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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
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Fatigue, How and Why
Physics of Fatigue Material Properties Similitude Fatigue Calculator
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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
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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
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Mechanisms Crack Nucleation
Nucleation in Slip Bands inside Grain Nucleation at Grain Boundaries Nucleation at Inclusions
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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
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Crack Nucleation
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Slip Band in Copper
Polak, J. Cyclic Plasticity and Low Cycle Fatigue Life of Metals, Elsevier, 1991
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Slip Band Formation
Loading Unloading
Extrusion
Undeformed material
Intrusion
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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
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2124-T4 Cracking in Slip Bands
N = 60
N = 2000 N = 1200
N = 300 N = 240
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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.
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2219-T851 Cracked Particle
10µm James & Morris, ASTM STP 811 Fatigue Mechanisms: Advances in Quantitative Measurement of Physical
Damage, pp. 46-70, 1983.
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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.
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7075-T6 Cracking at Inclusion
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Crack Initiation at Inclusions
Langford and Kusenberger, “Initiation of Fatigue Cracks in 4340 Steel”, Metallurgical Transactions, Vol 4, 1977, 553-559
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Subsurface Crack Initiation
Y. Murakami, Metal Fatigue: Effects of Small Defects and Nonmetallic Inclusions, 2002
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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
Tensile Strength, MPa
Fatig
ue S
treng
th, M
Pa
0.6
0.5
0.35
From Forrest, Fatigue of Metals, Pergamon Press, London, 1962
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Crack Nucleation Summary
Highly localized plastic deformation Surface phenomena Stochastic process
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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
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Stage I Stage II
loading direction
free surface
Stage I and Stage II
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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
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Small Cracks at Notches
D a crack tip plastic zone
notch plastic zone
notch stress field
Crack growth controlled by the notch plastic strains
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Small Crack Growth
1.0 mm
N = 900
Inconel 718 ∆ε = 0.02 Nf = 936
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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
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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
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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
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Stage II Crack Growth
Locally, the crack grows in shear Macroscopically it grows in tension
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Long Crack Growth
Plastic zone size is much larger than the material microstructure so that the microstructure does not play such an important role.
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Material strength does not play a major role in fatigue crack growth
Crack Growth Rates of Metals
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Maximum Load
monotonic plastic zone
σ
Stresses Around a Crack
σ
ε
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Stresses Around a Crack (continued)
Minimum Load σ
ε
cyclic plastic zone
σ
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Crack Closure
S = 250
b
S = 175
c
S = 0
a
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Crack Opening Load Damaging portion of loading history
Nondamaging portion of loading history
Opening load
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Mode Iopening
Mode IIin-plane shear
Mode IIIout-of-plane shear
Mode I, Mode II, and Mode III
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5 µmcrac
k gr
owth
dire
ctio
n
Mode I Growth
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crack growth direction
10 µm
slip bands shear stress
Mode II Growth
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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
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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
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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.
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Fatigue, How and Why
Physics of Fatigue Material Properties Similitude Fatigue Calculator
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Characterization
Stress Life Curve Fatigue Limit
Strain Life Curve Cyclic Stress Strain Curve
Crack Growth Curve Threshold Stress Intensity
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Bending Fatigue
F
stre
ss
time
stress amplitude
stress range
IcM
=σBending stress:
ω
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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
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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
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6061-T6 Aluminum Test Data
Sharpe et. al. Fatigue Design of Aluminum Components and Structures , 1996
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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
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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
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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 ∆∝
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Fatigue Limit 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
Tensile Strength, MPa
Fatig
ue S
treng
th, M
Pa
0.6
0.5
0.35
From Forrest, Fatigue of Metals, Pergamon Press, London, 1962
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Fatigue Limit Strength Correlation
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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
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Strain Controlled Testing
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Cyclic Hardening / Softening
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Stable Hysteresis Loop
∆σ
∆ε
∆εe ∆εp
Hysteresis loop
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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
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Cyclic Stress Strain Curve
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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
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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
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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
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Transition Fatigue Life
From Dowling, Mechanical Behavior of Materials, 1999
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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
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Crack Growth Testing
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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
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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
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Crack Growth Measurements
σ
σ
2a
Cycles
Cra
ck s
ize
dNda
a1
a2
σ2 σ1
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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
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Threshold Region
threshold stress intensity
πσ∆>∆
wafaKTH
operating stresses
flaw size
flaw shape
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Threshold Stress Intensity
From Dowling, Mechanical Behavior of Materials, 1999
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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≈
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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
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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
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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
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Aluminum Crack Growth Rate Data
Sharp, Nordmark and Menzemer, Fatigue Design of Aluminum Components and Structures, 1996
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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
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Things Worth Remembering
Method Stress-Life Strain-Life
Crack Growth
Physics Crack Nucleation
Microcrack Growth Macrocrack Growth
Size 0.01 mm
0.1 - 1 mm > 1mm
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Fatigue, How and Why
Physics of Fatigue Material Properties Similitude Fatigue Calculator
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Fatigue Analysis
Material Data
Component Geometry
Service Loading
Analysis Fatigue Life Estimate
?
