Transcript Document 7700432

MECHANICAL PROPERTIES
OF MATERIALS
Thursday, 3 March 2005
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2016
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MECHANICAL PROPERTIES AND
TESTING OF MATERIALS
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The Tensile Test
Compression
The Bend Test
The Hardness Test
The Impact Test
The Fatigue Test
The Creep Test
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Structure-Property-Processing
Relationship
Figure 1The three-part relationship between structure,
Properties, and processing
Method.
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Properties of A Material
 Mechanical Properties
How a material responds
 To an applied force, include strength and ductility.
 To a sudden, intense blow (impact)
 To a continually cycled through an alternating force
(fatigue)
 To a high temperatures (creep)
 To an abrasive conditions (wear)
Also determine the ease with which a material can be
deformed into a useful shape.
 Physical Properties: electrical, magnetic, optical,
thermal, elastic, and chemical behaviour depend on both
structure and processing of a material.
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The Tensile Test
 Measures the resistance
of a material to a static
or slowly applied force.
 Strength
 Ductility
 Toughness
 Elastic Modulus
 Strain Hardening
 Tension-test specimen:
Solid and round (ASTM:
lo=50mm and =12.5mm)
standard, flat-sheet or
tubular.
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Use of the Stress-Strain Diagram
 Tensile strength: the stress that corresponds to the
maximum load in a tensile test.
 Ductility: the ability of material to be permanently
deformed without breaking when a force is applied.
 Modulus of elasticity: Young’s modulus, or the slope of
the stress-strain curve.
 Toughness: a qualitative measure of the impact
properties of a material. A material that resists failure by
impact is said to be tough.
 Strain hardening:
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Stress-Strain Curves
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Mechanical Properties of Various
Materials at Room Temperature
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The Conversions from Load-Gage Length to
Stress-Strain
 Engineering stress =  = F/Ao
 Engineering strain =  = (l-lo)/lo
Where :
Ao = the original cross sectional area of the specimen before the
test begins
lo
= the original distance between the gage marks
l
= the distance between the gage marks after force F is applied
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Table 1 Tensile Test Data of a 0.505 in.
diameter Al-alloy test bar
Measured
Load (lb)
Calculated
Gage Length (in)
Stress (psi)
Strain (in./in.)
0
2.000
0
0
1000
2.001
5000
0.005
3000
2.003
15000
0.0015
5000
2.005
25000
0.0025
7000
2.007
35000
0.0035
7500
2.030
37500
0.0150
7900
2.080
39500
0.0400
8000 (max.load)
2.120
40000
0.0600
7950
2.160
39700
0.0800
7600 (fracture)
2.205
38000
0.1025
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Examples 1:
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a.
b.
c.

