Properties of Materials By S.Muthurajan Lecturer Department of Mechanical Engineering Salalah College of Technology Salalah
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Properties of Materials
By
S.Muthurajan
Lecturer
Department of Mechanical Engineering
Salalah College of Technology
Salalah
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The various engineering material properties are given as under.
Mechanical properties Chemical properties
Thermal properties
Electrical properties
Magnetic properties
Physical properties
When selecting a material for an engineering application, a primary concern is to
assure that its properties will be adequate for the anticipated operating conditions
Properties of materials
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Mechanical Properties
The mechanical properties of the metals are those which are associated
with the ability of the material to resist mechanical forces and load.
The main mechanical properties of the metal are strength, stiffness,
elasticity, plasticity, ductility, malleability, toughness, brittleness, hardness.
These properties can be well understood with help of tensile test and stress
strain diagram.
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Three types of forces or loads
Tensile force
Compressive force
Shear force
Example for Shear load
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Stress is a measure of the internal forces acting within a deformable body. It is a
measure of the average force per unit area of a surface within the body on whichinternal forces act. The internal resistance offered by a material to an externally
applied force is called stress.
Stress (s) = Force / Area of cross section
Unit of stress is N/mm2
1 Pa = 1 N/mm2
1 MPa = 1 N/m2
Strain is the deformation produced per unit length of a body due to the effect of
stress on it. It is the ratio of the change in length of the specimen to its original
length.
If L is the original length of the sample and l is the change in length, then
longitudinal strain, e = l / L . It has no units
Stress
Strain
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Tensile Test Stress – Strain Diagram
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It is the ability to resist the application of force without breaking. Forces may be tensile,
compressive or shear.
Tensile Strength = Maximum tensile force / Original Cross section Area
In this diagram, the point E represents Ultimate tensile strength of the material or
Maximum tensile strength
Strength
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Elasticity
Elastic deformation (OA): It is defined as the property of a material to regain
its original shape after deformation when the external forces are removed.
Within the elastic limit, Stress is directly proportional to Strain. This is called
Hooks Law.
Stress ∞ Strain
Stress / Strain = A constant ( E )
This constant E is called as the modulus of elasticity or Young’s Modulus
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Stiffness
It is defined as the ability of a material to resist deformation under stress.
The resistance of a material to elastic deformation or deflection is called
stiffness or rigidity.
Small strains under large stresses indicates good stiffness of the material.
This means that materials with bigger modulus of elasticity are stiffer.
Which one is Stiffer material?
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Ductility is termed as the property of a material enabling it to be drawn
into wire with the application of tensile load. A ductile material must be strongand plastic.
The ductility is usually measured by the terms of percentage elongation. If
the percentage of elongation is more then the material is more ductile.
The ductile material commonly used in engineering practice in order of
diminishing ductility are mild steel, copper, aluminium, nickel, zinc, tin and lead.
Ductility
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Brittleness
Brittleness is the property of a material opposite to ductility. It is the
property of breaking of a material with little permanent distortion.
Glass, cast iron, and ceramics are considered as brittle material.
The brittleness is usually measured by the terms of percentage elongation. If
the percentage of elongation is less then the material is more brittle.
Stress – Strain curve for Brittle materials
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Toughness
Ability to absorb energy up to fracture
Total area under the strain-stress curve up to fracture
If the material has more strength and more ductility, it is said to be toughness
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Different Stress – Strain Curves
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Hardness
Hardness is a very important property of materials.
Hardness indicates wear-resistance and resistance against abrasion or
scratching.
A hard material also offers resistance to penetration by another body.
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Electrical Properties of the Materials
Electrical conductivity
Electrical Resistivity
Di electric strength
Electrical Resistivity
Electrical resistivity is a measure of how strongly a material opposes the
flow of electric current.
A low resistivity indicates a material that readily allows the movement of
electric charge.
