FORE 307 – Wood Behavior (Wood Physics)

Wood physics is a branch of wood science which deals with the mechanical and non-mechanical
properties of wood.
Wood Science….that body of knowledge applicable to …. wood as a material, including its origin, properties,
composition and characteristics.

Wood Technology….the application of knowledge in the conversion, processing and the many uses of wood,
including the design, manufacture and marketing of wood products.

Wood— why do we care in the 21st Century?

 Money, jobs
▪ Keeps private land in forests
▪ Employs millions of people
▪ Supports many public services
 Essential to human existence as we know it

Wood Through Time

 The foundation on which early civilization was built

 Society’s principal fuel
▪ Fuel is still largest single use for wood
 Society’s principal building material
 Wood use is directly related to living standards and literacy

Fuel Wood Created:

 Heat from wood fires made cold climates habitable

 Inedible grains changed to major food sources
 Clay converted to pottery
 Metal could be extracted from ore
 Glass could be made from sand
 Steam for steamboats, locomotives and industry

Wood Built:

 Ships to explore new worlds

 Waterwheels to generate mechanical power
 Tools for everyday life
 Shelter from the elements and enemies
 Wagons, carts, chariots, and bridges
 Spear shafts and arrows

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  Economical ways to communicate and do business—books, newspapers, letters, etc

Why Use Wood?

 Renewable
 Available
 Economical
 Burns
 Biodegradable
 Aesthetic
 Cultural heritage
 Makes useful products
 Favorable properties
▪ Physical
▪ Mechanical
▪ Chemical
 Easy to work with simple tools
 Low energy consumption
 Source of fiber and chemicals
 To store carbon for long periods

Wood Science and Technology Goals

 Economic well-being of human communities

 Improved human quality of life
 Conservation of forest resources through:
▪ Efficient manufacture and use
▪ Intelligent consumption

Non-mechanical properties are characteristics of a material, such as wood, that can be observed without
altering the identity of the material. Some non-mechanical properties are extensive properties and others
are intensive properties. Non-mechanical properties include density, mass, volume and specific gravity.
Mechanical property: a property that involves a relationship between stress and strain or a reaction to an
applied force. Examples are MOR, MOE, tensile, compression, and shear strengths. Others are
malleability, ductility, hardness, etc.
Material properties: physical, chemical, or mechanical components of specific product that would
determine its functionality and manufacturability. This means that a product’s material properties would
specifically define the capabilities of the product in all aspects.
The mechanical properties of wood are a function of its physics, chemistry and biology.
The non-mechanical properties of wood are determined by the factors inherent in its structural
organization to wit:
1. Amount of cell wall substance
2. Amount of water in the cell wall
3. Amount of extractives
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4. Arrangement of cell wall materials
5. Kind and size of woody tissues

Non-Mechanical Properties
Hygroscopicity is the ability of wood to absorb and lose water.
This property is dependent on temperature and relative humidity of the surrounding atmosphere.
Wood is hygroscopic substance; therefore, it has an affinity for water.

Types of water according to:

1. Location of Water
a. Free water – found in the cell lumen, cavities, and intercellular spaces
b. Bound water – found associated in the cell wall
c. Water of constitution – found associated with chemical structure

2. Expression of Water in Wood

a. Moisture Content (MC) – the amount of water in wood expressed as thepercentage of its ovendry
weight
The Equations are:
i. Moisture Content
%MC = Wg – Wo x 100
Wo
%MC = Wi – Wf x 100
Wo
%MC = Ww x 100
Wo
Where: MC = Moisture content; Wg = Green weight; Wo = Ovendry weight/final weight; Wi = Initial
weight; Ww = Weight of water.
ii. Green Weight
Wg = Wo (1 + %MC)

iii. Ovendry Weight

Wo = Wg__
(1 + %MC)

Example: A piece of wood weighing 200 grams was placed inside an oven set at a temperature of
110oC. After few days, the wood attains a constant weight of 190 grams. Calculate the moisture
content of the wood.

