Fatigue, How and Why Physics of Fatigue Professor Darrell ... · Physics of Fatigue Professor Darrell F. Socie ... BS L65 Aluminum Loading: 63 ksi, ... 7075-T6 Cracking at Inclusion

Fatigue, How and Why Physics of Fatigue

Professor Darrell F. Socie Mechanical Science and Engineering

University of Illinois

© 2004-2013 Darrell Socie, All Rights Reserved

Fatigue and Fracture

Fatigue, How and Why © 2004-2013 Darrell Socie, All Rights Reserved 1 of 141

Fatigue, How and Why

Physics of Fatigue Material Properties Similitude Fatigue Calculator


10-10 10-8 10-6 10-4 10-2 100 102

Specimens Structures Atoms Dislocations Crystals

Size Scale for Studying Fatigue

Understand the physics on this scale

Model the physics on this scale

Use the models on this scale


The Fatigue Process

Crack nucleation Small crack growth in an elastic-plastic

stress field Macroscopic crack growth in a nominally

elastic stress field Final fracture


Mechanisms Crack Nucleation

Nucleation in Slip Bands inside Grain Nucleation at Grain Boundaries Nucleation at Inclusions


1903 - Ewing and Humfrey

Cyclic deformation leads to the development of slip bands and fatigue cracks

N = 1,000 N = 2,000

N = 10,000 N = 40,000 Nf = 170,000 Ewing, J.A. and Humfrey, J.C. “The fracture of metals under repeated alterations of stress”, Philosophical Transactions of the Royal Society, Vol. A200, 1903, 241-250


Crack Nucleation


Slip Band in Copper

Polak, J. Cyclic Plasticity and Low Cycle Fatigue Life of Metals, Elsevier, 1991


Slip Band Formation

Loading Unloading

Extrusion

Undeformed material

Intrusion


Slip Bands

Ma, B-T and Laird C. “Overview of fatigue behavior in copper sinle crystals –II Population, size, distribution and growth Kinetics of stage I cracks for tests at constant strain amplitude”, Acta Metallurgica, Vol 37, 1989, 337-348


2124-T4 Cracking in Slip Bands

N = 60

N = 2000 N = 1200

N = 300 N = 240


Crack at Particle

Material: BS L65 Aluminum

Loading: 63 ksi, R=0 for 500,000+ cycles, followed by 68 ksi, R=0 to failure. Cracks found

during 68 ksi loading.

S. Pearson, “Initiation of Fatigue Cracks in Commercial Aluminum Alloys and the Subsequent Propagation

of Very Short Cracks,” RAE TR 72236, Dec 1972.


2219-T851 Cracked Particle

10µm James & Morris, ASTM STP 811 Fatigue Mechanisms: Advances in Quantitative Measurement of Physical

Damage, pp. 46-70, 1983.


Crack at Bonded Particle

Material: BS L65 Aluminum

Loading: 63 ksi, R=0 for 500,000+ cycles, followed by 68 ksi, R=0 to failure. Cracks found

during 68 ksi loading.

S. Pearson, “Initiation of Fatigue Cracks in Commercial Aluminum Alloys and the Subsequent Propagation

of Very Short Cracks,” RAE TR 72236, Dec 1972.


7075-T6 Cracking at Inclusion


Crack Initiation at Inclusions

Langford and Kusenberger, “Initiation of Fatigue Cracks in 4340 Steel”, Metallurgical Transactions, Vol 4, 1977, 553-559


Subsurface Crack Initiation

Y. Murakami, Metal Fatigue: Effects of Small Defects and Nonmetallic Inclusions, 2002


Fatigue Limit and Strength Correlation

0 500 1000 1500 2000

250

500

750

1000

1250

Tensile Strength, MPa

Fatig

ue S

treng

th, M

Pa

0.6

0.5

0.35

0 500 1000 1500 2000

250

500

750

1000

1250


Fatig

ue S

treng

th, M

Pa

0.6

0.5

0.35

From Forrest, Fatigue of Metals, Pergamon Press, London, 1962


Crack Nucleation Summary

Highly localized plastic deformation Surface phenomena Stochastic process


100 µm

bulk surface

10 µm

surface

20-25 austenitic steel in symmetrical push-pull fatigue (20°C, ∆εp/2= ±0.4%) : short cracks on the surface and in the bulk

