Post on 24-Feb-2018
Fatigue Crack Propagation and Stable Tearing in Friction-Stir-Welded Aluminum Sheet
Presented by: Eui I. LimThe Boeing Company
At: ASIP 2006San Antonio, TexasNovember 28, 2006
ASIP 2006 2
Collaborative Effort Univ of So Carolina
Raphael MuzzoliniJean-Christophe Ehrstrom, PhD
Nathalie FuzierAnthony Reynolds, PhD
Kenneth K. ChanEui I. Lim
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Initiation
• Seemingly Consistent Crack Turnings– FCGR test on 7xxx-T7 Sheet– 3 Samples each, from 3 Lots
FSW sample Base metal
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Motivation
• Understand Phenomenon– Why did the Cracks Turned?– Can it be Captured, and
Controlled?• Via Experiments
– FCGR and stable tearing– C(t) and M(t) coupons– Cracks to FSW @ 90º & 45º
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Outline
1. Material and weld properties
2. Fatigue tests– Geometry– Crack path– da/dN curves– Residual stress measurements
3. Stable tearing tests– Crack path– Properties
4. Fractography
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Material and Welding Description• Aluminum 7xxx-T7 Sheet , t = 5mm.• Post Weld Aging heat treat to stabilize properties of the weld.
Profile of the Tool :R
etre
atin
g si
de
Adva
ncin
g sid
e
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Weld Properties
5.95477 (69)Weld (T-L)13.98.1552 (80)Base metal (T-L)
Elongation at rupture (%)
Elongation at UTS (%)
UTSMPa (ksi)
Weld UTS : 86% of base material
engineering strain (%)0 2 4 6 8 10 12 14 16
engi
neer
ing
stre
ss (M
Pa)
0
100
200
300
400
500
600
(ksi
)
0
20
40
60
80
welded sample
base material
TS
L
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Hardness Profile
Distance from the centerline of the weld (mm)-30 -20 -10 0 10 20 30 40
Mic
roha
rdne
ss(H
V)
100
120
140
160
180
200
(inches)
-1.0 -0.5 0.0 0.5 1.0 1.5
Advancing side Retreating side
ST
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Outline
1. Material and weld properties
2. Fatigue tests– Geometry– Crack path– da/dN curves– Residual stress measurements
3. Stable tearing tests– Crack path– Properties
4. Fractography
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C(T) Samples
Dimensions are in inches.
LT 90º orientation 45º orientation
L
T
dθ
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Crack Path 90º OrientationFatigue cycling at constant ΔK = 15 MPa*m1/2
Propagation along straight line, slight deviation angle.
R=0.1
Angle 13.7º 15º < 5º
Angle 7º < 5º < 5º
TL
R=0.5
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Crack Path 45º OrientationCurved crack path.
L T
R=0.1
32º (local max) 38º (local max) 10º
17º (local max) 21º (local max) 11.5º
R=0.5
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Fatigue Results
R=0.5 ΔK=15MPa*m1/2L
T
Distance from the centerline of the weld (mm)
-20 -10 0 10 20 30 40 50 60
da/d
N(m
m/c
ycle
)
0.0001
0.001
0.01
(inches)
-0.5 0.0 0.5 1.0 1.5 2.0
(inch
/cyc
le)
1e-5
1e-4
weld
parent
Cracking Direction
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Fatigue Results
R=0.1 ΔK=15MPa*m1/2
LT
weld
parent
Cracking Direction
Distance from the centerline of the weld (mm)
-30 -20 -10 0 10 20 30 40 50 60
da/d
N(m
m/c
ycle
)
1e-7
1e-6
1e-5
1e-4
1e-3
(inches)
-1 0 1 2
(inch
/cyc
le)
1e-8
1e-7
1e-6
1e-5
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Residual Stress Evaluation
Forward determination of the stress intensity factor due to residual stresses :
By “Cut Compliance” technique - Measure Δε at each additional cut
dad
aZEaK resI
ε)(')( =
where Z(a): a geometrical function
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Kresidual
True Kmax applied at the crack tip, Kmaxtotal, is a summation of Kresidualand Kmax generated by loading.
LT
(inch)
Slot cutting direction
K re
s (M
Pa*m
1/2 )
(ksi
*inch
1/2 )
Distance from the centerline of the weld (mm)
-40 -30 -20 -10 0 10 20 30 40 50-20
-15
-10
-5
0
5
10
15-1 0 1
-15
-10
-5
0
5
10
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Residual Stress Profile Calculated from K
Advancing side Retreating side
Distance from the centerline of the weld (mm)-40 -30 -20 -10 0 10 20 30 40 50 60
Res
idua
l stre
ss (
MP
a)
-100
-50
0
50
100
150
(inch)-1 0 1 2
(ksi
)
-10
0
10
20
LT
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Outline
1. Material and weld properties
2. Fatigue tests– Geometry– Crack path– da/dN curves– Residual stress measurements
3. Stable tearing tests– Crack path– Properties
4. Fractography
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Stable Tearing - Parent Material
Rolling Direction appears to be the Preferred Path for Crack Extension.
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Stable Tearing - Welded Samples
Crack turns so it extends along the L direction. Combined effect of the microstructure and the welding?
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Stable Tearing Data
0
2
4
6
8
10
12
0 5 10 15 20 25 30 35delta a (mm)
load
(kN
)
-0.3
0.2
0.7
1.2
1.7
2.2
2.7-0.1 0.1 0.3 0.5 0.7 0.9 1.1 1.3
(inch)
(kip
)
base metal
classic weld (12 ipm)
slow weld (4 ipm)
Series2
weld
LT
ASIP 2006 22
Stable tearing data
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Outline
1. Material and weld properties
2. Fatigue tests– Geometry– Crack path– da/dN curves– Residual stress measurements
3. Stable tearing tests– Crack path– Properties
4. Fractography
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Fractography – Fatigue R=0.1
SEM pictures of Fracture Surface
ST
100 μm
10 μm
Inside the nugget
Appears mostly inter-granular
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Inter-granular Fracture
Surface rougher than for R=0.1
Fractography – Fatigue R=0.5
ST
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Stable Tearing – Nugget
ST
R A
SEM pictures
100 μm
20 μm 5 μm
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Summary
• Weld characterization:– With appropriate welding parameters, weld UTS would be 86%
of the base material.
• Crack path:– For 90º oriented fatigue samples, propagation along a straight
line, with sometimes a slight deviation angle observed.
– For 45º oriented fatigue sample, curved crack path.
– In base metal stable tearing tests, the rolling direction is often a preferred path direction for the crack.
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Summary
• Fatigue crack growth rate : – Compared to parent material, reduced crack growth rate is
observed in the HAZ. This reduction is higher for lower R.
– Residual stress seem to have a predominant effect in this variation. Closure and microstructure might also be involved.
• Fractography:– In fatigue, fracture is mostly inter-granular. The surface is
rougher for higher R.
– In stable tearing, the fracture is also clearly inter-granular.
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Future Work
• Investigate Residual Stress Affect due to FSW
• Acquire Crack Growth Properties Along the Weld, HAZ from both the Advancing Side and the Retreating Side of the Weld
• Acquire Crack Growth Properties Across the Weld, HAZ from both the Advancing Side and the Retreating Side of the Weld
• Repeat Testing and Data Acquisitions for other Materials Commonly used in FSW