Basic Wood Properties
Transcript of Basic Wood Properties
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Basic Wood Properties
FOR 111
Terry Brown
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Wood and Water
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Wood & WaterImportance of Water in Wood
Wood Properties differ with Moisture Content (MC)
DimensionsStrengthThermal ConductivityDecay ResistanceElectrical ResistanceDielectric Properties
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Wood & WaterHygroscopicity
Wood is hygroscopic below the FSP. This means wood adsorbs and desorbs water from its environment until an equilibrium dependent on ambient temperature and humidity is reached.
Water Vapor
Water Vapor or Liquid
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Wood & WaterWood Moisture Measurement
The Oven Dry (OD) basis for calculating moisture content is used with lumber, plywood, particleboard and other composite products
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Wood & WaterWood Moisture Measurement
Oven dry (OD) basis calculations
%100.
..×
−=
wtODwtODwtWetMC
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Wood & WaterLocation of Water in Wood
Water Vapor
Free Water
Cell Wall withBound Water
Occurs in two locationsWithin cell lumen
Liquid or vaporCalled Free Water
Within the cell wallCaptured in cell wall matrix Called Bound Water
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Wood & WaterFiber Saturation Point
The Fiber Saturation Point (FSP) is the point at which there is no free water present in the cell lumens and there is the maximum amount of bound water present in the cell walls
Cell wall saturatedwith Bound Water
Water Vapor only
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30% Equilibrium Moisture Content
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Wood & WaterEquilibrium Moisture Content
The Equilibrium Moisture Content (EMC) is the MC of wood when it is in equilibrium with its environment’s temperature and humidity.
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6 % Equilibrium Moisture Content
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12 % Equilibrium Moisture Content
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Wood & WaterEquilibrium Moisture Content
Factors that affect EMCTemperature & Humidity
18.29.45.2130°19.810.55.990°20.511.06.270°21.011.36.330°90%60%30%
Relative Humidity %Temp. °F
EMC of wood at various temperature and humidity values
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Wood & WaterWood Shrinkage
When does wood shrink and swell?
Wood shrinks and swells only with changes in the bound water content, i.e. below the FSP. Free water content has no affect on wood dimensions.
0
5
10
15
0% 15% 30% 45% 60%
Moisture Content
PercentSwelling
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Wood & WaterAnisotropic Wood Shrinkage
Anisotropic shrinkage and swelling occur when the percentage change in dimension is different for the different directional axes found in wood
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Wood Shrinks Unevenly
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Wood & WaterAnisotropic Wood Shrinkage
Differences in transverse shrinkage and swelling rates can change cross-sectional shapes
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Cup
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End Checks
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Surface Checks
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Honeycomb
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Collapse
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Wood & WaterGreen Moisture Content
The MC in living trees varies by species. Differences between sapwood and heartwood also exist for hardwoods and softwoods.
Hardwoods exhibit no green MC pattern between heartwood and sapwood, species variation is extremely wideSoftwoods commonly have a heartwood MC near the FSP which, of course, is much lower than the MC of its sapwood
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Wood & WaterGreen Moisture Content
16098white fir24958w. red cedar
14840ponderosa pine
11537Douglas-fir11033loblolly pine
SapwoodHeartwoodSoftwoods11395Aspen7390black walnut13779sweetgum7864white oak4446white ash
Hardwoods SapwoodHeartwood
Moisture ContentSpecies
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Wood Density
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Wood Density Density Defined
Density is a substance’s mass per unit volume
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Wood Density Density and MC
Wood density varies with moisture content because MC can affect both mass and volume
To be meaningful, the MC during both mass and volume measurements must be specified.
MC varies MC varies
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Wood Density Density and MC
Possible densities for an eighteen foot 2x4 (lbs./ft3)
27.924.4Oven-dry(19 lbs.)
32.428.2Kiln-dry 15%(22 lbs.)
55.948.7Green(38 lbs.)
Oven-dry(0.68ft3)
Green(0.78ft3)
Volume BasisWeight Basis
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Wood DensitySpecific Gravity
Specific Gravity (SG) is the density of a substance relative to the density of water and is sometimes called relative density or basic density Substances with a SG of less than one will float
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Wood DensitySpecific Gravity
The specific gravity of wood is alwayscalculated using its oven-dry mass. Therefore, only the MC during volume measurement will affect specific gravity results.
MC oven-dry MC varies
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Wood DensityWood Porosity
Wood is a mixture of void volume and cell walls. Void volume directly relates to density because cell-wall specific gravity is constant.
