ORTHOTROPIC PROPERTIES OF WOOD 2012
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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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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
conditions use.
Figure 10 Typical moisture content – shrinkage curves
7.3 Testing Method for Radial and Tangential Shrinkage
The testing method for radial and tangential shrinkage for wood is based on
BS 373- 1957.
Radial and tangential shrinkage shall be determined on test pieces 1 in. × 1 in.
× 4 in., the 4 in. being the direction for which the shrinkage is to be determined. The
test piece shall be weighed and measured before, drying and after subsequent drying,
at both the air-dry and the oven-dry conditions. The green test pieces shall be allowed
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to dry on wire racks in well ventilated boxes until a uniform moisture content of
approximately 12 per cent is reached. Subsequently they shall be placed in an oven
and dried until the weight is constant at 100 – 105 °C (212 – 221 °F).
1) Data:
Width, green = Lg inches
Width, air-dry = La inches
Width, oven-dry = L0 inches
Weight, green = Wg grammes
Weight, air-dry = Wa grammes
Weight, oven dry = W0 grammes
2) Properties to be computed:
i. Percentage radial shrinkage Green to air-dry
ii. Percentage tangential shrinkage =
iii. Percentage radial shrinkage Green to oven-dry
iv. Percentage tangential shrinkage =
v. Percentage moisture content, green =
vi. Percentage moisture content, air-dry =
8.0 ORTHOTROPIC PROPERTIES OF WOOD AFFECTING STRENGTH
OF WOOD
Longitudinal properties are much different than transverse properties. While
radial and tangential properties generally do not differ greatly.
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Figure 11 Orthotropic properties of wood
Besides that, orthotropic behavior also results in dramatically different load
carrying capacities.
Figure 12 Comparison of strength parallel and perpendicular to grain
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8.1 Testing Method for Compression Test
The resistance to compression shall be determined both a) parallel to the
longitudinal grain, and b) perpendicular to the longitudinal grain.
a) Compression parallel to grain.
The form and dimensions of the test pieces shall be as given in Figure 13. The
methods by which the tests on both the 2 in. standard and the 2 cm standard
test pieces shall be made are shown diagrammatically in Figure 14 and Figure
15. The load shall be applied to both types of test piece in such a way that the
loading plates approach each other at a rate of 0.025 in. /min.
Figure 13 Form of test pieces for compression parallel to grain
Figure 14 Suitable arrangement for compression test parallel to grain (2 in. standard)
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Figure 15 Compression parallel to grain (2cm standard)
b) Compression perpendicular to grain.
The test piece shall be a cube of 2 in. side as shown in Figure 16. The test shall
be made by loading between parallel plates. It shall be made in both the radial
and tangential directions. The load shall be applied to the test piece at a
constant head speed of 0.025 in./min. The load compression curve shall be
plotted to the point when the compression of the test piece reaches 0.1 in.
Should a definite maximum load be reached at some lesser value of
compressive strain, the maximum load and its associated strain shall both be
recorded.
Figure 16 Test piece for compression perpendicular to grain
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8.2 Testing Method for Shear Parallel to Grain
The test piece shall be a cube of either 2 in. or 2 cm side as shown in Figure 17.
Suitable apparatus for making the test on the 2 in. test pieces is shown
diagrammatically in Figure 18. The load shall be applied at a constant rate of
crosshead movement of 0.025 in./min. A similar testing speed of 0.025 in./min is used
for the 2 cm test piece, which shall be tested in an apparatus of the type illustrated in
Figure 19. The direction of shearing shall be parallel to the longitudinal direction of
the grain. The test shall be made with the plane of shear failure parallel to the
tangential direction of the grain and also with the plane of shear failure parallel to the
radial direction.
Figure 17 Test pieces for shear parallel to grain
Figure 18 Test for shear parallel to grain (2 in. standard)
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Figure 19 Test for shear parallel to grain (2 cm standard)
8.3 Testing Method for Tensile Test
The resistance to tension when required shall be determined both a) parallel to
the grain, and b) perpendicular to the grain.
a) Tension parallel to grain.
The form and dimensions of the test piece used in one method for determining
the tension parallel to grain strength shall be as illustrated in Figure 20. The
test piece shall be so orientated that the direction of the annual rings at the
cuboidal section is perpendicular to the greater cross-sectional dimensions.