Convert the load-gage length data in Table 1 to
engineering stress and strain and plot a stress-strain
curve and calculate:
The tensile strength
The % elongation
The engineering stress at fracture
An aluminium rod is to widhstand an applied force of
45000 pounds. To assure a sufficient factor of safety,
the max. allowable stress on the rod is limited to 25000
psi. The rod must be at least 150 in. long but must
deform elastically no more than 0.25 in. when the force
is applied. Design an appropriate rod.
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Properties Obtained from the
Tensile Test
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Yield Strength
Tensile Strength
Elastic Properties
Ductility
True Stress and True Strain
Effect of Temperature
Effect of Deformation Rate
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Yield Strength
• The strength at which plastic deformation
becomes noticable.
• The stress required for dislocation to slip (in
metals).
• The stress that divides the elastic and plastic
behaviour of the material.
• When designing a part that will not plastically
deform:
- Select a material that has a high yield strength
- Make the component large so that the applied
force produces a stress that is below the yield
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strength.
2016
•
•
OFFSET YIELD STRENGTH –
in some materials, the
stress at which the material
changes from elastic to
plastic behaviour is not
easily detected.
The stress-strain curve for
certain low-carbon steels
displays a double yield
point.
Figure 2 (a) Determining the 0.2% offset yield strength in gray cast iron and
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(b)
Upper and lower yield point behaviour in low carbon steel.
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Tensile Strength (UTS)
 The stress obtained at the highest applied
force, which is the maximum stress on the
engineering stress strain curve.
 If the specimen is loaded beyond its UTS, it
begins to neck (locally deformed region).
As the test progresses, the engineering
stress drops further and the specimen
finally fractures at the necked region. The
engineering stress at fracture is known as
breaking or fracture stress.
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Elastic Properties
 The ratio of stress to strain in the elastic region:
modulus of elasticity or Young’s modulus (after T.
Young, 1773-1829), E.
 The modulus is a measure of stiffness of the material.
Hooke’s law (after R. Hooke, 1635-1703):
Modulus of elasticity, E = /
 Modulus of Resilience (Er):the area contained under
the elastic portion of a stress-strain curve, is the
elastic energy that a material absorbs during loading
and subsequently releases when the load is removed.
Er = yield strength/[(2)(strain at yielding)]
 Poisson’s ratio,  (after S. D. Poisson, 1781-1840)
 = - lateral/  longitudinal
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Ductility
 Measures the amount of deformation
that a material can withstand without
breaking.
 % elongation = [(lf-lo)/lo]x100
 % reduction in area = [(Ao-Af )/Ao]x100
Example 2:
The Al alloy in example 1 has a final gage length after failure of 2.195 in.
and final diameter of 0.398 in. at the fractured surface. Calculate the
ductility of this alloy.
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True Stress and True Strain
 True Stress is the ratio of the load F
to the actual (instantaneous) crosssectional area A of the specimen.
True stress,  = F/A
 True Strain,  = ln (l/lo)
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Strain Hardening
 We can represent the true stress-true strain curve by
the equation:
 = K n
Where
K = the strength coefficient.
n = the strain hardening (work hardening) exponent.
 Plotting the corrected curve in Fig. 2.5c on a log-log
graph – approximately a straight line (Fig. 2.5d). The
slope of the curve is equal to exponent n. Thus, the
higher the slope, the greater the strain hardening
capacity of the material – the stronger and harder it
becomes as it is strained.
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Effect of Temperature
Figure 3 The effect of temperature (a) on the stress-strain curve (b) on the tensile
Properties of an Al alloy.
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Effect of Deformation Rate
 Deformation rate is defined as the
speed at which a tension test is being
carried out, in units of, say m/s or
ft/min.
 The strain rate is a function of the
specimen length.
 Increasing the strain rate increases
the strength of the material (strainrate hardening).
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Compression
 Forging, rolling, extrusion are performed
with the workpiece subjected to
compressive forces.
 Compression test: compressing a solid
cylindrical specimen between two flat dies
(platens). The friction between the
specimen and the platens, the specimen’s
cylindrical surface bulges (barreling).
 True stress-true strain curves for the tensile
and compression tests for ductile material
coincide. But this comparability does not
hold true for brittle materials.
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Disk Test
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For brittle materials such as
ceramic and glasses. The disk is
subjected
to
compression
between
two
hardened
flat
platens. When the material is
loaded, tensile stress develop
perpendicular to the vertical
centerline along the disk, fracture
begins, and the disk splits in half
vertically. The tensile stress, is
uniform along the centerline and
can be calculated as
 = 2P/[dt]
P= load at fracture
d= diameter of the disk
t = thickness of the disk.
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The Bend Test
 In brittle materials, failure occurs at the
maximum load, where the tensile strength
and breaking strength are the same. How
about with ductile materials ?
 In many brittle materials- flaws at the
surface  Bend test
 By applying the load at 3 points and
causing bending, a tensile force acts on the
material opposite the midpoint. Fracture
begins at this location.
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 Flexural strength or
modulus of rupture
describes the
material’s strength.
Flexural strength
=3FL/(2wh2)
 Flexural modulus is the
modulus elasticity in
bending and is
calculated in the
elastic region.
Flexural modulus =
L3F/4wh3
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Figure 4 (a) The bend test often used for
Measuring the strength of brittle materials, and
(b) The deflection  obtained by bending.
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Hardness Test
 Measures the resistance to
penetration of the surface of a
material by a hard object.
 Brinell, Rockwell, Vickers, Knoop.
 Tensile strength (psi) = 500 HB
 Hardness correlates well with wear
resistance.
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Impact Test
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Impact Test
 To evaluate the brittleness of a material under a
sudden, intense blow, in which a strain rate is
extremely rapid.
 Charpy and Izod test.
 Izod for nonmetallic materials. The specimen may be
either notched or unnotched.
 Knowing the initial and final elevations of the
pendulum, we can calculate the difference in potential
energy (impact energy absorbed by the specimen
during failure).
 Toughness ??
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Properties Obtained from the
Impact Test
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Transition temperature
Results of a series of impact
tests performed at various
temperature.
Notch sensitivity
Caused by poor machining,
fabrication or design
concentrate stresses and
reduce the toughness of the
material. Compare the
absorbed energy between
notched and unnotched
specimen.
Relationship to the stressstrain diagram.
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Charpy V-notch properties for BCC
Carbon steel and a FCC ss. FCC crystal
Structure typically leads to higher
Absorbed energies and no transition
Temp.
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The area contained within the true stressTrue strain curve is related to the impact
Energy. Although material B has a lower
Yield strength, it absorbs a greater energy
Than material A.
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The Fatigue Test
The rotating cantilever beam
Fatigue test.
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 A component often
subjected to the
repeated application of
a stress below the
yield strength of the
material.
 Cyclical stress:
rotation, bending, or
vibration.
 Although the stress is
low, the material may
fail after a large
number of applications
of the stress.
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3 Stages of Fatigue
 Tiny cracks initiates at the surface.
 The crack gradually propagates as the
load continues to cycle.
 Sudden fracture of the material
occurs when the remaining cross
section of the material is too small to
support the applied load.
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Results of the Fatigue Test
Endurance limit: the stress
below which there is a
probability that failure by
fatigue will never occur –
design criterion.
 Fatigue life: how long a
component survives at a
particular stress
 Fatigue strength: max stress
for which fatigue will not
occur within a particular
number of cycles, such as
500000 cycles (for Al and
polymers which have no
endurance limit).
 In some materials, endurance
limit  half tensile strength.
Endurance ratio= endurance
limit/tensile strength  0.5

The stress-number of cycles to
Failure (S-N) curves for a tool
Steel and an Al alloy.
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The Creep Test
 CREEP: Plastic
deformation at high
temperatures = stress
+ high temperature.
 As soon as the stress
is applied, the
specimen stretches
elastically a small
amount depending on
the applied stress and
the modulus elasticity
o of the material at
high temperature.
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