Lesson 04 - Mechanical Properties

8/7/2019 Lesson 04 - Mechanical Properties

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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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Lesson 04 - Mechanical Properties