Mechanical Properties of Materials

1
Mechanical Properties of Materials
Section II
2
Talking Points

• Introduction
• Relevant concepts
• Classifications of materials
• Actual tests for the mechanical properties
• Physical constants of some materials

3
Introduction

• 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.

4
Introduction- Cont.

• Mechanical Properties of Materials
• Strength
• Toughness
• Ductility
• Brittleness

• Hardness
• Ductility
• Elasticity
• Etc

5
Relevant Concepts

• Stress the resistance of unit area to
  deformation i.e.
• where, s 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, e strain, dL 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, dA change of cross-sectional area
  of the body, and Ao the original
  cross-sectional area of the body.

Or in percentage form
6
Classifications of Materials

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

Classification I
Classification II
Elastic Material regains its original shape
on the removal of the external forces.
Plastic Material does not regain its
original shape on the removal of the external
forces.
Ductile Material can undergo a considerable
deformation without rupture.
Brittle Materials cannot undergo any
deformation when some external forces act on it
and it fails by rupture.
7
Actual tests for the mechanical properties

• Tensile test
• Compression test
• Fatigue test
• Hardness test
• Impact test

8
Tensile Test

• Point A is called the limit of proportionality or
  the elastic limit. Hookes Law is applied.
• E Constant of proportionality
• or Modulus of elasticity
• or Youngs 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.

Elastic Region
Plastic Region
Stress
Mild steel specimen
(Ductile Material)
Strain
9
Tensile Test Cont.
10
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.

11
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.

12
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.

13
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.

(For Steel)
(For Cast Iron)
14
Impact Test
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.

• 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.

15
Physical Constants of Some Materials