jinhee park (m.s. candidate) date of joining masters’ program : fall 2002
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Influence of Overload Induced Residual Stress Field on Fatigue Crack Growth in Aluminum Alloy. Jinhee Park (M.S. Candidate) Date of joining Masters’ program : Fall 2002 Thesis advisor : Dr. M. A. Wahab Dr. S. S. Pang. The experimental facility (MTS 810). - PowerPoint PPT PresentationTRANSCRIPT
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Influence of Overload Induced Residual Stress Field on Fatigue Crack Growth
in Aluminum Alloy
Jinhee Park (M.S. Candidate)
Date of joining Masters’ program : Fall 2002
Thesis advisor : Dr. M. A. Wahab
Dr. S. S. Pang
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The experimental facility (MTS 810)
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Instrumented Specimen in the Corrosion Cell
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< Overview >
• Introduction• Theoretical Review• Numerical Modeling (FEM)• Design of Experimental Methodology • Numerical Result• Comparison of Numerical Result with Theory• Conclusion & Comment• Further Work
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< Introduction >
• Concept of Fatigue Crack Growth
∆a
∆a
loading
unloading
loading
unloading
smax
smin
s
Time
RSfNa ,
)(maxmin
RatioStressSSR
)(minmax
RangeStressSSS
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• 2D - Modeling of Center Crack Specimen
W
a
Crack Tip
ASTM Standard E 1823 M(T)
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• Material Behavior Modeling (elastic-perfect plastic) Bilinear Inelastic
Isotropic Hardening
Linear elastic material
Elastic plastic material stress-strain loop
Load
ing
Unl
oadi
ng
Permanent Set
Dis
sipa
ted
Ene
rgy
y
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• Plastic Zone Size for Plane Stress
r p2
Crack
y
Distance from crack tip
At cyclic loading (+)
(--)
y
At cyclic unloading
(+)
: Irwin’s plastic zone radius (1960)
σK
π2ry
Ip
1
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• Compressive Residual Stresses ahead of Crack Tip after Overload
Distance from crack tip
Com
pre
ssiv
e S
tres
sT
ensi
le
Str
ess
(+)
(--)
Overload Plastic Zone
Cyclic loading plastic zone
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Cra
ck L
engt
h(a)
• Fatigue-life Extension due to Periodic Overloads
The slower crack growth continues until the crack grows beyond the overload plastic region(The wheeler model of crack growth inside an overload plastic zone). This beneficial residual stress effect of overload is called crack growth retardation.
Number of cycles(N)
Overload
Periodic overload retardation
Constant amplitude cycles
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• The wheeler model of crack growth inside an overload plastic zone
•
a’ '2r p
After second overload
•(-)
(+)
a
Before overload
•
a
After first overload
•
a’
r p2
After some crack growth
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< Theoretical Review >
• Consequences of Overload for Crack Tip Plastic Strain Loop
Before overload After overloadF
AE
A E
F
Cyclic elastic strain range
F
EA
E
F
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< Numerical Modeling >
Elastic Modulus (E)
Poisson Ratio ()
Yield Stress ( )
RoomTemp.(T)
Tangent Modulus (H)
70 0.33 200 20c 0
• Mechanical Properties of Aluminum Alloy
(2024-T3, 7050-T7451)
y
MPaGPa GPa
Cu Mn Mg Zn Cr
7075-T7451 1.6% 2.5% 5.6% 0.23%
2024-T3 4.4% 0.6% 0.45%
- Chemical Composition of Aluminum Alloy
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• Finite Element Model for Center-Crack Plate
Symmetric Boundary Condition
Element Size around crack tip : 0.5mm
Element type :
8 node PLANE 82
10mm
40mm
20
mm
40
mm
Crack Tip
Node just front crack tip
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• Cyclic loading condition with two overloads history
Overload ratio = 80 / 3080MPa
30MPa
t
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< Design of Experimental Methodology >
• Influence of low cycle fatigue(LCF) damage on high cycle fatigue(HCF) crack growth.
• Modeling of overloading effects on crack growth, and considering various load amplitudes.
• Formulating equations for lifetime prediction.
• Providing recommendations for service life extension and developing improved fatigue life assessment tools.
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< Numerical Results >
After one overload, cyclic strain range was considerably reduced and
the crack growth rate will decrease accordingly (Retardation).
Before overload
After overload (80MPa)
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• Stress-total strain curve with two overloads (80MPa, 60MPa)
After overload (60MPa)After overload
(80MPa)
Before overload
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• Stress-total strain curve with two overloads (80MPa, 100MPa)
After overload (100MPa)
After overload (80MPa)
Before overload
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• Stress-strain curve with two overloads (80MPa, 80MPa)
This curve didn’t show any more decreased strain after the second overload.
After overload (80MPa)
Before overload
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• Substep time-total strain curve with two overloads
0 5 10 15 20 25-0.015
-0.01
-0.005
0
0.005
0.01
0.015
Substep Time
Tot
al s
trai
n
80MPa - 100MPa
80MPa - 80MPa
80MPa - 60MPa
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0 5 10 15 20 25 30 35 4020
40
60
80
100
120
140
160
180
200
Distance from the crack tip (mm)
Von
-mis
es s
tres
s (M
Pa) Before overload
After first overload (80MPa) After second overload (100MPa)
• Von-mises stress distribution along the crack plane
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• Two overloads (80MPa - 100MPa)
The plastic zone disappeared after each overload.
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< Comparison of Numerical Results with Theory >
After overload
F
After overload (80MPa), strain range moves to the left. It becomes negative. The strain hardening should be considered.
After overload (80MPa)
Before overload
F
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< Conclusion and Comments >
• After one overload, strain was decreased.
(Crack growth retardation)
• After the second overload, the second reduced strain was recorded in 80MPa & 100MPa.
• Von-mises stress redistribution along the crack plane after first overload (80MPa) was reduced. After the second overload (100MPa), the stress redistribution was smaller than the first one.
• Two overloads effects on the crack plane and crack growth rate was not well checked in this work.
• This paper was accepted by ICCE 10, July 2003.
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< Further Work >
• Cyclic strain hardening and path dependent plasticity should be considered later on.
• Various loading conditions like overload ratio, stress ratio, and stress range should be conducted later on.
• To advance crack, the crack tip advance scheme (involving node release immediately after maximum load on each cycle) needs to be carried out.