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ENCN221 > Wood > Flashcards

Wood Flashcards

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
Q

parallel compression

A

very strong, 30-70 MPa

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26
Q

perpendicular compression

A

strength increases with deformation, max when wood compressed to about 1/3 original thickness
2 - 12 MPa

27
Q

parallel tension

A

also strong, about twice compressive stress, 70 - 150MPa

28
Q

perpendicular tension

A

relatively weak, 2-9 MPa

29
Q

bending

A

relatively strong, failure begins as compressive failure (stronger in tension)
flexural strength = 40-100MPa

30
Q

longitudinal shear

A

often a controlling a factor in beam design, 5 - 15 MPa

31
Q

hygroscopic material

A

evaporates or absorbs water until moisture content in equilibrium with surrounding air, function of temperature and relative humidity

32
Q

fibre saturation point (FSP)

A

cell cavity is empty (no free water) but cell walls are fully saturated (M.C. at FSP = 25-30%)

33
Q

free water

A

liquid filling the wood cell cavities

34
Q

bound water

A

liquid or vapour chemically bound by hydrogen bonding to the cellulose of the wood cell walls

35
Q

water content greater than FSP

A

no general change on dimensions of wood

36
Q

water content decreasing below FSP

A

reduction in volume, shrinkage (process is reversible)

37
Q

shrinkage

A
  • greatest in tangential direction
  • slightly less in radial direction (about 2/3 tangential)
  • very little in logitudinal
38
Q

Lumber Process

A
  1. Harvesting
  2. Sawing
  3. Seasoning (Drying)
  4. Surfacing (Planing)
39
Q

Harvesting

A

harvesting when trunk reaches optimal size for processing

40
Q

Sawing

A
  • live (plain) sawing: most rapid and economic
  • quarter sawing: maximum amount of prime (vertical) cuts
  • combination: most typical
41
Q

Seasoning (Drying)

A

methods of seasoning

  • air drying (cheap and slow)
  • kiln drying (fast and expensive)
  • usually a combination
42
Q

Surfacing (Planing)

A
  • takes approx 2-5mm from each side

- nominal sizes refer to rough-sawn dimensions of lumber

43
Q

Visible features in sawn timber caused by:

A
  • natural wood growth
  • seasoning too fast
  • wood diseases
  • animal parasites
  • faulty processing
44
Q

knots

A

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
Q

sloping grain

A

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
Q

corewood (juvenile wood)

A

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
Q

reaction wood

A

can lead to significant shrinkage and warping after sawing - density 30-40% greater than normal wood

48
Q

checks

A

cracks - an almost unavoidable consequence of differential drying in larger pieces of timber

49
Q

shake

A

separation occuring between annual growth rings

50
Q

wane

A

lack of wood at corner of a board

51
Q

Two types of grading

A
  • Pre-sorted (typically visual grading)

- Strength class (machine grading)

52
Q

visual grading

A

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
Q

machine grading

A
  • constant load plank grader
  • constant deflection plank grader
  • joist grader
  • acoustic grader
54
Q

major causes of deterioration

A
  • fire
  • decay
  • termites
  • insects and marine borers
  • mechanical abrasion
  • chemicals
  • UV light
  • moisture fluctuations
55
Q

Prevention of decay

A
  • moisture control (good construction practices)
  • exclusion of air
  • lower temperatures
  • wood preservatives
56
Q

How fire retardant treatment works

A
  • reduce amount of flammable gas released

- reduce amount of heat released in initial stages of fire

57
Q

methods of fire resistance

A
  • pressure impregnation with water soluble salts

- fire-retardant chemicals painted on surface

58
Q

Composite benefits

A
  • 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
Q

Composite drawbacks

A
  • emissions (panels)
  • mechanical properties
  • moisture/temp. effects
  • manufacturing energy
  • resource competition
60
Q

Laminated Veneer Lumber (LVL) advantage

A

spread any defects over length of material

  • less localised stresses
  • reduce uncertainty
61
Q

Four requirements for fungal growth

A
  1. air
  2. moisture (fungi inhibited below 20%)
  3. favourable temperature
  4. food
62
Q

Fungi Types

A
  1. Visual Health
    - moulds
    - stainers
  2. Strength
    - soft rot fungi
    - wood-rotting basidiomycetes
63
Q

Microstructure of Wood

A
  • 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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