engg mtls chapter2

8/3/2019 Engg Mtls Chapter2

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Properties of Materials

By

S.Muthurajan

Lecturer

Department of Mechanical Engineering

Salalah College of Technology

Salalah



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



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.



Three types of forces or loads

Tensile force

Compressive force

Shear force

Example for Shear load



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



Tensile Test Stress – Strain Diagram



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



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



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?



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



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



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



Different Stress – Strain Curves



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.



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



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



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.



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



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



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

http://upload.wikimedia.org/wikipedia/commons/0/00/Coloured_ceramic_thermal_barrier_coating_on_exhaust_component.jpg



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.

http://upload.wikimedia.org/wikipedia/commons/f/fa/Rust_Bolt.JPG



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.



How Magnetic induction occur

H = N I / L

B = μ H



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.

engg mtls chapter2

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