ORTHOTROPIC PROPERTIES OF WOOD 2012 Page | 1 TABLE OF CONTENT TITLE PAGE 1.0 INTRODUCTION 2 – 3 2.0 MECHANICAL PROPERTIES OF WOODS 4 3.0 ORTHOTROPIC PROPERTIES OF WOOD 4 – 8 4.0 MODULUS OF ELASTICITY OF WOOD 8 – 9 5.0 POISSON’S RATIO OF WOOD 10 6.0 MODULUS OF RIGIDITY OF WOOD 11 7.0 SHRINKAGE OF WOOD 11 – 19 8.0 ORTHOTROPIC PROPERTIES OF WOOD AFFECTING STRENGTH OF WOOD 19 – 25 9.0 ADHESIVE BONDING OF WOOD RELATED TO THE CHANGES IN DIMENSIONAL AND MOISTURE CONTENT 26 – 28 10.0 IMPROVE THE SHAPE STABILITY OF WOOD 28 – 34 11.0 CONCLUSION 34 12.0 REFERENCE 34
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ORTHOTROPIC PROPERTIES OF WOOD 2012
Page | 1
TABLE OF CONTENT
TITLE PAGE
1.0 INTRODUCTION 2 – 3
2.0 MECHANICAL PROPERTIES OF WOODS 4
3.0 ORTHOTROPIC PROPERTIES OF WOOD 4 – 8
4.0 MODULUS OF ELASTICITY OF WOOD 8 – 9
5.0 POISSON’S RATIO OF WOOD 10
6.0 MODULUS OF RIGIDITY OF WOOD 11
7.0 SHRINKAGE OF WOOD 11 – 19
8.0 ORTHOTROPIC PROPERTIES OF WOOD
AFFECTING STRENGTH OF WOOD 19 – 25
9.0 ADHESIVE BONDING OF WOOD RELATED
TO THE CHANGES IN DIMENSIONAL AND
MOISTURE CONTENT 26 – 28
10.0 IMPROVE THE SHAPE STABILITY OF WOOD 28 – 34
11.0 CONCLUSION 34
12.0 REFERENCE 34
ORTHOTROPIC PROPERTIES OF WOOD 2012
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1.0 INTRODUCTION
Throughout history, the unique characteristics and comparative abundance of
wood have made it a natural material for homes and other structures, tools, vehicles,
furniture and decorative objects. Today, wood is prized for a multitude of uses for the
same reasons.
Generally, all wood is composed of cellulose, lignin, hemicelluloses and
minor amounts (5 – 10%) of extraneous materials contained in a cellular structure.
Variations in the characteristics and volume of these components and also the
differences in cellular structure make woods heavy or light, hard or soft, and stiff or
flexible. In order to use wood to its best advantage and most effectively in
engineering applications, specific characteristics must be considered.
Historically, some species filled many purposes, while other less available or
less desirable species used for one or two needs only. For example, because white oak
is tough, strong and durable, it was highly prized for shipbuilding, bridges, cooperage,
barn timbers, farm implements, railroad crossties, fence posts and flooring. While
woods such as black walnut and cherry were used primarily for furniture and cabinets.
What the early builder or craftsman learned by trial and error became the basis for
deciding which species were appropriate for a given use in terms of their
characteristics. It was normally accepted that wood from trees grown in certain
location under certain condition was stronger, more durable and more easily worked
with tools than other wood from trees in other locations. Modern research on wood
has proven that location and growth conditions do significantly affect the properties of
wood.
Trees are divided into two broad classes, usually referred to hardwoods and
softwoods. These names can be confusing since some softwoods are actually harder
than some hardwoods, and some hardwoods are softer than some softwoods. For
example, softwoods such as longleaf pine and Douglas-fir are typically harder than
the hardwoods basswood and aspen. Botanically, hardwoods are Angiosperms where
the seeds are enclosed in the ovary of the flower. Anatomically, hardwoods are porous;
that is they contain vessel elements. A vessel element is a wood cell with open ends;
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when vessel elements are set one above another, they form a continuous tube or vessel
which serves as a conduit for transporting water or sap in the tree. Typically,
hardwoods are plants with broad leaves that, with few exceptions in the temperate
region, lose their leaves in autumn or winter.
Botanically, softwoods are Gymnosperms or conifers; the seeds are naked.