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 Solution:
Wg = 200g
Wo = 160g
MC =?
MC =200g – 160g x 100
160g
MC = 25%

b. Equilibrium Moisture Content (EMC) – the balance of moisture content attained by wood at any
given level of relative humidity and temperature of the surrounding atmosphere
c. Fiber Saturation Point (FSB) – a point when all water is evaporated from the cell cavities but the
cell walls are still fully saturated with moisture.
d. Maximum Moisture Content (MMC) – the total amount of water present in the cavities and cell
wall expressed in percent.

Dimensional and Volumetric Changes

These changes are the function of wood moisture and the amount of cell wall substance.
1. Swelling – the increase in wood dimension and volume as it gains moisture below FSP expressed in
percent. Swelling will only stop when FSP is attained (at least 30% MC).
%Sw =Vg – Vd x 100
Vd
%Sw =V1 – V2 x 100
V2
Where: Sw = percent of swelling; Vd = dry volume; Vg = green volume; V 1 = original volume; V 2 =
volume after moisture is loss.

Example: Compute the swelling in percent of Naga wood sample if its dry volume is1,000cc and volumes
at certain MC after exposing in the yard site becomes 1,200cc.
Solution:
Vd = 1,000cc
Vg = 1,200cc
Sw = ?
%Sw = Vg – Vd x 100
Vd
= 1,200cc – 1,000cc x 100
1,000cc
Sw = 20%

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2. Shrinkage – the reduction in wood dimension and volume as it losses moisture below FSP expressed in
percent. It will only stop when ovendry condition is met.

%Sh = Vg – Vd x 100
Vd
%Sh =V1 – V2 x 100
V2

Where: Sh = percent of shrinkage

Example: Calculate the percent of shrinkage of sapelle block, if the green volume is 800cc and dry volume
after exposing to warm atmosphere for a week is 700cc.

%Sh =Vg – Vd x 100

Vd
%Sh =800cc – 700cc x 100
800cc
Sh = 12.5%

Specific Gravity
Specific gravity refers to the ratio of weight of the substance to the weight ofan equal volume of
water.
Specific gravity of wood (G) = (OD weight of wood)/ (weight of displaced volume of water).
Wo
Mathematically, G=
Wv
In this equation the ovendry weight of the wood is always used as the numerator. The value of the
denominator, which depends on the volume of the wood, varies with the moisture content of the test
block, because of the dimensional changes that occur in wood below the fiber saturation point (FSP). For
this reason it is necessary to specify the moisture content of the wood at which the volume was
determined, when stating the specific gravity. As the volume becomes smaller, with decrease in the
moisture content, the denominator of the ratio becomes smaller and the specific-gravity value
correspondingly larger. The reverse is true as the moisture content of the test block increases. As a
result, the minimum value of the specific gravity is obtained when the green volume is used, and the
maximum when the volume of the wood is taken at the ovendry condition in determining the weight of
the displaced volume of water.

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Specific gravity of wood based on green volume, or basic specific gravity, is one of the most useful and
commonly cited values. The term basic is applied since both green volume and ovendry weight are as
nearly constant and reproducible measurements as can be obtained with wood.
Specific gravity (G) of dry solid wood substance, i.e., of the ovendry cell wall material, has been
determined to be about 1.5 (all species).G ≤ 0.36, light wood; 0.36 < G ≤ 0.50, moderately light/heavy
wood; G > 0.50, heavy wood
In general terms, the specific gravity of wood depends upon 1. The size of the cells, 2. The thickness of the
cell wall, and 3. The interrelationship between the number of cells of various kinds in terms of 1 and 2.
The methods of determining specific gravity of wood:

1. Gravimetric method – the weight and volume of regular-shaped wood are

considered.
2. Floatation method – the wood is allowed to float on water and submergedportion will
approximate the specific gravity.
3. Maximum moisture content method – the weight and volume of regular and irregular
shaped wood.
4. X-ray densiometry method – the wood is placed over the film and exposed toan x-ray and when
the film is developed, then the specific gravity is determined.
The equations are: 1.
Ovendry Condition
Wo
Go =
Vo x Dw

Where: Go = ovendry specific gravity;Wo = ovendry weight; Vo = ovendry volume;

Dw = density of water (1g/cc); 62.4 lb/ft3; 1000kg/m3salt water (1.125g/cc)
Example: At ovendry condition, a piece of wood weights 600 grams and has avolume of 1,000cc.
Calculate its ovendry specific gravity.

Solution:
Wo = 600g
Vo = 1,000cc
Go = ?
Wo
Go =
Vo x Dw

= 600g
(1000cc)(1g/cc)
Go = 0.6
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 2. At certain moisture content
Wo
Gm =
Vm x Dw

Where: Gm = specific gravity at certain MC; Vm = volume at certain MC

Example:A piece of wood is 200cc in volume weighs 120 grams at 20%MC. Calculate its specific gravity.

Wg
Wo = MC
(1+ )
100

120 g
=
1.2

Wo = 100g
Wo
Gm =
Vm x Dw

100 g
= g
200 cc (1 )
cc

Gm = 0.5

3. At Green condition
Wo
Gg =
Vg x Dw

Where: Gg = green specific gravity; Vg = green volume

Example: The ovendry weight of Abura block is 500 grams. Find the basic specific gravity if its green
volume is 900cc.
solution:
Wo
Gg =
Vg x Dw
= 500g
900cc(1g/cc)
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 Gg = 0.55

Example: What is the weight of 10,000 bdft of red lauan lumber and the amount of water present in
kilograms whose specific gravity is 0.4 and the moisture content is 12%.

a. V = 10,000bdft
424bdft/cu.m
V = 23.58cu.m

Wo
b. Gm =
Vm x Dw

Wo = Gm x Vm x Dw
Wo = 0.4 x 23.58m3 x 1g/cc x 1,000,000cc/m3 x 1kg/1,000cc
Wo = 9,432 kg.

c. Wg = Wo (1 + %MC)
= 9,432 kg (1.12)
Wg = 10,563.84 kg

d. Ww = Wg – Wo
= 10,563.84kg – 9,432kg
Ww = 1,131.84kg

Specific gravity can also be computed if MC and another value of specific gravity are known.
¿
1. Go = 1−(0.27)(¿)

Gm
2. Go =
1−(0.009)(Gm)( MC )

¿
3. Gm = 1+(0.009)(¿)(30−MC)

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 Go
4. Gm =
1+(0.009)(Go)( MC )

Gm
5. Gg =
1+(0.009)(Gm)(30−MC )

Go
6. Gg =
1+(0.27)(Go)

Gm2
7. Gm1 =
1+(0.009)(Gm 2)(MC 2−MC 1)

Gm1
8. Gm2 =
1+(0.009)(Gm 1)(MC 2−MC 1)

Where:
Go = ovendry specific gravity
Gm = specific gravity at certain MC
Gg = green specific gravity
MC = moisture content
MC1 = initial moisture content
MC2 = final moisture content

Density and Weight of Wood

Density is the mass or weight of wood per unit volume.Density of wood is a good indicator of the amount
of wood substance in a given block of wood. It is a reliable index of the strength properties of wood.
Weight density, or weight of wood per unit volume, is customarily calculated on the basis of both the
weight and the volume of the piece taken at the same moisture content.
Weight density of wood = (weight of wood with moisture)/volume of wood with moisture).

