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Materials Engineering
MMR - 2044
Lesson 4 Mechanical Properties
Strength
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Topic Outline Stress and Strain Tension
Compression
Shear
Torsion
Elastic Deformation
Plastic Deformation Yield Strength
Tensile Strength
Ductility
Toughness
Hardness
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Stress and Strain Stress is defined as force (F) per unit area (A).
Stress is used to express the loading in terms
of force applied to a certain cross-sectionalarea of an object.
It is measured inN/m2 and this unit isspecifically calledPascal (Pa)
1 N/m2 = 1 Pa
e
o
F
A
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Types of Loading
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Strain Strain is the response of a system to an
applied stress.
Strain: elongation change in dimension perunit length.
Stress and strain are positive for tensile
loads, negative for compressive loads.
0
0
L L
e L
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Stress-Strain Behavior When a material is loaded with a force, it
produces a stress, which then causes amaterial to deform.
Deformation Elastic Deformation Plastic Deformation
Elastic deformation: Reversible: when the stress is removed, thematerial returns to the dimension it had beforethe loading.
Plastic deformation Irreversible: when the stress is removed, the
material does not return to its previousdimension.
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Elastic Deformation
F
bondsstretch
return to
initial
Elastic means reversible!
F
Linear-elastic
Non-Linear-elasticReversible: when the stress is
removed, the material returnsto the dimension it had beforethe loading.
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Plastic Deformation
Plastic means permanent/Irreversible!
planes
stillsheared
F
elastic + plastic
bondsstretch& planesshear
plastic
Irreversible: when the stress is removed, thematerial does not return to its previousdimension.
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Tensile Test A tensile test is a fundamental mechanical
test that measure the applied load and the
elongation of the specimen over somedistance.
Tensile tests are used to determine: the modulus of elasticity
elastic limit Elongation
Reduction in area
tensile strength
yield point, yield strength and other tensileproperties.
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Tensile Test
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Stress Strain Curve The engineering
stress-strain curve
(
e- e) is obtainedfrom the load-elongation curve.
The yield point,
called the yieldstrength (y),signifies the start ofthe plastic region.
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Tensile Test: Elastic deformation In tensile tests, if the
deformation is elastic,
the stress-strainrelationship is calledHooke's law:
E is Young's modulus ormodulus of elasticity, has
the same units as ,N/m2 or Pa
Higher E higher stiffness
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Tensile Test: Plastic Deformation stress and strain
are notproportional
The deformation isnot reversible Deformation
occurs by breakingand re-
arrangement ofatomic bonds
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tensile stress,
engineering strain,
y
p = 0.002
YIELD STRENGTH,
y The yield stress is a measure of resistance to plastic
deformation
Stress at which noticeable (ep = 0.002) plastic deformation
has occurred. Yield strength y - is chosen
as that causing a permanentstrain of 0.002
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For a low-carbon steel, the stress vs. strain curveincludes both an upper and lower yield point.
The yield strength is defined in this case as the averagestress at the lower yield point.
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Maximum possible engineering stress in tension.
Metals: occurs when noticeable necking starts.
TENSILE STRENGTH, TS
strain
engineering
stress
TS
Typical response of a metal
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DUCTILITY Ductility is a measure of the deformation at
fracture
Lo LfAo
Af
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Ductility defined by:
Percent elongation (%EL)
Percent reduction in area (%AR)
%AR
Ao A fAo
x100
%EL
L f LoLo
x100
Note: %AR and %EL are often comparable.--Reason: crystal slip does not change material volume.--%AR > %EL possible if internal voids form in neck.
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Toughness Toughness = the ability to absorb energy up to fracture
= the total area under the strain-stress curve up tofracture
Units: the energy per unit volume, e.g. J/m3
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Materials EngineeringMMR - 2044
Lesson 4 Mechanical PropertiesHardness & Impact Behavior
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Hardness of Materials
Hardness is a measure ofresistance to localizedplastic deformation of materials and can berelated to the strength of materials.