Surface Damage

From Jacques Stolarz, Ecole Nationale Superieure des Mines Presented at LCF 5 in Berlin, 2003


Stage I Stage II

loading direction

free surface

Stage I and Stage II


Stage I Crack Growth

Single primary slip system

individual grain

near - tip plastic zone

S

SStage I crack is strongly affected by slip characteristics, microstructure dimensions, stress level, extent of near tip plasticity


Small Cracks at Notches

D a crack tip plastic zone

notch plastic zone

notch stress field

Crack growth controlled by the notch plastic strains


Small Crack Growth

1.0 mm

N = 900

Inconel 718 ∆ε = 0.02 Nf = 936


0

0.5

1

1.5

2

2.5

0 2000 4000 6000 8000 10000 12000 14000

J-603

F-495 H-491

I-471 C-399

G-304

Cycles

Cra

ck L

engt

h, m

m

Crack Length Observations


Crack - Microstructure Interactions

Akiniwa, Y., Tanaka, K., and Matsui, E.,”Statistical Characteristics of Propagation of Small Fatigue Cracks in Smooth Specimens of Aluminum Alloy 2024-T3, Materials Science and Engineering, Vol. A104, 1988, 105-115

10-6

10-7

0 0.005 0.01 0.015 0.02 0.025 0.03 0.025 0.02 0.015 0.01 0.005

A B C D

F

E

Crack Length, mm

da/dN, mm/cycle


Strain-Life Data

Reversals, 2Nf

Stra

in A

mpl

itude

∆ε

2

10-5

10-4

0.01

0.1

1

100 101 102 103 104 105 106 107

10-3

10µm 100 µm

1mm fracture Crack size

Most of the life is spent in microcrack growth in the plastic strain dominated region


Stage II Crack Growth

Locally, the crack grows in shear Macroscopically it grows in tension


Long Crack Growth

Plastic zone size is much larger than the material microstructure so that the microstructure does not play such an important role.


Material strength does not play a major role in fatigue crack growth

Crack Growth Rates of Metals


Maximum Load

monotonic plastic zone

σ

Stresses Around a Crack

σ

ε


Stresses Around a Crack (continued)

Minimum Load σ

ε

cyclic plastic zone

σ


Crack Closure

S = 250

b

S = 175

c

S = 0

a


Crack Opening Load Damaging portion of loading history

Nondamaging portion of loading history

Opening load


Mode Iopening

Mode IIin-plane shear

Mode IIIout-of-plane shear

Mode I, Mode II, and Mode III


5 µmcrac

k gr

owth

dire

ctio

n

Mode I Growth


crack growth direction

10 µm

slip bands shear stress

Mode II Growth


1045 Steel - Tension

Nucleation Shear

Tension

1.0

0.2

0

0.4

0.8

0.6

1 10 10 2 10 3 10 4 10 5 10 6 10 7

Fatigue Life, 2N f

Dam

age

Frac

tion

N/N

f

100 µm crack


Fatigue Life, 2N f

Dam

age

Frac

tion

N/N

f f

Nucleation

Shear

Tension

1 10 10 2 10 3 10 4 10 5 10 6 10 7

1.0

0.2

0

0.4

0.8

0.6

1045 Steel - Torsion


Things Worth Remembering

Fatigue is a localized process involving the nucleation and growth of cracks to failure.

Fatigue is caused by localized plastic deformation.

Most of the fatigue life is consumed growing microcracks in the finite life region

Crack nucleation is dominate at long lives.





Characterization

Stress Life Curve Fatigue Limit

Strain Life Curve Cyclic Stress Strain Curve

Crack Growth Curve Threshold Stress Intensity


Bending Fatigue

F

stre

ss

time

stress amplitude

stress range

IcM

=σBending stress:

ω


SN Curve

200

300

400 500

200

300

400

500 St

ress

Am

plitu

de, M

Pa

105 106 107 108 109

1x108 2x108 3x108 4x108 5x108 0

Cycles to Failure

Cycles to Failure

Monel Alloy

1 hour 1 day 1 month 1 year Testing time @ 30 Hz


Fatigue Strength

105 106 107 108 109

2014-T4 290 235 186 152 138 2024-T4 297 214 166 145 138 6061-T6 186 152 117 104 90 7075-T6 276 200 166 152 145

Fatigue Life Alloy


6061-T6 Aluminum Test Data

Sharpe et. al. Fatigue Design of Aluminum Components and Structures , 1996


SN Curve for Steel

100

1000

104

102

Cycles

Stre

ss A

mpl

itude

, MPa

103 104 105 106 107 108 109

fatigue limit

( )bf'f NS

2S

=∆

The fatigue limit is usually only found in steel laboratory specimens


Very High Cycle Fatigue of Steel

100

1000

104

Cycles

Stre

ss A

mpl

itude

, MPa

1010 103 104 105 106 107 108 109

conventional fatigue limit

surface failures large inclusions

internal inclusions


Fatigue Damage

100

1000

10000

Stre

ss A

mpl

itude

, MP

a

100

Cycles 101 102 103 104 105 106 107

( )bf'f NS

2S

=∆

1

10

b1

'f

f S2SN

∆=

10SDamage ∆∝


Fatigue Limit Strength Correlation

0 500 1000 1500 2000

250

500

750

1000

1250


Fatig

ue S

treng

th, M

Pa

0.6

0.5

0.35

0 500 1000 1500 2000

250

500

750

1000

1250


Fatig

ue S

treng

th, M

Pa

0.6

0.5

0.35

From Forrest, Fatigue of Metals, Pergamon Press, London, 1962


Fatigue Limit Strength Correlation


SN Materials Data

10 1 10 102 103 106 107 105 104

Fatigue Life, Reversals

100

1000

10000

Stre

ss A

mpl

itude

, MP

a

93 steels

17 aluminums


Strain Controlled Testing


Cyclic Hardening / Softening


Stable Hysteresis Loop

∆σ

∆ε

∆εe ∆εp

Hysteresis loop


Strain-Life Data σ − ε

0

100

200

300

400

500

600

0 0.004 0.008 0.012

Strain Amplitude

Stre

ss A

mpl

itude

∆ε ∆σ ∆σ2 2 2

1

= +

E K

n

'

/ '

During cyclic deformation, the material deforms on a path described by the cyclic stress strain curve

∆ε 2

∆σ

2


Cyclic Stress Strain Curve


Strain-Life Data ∆ε - 2Nf

10-5

10-4

0.001

0.01

0.1

1

Reversals, 2Nf

Stra

in A

mpl

itude

100 101 102 103 104 105 106 107

2 Reversals, 2Nf = 1 Cycle, Nf

∆ε

2


Elastic and Plastic Strain-Life Data

10-5

10-4

0.001

0.01

0.1

1

Reversals, 2Nf

Stra

in A

mpl

itude

100 101 102 103 104 105 106 107

∆ε

2 Plastic

Elastic


Strain-Life Curve

10-5

10-4

0.001

0.01

0.1

1

Reversals, 2Nf

Stra

in A

mpl

itude

100 101 102 103 104 105 106 107

cf

'f

bf

'f )N2()N2(

E2ε+

σ=

ε∆

c

b

'fε

E

'fσ

2Nt

∆ε

2


Transition Fatigue Life

From Dowling, Mechanical Behavior of Materials, 1999


εN Materials Data

Fatigue Life, Reversals

93 steels

17 aluminums

1 10 102 103 106 107 105 104 10-4

10-2

10-3

0.1

10

1

Stra

in A

mpl

itude


Crack Growth Testing


Stress Concentration of a Crack

2 a

ρ

ρ+=

a21KT

a ~ 10-3

for a crack

ρ ~ 10-9

KT ~ 2000

appliedlocal 2000σ=σ

Traditional material properties like tensile strength are not very useful for cracked structures


Stress Intensity Factor

σ

σ

2a

aK πσ=

K characterizes the magnitude of the stresses, strains, and displacements in the neighborhood of a crack tip

