unit 1 - fracture

8/10/2019 Unit 1 - Fracture

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EG-381Mechanical Properties 3(Fatigue and Fracture)

Dr. Richard Johnston


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Mechanical Properties 3

(Fatigue and Fracture)

Total credits = 10

2 hour examination (Jan 2012)Answer 3 from 4 questions

2 questions from each sub module:

1. Fracture mechanics Static (weeks 1 to 5, REJ)2. Fracture mechanics Fatigue (weeks 6-11, DHI)

EG-381 Credits & examination


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Lecturers: Units 1 to 4: Dr Richard Johnston, Rm 957

Units 5 to 7: Dr David Isaac, Rm 979

Recommended texts: 1 . Mechanical Metallurgy by G.E. Dieter 2. Fatigue of Materials by S. Suresh (2nd edition 1998)

3. Materials Science & Engineering by W. D. Callister

Web site:www.tech.plymouth.ac.uk/sme/interactive_resources/index.html

Course Videos:Last of the liberties , OU Facts on fracture , Welding Institute Living with cracks , OU

Course notes provided for revision purposes backgroundreading and attendance at lectures necessary to gainexperience in failure analysis and case studies.

EG-381 Course support

A. no you cant have a copyof my lecture presentations


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1. Micro-mechanisms of fracture

2. Cracks in structures: energy balance

3. Stress intensity factor: LEFM

4. Fracture toughness

5. Design against fatigue: mechanisms ofcyclic fracture

6. Stress and strain dependence of fatigue

7. Fatigue crack propagation

EG-381 Course units

STATIC}

CYCLIC

}


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1. Build on previous L1 and L2 mechanical properties courses

2. Extend studies to include fracture behaviour in metals andalloys at T


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Objectives 1. Classify types of fracture

2. Define their characteristic features

3. Factors which influence fracture behaviour

(strain rate, temperature, environment)

4. Fracture mechanism maps

EG-381 Unit 1 Micro-mechanisms of fracture


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Introduction

Q. Cylindrical laboratory test piece of any given engineeringmaterial what are the types of failure mode it could experience ?

Q. How are these influenced by the rate of loading, testtemperature and environment ?

Largely restrict our interest to uni-axial tension

Characteristic features associated with each of the failure modes

Basis for failure investigations of engineering components -common for a failure investigator to be given no information onstress axiality, magnitude, operating conditions they must reachconclusions from fracture surface examinations alone !

Develop fracture maps usually in terms of stress and temperature useful for comparisons with real failures


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Defined as the maximum stress required to promote failure ina perfect crystal ( i.e. one containing no defects ).

Crystal is made of perfect BCC/FCC/HCP units

s c determined by calculation of the tensile force required topull atoms apart

Ideal or theoretical strength, s c

BCC FCC HCP


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Approximation of upper limiting strength:

s c ~ E / p

where E=elastic or Youngs modulus

Generally measured fracture stress is significantlylower than the theoretical stress calculated to

separate atomic planes.The closest possible agreement with theoreticalvalues is found for very fine fibres or whiskers ..

Ideal or theoretical strength, s c


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Definitions:Whisker: L < 10d

Fibre: L > 10d(d = small)

Measured fracture strengths of fibres / whiskers

Material Modulus[ GPa ]

Theoretical s c [ GPa ]

Measured s c [ GPa ]

Silica fibres 97.1 30.9 24.1

Iron whiskers 295.2 93.9 13.1

Silicon whiskers 165.7 52.7 6.47

Alumina whiskers 496.2 157.9 15.2

Steel wire 200.1 63.7 2.75

Q. Why so low ?


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measurements are very sensitive to section sizeSame would apply to longer fibres probability of damage increases !

silica and zinc oxide whiskers

Whisker diameter [ m ]

0 10 20 30 40Fracture stress,

s f [ % of E]

0

1

2

3

4


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Pristine glass fibre


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Low fracture stresses measured in real materials ?

