Bachelor of Technology
Mechanical
Industrial Material
UOG
Lecture # 04
By: Jahangir Rana
Mechanical Properties of metal
Mechanical Properties of metal
• There are so many properties by which a metal can test.
Some are as follow:
 Engineering stress & Engineering Strain
 Shear Stress & Shear Strain
 Modulus of Elasticity
 Yield Strength
 Ultimate tensile strength
 Percent Elongation
 Percent Reduction in area
 Hardness
 Fatigue
 Ductility
 Forging
And many more!!!!!!!!
Stress (σ)
Stress is defined as force per unit area. It has the
same units as pressure, and in fact pressure is one
special variety of stress. However, stress is a much
more complex quantity than pressure because it
varies both with direction and with the surface it
acts on.
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•
•
•
•
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σ = Fn / A
(1)
where
σ = normal stress ((Pa) N/m2, psi)
Fn = normal component force (N, lbf)
A = area (m2, in2)
Strain (ε)
Strain is defined as the amount of
deformation an object experiences
compared to its original size and shape.
For example, if a block 10 cm on a side is
deformed so that it becomes 9 cm long,
the strain is (10-9)/10 or 0.1 (sometimes
expressed in percent, in this case 10
percent.) Note that strain is
dimensionless.
Types of stress
•
•
•
•
Compression stress
Tension stress
Normal stress
Shear stress
Compression stress
• Stress that acts to shorten an object.
Tension stress
Stress that acts to lengthen an object.
Normal stress
Stress that acts perpendicular to a surface. Can be either compression or
tensional.
Shear stress
• Stress that acts parallel to a surface. It can cause one object to
slide over another. Stress parallel to the plane is usually
denoted "shear stress" and can be expressed as
• τ = Fp / A
(2)
where
• τ = shear stress ((Pa) N/m2, psi)
• Fp = parallel component force (N, lbf)
• A = area (m2, in2)
• It also tends to deform originally rectangular objects into
parallelograms. The most general definition is that shear acts
to change the angles in an object.
Shear stress
Flexural stress
Torsional stress
Shear Stress & Shear Strain
Shear stress
• Shear force applied on the area over which shear force is
acted divided by the original area. It is donated by τ.
Τ = S/A
Shear strain
• The shear strain is defined as the amount of the shear
displacement ‘a’ divided by the height at which shear
force is applied is called shear strain.
γ = a/h
Stress Strain Curve
Modulus of Elasticity
• It is defined as when the metal is deformed
elastically or stress is directly proportional to
the strain.
stress α strain
stress E strain
‘E’ is young's modulus or modulus of
elasticity.
It is unit is pascal or PSI
Cont…
Yield Strength
• It is strength at which metals or alloys
shows the significant plastic deformation.
Ultimate tensile strength
• The maximum value of stress on stress
strain diagram is called ultimate tensile
strength.
Ductility & % Elongation
Ductility
• It is a mechanical property used to describe the extent to which
materials can be deformed plastically without fracture.
• The property of metal which permits it to be reduced in cross
sectional area without fracture. In a tensile test, ductile metals
show considerable elongation eventually failing by necking,
with consequent rapid increase in local stresses.
% Elongation
• The amount of elongation that a tensile specimen undergoes
during testing provide the value of ductility of metals. It is most
commonly expressed % elongation.
% Elongation
% Reduction in area
Hardness
• It is the property of a metal, which gives it the
ability to resist being permanently, deformed
(bent, broken, or have its shape changed),
when a load is applied.
• The greater the hardness of the metal, the
greater resistance it has to deformation.
• It is a measure of a material’s resistance to
localized plastic deformation (e.g., a small
dent or scratch).
Hardness Testing
•In quantitative hardness tests, a small indenter is
forced into the surface of a material to be tested,
under controlled conditions of load and rate of
application.
•The depth or size of the resulting indentation is
measured by an automated testing machine and
converted to a hardness number.