The SI unit of electrical resistivity is the ohm metre (Ωm). It is commonly
represented by the Greek letter ρ (rho). ρ = R * A / L
R = Resistance in ohm
A = Area of Cross SectionL = Length
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Conductivity is defined as the ability of the material to pass electric
current through it easily i.e. the material which is conductive will provide an easy
path for the flow of electricity through it.
Electrical Conductivity
K = 1 / ρ = L / R * A
1 / ohm = Siemen
Therefore Unit for conductivity is Siemen / meter
All metals have good electrical conductivity
Materials like ceramics, polymers, wood have low values of electricalconductivity
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Di electric Strength
It is a measure of the highest voltage that an insulating material can bearwithout breakdown
A material having high dielectric strength can withstand for longer time for
high voltage across it before it conducts the current through it.
Dielectric strength = Breakdown Voltage / Insulator Thickness
Unit for Dielectric strength is V/m.
For an example Alumina have a Dielectric strength of 13.4 MV/m
This means that a 1m thickness of alumina requires a voltage of 13.4 MV to
break.
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Thermal Properties
Coefficient of linear expansion
All materials are expand or increase in volume when heated to higher
temperature .
The linear thermal expansion coefficient relates the change in a material's
linear dimensions to a change in temperature.
α = δl / δt * l
δl = change in length
L = original length
δt = change in time
Unit of Coefficient of linear expansion is K-1
Eg : PVC = 50 - 250 x 106 / K
Brick = 3 - 9 x 106 / K
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Specific Heat Capacity
It is defined as the amount of heat energy required to raise the temperature
of 1kg of the material by 1°
Specific heat capacity = heat energy given / mass * temperature rise
Unit of Specific heat capacity is J/ Kg °K
In general metals have low value of specific heat capacity whereaspolymers and ceramics have higher values.
Eg : Iron = 437 J/ Kg °K
Polystyrene = 1300 J/ Kg °K
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Thermal Conductivity
It is the property of a material's ability to conduct heat
Thermal Conductivity, k = amount of heat energy / length * temperature
Unit for thermal conductivity is W/ m°K
Most of the metals having high thermal conductivity whereas ceramics
have low thermal conductivity
Ceramics can be used as heat insulators.
Ceramic coatings with low thermal
conductivities are used on exhaust systems toprevent heat from reaching sensitive components
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Chemical Properties
Corrosion Resistance
Corrosion is the disintegration of an engineered material into its constituent
atoms due to chemical reactions with its surroundings.
Formation of an oxide of iron due to oxidation of the iron atoms in solid
solution is a well-known example of electrochemical corrosion, commonly
known as rusting.
Aluminium and Stainless steels have excellent resistance.
Iron and carbon steels are very poor corrosion resistance.
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Magnetic Properties
Magnetic flux density or Magnetic Induction
A magnetic field can be produced by putting a current through a coil. Magnetic
induction occurs when the material is subjected to magnetic field. If the magnetic
field is applied to a solid medium, the magnetic induction in the solid is given by
B = μ H
μ= magnetic permeability . Its unit is Henry m-1
B is expressed as Weber / m2 or tesla
H= Magnetic field strength ( H ) is expressed in units of A-m – 1
Some materials are only temporarily magnetized. They are called Soft magnetic
materials . Examples are Iron and Ferrous alloys
Some other materials can be permanently magnetised. They are called hard magnetic
materials. Examples are metal oxides.
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How Magnetic induction occur
H = N I / L
B = μ H
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Physical Properties
Density
Mass per unit volume is called as density. In metric system its unit iskg/mm3. Because of very low density, aluminium and magnesium are preferred in
aeronautic and transportation applications. Density , ρ= mass / volume
Color
It deals the quality of light reflected from the surface of metal.
Size and shape
Dimensions of any metal reflect the size and shape of the material.
Length, width, height, depth, curvature diameter etc. determines the size. Shape
specifies the rectangular, square, circular or any other section.
Specific GravitySpecific gravity of any metal is the ratio of the density of a given metal to
the density of the water at a specified temperature.