1. Real materials contain various defects:

crystallographic point or line defects (dislocations)grain & low angle boundaries

2nd phase constituents e.g. precipitates, particles or fibres control of stiffness / toughness / ductility

processing defects e.g. pores, inclusions, lack of weld fusion



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2. Plastic deformation and the accumulation of permanent damage

leading to ultimate failure


strain

ac tual

s

s t r e s

s

Kt s max

ela s t ic m odulu s

= c ons tan t

m onot on ic s t res s -s t ra in curv e

s

yielding

Low fracture stressesmeasured in real materials ?


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Fracture behaviour sensitive to temperature

low temperature regime plasticity not affected by timeon load high temperature regime additional time dependent

creep effects

Plasticity affected by time dependent mechanisms aboveapprox. 0.3 T m

Pure Metal T m ( oC) 0.3 T m ( oC) Al 660 198Cu 1083 325

Ni 1453 435Fe 1536 460Ti 1670 500

Brittle / ductile behaviour is controlled by PLASTICITYbut can occur in either low or high temperature regimes

Low and high temperature fracture regimes

alloyingaffects T m


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In either temperature regime, crystalline solids subjected to tensileloading will fail in either a brittle or ductile fashion

Failure classifications under monotonic load

0

. 3 T

m


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Failure classifications: ductile fracture (idealised)

Polycrystaline(pure)

Single crystal

n.b. pure materials no inclusions or second phase particles

htt : www. outube.com watch?v=7kfbxJ 8z w
http://www.youtube.com/watch?v=7kfbxJy8zgwhttp://www.youtube.com/watch?v=7kfbxJy8zgw


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Failure examples: ductile cup & cone (reality)

matrix/particle de-cohesion coalescence necking - shear lips


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Failure examples: cup & cone (engineering alloys)

ultra high strength steel, 20 oC, UTS~2000 MPa, f ~20%


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Failure examples: ductile s curve

300M11: Engineering stress & strain

0

500

1000

1500

2000

2500

0 0.05 0.1 0.15 0.2 0.25

Strain

S t r e s s , M

P a

Easily avoided by design / ductile failures rare

i.e. operate within elastic region

Elastic ~ 1 %


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Ductile dimples: indicators of stressing mode

s 1

s 1

Equi-axed dimples = tension

Side view

Plan view


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s 1

s 1s 2

s 2

Elongated and opposing = shear

Side view

Plan view


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s 1

s2

s 3

Elongated and similar = bend

Side view

Plan view


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Failure examples: microscopic dimples

secondary electron back scattered

initiating particles identified


High chrome steel manganese sulphides


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Failure examples: ductile dimples around SiC p

initiating SiC particlesidentified



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Failure examples: elongated voids + carbides

BuRTi - shear or bend ???


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1. Brittle fracture mechanisms

2. Case study in brittle fracture Liberty ships

3. Creep fracture

Unit 1 Lecture #2


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Contrasting behaviour to ductile overload

Catastrophic, rapid event

Minimal or no plastic deformation preceding the failurei.e. no gross ductility / necking

Brittle fractures can be inter or transgranular

Dramatic service failures e.g. Liberty Ships

Brittle fracture: key characteristics


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Al-Zn-Mg alloy - secondary cracks common, 3D relief.No aggressive environment (i.e. not stress corrosion)De-cohesion between grains possibly due to impurity elements at the

boundaries - segregation and precipitation during extended service periods

Brittle fracture: intergranular


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Transgranular facets cleavage low energy crystallographic planescommon in BCC materials (iron), HCP (titanium or zinc), ioniccrystals (NaCl) and co-valent bonded materials (ceramics).FCC metals (copper and aluminium) are only prone to cleavageevents under extreme environmental conditions

Brittle fracture: transgranular

e.g TiAl intermetallic

20 m


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Failure examples: brittle s curve.

