2 mechanisms

7/31/2019 2 Mechanisms

1/34

FCP 1

Single primary slip system

S

S

F. V. Lawrence

Mechanisms of Fatigue Crack

Initiation and Growth


2/34

FCP 2

Fatigue Mechanisms

! Fatigue Crack Initiation Mechanisms! Fatigue Crack Growth Mechanisms


3/34

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


4/34

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


5/34

FCP 5

Stacking-fault energy effects

Planarslip inCu-Al

Wavy

slip insteel

Cu-Al alloys, Cu-Zn, Aust. SS

Ni, Cu, Al Fe


6/34

FCP 6

Development of cell structures

= 10-3

=10-5Dislocation cell structuresin copper


7/34

FCP 7

Planar and wavy slip materials

Wavy slip materials Planar slip materials


8/34


9/34

FCP 9

Cyclic Hardening


10/34

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


11/34

FCP 11

Crack initiation

Fatigue crack initiation at an inclusionCyclic slip steps (PSB)

Fatigue crack initiation at a PSB


12/34


13/34

FCP 13

High-cycle fatigue Strength

Strongermaterials resistcrack initiation

better.


14/34

FCP 14

Fatigue Mechanisms

! Fatigue Crack Initiation Mechanisms! Fatigue Crack Growth Mechanisms


15/34

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


16/34

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.


17/34

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


18/34

FCP 18

Stage II crack growth

Stage II crack growth (rc >> d)

Fatigue crackgrowing inPlexiglas


19/34

FCP 19

Ferritic-Pearliticsteels allhave aboutthe samecrack growthrates

Behavior of Structural Materials


20/34

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


21/34

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


22/34

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)


23/34

FCP 23

Crack Closure Mechanisms


24/34

FCP 24

Intrinsic, extrinsic crack closure

da

dn= C K( )m K max( )

p ExtrinsicIntrinsic


25/34

FCP 25

Aluminum - crack growth

Orientation of microstructural textureGrain sizeStrengthEnvironment


26/34

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


27/34

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.


28/34

FCP 28

Short Cracks, Long Cracks


29/34

FCP 29

Crack Growth at a Notch

Cracks growing from notches

dont know that that stressfield they are experiencing isconfined to the notch root.


30/34

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


33/34

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.


34/34

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.

2 mechanisms

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