Handout 3 Materials Corrosion

7/27/2019 Handout 3 Materials Corrosion

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CHE 3166: HANDOUT 3

Stresses, Deformation and Fracture

LEARNING OBJECTIVES: Part I

Stress and StrainElastic Deformation

Plastic Deformation

Ductility

Toughness


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Stress and Stress Types

Stress ( : Force (F) / Cross-sectional Area (A)

= F / A

States / Types of Stress

Tension

Compression Shear / Torsion


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True Stress and Strain

True stress, T:

Load Fdivided by the

instantaneous cross-sectional

area Ai(afterdeformation)

i

T

A

F

True strain, :0

lnl

li

T


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Elastic Deformation

1. Initial 2. Small load 3. Unload

F

bonds

stretch

return to

Initial

F Linear-elastic

Non-Linear-

elastic

Elastic Deformation

is reversible


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Plastic Deformation

Plastic Deformation

is NOT reversible

1. Initial 2. Small load 3. Unload

planesstillsheared

F

elastic + plastic

bondsstretch& planesshear

plastic

F

linearelastic

linearelastic

plastic


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Linear Elastic Behaviour

When stress ( ) is proportional to strain ( )

Linear-

elastic

EF

Fsimpletensiontest

Hooke's Law:

= E

E: Slope, a Constant, also known as:

Modulus of Elasticity or Youngs Modulus

Stiffness of the materials

Materials resistance to elastic deformation


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Youngs Modulus (E) of Different Material Types

0.2

8

0.6

1

Magnesium,Aluminum

Platinum

Silver, Gold

Tantalum

Zinc, Ti

Steel, Ni

Molybdenum

Graphite

Si crystal

Glass -soda

Concrete

Si nitrideAl oxide

PC

Wood( grain)

AFRE( fibers) *

CFRE*

GFRE*

Glass fibers only

Carbon fibers only

Aramid fibers only

Epoxy only

0.4

0.8

2

4

6

10

2 0

4 06 0

8 010 0

2 00

6 008 00

10 001200

4 00

Tin

Cu alloys

Tungsten

Si carbide

Diamond

PTFE

HDPE

LDPE

PP

Polyester

PSPET

CFRE( fibers) *

GFRE( fibers)*

GFRE(|| fibers)*

AFRE(|| fibers)*

CFRE(|| fibers)*

Metals

Alloys Ceramics PolymersComposites

/fibers

E

(GPa,

109 Pa)

Youngs

Modulus (E):

Metals:

40 400 GPa

Polymers:0.2 4GPa

Ceramics:

80 1200 GPa

1GPa = 103 MPa = 109 N/m2


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Effect of Temperature on Youngs Modulus (E)

E decreases with increase in temperature


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Yield strength

A plastically deformed structure, will

experience permanent change in shape and

may not be intended for good functionality.

Stress level at which plastic deformations

begins is known as yielding.

It is the point of linearity ofstress-strain

curve, shown as proportional limit.


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Yield Strength of Different Material Types

CeramicsMetals/Alloys

Composites/fibre

Polymers

Yieldstrength,

y(MPa)

PVC

Hardtomeasure

,

sinceintension,f

ractureusuallyoccursb

eforeyield.

Nylon 6,6

LDPE

70

20

40

6050

100

10

30

2 00

3 00

4 00

5 006 007 00

10 00

2 0 00

Tin (pure)

Al(6061)a

Al(6061)ag

Cu(71500)hrTa (pure)Ti (pure)aSteel (1020)hr

Steel (1020)cdSteel (4140)a

Steel (4140)qt

Ti (5Al-2.5Sn)aW(pure)

Mo (pure)Cu(71500)cw

Hardtomeasure,

inceramicmatrix

andepoxymatrixcomp

osites,since

intension,

fra

ctureusuallyoccursbeforeyield.

HDPEPP

humid

dry

PC

PET

Room Temp. Data

Based on data inTable B4,

Callister 7e.

a = annealed

hr = hot rolled

ag = aged

cd = cold drawncw = cold worked

qt = quenched &

tempered


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Tensile Strength (TS) or

Ultimate Tensile Strength (UTS)

y

strain

Typical response of a metal

F= fracture or

ultimate

strength

Neck acts

as stress

concentratorEngineering

TS

stress

Engineering strain

TS / UTS: Maximum stress on an engineering stress-strain curve

Adapted from Fig. 6.11,

Callister 7e.

Metals: when noticeable necking starts.


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Ductility

Ductility is a measure ofdegree of plastic

deformation that has been sustained at fracture.

A material that experiences very little or no

plastic deformation upon fracture is termed

brittle.

Ductility may be expressed quantitatively as

percent elongation orpercent reduction in area.

%EL is the percentage of plastic strain at fracture.


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Ductility

Plastic tensile strain at failure x 100L

LLEL%

oof

LfAo AfLo

Engineering tensile strain,

Engineering

tensile

stress,

smaller %EL

larger %EL

Another ductility measure: 100xA

AARA%

o

fo-

=


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Brittle fracture: elastic energy

Ductile fracture: elastic + plastic energy

Very low toughness:unreinforced polymers

Engineering tensile strain,

Engineeringtensile

stress,

Low toughness: ceramicsHigh toughness: metals

Toughness / Fracture Toughness

Energy to break a unit volume of material

Approximated by the area under the stress-strain curve

Why are metals/alloys

and reinforced plastic

so popular as structural

materials?


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Mechanical Properties and Testing

LEARNING OBJECTIVES: Part II

Materials response to:

Excessive Loading:Tensile Test

Localized Loading: Hardness Test

Sudden Intense Loading: Impact Test

Loading at High Temperatures: Creep Test Cyclic Loading: Fatigue Test


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Tensile Test

Tests are performed as per the ASTM, BS or

Australian Standards.

A tensile test measures the resistance of amaterial to a static or slowly applied force.

A machined specimen is placed in the testingmachine and load is applied.

A strain gage or extensometeris used tomeasure elongation.

The stress obtained at the highest applied forceis the Tensile Strength.


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Test providesdata:strength,stiffness,

ductility

Tensile Test


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Other Tensile Test Data

Yield Strength: The stress at which aprescribed amount ofplastic deformation(commonly, 0.2%) is produced.

Elongation: The extent to which thespecimen stretches before fracture.


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Tensile Properties: Effect of Temperature

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Handout 3 Materials Corrosion