Large hardness means:
Resistance to permanent indentation under static ordynamic loads
Energy absorption under impact loads (reboundhardness)
Resistance to scratching (scratch hardness)
Resistance to abrasion (abrasion hardness),
Resistance to cutting or drilling (machinability)
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Types of Hardness (3 Types) There are three general types of
hardness measurements
1. Scratch hardness : mainly used for minerals
2. Rebound or dynamic hardness: reboundheight measured in rebound test after adynamic load is dropped onto a surface -Scleroscope testing machine
increasing hardness
mostplastics
brassesAl alloys
easy to machinesteels file hard
cuttingtools
nitridedsteels diamond
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3. Indentation hardness : important for metallic
materials The hardness is obtained by forcing a hard indenter
into the material and measuring the dimensions ofthe indent left after the indenter is removed.
The most commonly used are the Brinell andRockwell hardness tests.
e.g.,10mm sphere
apply known force(1 to 1000g)
measure size
of indent afterremoving load
dDSmaller indentsmean largerhardness.
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Indentation hardness
According to the type of indenter andthe load applied, the indentation
hardness can be further divided into: Brinell Hardness
Rockwell Hardness/Superficial Rockwell
Hardness Vickers Microhardness
Knoop Microhardness
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2. Rockwell Hardness
The Rockwell Hardness test uses a small-diameter steel ballfor soft materials and a diamond cone for harder materials.
Indenter
spherical and hardened steel balls having diameters of 1/16, 1/8
and 1/4 inch & Conical diamond (Brale) indenter.
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Load
Hardness number is determined by applying of aninitial minor load followed by a larger major load.
The utilization of a minor load enhances test accuracy On the basis of the magnitude of both major and
minor loads, there are two types of tests:
Rockwell Hardness Scales (minor load is 10 kg) Superficial Rockwell Hardness Scales (minor load is 3
kg)Rockwell Hardness Scales
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Symbol / unit HR followed by the appropriate scale identification.
For example:
80 HRB represents a Rockwell hardness of 80 on the Bscale
60 HR30W indicates a superficial hardness of 60 on the30W scale.
Superficial Rockwell Hardness Scales
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3 & 4. Vickers and Knoop Microhardness
Two other hardness testing techniques are Knoop(pronounced nup) and Vickers (sometimes also calleddiamond pyramid)
The microhardness type hardness test leaves the leastamount of damage on the metals surface
Applied loads are much smaller than for Rockwell andBrinell, ranging between 1 and 1000 g.
The resulting impression is observed under a microscopeand measured; this measurement is then converted intoa hardness number
The Knoop and Vickers hardness numbers aredesignated by HK and HV, respectively.
Knoop and Vickers are referred to as microhardnesstesting methods on the basis of load and indenter size.
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Correlation between Hardness and Tensile Strength
Both hardness and tensile
strength are indicators of a
metals resistance to
plastic deformation.
TS (MPa) = 3.45xBHN
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Hardness Conversion
Example:30HRC = 300BHN
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(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning
is a trademark used herein under license.
Figure 6.26 The impact test: (a) The Charpy and Izodtests, and (b) dimensions of typical specimens
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Properties Obtained from the ImpactTest
Ductile to brittle transition temperature (DBTT)
- The temperature below which a material
behaves in a brittle manner in an impact test.
Notch sensitivity - Measures the effect of a
notch, scratch, or other imperfection on amaterials properties, such as toughness or
fatigue life.
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Results from aseries of Izodimpact tests for asuper-tough nylonthermoplastic
polymer
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The Charpy V-notch properties
for a BCC carbonsteel and a FCCstainless steel.
The FCC crystal
structure typicallyleads top higherabsorbed energiesand no transitiontemperature
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The area containedw ithin the true stress-true strain curve isrelated to the tensile
toughness. Althoughmaterial B has a loweryield strength, itabsorbs a greaterenergy than materialA. The energies fromthese curves may notbe the same as thoseobtained from impacttest data
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