FCP 1

Single primary slip system

S

S

F. V. Lawrence

Fatigue Overview

FCP 2

Fatigue Overview

! History of Fatigue! Fatigue Overview! The Process of Fatigue

FCP 3

Fatigue-prone Machine

FCP 4

Welded Ship - 2 sink each day

FCP 5

Service Enviromnent?

FCP 6

Determining Service Stresses

FCP 7

In Better Days

FCP 8

When is the next short course?

FCP 9

Early History1829 Albert Effects of repeated

loads

1839 Poncelet “Fatigue”

1843 Rankine Effect of stressconcentrations

1860 Wöhler Systematicinvestigations

1886 Bauschinger Reversed deformationeffects

1903 Ewing & Humfrey Nucleation of “fatigue”cracks

1910 Basquin Endurance limit

FCP 10

1844 - Rankine

Stress Concentrations- Railroad Axles, the Versailles Accident

William John Macquorn RankineBorn: 2 July 1820 in Edinburgh, ScotlandDied: 24 Dec 1872 in Glasgow, Scotland

FCP 11

1860 - Wöhler

107

106

105

104

103

10210 0

10 1

10 2

10 3

failuresrun-outs

Fatigue life, N (cycles)

Stre

ssra

nge,

S(k

si)

Fatigue Limit

Stress-Life (SN) Diagrams

FCP 12

1886 - Bauschinger

Cyclic behavior of materialsBauschinger effect

FCP 13

1903 - Ewings and Humphries

Cyclicdeformationleads to thedevelopment(initiation) offatigue cracks….

Mosaic structure

FCP 14

Origins of Fatigue Cracks

Surfacerougheningthrough cyclicplastic strainsleads to thedevelopmentof fatiguecracks

FCP 15

Recent Developments

1945 Miner Accumulation offatigue “damage”

1954 Coffin & Manson Plastic strains causefatigue

1961 Paris Growth of fatiguecracks correlated with²K

1970 Elber Crack closure

1975 Pearson Behavior of smallcracks

FCP 16

1954 - Coffin and Manson

Plastic strainscause theaccumulation of“fatiguedamage.”

FCP 17

1961 - Paris

Growth of fatiguecracks related to therange in stress

intensity factor.

da/dN = C(∆K)n

FCP 18

1970 - Elber

Critical importance of crack closureand the phenomena which cause it.

S

Crack openCrack closed

FCP 19

1975 - Pearson

Surprisingbehaviorof smallfatiguecracks

FCP 20

1980’s - UIUC

Development of the localstrain approach.

Fatigue caused by notch-root stresses and strains.

Fatigue life consists ofboth crack nucleationand growth.

FCP 21

Fatigue Mechanisms

! History of Fatigue! Fatigue Overview! The Process of Fatigue

FCP 22

Surface Effects in Fatigue

Surface finish Reducing surface roughness reduces the notch root stresses.Smoother is better!

Designed-innotches

Designed-in notches are a major source of fatigue problems;notches are sites of stress and strain concentration.

Fabricationdefects

Fabrication defects particularly crack-like (planar) as opposedto rounded (volumetric) defects are very damaging whenoriented perpendicular to the applied stress.

Absolute size Because most of fatigue life is spent in making a very smallcrack a little bigger, larger bodies have shorter fatigue livesbecause of the larger spatial extent of the high stresses in theirnotch-root stress fields.

Aggressiveenvironments

Chemical attack can create pits at which fatigue cracks start.Corrosion can greatly reduce the portion of fatigue lifedevoted to fatigue crack initiation and + growth and thusgreatly reduce the fatigue strength at long lives.

FCP 23

Material Property Effects

Tensile, yieldstrength

Higher strength materials resist plastic deformation andhence have a higher fatigue strength at long lives. Mostductile materials perform better at very short fatigue lives.

Temperature Temperature has little influence except for the ductile-to-brittle transition in BCC metals which phenomenon leads to avery much smaller final flaw size. At high temperatures,creep damage may be superposed on fatigue damage.

Quality ofMaterial

Metallurgical defects such as inclusions, seams, internal tears,and segregated elements can initiate fatigue cracks.

Rate of testing At high frequencies, the metal component may be self-heatedby the imposed plastic deformation. At low frequencies,environmental effects may become more important.

FCP 24

Stress Effects in Fatigue

Stress range The basic cause of plastic deformation and consequently theaccumulation of fatigue damage.

Mean andresidual stress

Tensile mean and residual stresses aid the formation andgrowth of fatigue cracks.

Stress gradients Bending is a more favorable loading condition than axialloading because (surface initiating) fatigue cracks propagateinto lower stress environments.

FCP 25

It depends on who you are….

Example Envir. Life Failure Perspective

Welded bridgegirder

NaCl, variableload histories

106 large crack Large preexisting defects

(?), life limited by fatiguecrack growth.

Nuclear reactorpressure vessel

H2O, high temps,constant

amplitude loadhistories

105

to10

8

small crack Small preexisting defects, lifelimited by fatigue crackgrowth.

Automotive spotweld in sheetsteel

Variable loadhistories

103

to10

8

visible crack Initiation of fatigue cracksimportant, only limitedfatigue crack growthpossible.

Engine crankshaft

Constantamplitude load

histories

108 sudden,

catastrophicfailure

Initiation of fatigue cracksimportant.

FCP 26

Past Events, Future Behavior?

Design Failure Analysis

An attempt to predict the futurebased on expectations: anticipatedservice loads, component design.

An attempt to explain the past based onevidence: metallography, fractography,computed loads, service records

Motive: save money but avoidpremature fatigue failure.

Motive: assignment of blame, understandcauses so future failure can be avoided.

Methods: fatigue testing, structuralanalysis, fatigue life prediction usingempirical or analytical methods.

Methods: Fractography, materials tests,fracture mechanics.

FCP 27

Analysis of pastperformance

Assignment of fault: Who’s going to buy the new bridge?

Silver point bridge failure

Failureanalysis

FCP 28Service life exhausted

Failure analysis

FCP 29Avoidance of future problems

Design

FCP 30

Fatigue Mechanisms

! History of Fatigue! Fatigue Overview! The Process of Fatigue

FCP 31

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 32

Intrusions and extrusions

Original Surface

Persistent Slip Band

Stage I Fatigue Crack

Gliding Dislocations

Intrusion

Extrusion

Onemechanismfor thedevelopment(initiation) ofa fatigue

crack

FCP 33

Intrusions and extrusions

Intrusions andextrusions on thesurface of a Nispecimen

FCP 34

Fatigue crack growth

Plastic wake New plastic deformation

S

S

Rem

ote

Str

ess,

S

Time, t

Smax

S , Sop cl

S

S

Rem

ote

Str

ess,

S

Time, t

Smax

S , Sop cl

a.

b.

S = 0

S = Sop

FCP 35

Fatigue crack growth

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 36

Fatigue fracture surface

Scanning electronmicroscope image -striations clearly visible

Schematic drawing ofa fatigue fracturesurface

FCP 37

Paris Power Law

+

+

+

+

++

+

+

+

+

+

+

+

+

+

++

+

+

+

+

++

+

+C

1 10 100

m∆K

th

K C

Paris Power Law

da

dN= C (∆K)

m

Log Range in Stress Intensity Factor, ∆K (MPa√m)

Log

Cra

ckG

row

thR

ate,

da/d

N(m

/cyc

le)

10

10

10

10

10

10

-11

-10

-9

-8

-7

-6

I II III I Sensitive tomicrostructureand environment

II Paris power Law

III Approachingfracture when

Kmax ≈ KIC.