Anatomically, softwoods are nonporous and do not contain vessels. Softwoods are
usually cone-bearing plants with needle or scale like evergreen leaves. Some
softwoods such as baldcypress and larches lose their needles during autumn or winter.
Figure 1 Principle structure of wood. (a) Structure of softwood consisting of
earlywood tracheids, latewood tracheids and uniseriate rays (b) Structure of hardwood
consisting of vessels, fibers and multiseriate rays.
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2.0 MECHANICAL PROPERTIES OF WOODS
Variability or variation in properties is common to all materials. Since woos is
a natural material and the tree is subject to many constantly changing influence such
as moisture, soil condition and growing space, wood properties vary considerably,
even in clear material. The mechanical properties of wood are such as orthotropic
properties of wood, elastic properties, strength properties, vibration properties and
others. Only orthotropic properties of wood will be explained in detailed in this paper.
3.0 ORTHOTROPIC PROPERTIES OF WOOD
An orthotropic material has two or three mutually orthogonal twofold axes of
rotational symmetry so that its mechanical properties are different along each axis.
Orthotropic materials are thus anisotropic where their properties depend on the
direction in which they are measured. An isotropic material has the same properties in
every direction.
One common example of an orthotropic material with two axis of symmetry is
polymer reinforced by parallel glass or graphite fibers. The strength and stiffness of
such a composite material will usually be greater in a direction parallel to the fibers
than in the transverse direction. Another example would be a biological membrane, in
which the properties in the plane of the membrane will be different from those in the
perpendicular direction. Such materials are sometimes called transverse isotropic.
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Wood may be described as an orthotropic material. It has unique and
independent mechanical properties in the direction of three mutually perpendicular
axes: longitudinal, radial and tangential. The longitudinal axis L is parallel to the fiber
or grain; the radial axis R is normal to the growth ring (perpendicular to the grain in
the radial direction); and the tangential axis T is perpendicular to the grain but tangent
to the growth rings. These axes are shown in Figure 2.
Figure 2 Three principal axes of wood with respect to grain direction and growth rings
Wood is a complicated composite of hard-celled cellulose microfibrils
(organic cells known as tracheids) embedded in a lignin and hemicellulose resin
matrix. The seasonal variation in the cell wall density of a tree in evident when
looking at the end of the cut trunk, where a concentric ring structure formed by the
walls of the long slender tracheids can be observed. Commonly referred to as growth
rings, this architecture composed of alternating layers of earlywood (formed in the
spring and summer) and latewood (formed at the end of the growing season) is
responsible for wood’s high anisotropic and viscoelastic behavior.
Woods are described as an orthotropic material because its mechanical
properties are independent and can be defined in there perpendicular axes that shown
in Figure 3. The longitudinal axis L is parallel to the cylindrical trunk of the tree and
therefore to the long axis of the wood fibres as well (parallel to the grain). The
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tangential axis T is perpendicular to the long grain and tangential to the annual growth
rings. Both the tangential and radial directions are referred to as being perpendicular
to the grain.
Figure 3 The principal axes useful for modeling wood as an orthotropic material. The
longitudinal axis L is parallel to the cylindrical trunk and the tangential axis T is
perpendicular to the long grain and tangential to the annual growth rings
Taking the tree trunk as a series of concentric cylindrical shells and cutting
thin radial slices, the growth ring curvature is negligible and occurs in straight parallel
lines orthogonal to both the longitudinal and tangential axis. In the case where the
long axis is parallel to the grain fibre orientation and the width is in the radial
direction, the piece is said to be quarter-sawn as shown in Figure 4. The wood used in
soundboards is almost always of quarter-sawn timber, which causes the speed of
sound to be higher and the value of damping to be lower than for wood cut at an angle
to the grain. In general, the mechanical properties vary the most between the
longitudinal grain and the other two radial and tangential directions.
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Figure 4 Figure show a log is converted to quarter sawn timber
Table 1 shows the some advantages of plain sawn and quarter sawn lumber.
Table 1 Some advantages of plain sawn and quarter sawn lumber
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The strength, the modulus of elasticity and other characteristics such as
shrinkage and swelling are different in the three directions. The mechanical properties
parallel to grain are greatly different from that perpendicular to grain. Compressive
strength parallel to grain may be 5 to 10 times as great as that perpendicular to grain,
and the difference in tensile strength will be much greater. The modulus of elasticity
parallel to grain is likely to be on order of 10 to 25 times that perpendicular to grain.
