Auto/Steel
Partnership
Fatigue of Spot-Welded Sheet Steel Joints:
Physics, Mechanics, and Process Variability
R. Mohan Iyengar1 , M. Amaya2, J. Bonnen3, K. Citrin2, H.-T. Kang4, S.
Laxman1, A. Khosrovaneh5, N. Schillaci6, and H.-S. Shih7
1 Severstal
North America Inc; 2Chrysler Corporation; 3 Ford Motor Company;4 University
of Michigan-Dearborn;5General Motors Corporation;6Arcelor-Mittal; 7 U. S. Steel
Great Designs in Steel – April 9, 2008
www.autosteel.org
Sheet Fatigue Team Members
Auto/Steel
Partnership
Car Company Members:
Mark Amaya
Chrysler Corp.
Ken Citrin
Chrysler Corp.
John Bonnen
Ford
A.K. Khosravaneh General Motors
Steel Company Members:
Rob Comstock
Raj Mohan Iyengar
Todd Link
H.-C. (Mike) Shih
Nick Schillaci
Benda Yan
AK Steel
Severstal NA
U.S. Steel
U.S. Steel
Arcelor-Mittal
Arcelor-Mittal
Staff Members
Bart Clark
A/SP
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Motivation
Auto/Steel
Partnership
•
Weight reduction
What are the implications of replacing heavier
gage components made of low carbon and HSLA
materials with thinner AHSS parts?
•
Over 5000 spot welds in a typical uni-body
structure fastens panels and components
together. Fatigue resistance of these joints
impart significant influence on durability of a
vehicle.
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Grades & Gages
Spot Weld Materials & Gages
Auto/Steel
Partnership
IF
GI
Gage
1.60
mm
YS,
DQSK CQSK
Bare
GI
1.83
1.74
1.59
HSL
A
340
GI
1.78
DP
600
DP
800
DP
980
GI
1.53
GA
1.61
Bare
1.55
TRI
P
600
TRI
P
800
RA
830
MS
1300
Bare
1.64
EG
1.53
GI
1.39
Bare
1.60
Boro
n
Bare
1.741.47
178
170
156
370
432
414
702
421
510
901
1156 1237
306
308
350
448
671
782 1057
672
839
895
1355 1382
UE,
%
21.7
20.5
14.5
15.9
13.6 12.7
7.1
20.6
22.3
0.8
3.2
5.55
TE,
%
32.2
32.5
22.7
31.7
22.1 19.5 11.4
29.3
27.2
6.7
5.1
5.55
MPa
UTS
MPa
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Spot Welding Parameters for All
Steels
Auto/Steel
Partnership
Squeeze time:
Weld time:
Hold time:
Electrode force:
Cap size, truncated:
99 cycles
22 cycles
90 cycles
1500 lb (DQSK, IF ~1100lb)
7.9mm
Target Button Size: 7.0 mm
Weld Current Adjusted for Each Material
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Fatigue Specimen Designs
Auto/Steel
Partnership
25mm
R = 2.5t
160mm
80mm
12.5
mm
38mm
Tensile Shear
Coach Peel
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t
All Spot Weld Results
(nominal -1.6mm)
Auto/Steel
Partnership
DQSK-R0.1
10000
DQSK-R0.3
CQSK-R0.1
CQSK-R0.3
Tensile Shear
IF-R0.1
IF-R0.3
HSLA340-R0.1
Load Amplitude (N)
Load, N
1000
HSLA340-R0.3
DP600-R0.1
DP600-R0.3
DP800-R0.1
DP800-R0.3
Coach Peel
DP980-R0.1
DP980-R0.3
100
TRIP600-R0.1
TRIP600-R0.3
TRIP800-R0.1
Thickness effect
No mean stress effect
10
1.E+02
TRIP800-R0.3
RA830-R0.1
RA830-R0.3
MS1300-R0.1
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
Cycles to Failure
Failure
cycles
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MS1300-R0.3
Boron-R0.1
Boron-R0.3
Location of Crack Initiation
and Growth
Auto/Steel
Partnership
1.78mm HSLA-340Y-GI sheet.
