faulty processing
44
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
45
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
46
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)
47
reaction wood
can lead to significant shrinkage and warping after sawing - density 30-40% greater than normal wood
48
checks
cracks - an almost unavoidable consequence of differential drying in larger pieces of timber
49
shake
separation occuring between annual growth rings
50
wane
lack of wood at corner of a board
51
Two types of grading
- Pre-sorted (typically visual grading) | - Strength class (machine grading)
52
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
53
machine grading
- constant load plank grader - constant deflection plank grader - joist grader - acoustic grader
54
major causes of deterioration
- fire - decay - termites - insects and marine borers - mechanical abrasion - chemicals - UV light - moisture fluctuations
55
Prevention of decay
- moisture control (good construction practices) - exclusion of air - lower temperatures - wood preservatives
56
How fire retardant treatment works
- reduce amount of flammable gas released | - reduce amount of heat released in initial stages of fire
57
methods of fire resistance
- pressure impregnation with water soluble salts | - fire-retardant chemicals painted on surface
58
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
59
Composite drawbacks
- emissions (panels) - mechanical properties - moisture/temp. effects - manufacturing energy - resource competition
60
Laminated Veneer Lumber (LVL) advantage
spread any defects over length of material - less localised stresses - reduce uncertainty
61
Four requirements for fungal growth
1. air 2. moisture (fungi inhibited below 20%) 3. favourable temperature 4. food
62
Fungi Types
1. Visual Health - moulds - stainers 2. Strength - soft rot fungi - wood-rotting basidiomycetes
63
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

ENCN221 (6 decks)

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