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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