Differences in the perpendicular to grain direction are likely to be minor between
properties parallel (tangent) to the growth rings and those perpendicular (radial) to the
growth rings. Directional differences in the mechanical properties must be taken into
account in the design of wood structures. The low levels of some properties must be
considered carefully in design, particularly where tensile stress perpendicular to grain
develops under service loads.
The properties of wood such as strength and stiffness along its grain and in
each of the two perpendicular directions are different. Hankinson's equation provides
a means to quantify the difference in strength in different directions.
4.0 MODULUS OF ELASTICITY OF WOOD
Elasticity implies that deformations produced by low stress are completely
recoverable after the load that applied is removed. When loaded to higher stress levels,
plastic deformation or failure will occurs. The three moduli of elasticity which are
denoted by EL, ER and ET respectively are the elastic moduli along the longitudinal,
radial and tangential axes of wood. These moduli are usually obtained from
compression tests; however, data for ER and ET are not extensive. Average values of
ER and ET for samples from a few species are presented in Table 1 as ratios with EL;
the Poisson’s ratios are shown in Table 2. The elastic ratios, as well as the elastic
constants, vary within and between species and with moisture content and specific
gravity.
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The modulus of elasticity determined from bending, EL rather than from an
axial test, may be the only modulus of elasticity available for a species. As tabulated,
EL includes an effect of shear deflection; EL from bending can be increased by 10% to
remove this effect approximately. This adjusted bending EL can be used to determine
ER and ET based on the ratios in Table 2.
Table 1 Elastic ratio for various species at approximately 12% moisture contenta
aEL may be approximated by increasing modulus of elasticity values in Table 3 by 10%
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5.0 POISSON’S RATIO OF WOOD
When a member is axially loaded, the deformation perpendicular to the
direction of the load is proportional to the deformation parallel to the direction of the
load. The ratio of the transverse to axial strain is called Poisson’s ratio. The Poisson’s
ratios are denoted by μLR, μRL, μLT, μTL, μRT and μTR. The first letter of the subscript
refers to direction of applied stress and the second letter refers to direction of lateral
deformation. For example, μLR is the Poisson’s ratio for deformation along the radial
axis caused by stress along the longitudinal axis. Average values of Poisson’s ratio for
samples of a few species are given in Table 2. Values for μRL and μTL are less
precisely determined than are those for the other Poisson’s ratio. Poisson’s ratios vary
within and between species and are affected by moisture content and specific gravity.
Table 2 Poisson’s ratio for various species at approximately 12% moisture content
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6.0 MODULUS OF RIGIDITY OF WOOD
The modulus of rigidity, also called as shear modulus indicates the resistance
to deflection of a member caused by shear stresses. The three moduli of rigidity
denoted by GLR, GLT and GRT are the elastic constants in the LR, LT and RT planes
respectively. For example, GLR is the modulus of rigidity based on shear strain in the
LR plane and shear stresses in LT and RT planes. Average values of shear moduli for
samples of a few species expressed as ratios with EL are given in Table 1. As with
moduli of elasticity, the moduli of rigidity vary within and between species and with
moisture content and specific gravity.
7.0 SHRINKAGE OF WOOD
Moisture content of wood is defined as the weight of water in wood expressed
as a fraction, normally a percentage, of the weight of oven dry wood. Weight,
shrinkage, strength and other properties depend upon the moisture content of wood.
In trees, moisture content can range from about 30% to more than 200% of the
weight of wood substance. In softwoods, the moisture content of sapwood is usually
greater than that of heartwood. In hardwoods, the difference in moisture content
between heartwood and sapwood is depends on the species of woods. The average
moisture content of heartwood and sapwood of some species is given in Table 3.
These values are considered typical, but these are considerable variation within and
between trees.
Moisture can exist in wood as liquid water (free water) or water vapor in cell
lumen and cavities and as water held chemically (bound water) within cell walls.
Green wood is often defined as freshly sawn wood in which the cell walls are
completely saturated with water; however, green wood usually contains additional
water in the lumens. The moisture content at which both the cell lumens and cell walls
are completely saturated with water is the maximum possible moisture content.