Two cracks with the same K will have the same behavior


Crack Growth Measurements

σ

σ

2a

Cycles

Cra

ck s

ize

dNda

a1

a2

σ2 σ1


Crack Growth Data

10-12

10-11

10-10

10-9

10-8

10-7

10-6

1 10 100

Cra

ck G

row

th R

ate,

m/c

ycle

mMPa,K∆

mKCdNda

∆=

∆KTH

Kc

m ~ 3


Threshold Region

threshold stress intensity

πσ∆>∆

wafaKTH

operating stresses

flaw size

flaw shape


Threshold Stress Intensity

From Dowling, Mechanical Behavior of Materials, 1999


Non-propagating Crack Sizes

a212.1KTH ππ

σ∆>∆

Small cracks are frequently semielliptical surface cracks

2TH

cK63.0a

σ∆∆

=

2

u

THc

K52.2a

σ

∆=

Smooth specimen fatigue limit 2

uσ≈


Non-propagating Crack Sizes

Ultimate Strength, MPa

Cra

ck S

ize,

mm mMPa5KTH =∆

0

0.2

0.4

0.6

0.8

1

0 500 1000 1500 2000


Stable Crack Growth

10-12

10-11

10-10

10-9

10-8

10-7

10-6

1 10 100

Cra

ck G

row

th R

ate,

m/c

ycle

mMPa,K∆

mKCdNda

∆=

∆KTH

Kc

Stable growth region


Crack Growth Data

( ) 0.312 mMPaK109.6dNda

∆×= −

( ) 25.210 mMPaK104.1dNda

∆×= −

( ) 25.312 mMPaK106.5dNda

∆×= −

Ferritic-Pearlitic Steel:

Martensitic Steel:

Austenitic Stainless Steel:

Barsom, “Fatigue Crack Propagation in Steels of Various Yield Strengths” Journal of Engineering for Industry, Trans. ASME, Series B, Vol. 93, No. 4, 1971, 1190-1196

5 10 100

10-7

10-6

10-8

Cra

ck G

row

th R

ate,

m/c

ycle

∆K, MPa√m

σyield 252 273 392 415


Aluminum Crack Growth Rate Data

Sharp, Nordmark and Menzemer, Fatigue Design of Aluminum Components and Structures, 1996


Crack Growth Data

0

10

20

30

40

50

Cra

ck L

engt

h, m

m

0 50 100 150 200 250 300 350 Cycles x103

Virkler, Hillberry and Goel, “The Statistical Nature of Fatigue Crack Propagation”, Journal of Engineering Materials and Technology, Vol. 101, 1979, 148-153



Method Stress-Life Strain-Life

Crack Growth

Physics Crack Nucleation

Microcrack Growth Macrocrack Growth

Size 0.01 mm

0.1 - 1 mm > 1mm





Fatigue Analysis

Material Data

Component Geometry

Service Loading

Analysis Fatigue Life Estimate

?


The Similitude Concept

Why Fatigue Modeling Works !


What is the Similitude Concept

The “Similitude Concept” allows engineers to relate the behavior of small-scale cyclic material test specimens, defined under carefully controlled conditions, to the likely performance of real structures subjected to variable amplitude fatigue loads under either simulated or actual service conditions.


Fatigue Analysis Techniques

Stress - Life BS 7608, Eurocode 3 Strain - Life Crack Growth


Life Estimation

Method Stress-Life BS 7608

Strain-Life Crack Growth

Physics Crack Nucleation

Crack Growth Microcrack Growth Macrocrack Growth

Size 0.01 mm

1 - 10 mm 0.1 - 1 mm

> 1mm


Stress-Life Fatigue Modeling

P

Fixed End

The Similitude Concept states that if the instantaneous loads applied to the ‘test’ structure (wing spar, say) and the test specimen are the same, then the response in each case will also be the same and can be described by the material’s S-N curve.

100

1000

10000

Stre

ss A

mpl

itude

, MP

a

100

Cycles 101 102 103 104 105 106 107


Fatigue Analysis: Stress-Life

Material Data

Component Geometry

Service Loading


SN curve Ka, Ks, …

Kf

∆S , Sm


Stress-Life

Major Assumptions: Most of the life is consumed nucleating cracks Elastic deformation Nominal stresses and material strength control

fatigue life Accurate determination of Kf for each geometry

and material


Stress-Life

Advantages: Changes in material and geometry can easily be

evaluated Large empirical database for steel with standard

notch shapes


Stress-Life

Limitations: Does not account for notch root plasticity Mean stress effects are often in error Requires empirical Kf for good results


BS 7608 Fatigue Modeling

The Similitude Concept states that if the instantaneous loads applied to the ‘test’ structure (welded beam on a bulldozer, say) and the test specimen (standard fillet weld) are the same, then the response in each case will also be the same and can be described by one of the standard BS 7608 Weld Classification S-N curves.