•The softer the material, the larger and deeper the
indentation, and the lower the hardness index
number.
Advantages of
Hardness Testing
Hardness tests are simple and inexpensive.
Ordinarily, no special specimen needs to be
prepared, and the testing apparatus is
relatively inexpensive.
Advantages of
Hardness Testing
•The test is non-destructive – the specimen is
neither fractured nor excessively deformed. A
small indentation is the only deformation.
•Other mechanical properties may be
estimated from hardness data.
Rockwell Hardness Tests
•The most common method used to measure
hardness.
•Several different scales may be utilized
from possible combinations of various
indenters and different loads, which permit
the testing of virtually all metal alloys (as
well as some polymers).
Rockwell Hardness Testing Machines
Rockwell Hardness Tests
Indenters include spherical and steel balls
having various diameters, and also a conical
diamond (Brale) indenter for testing the
hardest materials.
Rockwell Hardness Tests
•With this system, a hardness number if
determined by the difference in depth of
penetration resulting from the application of
a minor load followed by a larger major load.
•On the basis of the magnitude of both major
and minor load, two scales are used:
Rockwell and superficial Rockwell.
Rockwell Hardness Scales
Scale Symbol
A
B
C
D
E
F
G
H
K
Indenter
diamond
1.588 mm ball
diamond
diamond
3.175 mm ball
1.588 mm ball
1.588 mm ball
3.175 mm ball
3.175 mm ball
Major Load (kg)
60
100
150
100
100
60
150
60
150
Minor Load = 10 kg
Brinell Hardness Tests
•A hard, spherical indenter is forced into the
surface of the metal to be tested.
•The diameter of the hardened steel (or
tungsten carbide) indenter is 10.00 mm.
•Standard loads range between 500 and
3,000 kg in 500 kg increments.
Brinell Hardness Testing Machine
Brinell Hardness Tests
•During a test, the load is maintained
constant for a specified time (between 10
and 30s).
•The Brinell hardness number, HB, is a
function of both the magnitude of the load
and the diameter of the resulting indentation.
Fatigue
• Fatigue is the progressive and localized
structural damage that occurs when a
material is subjected to cyclic loading.
• Theses failure which occurs under
repeated or cyclic stressing are called
fatigue failure.
Changes Occur During Fatigue Process
The structural changes, which occur during
fatigue process are:
• Crack initiation
• Slip band crack growth
• Crack growth on metal due to high stress
• Ultimate ductile failure
Cont…
Crack initiation
• During this stage, the early damage
occurs and crack may no be seen.
Slip band crack growth
• Plastic deformation in one direction and
then alternatively in reverse direction
cause grooves appear on the surface is
called slip band growth.
Cont…
Crack growth on metal due to high stress
• During last stage crack may grow in metal only
a few grain diameter and in this stage, crack
growth will define at relatively rapid rate.
Ultimate ductile failure
• Finally when the crack covers a sufficient area,
so that the remaining metal can not support the
applied load then the metal ruptured by the
ductile area.
Factors that affect fatigue-life
• Cyclic stress state.
• Surface quality
• Environment.
Forging
• It is another primary method for
working metal into useful shapes.
• In the forging process the metal is
hammered or pressed into the desired
shape.
• Most forging operations are carry out in
the hot conditions.
• Some are carry out in the cold
conditions
( room temperature)
Types of forging
There are to types of forging process
• Hammer forging
• Press forging
Press fording is further divided in the
following types
• Open die forging
• Close die forging
Cont…
Closed die forging
Open die forging
• Open die forging is • In close die forging the
metal to be forged is
carried out b/w two flat
placed b/w two dies
dies with very simple which have he upper
shape such as vees or and lower impression of
semi circular cavities.
desired shape.
• It
is
useful
for • Close die forging can be
producing large parts carried out by using a
such as steel shafts for single pair of dies, or
impression
electric steam turbines multiple
dies.
and generator
Cont…
OPEN DIE FORGING
CLOSED DIE FORGING
Open die forging