The actual dimensions at the minimum cross-section shall be measured. The
load shall be applied to the 2 cm face of the ends of the test piece by special
toothed plate grips which are forced into the wood before the test piece
commenced. See Figure 21. These grips shall be designed so as to give axial
load. Load extension curves when required shall be taken for a 2 in., central
gauge length. The load shall be applied to the test piece at a constant head
speed of 0.05 in./min.
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Figure 20 Test piece for tension parallel to grain test
Figure 21 Grip ends for Figure 20 specimen
b) Tension perpendicular to grain.
The form and dimensions of the test piece shall be as given in Figure 22. Load
shall be applied through split grips with suitable precautions for ensuring axial
load. The load shall be applied to the test piece at a constant head speed of
0.01 in./min.
Figure 22 Test piece for tension perpendicular to grain
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9.0 ADHESIVE BONDING OF WOOD RELATED TO THE CHANGES IN
DIMENSIONAL AND MOISTURE CONTENT
Water occurs naturally in living trees; as free water in cell lumens and as
adsorbed water within cell walls. Total water content of wood can range well above
200% (based on oven dry weight), but when the free water is removed from cell
lumens by drying, approximately only 30% of water remains bound within cell walls.
Water has strong molecular attraction to wood, primarily through hydrogen bonding
with hydroxyl groups of wood cellulosic. Therefore, cell walls remain saturated with
moisture (called the fiber saturation point) until the moisture content of the
surrounding air falls below that of saturated cell walls. Actual moisture content at
fiber saturation point (roughly 30%) varies, depending on species, tree, temperature,
and pressure. This is the critical point where the wood begins to shrink. If wood has
dried below the fiber saturation point, then regains moisture, the wood will swell.
These dimensional changes different with the three principal directions, or grain
directions in wood, that is, longitudinal, radial, and tangential, with intermediate
changes varying with the angle between the principal directions. Longitudinal
dimensional change along the grain is least and amounts to less than 1% in drying
from fiber saturation point to oven dry. Dimensional change is greatest across the
grain, but the amounts differ with the direction; dimensional change varies with and
within species. As a rule of thumb, tangential dimensional change is about twice that
of the radial direction; but again, there are variations by species.
Dimensional changes that accompany changes in moisture content have broad-
ranging and significant consequences on performance of bonded joints. As wood in
bonded assemblies swells and shrinks, stresses develop that can be great enough to
rupture adhesive bond and wood. Ruptures may develop when adjacent pieces of
wood in a bonded joint differ in grain direction and shrinkage coefficients, for
example, radial grain bonded to tangential grain, or in the worst case, longitudinal
grain bonded to either tangential or radial grain. Even if moisture content levels in
adjacent pieces are equal, but changing, stresses could be severe. Moreover, if
moisture content in one piece is at equilibrium with surrounding air, that is, stable, but
the other piece with differing grain direction is shrinking as it approaches equilibrium
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moisture content (EMC), then resultant stresses would be compounded and almost
sure to rupture either the adhesive bond or the wood, whichever is weaker. Some
wood adhesives are elastic enough to yield to stresses so that fracture does not occur.
Structural wood adhesives have greater moduli of elasticity than wood and can
effectively transfer stresses from one adherend to the other without failure. However,
if stresses are great enough from extraordinary moisture content changes within
adjacent pieces of wood of differing shrinkage coefficients, then fracture in either
wood or a poor bond is almost unavoidable. Severe stresses on bond lines can be
minimized by bonding pieces of wood with compatible grain directions of low
shrinkage coefficients at a uniform moisture content equivalent to that which the
bonded assembly will encounter in service.
The amount of moisture in wood combined with water in adhesive will greatly
influence the wetting, flow, penetration, and even cure of aqueous wood adhesives. In
general, these adhesives bond satisfactorily across moisture content levels ranging
from 6% to 14% and even below and above this range when adhesives are formulated
for specialized processing. The optimum moisture content range for bonding a
specific product with a specific adhesive is determined from practical experience and
product performance. Aqueous adhesives tend to dry out when applied to wood below
6% moisture content. Wood absorbs water from the adhesive so quickly that adhesive
flow and penetration into the wood is drastically inhibited, even under high pressure.
