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Lecture # 20:Engineering Properties of Wood
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Engineering Properties of Wood
•Grain Orientation•Density•Moisture•Temperature•Duration, Rate of loading•Defects•Other•Variability
•MECHANICAL PROPERTIES
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Anisotropic Axes of Wood
Terms:•Slope of grain•Cross grain•Diagonal grain•Spiral Grain
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Mechanical Properties of Wood
•Parallel to the grain•Perpendicular to the grain
Modulus of ElasticityTensile StrengthCompressive StrengthFlexural StrengthShear Strength
•Large Variability
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Modulus of Elasticity
PL 60% Ult Load
PL 20 - 50% Ult Load
Parallel to Grain
L
R L
T R L
Decrease w/increase
in Temperature
Decrease 3% with 1%
increase in MC
E largest
E = 10% E
E = 50% E = 5% E
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Tensile Strength
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1 2
70 to 150 MPa - parallel to grain
(2 to 4) X compressive-parallel
Failure strains 1%
Failure occurs in the S layer or
S / S interface
2 to 9 MPa - perpendicular t
o grain
Cell Wall Structure
Figure 14.3
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Compressive Strength
25 to 60 MPa - parallel to grain
~ 50% tensile-parallel
buckling
3 to 10 MPa - perpendicular to grain
12 to 18% compressive-parallel
cells compress
decrease 4-6% with in
crease of 1% in MC
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Compressive Behavior
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Localized Buckling
65 angle
Comp Str // to grain 25 to 60
MPa (about half of Tensile Str)
Comp Str to grain 3 to 10
MPa
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Flexural Strength
Compression in top: Tension in bottom
Comp Str 1/2 Tensile Str
Comp failure occurs first then tensile failure in the bottom
Flex Str 40 to 100 MPa
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Shear Strength
Greatest Shear Strength- primary bond
Relative sliding of microfibrils - secondary bond5 to 15 MPa
Bundle of straws; 1/4th of the strength to grain
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Grain Orientation
sin cos
N = property @ grain angle
P = property - parallel
Q = property - perpendicular
n n
PQN
P Q
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Density (G)
G0 = Oven-dry specific gravityMC = Moisture content
0
0
62.4G 100 =
1+0.009 G 100
MCDensity
MC
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Variations in Density
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Moisture Content
Two states of moisture:
Fiber Satur
Free water
Bound water
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ation Point
Equilibri
to 30%
5 to 12%
f(T, %RH
um MC
)
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Moisture Effects on Mechanical Properties
12P 12
- 12 = LogP
M - 12gPM
LogP LogP
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P
g
P = property of interest
P = value of the property at a moisture
content of 12%
M = moisture content
M = moisture content below which property
changes due to drying 25%
P = value
p
of the property for moisture
contents above M
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Shrinkage
2 1
2 1
30M M G D
M MS S
i
G-D
M = Moisture Contents
S Shrinkage from a
green to a dry state
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Typical Shrinkage Values
Longitudinal = small
0.1 to 0.2%
Tangential = 5 to 12%
Radial = 1/2 of tangential
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Duration of
Loading
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Other Effects
creep strain ~ elastic strain; permanent
increased rate of loading = increased strength
properties decrease (1%) w/increase in T (1 C)
irreversible if long time @ high T
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Creep• Creep: The increase in strain or deformation with
time under constant stress.
Stress Strain
Time TimeStrain
Time
Creep
Elastic
Strain
Elastic
Time
Relaxation
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Creep
• Creep strain is permanent and:– is much more severe when loaded perpendicular to the
grain.– about 50% of elastic strain for seasoned wood that
remains dry or unseasoned wood that remains green.– about 3.5 times the elastic component for green wood that
is allowed to dry in service.– of plywood at 18% MC is about 3 - 4 times that of
specimens at 6% MC.
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Creep
•creep strain increases in a cyclic moisture environment•an increase in relative humidity from 65 to 80% increases the creep 50%.
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Grading
Density
Decay
Heartwood or Sapwood
Knots: shape, size, location
Slope of grain (cross grain)
Shakes, checks, and splits
Visual: 95%
Mech
anical
Other defects
Done by machine
Measure of
stiffness; strength is estimated
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Defects in Wood
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Defects in Wood
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Knot: Bottom
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Knot: Top
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Reduction in Strength
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Design
m
m
AllowableProperty = (5% exclusion limit) x adjustment factor
x (strength ratio) x (other factors)
5% exclusion limit - - 1.65s
= mean strength
s = standard devia
tion
strength ratio = strength w/defects/strength w/out defects
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Adjustment Factors: Clear Wood
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Minimum Strength Ratios
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Stress Modification Factors
: 3 or more parallel members on 24 in. centers
allowable bending stresses may be increased by 15%
: Allowable stresses are reduced 10%
: Large specimens are we
Load Sharing
Treatment Factor
Size Factor
aker in strength
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Duration of Load
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Service ConditionNote: a moisture content < 15% is dry
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Temperature