TiAl, 20C

0

100

200

300

400

500

600

700

800

900

1000

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

strain [%]

s t r e s s [ M

P a

]

Catastrophic failure

UTS at peak of curve


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Brittle fracture: transgranular crack growth

Crack grows on low energy cleavage planes

Grain orientations crack deviates

micro = shiny facets / macro = flat fractures


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Brittle fracture: transgranular crack growth

individual facets = grains

local growth direction river markings fan out


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Brittle fracture: thin sections

Chevrons point back to initiation site

10 mm

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Brittle fracture: classifications

Cleavage 1

(or BIF1)

No general plasticity, small,inherent flaws. Can occur atstresses below yield if largeflaw. Strength controlled bylargest flaw - greatest stressconcentrator

Cleavage 2

(or BIF2)

Pre-existing flaws extremelysmall scale or absent. Stressneeds to be at yield or above in

order to initiate its own defectthrough deformation. Micro-plasticity p remains approx. 1%or less

Cleavage 3

(or BIF3)

Preceded by substantial strain -1 - 10%. Fracture encouragedby work hardening - restricts

deformation. Or due toformation of large crack-likedefects following extensive slipover long slip band lengths.More prevalent at hightemperature.

Stress /

strain

Defect

size

increasing decreasing

BIF = brittle intergranular fracture


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The Liberty Ships

Last of the Liberties - Open University video T353/1 (Mr Peter Davies)

Classic example of brittle fracture

Salient points to consider during film:

Material grade

Temperature

Design featuresManufacturingDefects

Fractography

Brittle fracture: case study


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1. Impact test techniques

2. Ductile to brittle transition

3. Time dependent creep

4. Fracture maps

Unit 1: lecture 3


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Toughness material property:

absorption of energy during fracture

Laboratory / service - inconsistencies in behaviourImpact tests developed - replicate most severe in service conditionsoften associated with brittle fractures in the field:

1. Deformation at relatively low temperature2. High strain rate3. Tri-axial stress state (notch / s concentrator / constraint / plane

strain)

Two standards developed Charpy (CVN) and Izod Still employed to measure impact energy (or notchtoughness)

Impact testing: requirements


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Both employ square sectionbars + V notch

Difference in grip / supportand impact position

http://www.steeluniversity.org/content/html/eng/defau

lt.asp?catid=151&pageid=2081271964

Measure difference inpotential energy ofpendulum before and after

Impact testing: apparatus

ASTM D256


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Results from impact tests are qualitative involve both initiationand propagation of cracks.Used to rank materials

Results do not quantify the fracture toughness, K 1C (used fordefect tolerant design assumes crack pre-exists loading)

More complicated (and expensive) types of fracture toughnesstesting (e.g. plane strain compact tension to be introduced inlater Units)

Greatest use define ductile to brittle transitions in materials &temperature range for transitions

Impact testing: considerations


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Curve A defines narrow transition band

Greatest impact energy in high T regime

High energy ductile mechanism

Low energy brittle mechanism

Fracture appearance (curve B = % area shear features)

Transition curves

n.b.

relatively brittle at RT !

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Exact transition temperaturedifficult to define

Quote T at given impact energyfrom curve A

Quote T at 50% fractureappearance transitiontemperature (F.A.T.T.) fromcurve B

Neither are accurate B verysubjective

Conservative approach quoteT at initial reduction in energy

Transition temperature: definitions


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% fibrous c/w shiny ( microvoids vs facets)

Transition temperature: fracture appearance

Temperature increasing


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Ductility defined by degree of lateral expansion of CVN specimens

Transition temperature: alternative approach

I m p a c t e n e r g y

[ J ]


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FCCs remain ductile throughout T range NO TRANSITION

Steels relative improvement in toughness

Trade off with yield strength (i.e. encourages plastic deformation /ductile mechanism)

Transition behaviour: material variations

I m p a c t e n e r g y

[ J ]


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Generally low strain rate moves TT to lower values (curve shifts left) i.e.ductility encouraged by slow loading, remains ductile to lower TSlow strain rate plasticity / dislocations accommodated within crystal structures

Transition behaviour: controlling parameters

temperature

fastslow

temperature


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Specific effects of impurities:

Increasing Carbon detrimental

Increasing Manganese beneficial

Ignore rate/grain size/impurity examples in the notes !