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The weight of woods depends on:
1. Amount of wood substance present
2. Amount of extractives
3. Weight of absorbed water

The minimum values of weight density occur at the ovendry condition and the maximum when the wood
is fully saturated.
The equations are:

1. Ovendry condition
Do = Wo
Vo
Where: Do = ovendry density;Wo = ovendry weight; Vo = ovendry volume
Example:The ovendry weight of white lauan wood is 600 grams. What is its density if the final
volume is 600cc?
solution:
Do = Wo
Vo
Do = 600g
600cc
Do = 1g/cc

2. At certain moisture content

Dm = Wm
Vm

Where: Dm = density at certain MC;Wm = weight at certain MC; Vm = volume at certain MC

3. At green condition
Dg =Wg
Vg
Where: Dg = density at green condition; Wg = green weight;Vg = volume at green condition
Kosipo has a specific gravity of 0.656 based on ovendry weight and volume; the volumetric shrinkage from
the green size to ovendry condition is 15%, and the FSP for this wood is taken as 30% MC. Determine a.
weight density ovendry, b. maximum volume of swollen wood, c. weight density at FSP, d. weight density
at 86% MC.

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Buoyancy of Wood
The ability of a piece of wood to float is due to the buoyant force which develops as a result of the
difference between the density of the wood and that of the water displaced by the fully submerged
piece.It is sometimes called upward force.This principle was first stated by Archimedes (287 B.C – 212 B.C)
a Greek mathematicians and inventor. A body denser than water sinks because the downward force due
to the weights of the body is greater than the upward force A few kinds of wood have specific gravities
greater than 1.0; that is, they contain sufficient amount of dry solid cell wall material, plus extractives, to
sink even when the wood is ovendry. However, most woods when dry contain a great deal of air space
and this allow them to float. As these woods are soaked, the air spaces fill with water and the density of
the wood increases until it equals or exceeds that of the displaced water, and the block sinks.
. The equations are:
1. Bouyant Force

Fb = Wf – Wa

Where: Fb = buoyant force of floating volume

Wf= weights of fluid with a volume equal to the volume of the body
Wa = weight of the body in air

2. Percent of the Fraction the Body Submerged

%Fw = Wa x 100
Wf

Where: Fw = percent of the fraction the body submerged

If the body sinks, the equation is:
1. Weight of the Body under Water
Wu = Wa – Wf
Where: Wu = weight of the body under water

2. Weight of the Body in Air

Wg = Wa = Wo (1 + MC/100)

Where: Wg = green weight

Wo = ovendry weight
MC = moisture content
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Problem:
1. Calculate the buoyant force and the fraction of wood submerged whoseweight of water
equivalent to the volume of a block of wood is 3,850 kg and the specific gravity at 30%MC is 54
with an estimated volume of 4cubic meters.

Solutions:

a. Wo = Gm x Vm x Dw
= 0.54 x 4m3 x 1g/cc x 1,000,000 cc/m3 x 1kg/1,000g
Wo= 2,160 kg

b. Wa = Wo (1 + MC/100)
= 2,160 kg (1.3)
Wa = 2, 808 kg

c. Fb = Wf – Wa
= 3,850 kg – 2,808 kg
Fb = 1,042 kg

d. %Fw = Wa x 100
Wf
= 2, 808x 100
3,850
Fw = 72.93%

Thermal Properties of Wood

Thermal property of wood is a property that is in relation to heat.Heat was originally measured by noting
the rise in temperature of a measured quantity of water which absorbed the heat. The units used in
measuring heat are:
1. Calorie – the quantity of heat required to raise the temperature of onegram (g) of water through
one degree Celsius (0C).

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2. British Thermal Unit (BTU) – the quantity of heat required to raise thetemperature of one pound
mass of water through one degree Fahrenheit (1 BTU = 252 calories)
3. Kilocalorie – the amount of heat necessary to raise the temperature of onekilogram of water
through one degree Celsius.

