unit 1 - fracture
TRANSCRIPT
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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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Measured fracture strengths of fibres / whiskers
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
Measured fracture strengths of fibres / whiskers
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2. Plastic deformation and the accumulation of permanent damage
leading to ultimate failure
Measured fracture strengths of fibres / whiskers
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
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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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Ductile dimples: indicators of stressing mode
s 1
s 1s 2
s 2
Elongated and opposing = shear
Side view
Plan view
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Ductile dimples: indicators of stressing mode
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
Equi-axed dimples = tension
High chrome steel manganese sulphides
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Failure examples: ductile dimples around SiC p
initiating SiC particlesidentified
Equi-axed dimples = tension
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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
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
Examination questions Unit 1
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.