0.00120.075Moist Air
1.062.4Water
1.5*93.6*Cell Wall
Specific Gravity
Density (lbs./ft3)Material
* Constant for all species
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Wood DensityWood Porosity
Wood density varies among species because different cell types and sizes occur in differing proportions.
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Wood DensityExtractives
Wood often contains organic compounds such as resins, oils, and polyphenols; and inorganic compounds such as silicates, carbonates, and phosphates.
Often found in cell walls Increases overall densityMore found in heartwood, so its density tends to be slightly higher
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Wood DensityGrowth Rate
Hardwoods Only affects ring porous species
where wide growth rings produce significant increases in latewood and, therefore, density.
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Wood DensityGrowth Rate
Softwoods Species with wide latewood bands are more dense than those without, but such species do not produce proportionally more latewood with faster growth.
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Wood DensityDensity Variability
The factors that are known to affect wood density within a species.
Location within the treeLocation in its rangeSite conditionsGenetic factors
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Wood DensityDensity Variability
0.31-0.450.38ponderosa pine
Softwoods
0.48-0.720.60white ash0.45-0.670.56sugar maple0.45-0.670.56red oak0.41-0.610.51black walnut
Normal SG rangeAvg. green SGHardwoods0.28-0.420.35w. white pine
0.38-0.560.47loblolly pine0.36-0.540.45Douglas-fir
Normal SG rangeAvg. green SGSpecies
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Mechanical Properties
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Mechanical Properties
Mechanical or strength properties have far-ranging impacts on the use of wood in many applicationsWood, like steel or concrete, is engineered and products designed based on these mechanical properties
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Mechanical PropertiesFactors Affecting Strength
Anisotropic nature of wood
Longitudinal properties are much different than transverse propertiesRadial and tangential properties generally do not differ greatly
Parallel to the grain
RadialDirection
TangentialDirection
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Mechanical PropertiesFactors Affecting Strength
Anisotropic behavior results in dramatically different load carrying capacities
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Strength Measurements
MOR – modulus of ruptureMeasures the ultimate strength of wood
May be in bending, tension, or compression
MOE – modulus of elasticityMeasures the bending strength of wood
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Mechanical PropertiesFactors Affecting Strength
Specific GravityThe most important predictor of wood strength is specific gravitySome property relationships are nearly linear and others are exponential
Bending MOR
05000
1000015000
0 0.1 0.2 0.3 0.4 0.5 0.6Specific Gravity
psi
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Mechanical PropertiesFactors Affecting Strength
Defects that disturb fiber continuity and direction, most notably knots, adversely affect the strength of individual pieces.
A 2” knot on the edge of a 2 X 8reduces its strength by 43% .
A 2” knot in the center of a 2 X 8reduces its strength by only 24% .
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Mechanical PropertiesFactors Affecting Strength
Environmental FactorsMoisture Content
Mechanical properties increase in value below the FSP The relationships are exponential
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Mechanical PropertiesFactors Affecting Strength
Environmental Factors – continued
Time Creep – added deflection occurring over time under constant load. It is accelerated by high MC and cycling.
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Mechanical PropertiesFactors Affecting Strength
Environmental Factors – continued
Cyclic loading or Fatigue – failure after repeated cycles of loading and unloading. For example, clear straight grained wood will retain only 60% of its original strength after 2 million cycles. (Ag. Handbook #72 – Wood handbook, 1974)
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Mechanical PropertiesFactors Affecting Strength
Environmental Factors – continued
Temperature – Wood weakens with increased temperature, especially above 100°C. The damage is accumulative. Wood is generally stronger at lower temperatures.
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Mechanical PropertiesFactors Affecting Strength
Environmental Factors – continued
Chemical exposure – Wood strength decreases with exposure to severe acidic or alkaline environments, but it is more resistant than steel to acidic conditions. Less permeable woods are more resistant.
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Mechanical PropertiesFactors Affecting Strength
Environmental Factors – continued Biological activity – Wood strength decreases with only small weight losses due to fungal decay. Impact strength is affected more rapidly than static strength. Failure mode is brash.
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Mechanical PropertiesLumber Strength Evaluation
Grading MethodsThere are two methods currently in use for grading lumber and veneer
Visual gradingMSR – machine stress rating
Grading of lumber and veneer (plywood) determines the load carrying capacity of the productsThis information is used to design wood structures
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What’s Next?
We have looked at the basic wood science aspect of woodWe have looked at basic physical properties of woodNext we will look at the products and processes that are made from wood
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The End