Physical and Mechanical Properties of Wood
Physical properties are defined as, “The properties which are determined without any change in size, shape, and chemical composition of wood.”
The physical properties of wood are discussed as under:
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COLOR OF WOOD:
The color of the wood is very important from the practical point of view or decorative point of view eg walnut. Distinctive coloration of wood is due to various extractives present in cell wall of heartwood. The color of the wood may be changed due to the following reasons:
- Many timbers darken with age.
- Some timber is changing color by the impact of sunlight
- Moist heat applied during kiln seasoning change color of wood and turn lighter colored to darker.
- The color of wood also change4s by applying different chemical on it. Eg liming lightens the color etc
- The color of wood may be changed due to any fungal attack on timber. Due to fungus attack, bluish or brownish stains appears commonly in the sapwood.
- The color of wood also changes when we observe tree from down to upward direction.
The luster of wood may be defined as, “Property of wood elements and its contents which they have the tendency to reflect light.”
Mulberry, Farash, Conocarps usually give a shiny surface. Importance of color is from a decorative point of view and use for the identification of wood.
Following factors affect the luster of wood:
- Angle of observation
- The plane of light falling
- The ability of cell wall to reflect to reflect the light.
ODOR AND TASTE:
- Many timbers have the characteristic odor, which is more effective during fresh conditions. But gradually disappears with the passage of time.
- Perhaps the most outstanding properties are the resinous odor of pines; spicy aroma of sandalwood, deodar, teak, juniper, etc.
- Similarly, Quercus and walnuts have also objectionable odor which gradually disappears with the passage of time.
- The taste of wood is greatly related to the odor probably traced to the same constituents.
- Both properties compel us to use wood in proper place.
- Bad smelling wood should not be used as food containers.
- The flavor of tobacco greatly changes along with storage.
- Quality gradually improves the passage of time.
DENSITY OF WOOD:
- Density may be defined as, “mass per unit volume.”
- Density is usu obtained by dividing weight/mass by volume. Ie, Density = weight / volume
- While weight is determined by balance; while vol can be calculated by multiplying length, breadth, and height.
- The recommended standard size of the wood piece should be 5-6cmm × 2cm × 2cm. This is the case when we have to find out the density of regular shape.
- So volume always is 6cm × 2cm × 2cm = 24 cubic cm.
- While weight may be varied this is usually 2% of sample or 20gm.
- The unit of density is gm/cubic cm.
- The density of irregular shapes may be found by following two methods:
- With the help of volume meter
- With the help of weight, balance, beaker
- In the case of the 1st method, vol meter is used this is of the following shape.
- First of all initial reading of mercury is taken. After put the wood sample in volume meter containing mercury. The final reading is taken by adjusting the reading by moving screw. The difference in initial and final reading will give us vol.
- In the second method, the simple scale is used containing two panes. One pane serving for weight and other for the beaker containing water. The initial reading is taken or simply weight of beaker containing water founded. The piece of wood, first of all, part in molten wax and in this way pores are filled up and excessive wax is scraped. Then put in the water in the beaker. The difference in b/w the two readings give us the reading. Finally the density of the wood taken.
Classification of Density:
- When density or specific gravity is 36 wood called very light
- When density or specific gravity is = 0.36 wood is called light.
- When density or specific gravity is 0.36 – 0.05, wood is moderately heavy.
- When density or specific gravity is > 0.05, wood is heavy.
- Hardness is defined as, “the resistance offered by the wood to indentation (to make a dent)”
- Resistance is checked against a hard steel rod called as Janka which is the type of electronic device. The standardized wood sample is of the following size.
10cm × 2cm × 2cm
- For determination of hardness two methods are generally used:
- Janka Method (for wood determination)
- Brinell method of matters
- According to Janka method dia of steel bar is 11.28 mm. the Janka penetrates to a specific depth of 5.04 mm. The hardness of wood sample measured along two sides ie Radial ‘R’ and Tangential ‘T’ while side resistance = (R + T) / 2
- Before discussing shrinkage and swelling it would be better to discuss following points
Fiber Saturation Point:
- It is denoted by FSP which is defined as, “theoretical stage at which all moisture is removed from all cavity and is still retaining in the cell wall.” The moisture content of a wood at 25-30% this is also called as a green condition of wood or fiber saturation point.