Transition behaviour: controlling parameters


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In either temperature regime, crystalline solids subjected to tensileloading will fail in either a brittle or ductile fashion

Failure classifications under monotonic load

< 0

. 3 T

m

t e m p e r a

t u r e

> 0

. 3 T

m


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Time dependent creep fracture: deformation curve

Static stress, intergranular & transgranular modes

High temperature regime T > 0.3*T m

Time independentT < 0.3Tm


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0

200

400

600

800

1000

1200

1400

0.1 1 10 100 1000 10000Time to failure, hr

S t r e s s

( M

P a

)

650700

750

760

790

850

Stress rupture data


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Creep fracture: grain boundary mechanisms

Triple point cracking: high stresses

Cavitation: low & high stress


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Creep fracture: cavitation micro mechanisms

All mechanisms invoke shear +/- multi-axial stresses


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Creep fracture: metallographic sections


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Creep damage: RR1000, 750 oC


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Creep damage: RR1000, 750 oC


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b


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fanintake

-60 oC

IPcompressor

300 oC

HP

compressor600 oC

combustor1200 oC

TET >1200 oC

Gas turbine: creep sensitive regions

G bi i i ll


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Gas turbine: creep sensitive alloys

C f bi bl d f il


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Creep fracture: turbine blade failures

Intergranular HT creep failure - BRITTLE

Nickel HPT alloy

F il l ifi i f


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a. Ashby (Cambridge University)method of displaying characteristic failure modes as a function ofthe dominant variables stress and temperature

one important difference NO BRITTLE / CLEAVAGE in FCC

Failure classification: fracture maps

FCC BCC

b-d

F il l ifi ti f t ( l i )


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Largely based on literature searches

Failure classification: fracture maps (e.g. alumina)

Alt ti f il d


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>80% of engineeringcomponents fail due to fatigue

the cyclic application of strainor stress at levels below UTS,characterised by initiation andprogressive growth of cracks

stress corrosion progressivegrowth of cracks under aconstant stress (or strain) andexposed to aggressiveenvironments (e.g. common insteels & aluminium / saline &humid conditions) : Canberra

bomber case study from EG-283

Alternative failure modes

10

m

C l i t U it 1


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Conclusions to Unit 1

1. Defined low & high temperature fracture regimes

2. Brittle & ductile failure mechanisms prevail in each (i.e. bothfound below 0.3 * Tm)

3. Ductile - gross plastic deformation , localised slip, reduction inarea, microvoid formation around precipitates/particles,dull/fibrous appearance

4. Void forms stress mode indicators

5. Brittle minimal plastic deformation , cleavage/facets,transgranular, flat/shiny appearance

6. Case study for brittle fracture The Liberty Ships7. Impact tests define ductile to brittle transitions comparative

measurements of toughness

8. Failures classified using fracture maps (stress & temperature)

E amination q estions Unit 1


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Describe the characteristic form of fracture demonstrated by the LibertyShips. How did the prevailing social and economic factors contribute to thedesign and manufacture of these vessels and ultimately compromise theirmechanical performance.

It is possible that the Liberty Ship failures would have been avoided if theambient in-service temperatures had been higher. Describe the transition inthe fracture behaviour of certain materials which supports this statement.

Explain why the measured strengths of common engineering materials aresignificantly lower than those predicted from elasticity theory.

Describe a practical impact technique for the measurement of toughnessand how this technique is employed to characterise the ductile to brittletransition noted in certain materials.

Examination questions Unit 1



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The following data were measured for two specific steels designated A and B:

Plot the data in a suitable format to illustrate the transition from ductile to brittlebehaviour for each steel

Assuming the ductile to brittle transition temperature T t is given by the average ofthe maximum and minimum impact energies, quote the transition temperature foreach steel. Suggest a more conservative criterion for defining T t in each steelWhich of the two steels would be suitable for operation in environments wheretemperatures occasionally dropped below room temperature, justifying yourselection


Steel A Steel B

Temp ( oC) Impact Energy (J) Temp ( oC) Impact Energy (J)

30 104 75 76

-15 104 50 76

-50 103 35 71

-75 97 25 58

-100 63 10 38

-113 40 0 23-125 34 -10 14

-150 28 -20 9

-175 25 -30 5

-200 24 -40 1.5

Video: Facts on Fracture (TWI)


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Video: Facts on Fracture (TWI)

Points to note:

1. Charpy tests brittle fracture appearance = shiny

2. Constraint notches / section size / plane strain & plane strain

3. Alternative test methods 3 point impact / large plates (cost !)

4. Crack opening displacement (COD) remember for later units.

unit 1 - fracture

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