Wood may become so dry below 3% moisture content that it temporarily resists
wetting by the adhesive because insufficient water remains bound to the wood to
establish intermolecular attraction forces with water in the adhesive.
When wood contains excess amounts of moisture, then less water and
adhesive can be absorbed by the wood. This leads to excessive adhesive mobility,
followed by squeeze-out when pressure is applied. Control of moisture content is
particularly critical to bonding in hot presses because excess moisture increases
adhesive mobility, followed by over penetration of the adhesive. Furthermore, high
vapor pressure builds internally as water boils, and on release of platen pressure,
sudden release of internal pressure actually separates laminates along the bond lines,
called blows. Even if blows do not occur, excess moisture within thermosetting
adhesives can prevent complete cross-linking with accompanying weakened adhesive
film and bond. Appropriate moisture content levels of wood for bonding by hot-press
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methods are well known, as are target moisture content levels for satisfactory service
of wood products. However, control of moisture content in bonding wood materials is
not easily achieved.
10.0 IMPROVE THE SHAPE STABILITY OF WOOD
Sawn wood is a renewable material that is inexpensive and has a very high
strength to weight ratio. However, it is an orthotropic material and it is affected by
changes in environment condition, especially moisture levels. Hence, it may be
deformed during drying. This is potentially damaging, since wooden studs and boards
must be straight to be useable for construction, and must remain straight as long as
they are in service. Wood that is susceptible to such deformation (bow, crook, twist
and cup as illustrated in Figure 23 is said to have poor shape stability.
Figure 23 Illustration of the shape stability defects bow, crook, twist and cup
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Wood shape stability traits are very important for many applications of long
pieces or sheets of sawn wood, for example joinery, glulam, veneers and timber used
in building construction.
10.1 Heat Treated Wood Flooring
Heat treated wood (HTW) is a material with changed chemical composition,
cell wall structure and physical properties. The process is generally conducted under
the influence of heat and pressure. Temperature during thermal treatment usually
range from 120˚C to 280˚C, treatment time spans between 15 minutes and 24 hours,
depending on the type of the process, wood species, stock dimensions, initial moisture
content and the desired level of alteration of mechanical properties, resistance against
biological deterioration and dimensional stability of the product. The presence of air
or other oxidative medium can accelerate the degradation process of wood
components during heat treatment and this is why the process is usually carried out in
a protective gaseous medium (nitrogen, steam, CO2) or immersed in various oils.
Change in cell wall chemistry cause the reduction of water uptake and consequently
improvement in dimensional stability. Heat treatment wood increases its moisture
resistance, improves dimensional stability, enhances resistance against biological
deterioration and contributes to uniform color change from original to dark brownish
tones. This material also exhibits some shortcomings, such as reduced tensile and
bending strength, unstable color in exterior exposure (unless the surface is coated),
appearance of surface checking and increased brittleness. Besides, after thermal
treatment some wood species have a burnt smell for months.
Heat treatment process was developed with the intention to use cheep
softwoods for cladding and decking in outdoor use. Heat treated wood can be used as
a substitute for tropical species. Better dimensional stability in variable climatic
conditions is an additional reason for the use of this material for parquet production.
Equilibrium moisture content of heat treated specimens after 3 years of natural
exposure was 40 to 60% lower compared to untreated wood, regardless of surface
protection system, which indicates permanent improvement in dimensional stability.
However, the improvement in dimensional stability does not correlate well with the
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form stability of heat treated wood. In other words, although HTW parquet will shrink
and swell considerably less, it will still cup and twist due to the same ratios of radial
to tangential properties as would native wood do. Heat treated wood is an excellent
substrate for finishing as it is dry and free of resin which run out during heating. At
temperature above 180˚C, oils and waxes are extracted from sapwood and later they
cause no problems with adhesion. The reduction in dimensional changes of heat
treated wood compared to untreated wood was expressed by volumetric shrinking.
For the experimental purposes, 10 replicates were prepared to form sample of
each of the following variables: wood species, ring orientation and treatment level
according to Table 5. Material for testing was commercially heat treated wood at two
temperature level – mild at 190˚C and intensive at 210˚C in water vapor atmosphere.
