Chapter 10 Mechanical properties of
wood
木材力学性能
Ⅰ. Definition of terms
Ⅱ. Effect of specific gravity on strengthen of wood
Ⅲ. Effect of moisture content on strength of wood
Ⅳ. Anisotropic behavior of wood
Ⅴ. Nondestructive stress determination in lumber
Ⅵ. Time-dependent properties of wood
Ⅶ. Degradative changes in strength of wood
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Ⅰ. Definition of terms /术语定义
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The mechanical properties of wood are an expression of its
behavior under applied forces.
stress (应力 σ): force on unit area or volume
There are compressive stress, tensile stress, shear stress,
bending stress.
strain(应变ε): deformation per unit length, area or volume
Each different type of stress produces a corresponding strain.
modulus of elasticity (弹性模量 E): the proportionality constant between
stress and strain, .
MOE = σ/ ε.
Usually,The modulus of elasticity for compressive and tensile
tresses is known as Yong’s-modulus (Y), and the modulus of
bending elasticity is commonly indicated as E.
modulus of rupture (断裂模量 R)：the stress required to cause failure
R = σmax
proportional limit(比例极限 σp): the maximum stress beyond which
the σ/ ε ratio doesn’t keep constant.
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The elastic behaviorof
wood is illustrated by the
straight-line portion of the
curve for load and
deformation, as shown in
the figure. The area under
the straight-line portion of
the curve represents the
potential energy, or
recoverable work, and is a
measure of the resilience
of the material. The
steepness of the slope of
the elastic line is a
measure of the magnitude
of the elastic modulus; i.e.,
the steeper the slope, the
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Ⅱ. Effect of specific gravity on strength of
wood
The specific gravity of wood, because it is a measure of the
relative amount of solid cell wall material, is the best index that
exists for predicting the strength properties of wood. In general
terms, without regard to the kind of wood, the relationship
between specific gravity and strength can be expressed by the
following equation:
S = K (G) n
S is any one of the strength properties
K is a constant differing for each strength property
G is the specific gravity
n is an exponent that defines the shape of the curve
representing the relationship.
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1. Conception of specific strength
A measure of the efficiency of the wood to resist stress is
given by an index called the specific strength, which is the
ratio of strength to specific gravity. This index is often
referred to in general terms as the weight-strength ratio.
In comparison with other structural materials the
weight-strength ratio for wood is very favorable for some
applications.
2. Superior as bending members
The dispersal of the cell wall material as thin shells has
an important effect on the flexural rigidity of wood. For
this reason wood is well suited for long beams and
columns or stressed skin construction.
3. Inferior in compression and shear
The dispersal of the cell wall material as thin shells
reduces the efficiency of shear along the grain and
compression across the grain. So wood is inferior to metals
in comparison and shear.
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Ⅲ. Effect of moisture content on strength of
wood
Most of the strength properties and elastic characteristics of
wood vary inversely with the moisture content below the fiber
saturation point, and keep constant above the fiber saturation
point, as shown in the following figure.
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Ⅳ. Anisotropic behavior of wood /木材的各向
异性
1. Conception
A material, which has different physical properties in the
directions of the various structural axes, is said to be
anisotropic. Wood is anisotropic in mechanical strength.
2. C‖> C⊥
Wood is 4 to 12 times stronger in compression parallel
to the grain than it is perpendicular to the grain.
3. σr > σt
Many kinds of mechanical properties of wood also vary
somewhat between the radial and tangential axes
because of the orientation of the rays in the radial
direction.
Usually, σr > σt, especially for wood with large wood rays.
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Ⅴ. Nondestructive stress determination in
lumber
木材非破坏性应力测定
1. The Principle
• There are very strong correlations between modulus of
rupture and elasticity in bending, maximum compressive
strength parallel to the grain and Yong’s modulus in
compression, and also between maximum ensile strength and
modulus of elasticity in tension.
• The determination of elasticity in pieces of wood of structural
size is quite simple and can be performed without damage of
the wood using either static or vibrational methods.
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2. Practical use
This theory is used to determine stress grades in dimension
lumber. The most common systems employ equipment, which
bends the lumber slightly as the piece passes through a series
of rollers. The load and deflection at the rolls is measured
electrically; a small computer calculates the elasticity and
converts it to bending strength. The effects of all the factors
influencing the allowable strength and elasticity are
automatically integrated and accounted for in the stress value
which the machine stamps on the piece.
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Ⅵ. Time-dependent properties of wood
木材的依时性能
Deformation in wood under stress is the result of two
independent components operating simultaneously.
• The first component is the elastic deformation，which
occurs as the result of elastic response to load of the cellulose
microfibils.
• The second component is the plastic deformation of wood
with time, which occurs as the result of the flow properties of
the lignin fraction of the cell walls under load.
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1. Creep
Under a constant magnitude of load, wood deforms
plastically in direct relation to the duration of the load
application. Keep σ constant, ε↑with time.
e.g.: a book shelf with heavy books.
2. Relaxation
Under a constant deformation, wood shows a
decreasing magnitude of stress to the deformation with
increasing time. Keep ε constant, σ ↓with time.
e.g.: the veneer load in the hot press under pressure.
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Ⅶ. Degradative changes in strength of
wood
木材强度的降等
Wood in service can be subjected to a wide range of
conditions, which may result in degradative chemical
changes in the wood. The most important of the
degradative reactions affect the cellulose and depend on a
number of interrelated factors, such as:
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Temperature;
Length of time of exposure to the temperature;
Moisture content of the wood;
pH of the system in which wood is maintained.
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1. Immediate temperature response of wood
• Cases with short time effect of temperature
The short-term response in strength of wood to changes in
temperature is degradative only for extremely low temperature
at high moisture contents and for temperatures at or above the
ignition point of wood.
• The order of decreasing influence of temperature
Both the strength and elastic properties of wood vary inversely
with temperature at a given moisture content. The various
mechanical properties are affected to different degree.
C‖> MOR > shear > bending > tensioning
• Practical implication
— Steam treating of bolts for veneer peeling
— Steam treating lumber for bending
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2. Permanent changes in wood with temperature and time
Wood which is heated in the temperature range between
65oC and the ignition point for any appreciable length of
time, and subsequently tested at room temperatures, will
show a permanent loss of strength and elastic properties.
These changes in the mechanical properties of wood so
treated are the result of hydrolysis of the cellulose.
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3. Anaerobic decomposition of wood
Wood that is completely buried under conditions which
prevent free interchange of atmospheric gases, will
hydrolyze slowly.
Estimates of the time required to produce a 50 percent
reduction in the cellulose content in wood completely
submerged in water are 1500 to 2000 years for softwoods
and 200 to 420 years for hardwoods.
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4. Degradation of wood by chemicals
Wood is remarkably resistant to degradation when used in
contact with acids, but it is less resistant to basic solutions.
For example, a group of hardwoods and conifers after
soaking in 2 percent HCl for 32 days showed only minor loss
in modulus of rupture, while Soaking in 2 percent NaOH for
32 days at 20℃ resulted in 50 percent or more loss in bending
strength.
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Reflection and practice:
1. Conceptions of stress, strain, modulus of elasticity, modulus
of rupture and proportional limit?
2. Why wood is a more suitable material used for beams than
other construction materials?
3. Why wood usually has higher strength parallel to grain
than perpendicular to grain?
4. What kind of wood will have higher radial strength than
tangential strength?
5. The mechanism of nondestructive stress determination?
6. Conceptions of wood creep and relaxation?
7. What is specific strength?
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