fatigue overview - illinois
TRANSCRIPT
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FCP 1
Single primary slip system
S
S
F. V. Lawrence
Fatigue Overview
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FCP 2
Fatigue Overview
! History of Fatigue! Fatigue Overview! The Process of Fatigue
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FCP 3
Fatigue-prone Machine
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FCP 4
Welded Ship - 2 sink each day
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FCP 5
Service Enviromnent?
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FCP 6
Determining Service Stresses
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FCP 7
In Better Days
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FCP 8
When is the next short course?
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FCP 9
Early History1829 Albert Effects of repeated
loads
1839 Poncelet “Fatigue”
1843 Rankine Effect of stressconcentrations
1860 Wöhler Systematicinvestigations
1886 Bauschinger Reversed deformationeffects
1903 Ewing & Humfrey Nucleation of “fatigue”cracks
1910 Basquin Endurance limit
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FCP 10
1844 - Rankine
Stress Concentrations- Railroad Axles, the Versailles Accident
William John Macquorn RankineBorn: 2 July 1820 in Edinburgh, ScotlandDied: 24 Dec 1872 in Glasgow, Scotland
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FCP 11
1860 - Wöhler
107
106
105
104
103
10210 0
10 1
10 2
10 3
failuresrun-outs
Fatigue life, N (cycles)
Stre
ssra
nge,
S(k
si)
Fatigue Limit
Stress-Life (SN) Diagrams
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FCP 12
1886 - Bauschinger
Cyclic behavior of materialsBauschinger effect
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FCP 13
1903 - Ewings and Humphries
Cyclicdeformationleads to thedevelopment(initiation) offatigue cracks….
Mosaic structure
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FCP 14
Origins of Fatigue Cracks
Surfacerougheningthrough cyclicplastic strainsleads to thedevelopmentof fatiguecracks
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FCP 15
Recent Developments
1945 Miner Accumulation offatigue “damage”
1954 Coffin & Manson Plastic strains causefatigue
1961 Paris Growth of fatiguecracks correlated with²K
1970 Elber Crack closure
1975 Pearson Behavior of smallcracks
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FCP 16
1954 - Coffin and Manson
Plastic strainscause theaccumulation of“fatiguedamage.”
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FCP 17
1961 - Paris
Growth of fatiguecracks related to therange in stress
intensity factor.
da/dN = C(∆K)n
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FCP 18
1970 - Elber
Critical importance of crack closureand the phenomena which cause it.
S
Crack openCrack closed
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FCP 19
1975 - Pearson
Surprisingbehaviorof smallfatiguecracks
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FCP 20
1980’s - UIUC
Development of the localstrain approach.
Fatigue caused by notch-root stresses and strains.
Fatigue life consists ofboth crack nucleationand growth.
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FCP 21
Fatigue Mechanisms
! History of Fatigue! Fatigue Overview! The Process of Fatigue
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FCP 22
Surface Effects in Fatigue
Surface finish Reducing surface roughness reduces the notch root stresses.Smoother is better!
Designed-innotches
Designed-in notches are a major source of fatigue problems;notches are sites of stress and strain concentration.
Fabricationdefects
Fabrication defects particularly crack-like (planar) as opposedto rounded (volumetric) defects are very damaging whenoriented perpendicular to the applied stress.
Absolute size Because most of fatigue life is spent in making a very smallcrack a little bigger, larger bodies have shorter fatigue livesbecause of the larger spatial extent of the high stresses in theirnotch-root stress fields.
Aggressiveenvironments
Chemical attack can create pits at which fatigue cracks start.Corrosion can greatly reduce the portion of fatigue lifedevoted to fatigue crack initiation and + growth and thusgreatly reduce the fatigue strength at long lives.
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FCP 23
Material Property Effects
Tensile, yieldstrength
Higher strength materials resist plastic deformation andhence have a higher fatigue strength at long lives. Mostductile materials perform better at very short fatigue lives.
Temperature Temperature has little influence except for the ductile-to-brittle transition in BCC metals which phenomenon leads to avery much smaller final flaw size. At high temperatures,creep damage may be superposed on fatigue damage.
Quality ofMaterial
Metallurgical defects such as inclusions, seams, internal tears,and segregated elements can initiate fatigue cracks.