Equilibrium Moisture Content:
- It is denoted by EMC. Air dry condition or seasoned wood are other names of it. EMC is defined as, “Value of moisture content corresponding to the given combination of temperature and relative humidity of the atmosphere. Whenever moisture content is 12% called EMC.”
% of humidity = (Dry wood molecules / fully saturated molecules) × 100
= (Dry vapor of air / saturated vapor of air) × 100
Shrinkage and Swelling:
- Whenever softwoods are dried from Fiber Saturation Point to oven dry condition
Shrinkage in the longitudinal direction = 00.1 percent
Shrinkage along radial direction = 5%
Shrinkage along tangential direction = 10%
- Shrinkage for common hardwood spp is from FSP (25 – 30%) to EMC (12% moisture content.) Due to loss of moisture content size of wood changes.
- Difference is shrinkage value b/w three directions are due to following reasons:
- Restriction effect of ray on radial plane
- Difference in the lignifications value
α% age = [Dim t(max) – Dim t(min)] / Dim t(max) × 100
WOOD MOISTURE RELATION:
We can classify wood into four groups whenever we take into consideration moisture contents:
- Oven Dry
- Kiln Dry
- Air Dry
- Greenwood having a moisture content of 25-50%
Green Condition of Wood
|1. Free moisture in sap
2. Present in cell cavity or lamina
3. Easy to evaporate
4. Easy for seasoning
5. No adverse effect on size, shape.
|1. Bounded moisture
2. Present in cell wall
3. Vice versa
4. Vice versa
5. The adverse effect is great.
- The adverse effect of sound moisture in cell wall during seasoning causing deterioration, deformation, degradation (seasoning defects.)
- The phenomenon is called as case hardening.
Determination of Moisture Content:
- Freshly felled timber contains a large amount of moisture
- Some time the proportion of moisture is even more than the solid material of wood composition
- Water has a profound effect on the properties of wood. And also on a certain unit of moisture content fungi or insects can attack timber very easily. Therefore determination of moisture is very important.
- Following are the methods of determination of moisture content:
Oven dry method:
- In this method moisture content of wood is determined by the following formulae:
MC % = (initial w.t of sample – final wt) / final or dry wt of sample × 100
MC % = w1 – w2 / w2 × 100
- Here loss of moisture is determined by the comparison to dry w.t
Water content % = w1 – w2 / w1 × 100
- Here the percentage of water is calculated as compared to green wood.
- The selection of the piece should be done very carefully.
- The sample should be free of knots
- Once sample block is cut from timber it should be weighed suddenly to avoid loss of moisture.
- After initial weighing, the sample wood should be transferred to the oven or drying oven.
- Firstly, the wood is cleaned for few hours of the temp of 60 C. the temp of the oven should be increased up to temp 105 C for a night. After this can be seasoned for a few hours. Here precaution is that the wood after bringing out should be immediately weighed to avoid absorption of moisture.
- This method is more suitable for hardwood spp.
- In this method, 50 gm of chips or sawdust are taken into a round bottom flask.
- The flask is already containing a water-insoluble solvent having a higher boiling point than water commonly xylor is used for this purpose having a boiling point of 137 C. xylor also is soluble for resinous material and organic compounds of wood.
- On heating, vapors come in condenser after passing from glass tube attached to flask. Water condenser having an outlet and inlet of water.
- The vapors of wood are condensed in the form of water and are poured into the graduated beaker. The beaker is generally graduated in centimeter or cubic centimeter. The water in cubic centimeter converted into gm by relationship. (one cubic cm = one gm)
- This method is more accurate as compared to oven dry method. But the only difficulty is that during chip making or sawdust making some amount of moisture may be wasted in the form of moisture vapors. So the calculated amount of moisture should be lesser.
