2 mechanisms
TRANSCRIPT
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FCP 1
Single primary slip system
S
S
F. V. Lawrence
Mechanisms of Fatigue Crack
Initiation 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 initiation
Stage I crack growthStage II crack growthFailure
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FCP 4
Planar or wavy slip?
d =G b2 b3( )
2
Material Stacking Fault Energy ergs cm- 2
Aluminum 250
Iron 200
Nickel 200
Copper 90
Gold 75
Silver 25
Stainless Steel
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FCP 5
Stacking-fault energy effects
Planarslip inCu-Al
Wavy
slip insteel
Cu-Al alloys, Cu-Zn, Aust. SS
Ni, Cu, Al Fe
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FCP 6
Development of cell structures
= 10-3
=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 9
Cyclic Hardening
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FCP 10
Events leading to crack initiationDevelopment of cell structures (hardening)
Increase in stress amplitude (under strain control)Break down of cell structure to form PSBsLocalization 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 13
High-cycle fatigue Strength
Strongermaterials resistcrack initiation
better.
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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 initiation
Stage I crack growthStage II crack growth
Failure
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FCP 16
Cyclic plastic zone size
rc =1
KI
2y'
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-Pearliticsteels allhave aboutthe samecrack growthrates
Behavior of Structural Materials
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FCP 20
The fatigue crack growth rates for Al and Ti are much more rapid than
steel for a given K. However, when normalized by Youngs Modulus all
metals 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
RemoteStre
ss,S
Time, t
Smax
S , Sop cl
S
S
RemoteStress,S
Time, 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=
1
1 R1
Sopen
Smax
Initial crack length
A''A A'
A, A', A'' Crack tip positions
Plastic zones for crack
positions 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
da
dn= C K( )m K max( )
p ExtrinsicIntrinsic
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FCP 25
Aluminum - crack growth
Orientation of microstructural textureGrain sizeStrengthEnvironment
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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 cracks
grow 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 notches
dont 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 based
on 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 life
dominated by initiation so pick small, high-strength microstructures
Cracked specimen (Kt > 5) - in the absence
of 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 lower
strength 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 at
stress concentrators which are most frequently at thesurface of a component.
! Fatigue is a localized process involving the nucleation and
growth 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 spent
nucleating and growing a fatigue crack to a length atwhich it can be detected.
! The range of effective stress intensity factor, that is, the
idea 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.