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w w w . a u t o s t e e l . o r g
Manufacturing (Bending-Unbending-Stretching) Effects
on AHSS Fracture Strain
Hong Zhu
ArcelorMittal Global R & D - East Chicago
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Background
• Component fracture occurs during
crash
• Energy absorption predictions for
AHSS components are affected by
component fracture
50
100
150
200
250
300
300 600 900 1200 1500
Tensile Strength (MPa)
Cru
sh
Dis
tan
ce (
mm
)
HSLA350
DP590/HSLA350DP590
HFT590
DP965
DP980-SFM1310
FEA Simulation
From SAE 2005-01-0837, B. Yan et al.
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Background (cont’d)
• Fracture criteria needed for fracture prediction
• Fracture behavior of AHSS is also affected by prestrain during manufacture, such as stamping
• Understanding the manufacturing effect on AHSS fracture is important
• Current research activities in that area:
– Non-linear strain paths project, funded by DOE / ASP, is focusing on in-plane pre-strain modes.
– MIT industrial fracture consortium (2nd phase) is investigating some pre-strain modes (e.g., punching)
• However, the most common mode, bending-unbending-stretching, is not covered by either of them.
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Purpose of Present Work
• To fill the knowledge gaps concerning manufacturing effect
on AHSS fracture
• Investigate the bending-unbending-stretching effects on
AHSS fracture.
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Experiment
• Hat-section channels were stamped (drawing
mode)
– Side wall – undergoing bending-unbending-
stretch
– Flange – as-received condition
• Specimens were cut from the side walls and
flanges
– Subsize tensile
– Mini three-point bending
– Notched tensile – machined
– Notched tensile – punched
L direction
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AHSS Grades Tested
• Nominal thickness: 1.5 mm
Material Coating
Microstructure YS
(MPa )
TS
(MPa)
UE
(%)
TE
(%)
DP780 Uncoated
F+M 508 803 11.9 21.1
TRIP780 GA
F+B+RA 464 809 18.0 24.0
w w w . a u t o s t e e l . o r g 7
Channel Draw: Determining Process Conditions
• Variables: die radii, punch radii
and binder pressure
• Final process conditions:
− Strip size: 508 mm x 100 mm
− Draw distance: 101.6 mm
− Channel width: 80 mm
− Punch radius: 2t
− Die radius: 2t
− Binder pressure:
√ 202.2 kN for DP780
√ 244.7 kN for TRIP780
− Drawing directions: L
Objective: Minimize side wall curl and avoid fracture
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Manufacturing Strain Estimation
• Outer fiber strain history − Punch side
√ Tension through bending √ Compression through unbending √ Tension through stretching
− Die side
√ Compression through bending √ Tension through unbending √ Tension through stretching
• Effective plastic strain due to bending-unbending:
− = 2 * t/R ~ 0.4
• Stretch was measured by thinning strain (from
ultrasonic thickness gauge measurement)
Punch Die
Stretch pleff .
w w w . a u t o s t e e l . o r g 9
Manufacturing Strain
Material Locations Thickness Thinning /
Stretch Strain
Due to B-U
DP780 Uncoated
flange 1.54 0.0 0.0
side wall 1.25 0.194 ~ 0.4
TRIP780 GA
flange 1.49 0.0 0.0
side wall 1.32 0.154 ~ 0.4
pleff .
Average of all channel parts
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Subsize Tensile Test: Specimen
• Specimen configuration: 25 mm gage length, 6 mm width
(ASTM-E8/E8M -09)
• 3 replicates each
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Subsize Tensile Test: Stress-Strain Curves
DP780 TRIP780
Flange Wall
DP780 TRIP780
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Subsize Tensile Test: Stress-Strain Curves (2)
The true stress/strain curves were shifted by the
amount of stretch strain
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Summary of Subsize Tensile Test Results
Grades Locations
Prestrain
YS
(MPa)
TS
(MPa) UE TE
Fract.
Strain Thinning
/Stretch
due to B-U
DP780
Uncoated
flange 0.0 0.0 508 803 11.9 21.1 0.403
side wall 0.172 ~0.4 862 970 3.6 9.1 0.270
TRIP780
GA
flange 0.0 0.0 464 809 18.0 24.0 0.400
side wall 0.152 ~0.4 823 967 5.4 11.7 0.288
• There is no effect of bending/unbending on fracture strain
• The fracture is affected by stretch strength
pleff .