- But the advantage is that it is a rapid method completes in 3-4 hours.
Hygrometric method (Moisture Meter):
- Here moisture content is measured indirectly by measuring other properties. Eg moisture content of wood can be calculated by the hygrometer.
- Hygrometer has very simple construction it contains a metallic plated over which two thermometers are attached.
- Among these two thermometers, one is fixed in the vapor containing structure. Actually, bulb of that thermometer is supplied with vapors so that it can calculate net temperature.
- While other thermometer serving for calculating dry temp.
- Let dry temp T1 while wet temp is T2 (T1 > T2)
- T1 – T2 called wet bulb depression.
- Temp difference can be calculated.
- The relative humidity can be calculated as shown in the table:
|T1||T1 – T2|
- Relative Humidity is now plotted in the graph with moisture content.
Electric moisture method:
- The electrical resistance of wood and capacitance of wood are very useful for the measurement of M.C of wood. Moisture meters have a max capacity of measuring moisture up to 100%.
- So moisture meter is suitable to use for wood containing less moisture content or for dry wood.
- For dry woods, error possibility is ±2% of moisture content while for green wood error may be ± 1.5%.
THERMAL AND ELECTRICAL CONDUCTIVITY:
- The ability of the wood sample to conduct heat is called the Conductivity.
- Wood conduct heat comparatively slowly (due to the porous nature), it is one of the properly due to which timber is used in building material, furniture, and other materials.
- A wooden wall allows much less heat to pass through it than iron, concrete, brick or stone wall.
- The rate of flow of heat within a wood depends upon the direction of fiber/ grain and density of the wood material.
- Under the similar condition, 2-3 time more heat is conducted parallel to the grain as compared to across the or perpendicular to the grain.
- In the piece of wood (A) there is 2-3 times more heat conduction than the piece of wood (B)
- In other words, there is more heat conduction parallel to the grain than across the grain.
HEAT/ CALORIFIC VALUE OF WOOD:
- Definition: It is the process in which the quantity of heat produced by complete combustion of unit mass wood substance.
- The wood with excessive moisture contents has less calorific value and vice versa.
- Following cases of heat/ calorific value of wood study in relation to a moisture content within the wood.
- CASE I. Calorific heat value is minimum in excessive moisture content conditions (above the fiber saturation point FSP) ie above 30% moisture content.
- CASE II: Air dry condition calorific heat value is 1-2 time greater than the CASE I, at the FSP ie at 30% moisture content.
- CASE III: Oven dry condition calorific heat value is 2-3 times greater than the CASE I, below the FSP ie at 0% moisture content.
LATENT HEAT OF VAPORIZATION:
Definition: “The minimum amount of heat required to evaporate the unit mass of water”.
In the wood of excessive moisture contents (EMC), the most of the heat is utilized to evaporate the moisture content within the wood and this phenomenon is known as lowering the heat of combustion.
The case I: In the air dry wood, the evaporation of moisture along with the combustion with the release of gases takes place.
Case II: In the oven dry wood the combustion takes place with the maximum release of heat (due to minimum moisture contents within it).
The calorific heat value in all of our indigenous spp range from 4500 – 5500 kcal/kg
Conditions affecting the heat value of wood:
The chief conditions that affect the heat value of wood are:
- Dryness of wood: Moisture contents affect the heat/ calorific value of wood
- Anatomical structure: Porous wood burns more readily than dense wood. A steady heat is given by dense wood than porous wood.
- Soundness: Unsound wood (means the wood attacked by insects or fungi) give less heat than sound wood. This unsound or decayed wood takes fire readily and burns slowly without flame.
- Presence of extraneous substance: The presence of extraneous/ inflammable/ organic substances such as resin, etc increase the combustibility of wood.
- Size of a piece of wood: Smaller the piece of wood used the quicker the combustion process and greater heat is given out in short time, and vice versa.