250μm
1mm
Untested Nugget
Typical Fatigue fracture
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Thickness Effects
Auto/Steel
Partnership
10000
Tensile Shear
HSLA340TS01 1.78mm
HSLA340TS03 1.78mm
HSLA340CP01 1.78mm
HSLA340CP03 1.78mm
HSLA340TS01 1.00mm
HSLA340TS03 1.00mm
HSLA340CP01 1.00mm
HSLA340CP03 1.00mm
DP600TS01 1.53mm
DP600TS03 1.53mm
DP600CP01 1.53mm
DP600CP03 1.53mm
DP600TS01 0.83mm
DP600TS03 0.83mm
DP600CP01 0.83mm
DP600CP03 0.83mm
Load Amplitude (N)
Load, N
1000
100
Coach Peel
HSLA 340 – 1.78 mm
DP 600 – 0.83 mm
10
1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
Failure
Cycles cycles
to Failure
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1.E+07
Effects of Prestrain
Auto/Steel
Partnership
Raw Fatigue Data
10000
Minimal effect of pre-strain
when thickness of formed sheet
is accounted for
Max. Load (N)
Load, N
R=0 loading
1000
Normalized using Rupp parameter
DP600 As-Received, 1.29mm thick, 7.0mm Dia. button
DP600 Prestrained, 1.19mm thick, 7.3mm Dia. button
1000
DP600
1.E+06
Cycles to Failure
Failure cycles
1.E+07
1.E+08
T.M. Link, 45th Mechanical Working and
Steel Processing (MWSP) Conference
Proceedings, Vol. XLI, 2003
Max. Stress (MPa)
1.E+05
Load, N
100
1.E+04
DP600 As-Received, 1.29mm thick, 7.0mm Dia. button
DP600 Prestrained, 1.19mm thick, 7.3mm Dia. Button
100
(2)
DP600
10
1.E+04
1.E+05
1.E+06
1.E+07
Failure cycles
Cycles to Failure
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1.E+08
Spot Weld, Bond-Only, & Weld
Bonded
Auto/Steel
Partnership
1.E+04
HSLA340TS01 SW
SW = Spot Weld
Bond = Bond Only
WB = Weld Bond
blue – weld-bonded
HSLA340TS03 SW
HSLA340CP01 SW
HSLA340CP03 SW
HSLA340TS01 Bond
HSLA340TS03 Bond
HSLA340CP01 Bond
Tensile shear
HSLA340CP03 Bond
Load Amplitude (N)
Load, N
Tensile Shear
HSLA340TS01 WB
HSLA340TS03 WB
Coach Peel
1.E+03
HSLA340CP01 WB
Coach peel
HSLA340CP03 WB
DP600TS01 SW
DP600TS03 SW
DP600CP01 SW
DP600CP03 SW
red – bond only
DP600TS01 Bond
DP600TS03 Bond
DP600CP01 Bond
DP600CP03 Bond
black – spot weld
DP600TS01 WB
DP600TS03 WB
1.E+02
1.E+02
DP600CP01 WB
1.E+03
1.E+04
1.E+05
Failure cycles
1.E+06
1.E+07
Cycles to Failure
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DP600CP03 WB
Multiparameter Study:
Auto/Steel
Partnership
Hold Time, Heat Treatment, w/ Weld-bond
HSLA 340 1.78 mm – Button Dia: 4.9 mm
Heat Treatment: Paint & Bake Cycle
Tensile Shear Specimens, 1.78 mm HSLA 340-GI, R = 0.1
Hold Time: Thought to be detrimental for AHSS
10000
4.9 mm Dia, 1 cycle HT
4.9 mm Dia, 90 cycle HT
4.9 mm Dia, 1 cycle HT, heat treated
4.9 mm Dia, 90 cycle HT, heat treated
4.9 mm Dia, 1 cycle HT, weld bonded
4.9 mm Dia, 90 cycle HT, weld bonded
HSLA 340 (1.78mm) – Button Dia: 7 mm
Tensile Shear Specimens, 1.78 mm HSLA 340-GI, R = 0.1
10000
Load Amplitude, N
Load, N
7.0 mm Dia, 1 cycle HT
7.0 mm Dia, 90 cycle HT
7.0 mm Dia, 1 cycle HT, heat treated
7.0 mm Dia, 90 cycle HT, heat treated
7.0 mm Dia, 1 cycle HT, weld bonded
7.0 mm Dia, 90 cycle HT, weld bonded
(4)
1000
1.E+03
b)
(3)
1000
1.E+03
a)
Load Amplitude, N
Load,
N
Weld Bond: Load transfer over a larger region
1.E+04
1.E+05
1.E+06
Cycles to cycles
Failure
Failure
1.E+07
1.E+04
1.E+05
1.E+06
1.E+07
Cycles to Failure
Failure
cycles
For HSLA 340:
•Effect of HT or heat treatment – small
for 7.0 mm & more (longer HT – better
lives) for 4.9 mm welds
•More scatter for 4.9 mm welds
•Weld bond improves fatigue life
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Multiparameter Study:
Auto/Steel
Partnership
Hold Time, Heat Treatment, w/ Weld-bond
DP600 (1.53 mm) – Button Dia: 7 mm
Tensile Shear Specimens, 1.53 mm DP 600-GI, R = 0.1
1.E+04
Lo ad Amp litu d e, N
Load,
N
7.0 mm Dia, 1 cycle HT
7.0 mm Dia, 90 cycle HT
7.0 mm Dia, 1 cycle HT, heat treated
7.0 mm Dia, 90 cycle HT, heat treated