Table 5 Specimen preparation scheme
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Fibre saturation point was estimated in such specimen condition when their
dimensions reached their maximum after soaking. After complete saturation and
through gradual drying period, to final oven drying, the relation was determined
between the moisture content and corresponding dimensions in various stage of the
hygroscopic range.
Figure 24 Estimation of fibre saturation point
The value of shrinkage 𝛽 represents the ratio of the difference between the
dimensions of fully saturated wood (DV) and those of absolutely dried wood (D0)
compared to fully saturated DV wood, and it was calculated according to equation
𝛽 ( )
Volume shrinkage (𝛽V) was calculated as a product of linear dimensional
changes on separate radial and tangential texture samples, since it allowed to get more
precise dimension measurements over the width of the specimens.
Figure 25 shows that the estimated fibre saturation point (FSP) values are
somewhat higher than those quoted in the reference literature for sample of native
wood. FSP of mild heat treated beech samples is about 50% lower compared to native
wood, and intensive heat treated wood shows about 70% lower FSP value. Mild heat
ORTHOTROPIC PROPERTIES OF WOOD 2012
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treated ash exhibits for about 35% lower FSP and intensively treated about 40%. This
means that the intensity of the treatment (level of temperature, duration and other
parameters) influence the intensity of changes, but that different species do not react
equally to the regime parameter.
Measured equilibrium moisture content (EMC) (Figure 26) at room
temperature (23±2˚C and 50±5% relative humidity, RH) amounts to 8% for native
beech and 10% for ash, while the reference literature value is 9%. Mild treated beech
exhibits 15% lower EMC, mild treated ash 35% while both intensive treated species
attain nearly 50% lower EMC than native wood. This means that in the same ambient
condition the heat treated wood absorbs almost 50% less water which of course,
affects the reduction in dimensional changes, but also aggravates the reliable
measurements with electrical moisture meter. It is interesting to see that the EMC
established on the tangential panels, exhibits a fraction higher values than those
determined on radial samples, although both sets of panel were conditioned to
constant mass.
Figure 25 Fibre saturation pint (FSP) for beech and ash for two treatment intensities,
lit mark refers to literature values
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Figure 26 Equilibrium moisture content (EMC) at ambient conditions for beech and
ash, for two levels of treatment intensities
Reduction in shrinkage (Figure 27) results in better dimensional stability of
heat treated wood, expressed as Anti-Shrink Efficiency (ASE). Heat treating at lower
temperature (190˚C) resulted in improvement of dimensional stability of 27% for
beech and 35% for ash, while treatment on higher temperature (210˚C) resulted in
better dimensional stability of 54% for beech and even 62% for ash samples.
Figure 27 Volume swelling (𝛽V) and anti-shrink efficiency (ASE)
The results of laboratory test show that the heat treated wood, when compared
to genuine wood, shows a significant reduction of fibre saturation point (up to 15% in
average), lower equilibrium moisture content in room conditions (3.5 to 5%), and
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improvements in dimensional stability (up to 60%) expressed as ASE. This applies to
both wood species, but it should be mentioned that better effects were achieved with
ash compare with beech samples. Higher level of treatment temperatures yielded
proportionally greater stabilization effects. Although the flooring elements of HTW
may exhibit better dimensional stability than native wood elements, the ratio of radial
to tangential properties remains nearly the same. Therefore, the distortions of HTW
elements due to the R/T ratio will be similar as with the native wood, exhibiting
similar shape stability as native flooring elements in conditions of changing humidity.
11.0 CONCLUSION
As the conclusion, mechanical or strength properties have far-ranging impacts
on the use of wood in many applications. Wood, like steel or concrete, is engineered
and products designed based on these mechanical properties. Mechanical properties
such as orthotropic properties of wood must take into account for the design
consideration.
12.0 REFERENCES
BS 373 – 1957. Methods of testing small clear specimens of timber.
Keith F. Faherty, Thomas G. Williamson. Wood Engineering and
Construction Handbook. Second Edition. 1995. R. R. Donnely & Sons
Company.
United States Department of Agriculture. Wood Handbook - Wood as an
Engineering Material. 1999
Drvna Indusrija. 2008. Dimensional stability of heat treated wood floorings.