Fatigue Mechanisms

8/12/2019 Fatigue Mechanisms

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Single primary slip system

S

S

II Fatigue Mechanisms


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Current Theory of Fatigue

3. Intrusions,

extrusions

2. Persistent

slip bands

4. Stage I

(shear) fatigue

crack

5. Stage II

fatigue crack

1. Cyclic

slip


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Outline

n 1. Cyclic slip

n 2. Persistent slip bands (PSB)

n 3. Intrusions and extrusions

n 4. Stage I crack growth

n 5. Stage II crack growth


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Process of fatigue

3. Intrusions,

extrusions

2. Persistent

slip bands

4. Stage I

(shear) fatigue

crack

5. Stage II

fatigue crack

1. Cyclic

slip


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1. Cyclic slip

First cycle

Many cycles

later - cyclic

hardening

Cyclic slip occurs within agrain and therefore operates on

an atomic scale and are thus is

controlled by features seen at

that scale.


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1. Cyclic slip

d =G b2 b3( )

2

Material Stacking Fault Energy ergs cm-2

Aluminum 250

Iron 200

Nickel 200

Copper 90

Gold 75

Silv er 25

Stainl ess Steel


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1. Cyclic slip

Planar

slip in

Cu-Al

Wavy

slip insteel

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

Ni, Cu, Al Fe

n Stacking-fault energy effects


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1. Cyclic slip

Wavy slip materials Planar slip materials

n Planar and wavy slip materials


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1. Cyclic slip

= 10-3

=10-5

Dislocation cell structures

in copper

n Development of cell structures


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Example

5m

60 cycles

1,000 cycles

5,000 cycles

20,000 cycles

80,000 cycles

(failure)

OM surface OM etch pit

TEM

Stress range = 25 ksi.

n Fatigue of an Fe single crystal


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Outline

n 1. Cyclic slip

n 2. Persistent slip bands

(PSB)


n 4. Stage I crack growthn 5. Stage II crack growth


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2. Persistent slip bands (PSB)

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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Outline

n 1. Cyclic slip






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3. Intrusions and extrusions

Intrusions and

extrusions on the

surface of a Ni

specimen


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Cyclically hardened material

Extrusion


Intrusion



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Fatigue crack initiation at an inclusion

Cyclic slip steps (PSB)

Fatigue crack initiation at a PSB


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Outline

n 1. Cyclic slip






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4. Stage I crack growth

Single primary slip system

individual grain

near - tip plastic zone

S

S

Stage I fatigue cracks are the sizeof the grains and are thus

controlled by features seen at that

scale: grain boundaries, mean

stresses, environment.


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rc = 1 KI

2y'

2

Cyclic plastic zone is the region ahead of a growing fatigue crack

in which slip takes place. Its size relative to the microstructure

determines the behavior of the fatigue crack, i.e.. Stage I and

Stage II behavior.


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n Short Cracks, Long Cracks


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Cracks growing from notches

dont know that that stress

field they are experiencing isconfined to the notch root.


n

Crack Growthat a Notch


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Here the ?K is the remote stress intensity factor based on remote stresses.

n Growth of Small Cracks


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Outline

n 1. Cyclic slip






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5. Stage II crack growth

Plexiglas

Stage II fatigue cracks much largerthan the grain size and are thus

sensitive only to large scale

microstructural features - texture,

global residual stresses, etc.


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Example

n Stage II fatigue crack in a weldment


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Scanning electron

microscope image -

striations clearly visible

Schematic drawing of

a fatigue fracture

surface


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Plastic wake New plastic deformation

S

S

RemoteStress,

S

Time, t

Smax

S , Sop cl

S

S

R

emoteStress,S

Time, t

Smax

S , Sop cl

a.

b.

S = 0

S = Sop


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Plastic wake New plastic deformation

S

SR

emoteStress,S

Time, t

Smax

S , Sop cl

S

S

R

emoteStress,

S

Time, t

Smax

S , Sop cl

c.

d.

S = Smax

S = 0


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Elastic stresses near a crack tipS

c

x

y

xy

x

y

R

x =S c

2Rcos

2

1 sin2

sin32

=

KI2R

f1 ( )

y = S c2Rcos 2 1+ sin 2 sin 32 = KI

2Rf2 ( )

xy =S c

2Rcos

2

sin2

cos2

sin32

=

KI2R

f3 ( )

The magnitude stress field near a crack tip depends upon the stress intensity factor (KI).

Wouldnt it be nice if this quantity correlates with the speed with which fatigue cracks

grow? Lets see if it works! Rather, lets see if we can MAKE it work!

KYS aRange of stress intensity factor


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Geometry correction factor (Y)

2c

2c2c

c

S S

S S

S

SS

S

Y=1

Y=1.12

Y=2/p

W

Y = secc

W

Infinite width

center cracked

panel

Finite width

center cracked

panel

Edge

cracked

panel

Disc

shaped

crack in an

infinitebody

B


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Crack growth

rate (da/dN) is

related to the

crack tip stress

field and is thusstrongly

correlated with

the range of

stress intensity

factor:

(? K=Y? Svpa).

da

dN= C K( )n

Paris power law



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n Crack

ClosureMechanisms


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da

dn= C K( )m Kmax( )

p Extrinsic

Intrinsic


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Example

Orientation of microstructural texture

Grain size

Strength

Environment n Aluminum - crack

growth


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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 Dn Effects of

Environment


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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.

Example

n Crack Growth Rates of Metals


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Summary

n Fatigue is a complex process involving

many steps but it may be broken down

into the initiation and growth of fatigue

cracks.

n The growth of fatigue cracks is often

considered to be the most important

feature of fatigue from an engineeringperspective.

Documents

Fatigue Mechanisms