7.0 mm Dia, 1 cycle HT, weld bonded
7.0 mm Dia, 90 cycle HT, weld bonded
(3)
1.E+03
1.E+03
b)
1.E+03
1.E+04
1.E+05
1.E+06
Failure
cycles
Cycles to
Failure
1.E+04
Failure
cycles
Cycles to Failure
1.E+05
1.E+06
1.E+07
For DP 600:
(4)
1.E+03
a)
Load, N
1.E+04
L o a d A m p litu d e , N
DP600
(1.53 mm) – Button Dia: 7 mm
Tensile Shear Specimens, 1.53 mm DP 600-GI, R = 0.1
4.9 mm Dia, 1 cycle HT
4.9 mm Dia, 90 cycle HT
4.9 mm Dia, 1 cycle HT, heat treated
4.9 mm Dia, 90 cycle HT, heat treated
4.9 mm Dia, 1 cycle HT, weld bonded
4.9 mm Dia, 90 cycle HT, weld bonded
1.E+07
•Effect of HT or heat treatment – small
for 7.0 mm & more (shorter HT – better
lives) for 4.9 mm welds
•More scatter for 4.9 mm welds
•Weld bond improves fatigue life
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Evaluation of Fatigue Damage
Parameters
Auto/Steel
Partnership
1.E+08
1.E+08
1.E+06
1.E+05
1.E+04
Rupp
x5
Rupp
Rupp
1.E+03
Measured Life
Tensile Shear
Coach Peel
1.E+07
1.E+05
Dong
1.E+04
x5
1.E+07
Dong
x5
1.E+02
1.E+02
1.E+08
Predicted Life
1.E+04
1.E+05 1.E+06
Experimental Fatigue Life (Nf)
1.E+06
1.E+03
1.E+08
1.E+03
Coach Peel
Dong
x5
1.E+02
1.E+02
Tensile Shear
1.E+07
Predicted Life
Coach Peel
Predicted Fatigue Life (Nf)
Uncertainty of Analysis
or
Scatter of Data due to
Geometric Variability
of Spot Welds Joints?
Tensile Shear
Predicted Fatigue Life (Nf)
Predicted Fatigue Life (Nf)
Predicted Life
1.E+07
Tensile Shear
1.E+04 1.E+05 1.E+06 1.E+07
Measured
Life
Experimental Fatigue Life
(Nf)
1.E+03
Coach Peel
1.E+06
1.E+05
Bonnen et al.,
SAE Paper No. 200601-0978.
1.E+04
1.E+03
1.E+02
1.E+02
x5
Kang
x5
1.E+03
Kang
Kang
1.E+04
1.E+05 1.E+06
Experimental Fatigue Life (Nf)
1.E+07
Measured Life
1.E+08
http://www.a-sp.org/publications.htm.
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1.E+08
Fatigue Strength Variability
Auto/Steel
Partnership
Minimal effect of center-to-grip distance variation for current data
1.E+05
160mm
80mm
38mm
25mm
R=
2.5t
12.5
mm
t
M a x im u m A p p lie d L o a d , N
TS Specimens: R2 = 0.96
1.E+04
1.E+03
DP600 - 1.49, TS
1.E+02
CP Specimens: R2 = 0.96
DP800 - 1.59, TS
DP600 - 1.53, CP
1.E+01
1.E+02
RA830 - 1.39, CP
1.E+03
1.E+04
1.E+05
Cycle to Failure
Center to Grip Distance
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1.E+06
1.E+07
Fatigue Strength Variability
Auto/Steel
Partnership
Effect of thickness variation for current data
M a x im um A pplied L o a d, N
1.E+05
TS Specimens: R2 = 0.76
1.E+04
1.E+03
1.E+02
CP Specimens: R2 = 0.38
DP600 - 0.93
DP600-1.54
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
Cycle to Failure
Sheet Thickness
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1.E+06
1.E+07
Fatigue Strength Variability
Auto/Steel
Partnership
1.E+05
DP 600 – 0.93 mm
Load (N)/Moment (Nmm)
R2 = 0.98
1.E+04
1.E+03
R2 = 0.94
1.E+02
Load vs Life
160mm
80mm
38mm
25mm
R=
2.5t
12.5
mm
t
Moment vs Life
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
Cycles to Failure
Flange Length in CP specimens
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1.E+06
1.E+07
Crucial Geometric Parameters
Auto/Steel
Partnership
Von Mises Stress
σ M = (σ + 3τ
2
x
)
F
2 12
xz
Tensile Shear
τ xz
Axial Stress
z
6M
σx = 2
bt
Coach Peel
6M
σx = 2
bt
e
e
Tensile Shear
2r
α
2/
t
4Q
=
πD2
Coach Peel
x
t/2
Shear Stress
Q
P
M
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P
τ xz =
Dt
Fatigue Strength Variability
Auto/Steel
Partnership
1.E+06
DP600-0.93
TRIP600-1.64
RA830-1.35
IF-1.60
DP800-1.60
Maximum Applied Load, N
R2 = 0.86
1.E+05
DP600-1.54
HSLA-1.78
MS1300-1.60
DQSK-1.60
1.E+04
1.E+03
1.E+02
1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
Cycles to failure, cycles
Maximum Load - Tensile Shear
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Fatigue Strength Variability
