Fracture Control of Steel Structures 2003

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Fatigue Crack Propagation
Fracture Control of Steel Structures 2003
Chitoshi Miki
Lecture 6
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The Significance of Discussions on The Crack
Propagation
The life of structural components may be governed
by the sub-critical crack propagation.
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1. Crack Propagation Tests
CT
CCT
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1. Crack Propagation Tests
CT
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Crack Size-Number of Load Cycles
?s3
?s2
?s1
Crack Length, a
For the same length
Number of Cycles, N
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Crack Size-Number of Load Cycles
l3
l2
l1
Crack Length, a
?s
For the same
l3
gtl2
gtl1
Number of Cycles, N
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Application of Stress Intensity Factor Range, ?K
Log (da/dN)
by Paris and Erdogan
Log ?K
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Plastic Zone Development at Crack Tip
?
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Plastic Zone Development at Crack Tip
?
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Plastic Zone Development at Crack Tip
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Crack Propagation Threshold, ?Kth
Log (da/dN)
Log ?Kth
Log ?K
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Effective Stress Range, ?seff
R0.5
R0
R-1.0
?seff
smax
s
seff
Crack Closure
smin
Time
?K
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Effective Stress Intensity Factor Range, ?Keff
K
Log (da/dN)
Kmax
?Keff
Crack Closure
Kop
Kmin
Time
Log ?Keff
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Fatigue Crack Closure
Plasticity-induced Closure
Roughness-induced Closure
Oxide-induced Closure
No Closure Welded Joints because of tensile
residual stress
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Variation of Fatigue Crack Growth
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Fatigue Striations
Constant Loading
TEM
SEM
Random Loading
TEM
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Ductile and Brittle Striations
Ductile Striations
TEM
Brittle Striations
TEM
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Change of Fracture Surface Appearance
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2. da/dN-K for Fatigue Propagation Life
Prediction
Fatigue Propagation Life
Initial Crack Size
Final Crack Size
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Fatigue Propagation Life Prediction
JSSC
C
m
?Kth
Conservative
Mean
OK
Conservative Estimation