Mechanisms of Fatigue Crack Initiation and Growth

FCP 1

Single primary slip system

S

S

F. V. Lawrence

Mechanisms of Fatigue CrackInitiation and Growth

FCP 2

Fatigue Mechanisms

! Fatigue Crack Initiation Mechanisms! Fatigue Crack Growth Mechanisms

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

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

FCP 5

Stacking-fault energy effects

Planarslip inCu-Al

Wavyslip insteel

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

Ni, Cu, Al Fe

FCP 6

Development of cell structures

γ = 10-3

γ =10-5Dislocation cell structuresin copper

FCP 7

Planar and wavy slip materials

Wavy slip materials Planar slip materials

FCP 8

Cyclic Slip - initial arrangements

FCP 9

Cyclic Hardening

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

FCP 11

Crack initiation

Fatigue crack initiation at an inclusionCyclic slip steps (PSB)Fatigue crack initiation at a PSB

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

FCP 13

High-cycle fatigue Strength

Strongermaterials resistcrack initiationbetter.

FCP 14

Fatigue Mechanisms

! Fatigue Crack Initiation Mechanisms! Fatigue Crack Growth Mechanisms

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

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.

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

FCP 18

Stage II crack growth

Stage II crack growth (rc >> d)

Fatigue crackgrowing inPlexiglas

FCP 19

Ferritic-Pearlitic steels all have about the same crack growth rates

Behavior of Structural Materials

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

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

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)

FCP 23

Crack Closure Mechanisms

FCP 24

Intrinsic, extrinsic crack closure

dadn

= C ∆K( )m K max( )p ExtrinsicIntrinsic

FCP 25

Aluminum - crack growth

•Orientation of microstructural texture•Grain size•Strength•Environment

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

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.

FCP 28

Short Cracks, Long Cracks

FCP 29

Crack Growth at a Notch

Cracks growing from notchesdon’t know that that stressfield they are experiencing isconfined to the notch root.

FCP 30

Growth of Small Cracks

Here the ∆K is the remote stress intensity factor basedon remote stresses….

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

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

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.

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.