orthotropic properties of wood

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


Page | 2

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;


Page | 3

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.


Page | 4

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.


Page | 5

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


Page | 6

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.


Page | 7

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


Page | 8

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.


Page | 9

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%


Page | 10

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


Page | 11

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


Page | 12

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


Page | 13

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.


Page | 14

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


Page | 15

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


Page | 16

Figure 8 End checks

Figure 9 Surface checks


Page | 17

Table 4 Shrinkage values of woods


Page | 18

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


Page | 19

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.


Page | 20

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


Page | 21

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)


Page | 22

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


Page | 23

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)


Page | 24

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.


Page | 25

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


Page | 26

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


Page | 27

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


Page | 28

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


Page | 29

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


Page | 30

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


Page | 31

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


Page | 32

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


Page | 33

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


Page | 34

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.

orthotropic properties of wood

Documents

Transcript of orthotropic properties of wood