- Hardwoods are better fuel-wood than softwood.
Determination of Ash Content of Wood:
The following procedure is adopted for the determination of ash contents of a wood.
- A piece of wood with certain moisture contents
- Oven dries that piece of wood up to 0% moisture content and weight it.
- After oven dry, don the complete combustion of that piece of wood.
- After the complete combustion of that piece of wood, weigh the amount of ash leaving behind.
- Then apply the following formula to determine the ash contents in percentage.
Units: The calorific heat value is expressed in:
- i) Cals/ gm or, Calories/ gram
- ii) Kcal/ kg or, Kilo-calories/ kilogram
iii) Btu/ lb or, British thermal unit/ pound
(1 Btu = 252 calories)
Determination of Calorific value by Empirical Method or Indirect Method:
For the determination of calorific of the calorific value of a piece of a wood of a particular spp, we have to determine the amount of volatile matter present in it.
To determine the amount of volatile matter, the following procedure is adopted:
- Take a piece of wood with certain Moisture content and weigh it, this w.t is known as the initial w.t of the wood.
- Then oven dries this piece of wood and again weigh it, this w.t is known as final w.t or oven dry w.t.
- Now calculate the loss in w.t from initial w.t and final w.t, as follow:
Loss in w.t = Initial w.t – Final w.t (oven dry)
- Then the amount of volatile matter is calculated:
Volatile matter (V.M) = Loss in w.t – m.c %
Here; loss in weight = initial w.t – Oven dry w.t
- The m.c % can be determined by the following formula:
m.c % = w1 – w2 / w2 * 100
Where; w1 = w.t of a piece of wood with certain moisture content; w2 = w.t of that piece of wood after oven drying.
Determination of Fixed Carbon Percentage:
For the determination of fixed carbon % of a piece of wood, following formula is applied:
Fixed C % = 100 – [V.M % + m.c % + A.C %]Where; V.M % = volatile matter %; m.c % = moisture content %; A.C % = ash content %
Calculation of CALORIFIC VALUE:
After determining the fixed carbon percentage (F.C %), moisture content percentage (m.c %), ash content percentage (A.C %) and volatile matter percentage (V.M %), the calorific value of that piece of wood can be calculated from the final formula:
Q = 82 * F.C % + α * V.M %
Where; Q = amount of heat/ calorific value and ‘α’ is constant factor taken as 82.
The practical applicability of determination of Calorific Value:
- Industrial Purposes
- Fore domestic usage
- For the preparation of Charcoal in improved form
- For the timber and fuelwood grading
- The properties which are observed by applying an external force. Than establishment of data, from this data proper use is estimated. The mechanical properties of wood are also called as strength properties. There are some standards which are:
PSI (Pakistan Standard Institution)
ISO (International standard institution)
ISI (Indian standard Institution)
BSI (British standard Institution)
- Basic terminology related to strength properties are given:
- Stress = wood / unit area or ‘force on unit area (kg / cm2)’.
Force or load is in the form of load.
- Strain = deformation / original length
Or; = change in length / original length (length in longitudinal direction)
- MOE = Modulus of elasticity is defined as, ‘the fiber stress at elastic limit.’ This can be understood by Hook’s law.
Stress α Strain
Stress = Y Strain
Y = Stress / Strain
= Kg cm-2 or Kg / cm2
‘Y’ is called Young’s modulus or modulus of elasticity ‘MOE’. Rate of stress and strain with the help of graph.