10

100

1000

105

Cycles

Stre

ss R

ange

, MP

a

108 107 106


Weld Classifications

D E

F2 G


Fatigue Analysis: BS 7608

Material Data

Component Geometry

Service Loading


Weld SN curve

Class

∆S


BS 7608

Major Assumptions: Crack growth dominates fatigue life Complex weld geometries can be described by a

standard classification Results independent of material and mean stress

for structural steels


BS 7608

Advantages: Manufacturing effects are directly included Large empirical database exists


BS 7608

Limitations: Difficult to determine weld class for complex

shapes No benefit for improving manufacturing process


Strain-Life Fatigue Modeling

The Similitude Concept states that if the instantaneous strains applied to the ‘test’ structure (vehicle suspension, say) and the test specimen are the same, then the response in each case will also be the same and can be described by the material’s e-N curve. Due account can also be made for stress concentrations, variable amplitude loading etc.

10-5

10-4

0.001

0.01

0.1

1

Reversals, 2Nf

Stra

in A

mpl

itude

100 101 102 103 104 105 106 107


Fatigue Analysis: Strain-Life

Material Data

Component Geometry

Service Loading


εN curve σε curve

Kf

∆S , Sm


Strain-Life

Major Assumptions: Local stresses and strains control fatigue

behavior Plasticity around stress concentrations Accurate determination of Kf


Strain-Life

Advantages: Plasticity effects Mean stress effects


Strain-Life

Limitations: Requires empirical Kf

Long life situations where surface finish and processing variables are important


Crack Growth Fatigue Modeling

The Similitude Concept states that if the stress intensity (K) at the tip of a crack in the ‘test’ structure (welded connection on an oil platform leg, say) and the test specimen are the same, then the crack growth response in each case will also be the same and can be described by the Paris relationship. Account can also be made for local chemical environment, if necessary.

10-12

10-11

10-10

10-9

10-8

10-7

10-6

1 10 100

Cra

ck G

row

th R

ate,

m/c

ycle

mMPa,K∆


Fatigue Analysis: Crack Growth

Material Data

Component Geometry

Service Loading


da/dN curve

K

∆S , Sm


Crack Growth

Major Assumptions: Nominal stress and crack size control fatigue life Accurate determination of initial crack size


Crack Growth

Advantage: Only method to directly deal with cracks


Crack Growth

Limitations: Complex sequence effects Accurate determination of initial crack size


Choose the Right Model

Similitude Failure mechanism Size scale


Design Philosophy

Safe Life Damage Tolerant


Safe Life

0

100

200

300

400

500

104 105 106 107 108 109

Stre

ss A

mpl

itude

, MPa

Fatigue Life

99 90 11050Percent Survival

0

100

200

300

400

500

104 105 106 107 108 109

Stre

ss A

mpl

itude

, MPa

Fatigue Life

99 90 1105099 90 11050Percent Survival

Choose an appropriate risk and replace critical parts after some specified interval


Damage Tolerant

Inspect for cracks larger than a1 and repair Cycles

Cra

ck s

ize

a1

a2

Safe Operating Life

Inspection


Inspection

A Boeing 777 costs $250,000,000

A new car costs $25,000

For every $1 spent inspecting and maintaining a B 777 you can spend only 0.01¢ on a car



Questions to ask Will a crack nucleate ? Will a crack grow ? How fast will it grow ?

Similitude Failure mechanism Size Scale



Physics of Fatigue Material Properties Similitude eFatigue


Some Observations

Most fatigue failures are not the result of an expert using the wrong analysis etc.

Most fatigue failures are a result of a non-expert not considering fatigue because it is too complicated, not enough data etc.

Fatigue will no longer be taught in the major research universities as they focus on new science.


Prof Yukitaka Murakami

Science in the Sunlight Science in the Shade


Science in the Sunlight


Science in the Shade


A Common Viewpoint (controversial)

Fatigue is reasonably well understood, major problems are solved and current research is applications driven towards investigating special cases and improving the accuracy of our evaluations.

Fatigue is assessment is just like finite element

analysis, buy some software and make a color plot.


Fatigue Calculators

There is a need for some fatigue analysis tools that take only a few minutes to learn so non-experts can reliably conduct a fatigue assessment.


www.eFatigue.com

























Summary

eFatigue – Bring fatigue assessment out of the shade into the sunlight where many people can have access fatigue technology on demand.

Fatigue and Fracture

Documents

Fatigue, How and Why Physics of Fatigue Professor Darrell ... · Physics of Fatigue Professor Darrell F. Socie ... BS L65 Aluminum Loading: 63 ksi, ... 7075-T6 Cracking at Inclusion