Specific gravity is the major determinant of maximum moisture content. Lumen
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volume decreases as specific gravity increases, so maximum moisture content also
decreases as specific gravity increases because there is less room available for free
water.
Table 3 Average moisture content of greenwood, by species
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Conceptually, the moisture content at which only the cell walls are completely
saturated (all bound water) but no water exists in cell lumens is called the fiber
saturation point. While a useful concept, the term fiber saturation point is not very
precise. In concept, it distinguishes between the two ways water is held in wood. In
fact, it is possible for all cell lumens to be empty and have partially dried cell walls in
in one part of a piece of wood, while in another part of the same piece, cell walls may
be saturated and lumens partially or completely filled with water. It is even possible
that a cell wall will begin to dry before all the water has left the lumen of that same
cell. The fiber saturation point of wood averages about 30% moisture content, but in
individual species and individual pieces of wood, it can vary by several percentage
points from that value. The fiber saturation point also is considered as that moisture
content below which the physical and mechanical properties of wood begin to change
as a function of moisture content.
Wood is dimensionally stable when the moisture content is greater than the
fiber saturation point. Wood changes dimension as it gains or loses moisture below
that point. It shrinks when losing moisture content from the cell walls and swells
when gaining moisture in the cell walls. The shrinking and swelling can result in
warping, checking, splitting and loosening of tool handles, gaps in strip flooring or
performance problems that detract from the usefulness of the wood product. Therefore,
it is important that these phenomena be understood and considered when they can
affect a product in which wood is used.
With respect to the shrinkage properties, wood is an anisotropic material. It
shrinks most in the direction of the annual growth rings (tangentially) (varying from
4.4 to 7.8%), about half as much across the rings (radially) (varying from 2.2 to 5.6%)
and only slightly along the grain (longitudinally). This is shown in Figure 5. The
combined effects of radial and tangential shrinkage can distort the shape of wood
pieces because of the difference in shrinkage and the curvature of annual rings. The
major types of distortion as a result of these effects are illustrated in Figure 6.
Shrinkage values, expressed as a percentage of the green dimension, are listed in
Table 4.
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Figure 5 Wood shrinks unevenly
Figure 6 characteristic shrinkage and distortion of flat, square and round
pieces as affected by direction of growth rings. Tangential shrinkage is about twice as
great as radial
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The shrinkage of wood is affected by a number of variables. Generally, greater
shrinkage is associated with greater density. The size and shape of a piece of wood
can affect shrinkage and also the rate of drying for some species can affect shrinkage.
Transverse and volumetric shrinkage variability can be expressed by a coefficient of
variation of approximately 15%.
7.1 Longitudinal
Longitudinal shrinkage of wood (shrinkage parallel to the grain) is generally
quite small. Average values for shrinkage from green to oven dry are between 0.1%
and 0.2% for most species of wood. However, certain types of wood exhibit excessive
longitudinal shrinkage, and these should be avoided in uses where longitudinal
stability is important. Reaction wood, whether compression wood in softwoods or
tension wood in hardwoods, tends to shrink excessively parallel to the grain. Wood
from near the center of trees (juvenile wood) of some species also shrinks excessively
lengthwise. Reaction wood and juvenile wood can shrink 2% from green to oven dry.
Wood with cross grain exhibits increased shrinkage along the longitudinal axis of the
piece.
Reaction wood exhibiting excessive longitudinal shrinkage can occur in the
same board with normal wood. The presence of this type of wood, as well as cross
grain can cause serious warping, such as bow, crook or twist and cross breaks can
develop in the zones of high shrinkage.
Figure 7 Cupping of wood
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Figure 8 End checks
Figure 9 Surface checks
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Table 4 Shrinkage values of woods
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7.2 Moisture-Shrinkage Relationship
The shrinkage of a small piece of wood normally begins at about the fiber
saturation point and continues in a fairly linear manner until the wood is completely
dry. However, in the normal drying of lumber or other large piece, the surface of the
wood dries first. When the surface gets below the fiber saturation point, it begins to
shrink. Meanwhile, the interior can still be quite wet and not shrink. The result is that
shrinkage of lumber can begin before the average moisture content of the entire piece
is below the fiber saturation point, and the moisture content – shrinkage curve can
actually look like the one in Figure 9. The exact form of the curve depends on several
variables, principally size and shape of the piece, species of wood and drying