Thermal property of wood is further characterized by the following:

Thermal conductivity of wood (K) = amount of heat, in British Thermal Unit (Btu), that will flow in 1 hr
through a homogeneous material 1 inch thick and 1 foot square, when a 1◦F temperature difference is
maintained between the surfaces. In metric system, thermal conductivity of wood is a measure of heat in
calories that will flow through homogenous material per second per square centimeter for a temperature
gradient of one degree celsius per centimeter of thickness. The thermal conductivity of wood is the
measure of the heat flow and is dependent on three factors: a. direction of heat flow with respect to the
axis of grain orientation in the wood, b. the MC of the wood, and c. the specific gravity of the wood. It is
nearly the same in the two transverse directions, but in the longitudinal axis it is 2 ¼ to 2 ¾ times greater.
Thermal conductivity of wood for any direction is approximately one-third greater for moisture contents
above 40% than it is for wood that is drier. Specific gravity of the wood causes the thermal conductivity of
wood to vary directly in a straight-line relationship.
The three factors on which heat conductivity of wood depends may be combined in a single expression to
show the transverse thermal conductivity K:
K = G[1.39 + C(MC)] + 0.165
G = specific gravity at a given moisture content
MC = moisture content in percent
C = constant dependent on MC, with value of 0.028 below 40% MC, and 0.038 above.
The values of the thermal conductivity for wood are quite low compared with the metals and rate
favorably with those of the insulating materials. These low transmission rates for heat in wood explain
why wood furniture seems so “warm” to the touch and point out one of the reasons for the continued
preference in chairs and tables.

There are three ways by which heat can be transferred:

1. Conduction – the transfer of heat from molecules to molecules

2. Convection – the transfer of heat by the mass movement of heated particles asin air,
gas, or liquid currents.
3. Radiation – the process in which energy in the form of rays of light or heat is sent out
from atoms and molecules as they undergo internal change.

Thermal conductivity of wood is dependent on three factors:

1. Direction of heat flow

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 2. Moisture content
3. Specific gravity

Thermal insulating value of wood (R). This value is the reciprocal of the conductivity. It is therefore
apparent that the insulating value of wood is inversely proportional to the specific gravity and moisture
content. This relationship explains the use of low-density dry wood for insulating purposes.

Thermal Expansion of Wood

Thermal expansion of wood is very small in comparison with most materials, and for practical purposes
can usually be ignored. In addition, the effects of the thermal expansion that arise from normal
atmospheric temperature changes are masked by the changes in dimension produced by the normal
swelling or shrinkage of the wood, due to moisture-content changes. The measurement of the
dimensional changes of wood caused by temperature differences is called the coefficient of thermal
expansion (α). These coefficients vary in wood in inverse order as to those of thermal conductivity.
According to Weathermax and Stamm, the coefficient of thermal expansion for the longitudinal direction
(αL), in the temperature range from -50 to +500C averages 3.39 X 10 -6 per degree Celcius, regardless of
kind of wood and its specific gravity. In the transverse direction, for an average specific gravity of 0.46,
the coefficient of radial expansion (αr) is 27.7 X 10 -6 per degree Celcius, while that for tangential
expansion (αt) is 34.8 X 10-6 per degree Celcius. The values of the thermal coefficients in the radial and
tangential directions vary directly in a straight-line relationship to the specific gravity of the wood.
Ignition of Wood
The ignition of wood begins at 2730C when oxygen is limited or controlled, it is the temperature at which
gases begin to evolve from destructive distillation of the wood. The speed at which combustion is
initiated is dependent upon the rate of accumulation of heat at the surface of the wood. Factors that
influence the rate include a) the size of the piece of wood, b) the rate of heat loss from the surface to the
interior, c) the presence of thin outstanding edges, and d) the rate at which heat is supplied to the surface
of the wood. Small pieces with sharp projecting edges ignite more easily than large and round pieces of
wood.