Formula of MOE;
MOE = PL3 / 4DBT3 = kg/cm2
(Where; P = load at E.L; L = length of elastic strain; D = deflection/ deformation of strain (28cm); B = breadth of sample; T = thickness of sample)
- MOR = Modulus of rupture is defined as, fiber stress at maximums load which breaks the specimen. (graph position same as in case of MOE)
MOR found by the following formula;
MOR = 3PL / 2BT2 = kg/cm3
(Where; P = max load; L = length of specimen; B = breadth of specimen; T = thickness of specimen)
1) Athletics and supporting material
2) Wood as Beam
3) Tools handle
5) Railway sleeper
TESTS OF BENDING STRENGTH:
- Compression may be defined as a set of force acting on a similar piece of wood equal in magnitude and acting in the similar direction
- Compression may be of two kinds depending on grain direction
- Compression parallel to grain
- Compression perpendicular to the grain
- Parallel to grain:
- One of the precaution for this simple process is that it should be ensured that
- it should be protected from bending
- uniform distribution of load or cross-sectional area
- Size of specimen is 5-6cm × 2cm × 2cm. Rate of load applied is 60mm / sec or 1-2 minutes / test
- Max compression parallel to grain = Maximum load / cross-sectional area = kg/cm2
- Compression parallel to grain up to elastic limit = load at E.L / area = kg/cm2
- It may be carefully noted that the value of compression at the elastic limit (before deformation) is different from that value of maximum load (after deformation).
- The graph obtained by machine is given below.
- Load values at E.L and at maximum load are different. Eg E.L having a value of 1700 kg. While ML (maximum load) having a value of 3000 kg.
- Spokes of tonga wheel
- Botted timber
- Tool handles
- Railway sleepers
- Perpendicular to grain:
- This test seldom carried out by crushing wood b/w two steel plates in the UK.
- The standardized size used for this test is (5cm × 5cm × 5cm)
- Compression is perpendicular to grain at E.L = Load at E.L / Area (kg / cm2 )
- Wooden wedges
- Bearing blocks
IMPACT BENDING/ TOUGHNESS/ DYNAMIC PROPERTY:
- Impact bending is defined as, “resistance by a wood sample to certain shocks.”
- The size of the test piece is 30cm × 2cm × 2cm
- The weight of the hammer is 8.5 kg. While ‘D’ height from which a hammer is dropped is 1.2 meter.
- Energy or work = w = F × D = 8.5 × 1.2 = 10 m/kg
- Here energy means the striking energy of hammer. The energy by which the hammer strikes the wood sample and resistance is offered by wood sample called as residual energy.
- If there is no sample in way of a hammering than the amount of striking energy is 10 meters/kg.
- If the sample is present in a way that some amount of energy is transferred.
- Two kinds of energies are present:
- Striking energy
- Residual energy
- Tool handles
- Shear is defined as, “resistance offered by the wood sample to slipping or sliding of one position upon other.”
- Shear is found parallel to grain and perpendicular to grain as well.
- For a parallel to the grain size of the test, the piece is 5-inch × 2-inch × 2 inch
- For perpendicular grain
Maximum shear = max load / area = kg/ cm2
Duration of each test = 1-2 minutes
- Similar is the case which we apply load on wood.
- Cleavage is defined as, “resistance offered by the wood sample to splitting.” Standardized size for this test is 4.5cm × 2cm × 2cm is a cross section
- Max load at the time of cleavage is found. The splitting force is applied at a rate of .04mm/s and resistance to cleavage expressed force per unit width.
Cleavage = maximum load / width (2cm)
Time = 1 – 1.5 minutes / test
- Bolted timber
- Bones or crates
- Furnisher making
- Tool handles
NAIL OR SCREWING RESISTANCE:
- It may be defined as, “resistance offered by the wood sample to withdraw a nail or screw from its surface.”
- First of all nail or screw is penetrated into a wood sample of following dimension 15cm * 5cm * 5cm and then withdraw by force in kg or Newton.
- Resistance depending upon the shape of the nail
- Precaution: put nail always at a right angle
- Moisture content affects greatly to the nail strength property
- NWR is directly proportional to the density
- Factors affecting NWR (Nail Withdraw Resistance):
- Shape of nail
- The depth of nail driven into the wood
- Direction of loading
- Moisture content percentage
FACTORS AFFECTING STRENGTH OF WOOD:
- Density of wood
- Moisture content percentage
- Time of loading
- Direction of grain
- Percentage of early and late wood
- Ring width
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