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Mechanical Properties of

Materials

Section II

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Introduction Relevant concepts Classifications of materials Actual tests for the mechanical

properties Physical constants of some

materials

Talking Points

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In this modern age, while designing a structure, a special care is always taken, that the structure under design should be able to withstand the stresses; under the various load conditions, without failure.

Introduction

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Mechanical Properties of Materials: Strength Toughness Ductility Brittleness

Introduction- Cont.

Hardness Ductility Elasticity Etc…

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Stress: the resistance of unit area to deformation; i.e.

where, = stress, P = load or force acting on body, and A = the cross-sectional area of the body.

Strain: the deformation per unit length; i.e.

where, = strain, L = change of length of the body, and Lo = the

original length of the body. The percentage form is usually called the percentage elongation. There is a term called percentage reduction in area:

where, A = change of cross-sectional area of the body, and Ao = the original cross-sectional area of the body.

Relevant Concepts

A

P

oL

L % 100L

L

o

Or in percentage

form

% 100A

Aareain reduction Percentage

o

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Classifications of Materials

Elastic Material: regains its original shape on the removal of the external forces.

Classification I

When some external systems of forces acts on a body, this body undergoes some deformation. The deformation increases as the forces increase.

Plastic Material: does not regain its original shape on the removal of the external forces.

Ductile Material: can undergo a considerable deformation without rupture.

Classification II

Brittle Materials: cannot undergo any deformation when some external forces act on it and it fails by rupture.

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Actual tests for the mechanical properties

Tensile test Compression test Fatigue test Hardness test Impact test

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Tensile Test Point A is called the limit of

proportionality or the elastic limit. Hooke’s Law is applied.

E = Constant of proportionality

or Modulus of elasticity

or Young’s Modulus. The stress corresponding to

point B is called yield stress (Sy).

At Point C, the stress, which has a maximum value, is called the ultimate stress (Su).

Neck is formed beyond point C which decreases the cross-sectional area.

The stress corresponding to point E is called the fracture or breaking stress (Sf).

Only ductile materials behave this way.

Strain

Stre

ss

Mild steel specimen

EPlastic Region

(Ductile Material)

Elastic Region

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Tensile Test – Cont.

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Tensile Test – Cont.

In the tensile test of a typical brittle material, there is no plastic deformation region like the one for the ductile material.

No neck is formed.

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Compression Test

The compression test is opposite to the tensile test with respect to the direction of the applied load.

The test is generally performed for testing brittle materials.

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Fatigue Test

Some of the machine parts such as shafts, springs, crank-shafts, connecting rods are subjected to varying stresses. It includes the variation in the intensity of the same type of stresses as well as different types of stresses.

The most common test is a rotating bar test of cantilever type or beam type.

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Hardness Test

The resistance of material to penetration by a pointed tool is called hardness. This hardness can be measured.

The most known methods of measuring hardness are Rockwell and Brinell tests.

In Brinell test: an indenting tool (a ball) applies a force on a sample material. The hardness number (HB) is found as a number equals to the applied force divided by the spherical surface area of the indentation.

Brinell has an advantage that it is directly related to the ultimate strength of the material.

MPa 10.3 Bu HS

MPa 8658.1 Bu HS

(For Steel)

(For Cast Iron)

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Impact Test

Many machines or machine components are subjected to a suddenly applied load, which is called impact blow. For determining the suitability of a material to resist the impacts, Izod and Charpy notched-bar tests are generally carried out. The test enables us to establish the property of toughness of a material.

Specimen is struck by a pendulum released from a fixed height, and the energy absorbed by the specimen, called the impact value, is computed from the height of swing after fracture .

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Physical Constants of Some Materials