Auto/Steel
Partnership
1.E+05
DP600-0.93
TRIP600-1.64
RA830-1.35
IF-1.60
DP800-1.60
R2 = 0.89
von Mises Stress, MPa
1.E+04
DP600-1.54
HSLA-1.78
MS1300-1.60
DQSK-1.60
1.E+03
1.E+02
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
Cycles to failure, cycles
Effective Stress -Tensile Shear
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Fatigue Strength Variability
Auto/Steel
Partnership
Stress Intensity Range ( Δ KI), N/mm^1.5
1.E+04
R2 = 0.90
1.E+03
1.E+02
1.E+01
1.E+00
1.E+02
DP600-0.93
TRIP600-1.64
RA830-1.35
IF-1.60
DP800-1.60
1.E+03
DP600-1.54
HSLA-1.78
MS1300-1.60
DQSK-1.60
1.E+04
1.E+05
1.E+06
1.E+07
Mohan Iyengar et al.,
Cycles to failure, cycles
SAE Paper No. 2008-01- Stress Intensity Factor Range -Tensile Shear
0698.
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Fatigue Strength Variability
Auto/Steel
Partnership
Maximum Applied Load, N
1.E+05
DP600-0.93
DP600-1.54
TRIP600-1.64
HSLA-1.78
DQSK-1.60
DP800-1.60
R2 = 0.49
1.E+04
1.E+03
1.E+02
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
Cycles to Failure
Maximum Load - Coach Peel
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Fatigue Strength Variability
Auto/Steel
Partnership
1.E+05
DP600-0.93
DP600-1.54
TRIP600-1.64
HSLA-1.78
DQSK-1.60
DP800-1.60
von Mises Stress1, MPa
R2 = 0.81
1.E+04
1.E+03
1.E+02
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
Cycles to Failure
Effective Stress - Coach Peel
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Fatigue Strength Variability
Auto/Steel
Partnership
Stress Intensity Ra ng e ( Δ KI), N/m m ^ 1 .5
1.E+05
1.E+04
DP600-0.93
DP600-1.54
TRIP600-1.60
HSLA-1.78
DQSK-1.60
DP800-1.60
R2 = 0.83
1.E+03
1.E+02
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
Cycles to failure, cycles
1.E+07
Stress Intensity Factor Range - Coach Peel
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Conclusions
Auto/Steel
Partnership
1. Spot weld fatigue performance of studied steels (AHSS,
HSLA, low carbon) appears to be insensitive to base
metal composition, microstructure, and strength.
2. Spot weld fatigue behavior is mainly controlled by
geometric factors such as sheet thickness and weld
diameter.
3. Spot weld fatigue behavior is largely mean stress
insensitive, for the mean stresses examined.
4. No effect of weld hold time (between 1 and a 90 cycles).
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Conclusions
Auto/Steel
Partnership
5. No effect of paint bake cycle.
6. Adhesive bonding and weld bonding significantly
improve fatigue behavior over spot welding alone,
although this improvement is in keeping with the
actual increase in joint area gained by the addition of
the adhesive layer.
7. Prestraining or stretch-forming the parent metal before
spot welding has no impact on the fatigue performance
of spot welded joints.
8. Principal spot weld damage parameters (Rupp, Dong,
Swellam, Kang, & Sheppard) predict A/SP data equally
well.
9. Crucial parameters controlling the mechanics and
physics uncovered, thus reducing uncertainty in
prediction.
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Auto/Steel
Partnership
More information?
A/S-P spot weld fatigue knowledge base
(including detailed fatigue data, report,
microstructure, tensile data, etc.)
freely available at:
http://www.a-sp.org
http://www.a-sp.org/database/custom/ASP%20Spot%20Weld%20Fatigue%20Project%202-7-06v1c.exe
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Auto/Steel
Partnership
Acknowledgements
Thanks to other members of the sheet steel
fatigue team for work on spot welds:
• Todd Link – US Steel
• Ron Soldaat - Arcelor-Mittal
• Benda Yan - Arcelor-Mittal
Thanks to the Department of Energy for
their support of this program.
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