Rate of testing At high frequencies, the metal component may be self-heatedby the imposed plastic deformation. At low frequencies,environmental effects may become more important.
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FCP 24
Stress Effects in Fatigue
Stress range The basic cause of plastic deformation and consequently theaccumulation of fatigue damage.
Mean andresidual stress
Tensile mean and residual stresses aid the formation andgrowth of fatigue cracks.
Stress gradients Bending is a more favorable loading condition than axialloading because (surface initiating) fatigue cracks propagateinto lower stress environments.
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FCP 25
It depends on who you are….
Example Envir. Life Failure Perspective
Welded bridgegirder
NaCl, variableload histories
106 large crack Large preexisting defects
(?), life limited by fatiguecrack growth.
Nuclear reactorpressure vessel
H2O, high temps,constant
amplitude loadhistories
105
to10
8
small crack Small preexisting defects, lifelimited by fatigue crackgrowth.
Automotive spotweld in sheetsteel
Variable loadhistories
103
to10
8
visible crack Initiation of fatigue cracksimportant, only limitedfatigue crack growthpossible.
Engine crankshaft
Constantamplitude load
histories
108 sudden,
catastrophicfailure
Initiation of fatigue cracksimportant.
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FCP 26
Past Events, Future Behavior?
Design Failure Analysis
An attempt to predict the futurebased on expectations: anticipatedservice loads, component design.
An attempt to explain the past based onevidence: metallography, fractography,computed loads, service records
Motive: save money but avoidpremature fatigue failure.
Motive: assignment of blame, understandcauses so future failure can be avoided.
Methods: fatigue testing, structuralanalysis, fatigue life prediction usingempirical or analytical methods.
Methods: Fractography, materials tests,fracture mechanics.
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FCP 27
Analysis of pastperformance
Assignment of fault: Who’s going to buy the new bridge?
Silver point bridge failure
Failureanalysis
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FCP 28Service life exhausted
Failure analysis
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FCP 29Avoidance of future problems
Design
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FCP 30
Fatigue Mechanisms
! History of Fatigue! Fatigue Overview! The Process of Fatigue
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FCP 31
Process of fatigue
Stage II fatigue crack
Stage I fatigue crackIntrusions andextrusions(SurfaceRoughening) Persistent Slip Band
(Embryonic Stage I Fatigue Cracks)
Cyclic slipCrack initiationStage I crack growthStage II crack growthFailure
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FCP 32
Intrusions and extrusions
Original Surface
Persistent Slip Band
Stage I Fatigue Crack
Gliding Dislocations
Intrusion
Extrusion
Onemechanismfor thedevelopment(initiation) ofa fatigue
crack
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FCP 33
Intrusions and extrusions
Intrusions andextrusions on thesurface of a Nispecimen
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FCP 34
Fatigue crack growth
Plastic wake New plastic deformation
S
S
Rem
ote
Str
ess,
S
Time, t
Smax
S , Sop cl
S
S
Rem
ote
Str
ess,
S
Time, t
Smax
S , Sop cl
a.
b.
S = 0
S = Sop
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FCP 35
Fatigue crack growth
Plastic wake New plastic deformation
S
S
Rem
ote
Str
ess,
S
Time, t
Smax
S , Sop cl
S
S
Rem
ote
Str
ess,
STime, t
Smax
S , Sop cl
c.
d.
S = Smax
S = 0
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FCP 36
Fatigue fracture surface
Scanning electronmicroscope image -striations clearly visible
Schematic drawing ofa fatigue fracturesurface
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FCP 37
Paris Power Law
+
+
+
+
++
+
+
+
+
+
+
+
+
+
++
+
+
+
+
++
+
+C
1 10 100
m∆K
th
K C
Paris Power Law
da
dN= C (∆K)
m
Log Range in Stress Intensity Factor, ∆K (MPa√m)
Log
Cra
ckG
row
thR
ate,
da/d
N(m
/cyc
le)
10
10
10
10
10
10
-11
-10
-9
-8
-7
-6
I II III I Sensitive tomicrostructureand environment
II Paris power Law
III Approachingfracture when
Kmax ≈ KIC.