Fuel Value and Specific Heat of Wood

Fuel value of wood. Fuel value of wood is primarily determined by the density and moisture content of
the wood. It is modified by the variations in lignin content and to much greater extent by the presence of
extractives such as resins and tannins. The heat of combustion (H), heat in Btu produced by burning 1
pound of ovendry wood, averages about 8500 Btu for hardwoods and 9000 Btu for softwoods.
Actual heat produced by burning wood containing some moisture is lower than the value of H given
above, because part of the heat is lost in removing the water and vaporizing it. An approximation of the
actual fuel value for wood is given by the equation

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 MC
100−()
Btu per pound of wood = H X 7
100+ MC
where H is the heat of combustion of the wood and MC is the moisture content in percentage
Specific heat of a dry wood substance increases as the temperature goes higher. It does not vary with
wood species since it is determined on the basis of mass instead of volume. Wood with high resin content
may show higher specific heat or thermal capacity because of the presence of extractives.

Electrical Properties of Wood

Direct-current (D-C) electrical properties of wood

The electrical properties of wood are measured by its “resistivity or specific resistance” or by its reciprocal
electrical conductivity.Dry wood is an excellent insulating material whereas, wet wood is a good electrical
conductor. For ovendry wood, the d-c resistivity ranges from 3 X 1017 to 3 X 1018 ohm-centimeters.
Resistivity is inversely proportional to moisture content up to fiber saturation point (FSP). Very little
change occur in resistivity from FSP to maximum moisture content.Resistivity decreases with increasing
temperature.Wood of high density are somewhat more resistant to passage of direct current than those
with low density.The d-c resistivity of wood in the longitudinal direction is only about half of that in the
lateral direction.Resistivity may be affected by the presence of extractives and minerals in the wood.
Electrical-resistance moisture meters: a) useful for moisture content determinations in the range from 7
to 25 % MC in the outer shell of the wood b) meter must be calibrated for each kind of wood and for
temperature range that is expected c) presence of salts changes the resistance hence resistance meters
are unreliable when used on salt-treated wood.

Alternating-current (A-C) electrical properties of wood

The measure of the insulating capacity of a material under alternating current is its dielectric constant (ε).
This is expressed as the ratio of the charge held by a condenser in which the electrodes are separated by a
dielectric material such as wood, to the charge held by the condenser with the electronodes separated by
a vacuum at a given voltage. In wood, ε varies directly with specific gravity, moisture content, and
inversely with the frequency of the current. For ovendry wood it is a straight-line relationship, but at
higher moisture contents the curve is exponential. In dry wood the ε is from 30 to 50% greater in the
longitudinal direction than it is in the transverse direction. Along the grain, it is uniform when the cellular
structure shows no major differences in various parts of the cross section, but exhibits much greater
values for the large-pored early wood zones in ring-porous woods.
Basically, dry wood substance is a nonconductor with a dielectric constant of about 2. The addition of
water to the wood causes the wood-water system to become more conductive, in proportion to the
actual amount of water added. Above the FSP, the dielectric constant approaches that of water, which
has a value of 81.

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Wood Permeability
Permeabilityin wood is related to the sizes of the passages that are available for flow of liquid or gases.

Acoustic Property
Acoustic property refers to the ability of wood to absorb or reflect sound or the ability to dampen
vibrations.
Wood has higher damping capacity, hence it is preferred materials for structural components in which
vibration is undesirable.

Wood is acoustic due to the presence of:

1. Pores, vessel lines, and microcapillaries
2. Fabricated holes
3. Fabricated striations
Decibel – unit of sound measurement – 50 decibel would destruct eardrums

WOOD MECHANICAL PROPERTIES

TIMBER MECHANICS is a branch of wood science which deals with the energy and forces and their effects on wood
structure. The expression of the behavior of wood under applied forces is called MECHANICAL PROPERTY/
strength property.

The factors affecting the mechanical or strength property of wood are:

1. Wood defect
2. Specific gravity
3. Moisture content
4. Temperature
5. Duration of load

WOOD ADVANTAGES AS BUILDING MATERIALS

1. Strong for its weight

2. Easy to cut in various shapes
3. Easy to fasten with nails, bolts, etc.
4. Resilient, tough, and elastic

Anisotropic character of wood

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Strength along the grain is 4 times higher than across the grain for hardwood but 12 times high for softwood.

