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FCP 1
Single primary slip system
S
S
F. V. Lawrence
Mechanisms of Fatigue CrackInitiation and Growth
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FCP 2
Fatigue Mechanisms
! Fatigue Crack Initiation Mechanisms! Fatigue Crack Growth Mechanisms
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FCP 3
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 4
Planar or wavy slip?
d =G b2 b3( )
2π γ
Material γ Stacking Fault Energy ergs cm- 2
Aluminum 250
Iron 200
Nickel 200
Copper 90
Gold 75
Silver 25
Stainless Steel <10
α Brass <10
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FCP 5
Stacking-fault energy effects
Planarslip inCu-Al
Wavyslip insteel
Cu-Al alloys, Cu-Zn, Aust. SS
Ni, Cu, Al Fe
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FCP 6
Development of cell structures
γ = 10-3
γ =10-5Dislocation cell structuresin copper
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FCP 7
Planar and wavy slip materials
Wavy slip materials Planar slip materials
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FCP 8
Cyclic Slip - initial arrangements
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FCP 9
Cyclic Hardening
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FCP 10
Events leading to crack initiation•Development of cell structures (hardening)•Increase in stress amplitude (under strain control)•Break down of cell structure to form PSBs•Localization of slip in PSBs
PSB
cyclic hardening cyclic softening
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FCP 11
Crack initiation
Fatigue crack initiation at an inclusionCyclic slip steps (PSB)Fatigue crack initiation at a PSB
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FCP 12
Effects of strength and ductility
0.001
0.01
0.1
1
1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05 1.00E+06 1.00E+07
A36
5456-H311
Ti-6AL-4V
HY-80
Reversals, 2Nf
Str
ain
Am
pitu
de,²
e/2
1.00E-04
1.00E-03
1.00E-02
1.00E-01
1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05 1.00E+06 1.00E+07
1100
2014
2024
5456
7075
1015
4340
Reversals, 2Nf
Str
ain
Am
pltu
de,²
et/2
•Strong materialsgive the best fatigueresistance at longlives; whereas, ductilematerials give thebest fatigue resistanceat short lives
Straincontrolledtest onsmoothspecimen
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FCP 13
High-cycle fatigue Strength
Strongermaterials resistcrack initiationbetter.
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FCP 14
Fatigue Mechanisms
! Fatigue Crack Initiation Mechanisms! Fatigue Crack Growth Mechanisms
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FCP 15
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 16
Cyclic plastic zone size
rc =1π
∆KI
2σy'
2
Cyclic plastic zone is the region ahead of a growing fatigue crackin which slip takes place. Its size relative to the microstructuredetermines the behavior of the fatigue crack, i.e.. Stage I andStage II behavior.
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FCP 17
Stage I crack growth
Single primary slip system
individual grain
near - tip plastic zone
S
S
Stage I crack growth (rc ≤ d) is stronglyaffected by slip characteristics,microstructure dimensions, stress level,extent of near tip plasticity
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FCP 18
Stage II crack growth
Stage II crack growth (rc >> d)
Fatigue crackgrowing inPlexiglas
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FCP 19
Ferritic-Pearlitic steels all have about the same crack growth rates
Behavior of Structural Materials
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FCP 20
The fatigue crack growth rates for Al and Ti are much more rapid thansteel for a given ∆K. However, when normalized by Young’s Modulus allmetals exhibit about the same behavior.
Crack Growth Rates of Metals
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FCP 21
Crack closure
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 22
Crack closure
U =∆K eff
∆K=
Smax − Sopen
Smax − Smin=
11− R
1−Sopen
Smax
Initial crack length
A''A A'
A, A', A'' Crack tip positions
Plastic zones for crackpositions A...A”
Plastic wake
∆Keff = U ∆K
Plasticity induced crack closure (PICC)
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FCP 23
Crack Closure Mechanisms
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FCP 24
Intrinsic, extrinsic crack closure
dadn
= C ∆K( )m K max( )p ExtrinsicIntrinsic
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FCP 25
Aluminum - crack growth
•Orientation of microstructural texture•Grain size•Strength•Environment
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FCP 26
Subcritical Crack Growth
! Subcritical Crack Growth! Measuring Crack Growth! Use of Paris Power Law! Variable Amplitude Loads! Crack Closure! Small Cracks! Environmental Effects
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FCP 27
Long cracks, short cracks
mechanically short crack - no closure
long crack
nucleation - coalescence
roughness induced crack closure
How fatigue cracksgrow andparticularly the 3-Daspects of fatiguecrack growth is notfully understood.
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FCP 28
Short Cracks, Long Cracks
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FCP 29
Crack Growth at a Notch
Cracks growing from notchesdon’t know that that stressfield they are experiencing isconfined to the notch root.
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FCP 30
Growth of Small Cracks
Here the ∆K is the remote stress intensity factor basedon remote stresses….
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FCP 31
Effects of Environment
A. Dissolution of crack tip.
B. Dissolution plus H+acceleration.
C. H+ acceleration
D. Corrosion products mayretard crack growth at low∆K.
A B
C D
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FCP 32
Optimum microstructure?Smooth specimen (Kt ≈ 1) - at long lives lifedominated by initiation so pick small, high-strength microstructures
Cracked specimen (Kt > 5) - in the absenceof tensile residuals and for near conditions,large grain size preferred
Notched Specimen (Kt ≈ 2) - at long livesinitiation and crack growth equally important.Avoid high tensile residuals therefore use lowerstrength materials
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FCP 33
Summary! Fatigue may be thought of as a failure of the average
stress concept; consequently, fatigue usually begins atstress concentrators which are most frequently at thesurface of a component.
! Fatigue is a localized process involving the nucleation andgrowth of cracks to failure.
! Fatigue is caused by plastic deformation.
! The cyclic deformation of metals is fundamentally differentfrom the monotonic deformation.
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FCP 34
Summary
! The greatest portion of the fatigue life is spentnucleating and growing a fatigue crack to a length atwhich it can be detected.
! The range of effective stress intensity factor, that is, theidea of crack closure allows the growth of fatigue cracksto be rationalized.
! The behavior of small cracks is in many respects quite
different from long cracks.