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The Similitude Concept
Why Fatigue Modeling Works !
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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.
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Fatigue Analysis Techniques
Stress - Life BS 7608, Eurocode 3 Strain - Life Crack Growth
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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
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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
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Fatigue Analysis: Stress-Life
Material Data
Component Geometry
Service Loading
Analysis Fatigue Life Estimate
SN curve Ka, Ks, …
Kf
∆S , Sm
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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
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Stress-Life
Advantages: Changes in material and geometry can easily be
evaluated Large empirical database for steel with standard
notch shapes
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Stress-Life
Limitations: Does not account for notch root plasticity Mean stress effects are often in error Requires empirical Kf for good results
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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
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Weld Classifications
D E
F2 G
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Fatigue Analysis: BS 7608
Material Data
Component Geometry
Service Loading
Analysis Fatigue Life Estimate
Weld SN curve
Class
∆S
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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
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BS 7608
Advantages: Manufacturing effects are directly included Large empirical database exists
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BS 7608
Limitations: Difficult to determine weld class for complex
shapes No benefit for improving manufacturing process
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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
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Fatigue Analysis: Strain-Life
Material Data
Component Geometry
Service Loading
Analysis Fatigue Life Estimate
εN curve σε curve
Kf
∆S , Sm
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Strain-Life
Major Assumptions: Local stresses and strains control fatigue
behavior Plasticity around stress concentrations Accurate determination of Kf
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Strain-Life
Advantages: Plasticity effects Mean stress effects
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Strain-Life
Limitations: Requires empirical Kf
Long life situations where surface finish and processing variables are important
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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∆
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Fatigue Analysis: Crack Growth
Material Data
Component Geometry
Service Loading
Analysis Fatigue Life Estimate
da/dN curve
K
∆S , Sm
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Crack Growth
Major Assumptions: Nominal stress and crack size control fatigue life Accurate determination of initial crack size
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Crack Growth
Advantage: Only method to directly deal with cracks
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Crack Growth
Limitations: Complex sequence effects Accurate determination of initial crack size
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Choose the Right Model
Similitude Failure mechanism Size scale
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Design Philosophy
Safe Life Damage Tolerant
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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
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Damage Tolerant
Inspect for cracks larger than a1 and repair Cycles
Cra
ck s
ize
a1
a2
Safe Operating Life
Inspection
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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
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Things Worth Remembering
Questions to ask Will a crack nucleate ? Will a crack grow ? How fast will it grow ?
Similitude Failure mechanism Size Scale
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Fatigue, How and Why
Physics of Fatigue Material Properties Similitude eFatigue
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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.
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Prof Yukitaka Murakami
Science in the Sunlight Science in the Shade
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Science in the Sunlight
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Science in the Shade
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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.
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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.
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www.eFatigue.com
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Summary
eFatigue – Bring fatigue assessment out of the shade into the sunlight where many people can have access fatigue technology on demand.
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Fatigue and Fracture