PQ
For distorted grain, we use Hankinson’s formula : N¿
Psin 2θ+Q cos 2 θ
P=strength parallel to the grain; Q=strength perpendicular to the grain; θ=angle of deviation.

i. Effect of growth rings : 00> 900> 450

ii. Slope grain

Tensile strength is most sensitive to slope grain, 1 : 25; bending, 1 : 20; compression, 1 : 15

iii. Knots : knots are most sensitive to tension and bending. They are deviations.

Time dependent strength property of wood

Duration of stress : the period of time over which a constant load is applied. The application of stress for a longer
period of time leads to increase in strain for the same stress.

Creep – the response of wood specimen due to application of load over time.

Fatique resistance : the ability of a specimen to withstand repeated, reversed or cyclic loads of short duration
without failure.

TIMBER MECHANICS GLOSSARY OF TERMS

1. Bending strength – a measure of the ability of wood to resist load causing it to bend
2. Brashness – an abnormal condition that causes the wood to break suddenly and completely across the grain
at stress level lower than expected.
3. Cleavage – a measure of the ability of wood to resist from splitting.
4. Compression failure – the localized buckling of fibers and other elements produced by the compression of
wood along the grain beyond its proportional limits
5. Compressive strength – a measure of the resistance of wood to an externally applied force tending to
shorten the wood elements
6. Creep – a dimensional change with time of the wood under load following the initial deformation
7. Deflection – the deformation that resulted when a force acts on a member in such a manner that it tends to
cause bending
8. Durability – refers to the degree of resistance of wood to decay or biodeterioration
9. Honeycombing – a seasoning defect that is characterized by internal checking and splitting along the rays as
the wood is dried
10. Impact bending – a type of bending that is caused by a forceful contact or collision of the load to the wood
11. Isotropic property – a property of wood that exhibits identical values when tested along the axes in all
directions
12. Load – a magnitude of pressure due to the superimposed weight or due to the external forces acting on the
beam
13. Maximum crushing strength – a strength value that used to measure the ability of wood to withstand loads
in compression parallel to the grain up to the point of failure
14. Modulus of elasticity – a strength value that used to measure the ability of wood to recover its original
shape and size after stress is removed

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15. Modulus of rupture – a strength value that used to measure the ability of wood to resist to its tolerable
bending and when excess stress is applied might cause failure
16. Moment – refers to the reactions which the beam has obtained because of applied forces
17. Resonance – the capacity of wood to withstand induced vibration by sound waves transmitted from the
outside forces
18. Shearing strength – a measure of the ability of wood to resist a force causing one part of the material to slip
on another adjacent part
19. Stiffness – a measure of the ability of wood to retain its natural size and shape when acted on by an
externally applied load
20. Strain – any deformation on the wood made under the action of applied forces
21. Tensile strength – a measure of the ability of wood to resist deformation due to an externally applied force
tending to pull the wood apart
22. Tension wood – a specialized type of wood that is formed in the upper side of the branches and leaning
stems of hardwood
23. Timber fastener – any device that is used to hold two or more pieces of wood together
24. Timber mechanics – a branch of wood technology which deals with the action of outside forces and their
effect on wood structure
25. Toughness – a measure of the ability of wood to absorb shock energy
26. Ultimate strength – a measure of the ability of wood to resist the greater static load which a body can
support when tested to complete failure
27. Wood – an organic material generally used in its natural state or the xylary portion of the fibrovascular
tissues of trees
28. Wooden beam – a mechanical device or structural element that carry transverse load and is point-
supported in contrast with full bearing that occurs with a footing.
29. Rheology – relationship between stress and strain.
30. Elastic deformation - instantaneous recoverable deformation
31. Stress – force per unit area

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