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Mini Three-Point Bending Test
• Specimen: 50 mm x 40 mm
• Bending direction: √ Outer fiber on punch side
√ Bending-unbending-stretching-
bending
• Testing setup: Punch blade radius: 1*t
Roller diameter: 30mm
Gap between rollers: 3*t
• Test stopped at 5% load drop
• Microcracks were observed
• Fracture strain measured by 2mm grids
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Mini Three-Point Bending Test: F-D curves
TRIP780
DP780
Wall
Flange
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Mini Three-Point Bending Test: Fracture Strain
Grades Locations
Prestrain Surface
Fracture
Strain Thinning
Strain Due to B-U
DP780
Uncoated
flange 0.0 0.0 0.427
side wall 0.194 ~ 0.4 0.208
TRIP780
GA
flange 0.0 0.0 0.438
side wall 0.154 ~ 0.4 0.281
• Bending-unbending effect: not significant
• Stretching effect: significant
• Fracture is controlled by total strain
pleff .
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Machined Notch Tensile Test
• Specimen configuration
– Notch diameter: 20 mm
– Effective width at the
notch: 20 mm
– Machined
• Fracture strain was measured
by DIC system (15 fps)
− 2 mm gauge length
− 1 mm away from hole
edge
w w w . a u t o s t e e l . o r g 18
Machined Tensile Test: F-D Curves
• Peak load similar due to thinning
• Nominal stress increases due to hardening
• Displacement decreases after prestrain
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Machined Notch Tensile Test: Fracture Strain
Grades Locations
Prestrain
Fracture
Strain Thinning /
Stretch due to
B-U
DP780
Uncoated
flange 0.0 0.0 0.268
side wall 0.194 ~ 0.4 0.146
TRIP780
GA
flange 0.0 0.0 0.297
side wall 0.155 ~ 0.4 0.140
• Bending-unbending effect: not sensitive
• Stretching effect: significant
pleff .
w w w . a u t o s t e e l . o r g 20
Punched Notch Tensile Test
• Specimen configuration
− Notch diameter: 20 mm
− Width of notch center:
20 mm
− Punched with a
clearance of 0.1 t
• Fracture strain was
measured by DIC (15 fps)
• Gage length: 2mm
• Measured 1.0 mm away
from punch edge
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Punched Notch Tensile Test: F-D Curves
• Peak load increase after prestrain not significant due to thinning
• Nominal stress increases after prestrain
• Displacement decreases after prestrain
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Punched Notch Tensile Test: Fracture Strain
• Bending-unbending effect: not sensitive
• Stretching effect: sensitivity is reduced by punch damage
Grades Locations
Pre Strain Fracture
Strain Thinning /
Stretch
Due to B-U
DP780
Uncoated
flange 0.0 0.0 0.186
side wall 0.209 ~ 0.4 0.119
TRIP780
GA
flange 0.0 0.0 0.117
side wall 0.146 ~ 0.4 0.133
pleff .
w w w . a u t o s t e e l . o r g 23
Summary: Average Fracture Strain
Uniaxial Bending Machined Punched
Pre
Stretch
Fracture
Strain
Pre
Stretch
Fracture
Strain
Pre
Stretch
Fracture
Strain
Pre
Stretch
Fractur
e Strain
DP780
Uncoated
F 0.0 0.403 0 0.427 0 0.268 0 0.186
SW 0.172 0.270 0.194 0.208 0.194 0.146 0.209 0.119
TRIP780
GA
F 0.0 0.400 0 0.438 0 0.297 0 0.117
SW 0.152 0.288 0.154 0.281 0.155 0.140 0.146 0.133
w w w . a u t o s t e e l . o r g 24
Discussion: Fracture Strain
• Bending fracture strain is larger than in-plane test due to
strain gradient
• The effect of in-plane prestrain is significant for
uniaxial tension, bending and machined notch
specimens. However, the effect is reduced for punched
notched specimen due to punch damage.
• Effective Plastic Strain is not a valid index for fracture
of AHSS when bending-unbending is involved, “Total
Strain” may be a better parameter
w w w . a u t o s t e e l . o r g 25
Conclusions
Manufacturing effects (bending-unbending-stretching) on AHSS
fracture were investigated for DP780 Uncoated and TRIP780 GA
and the following conclusions were drawn:
• Bending-unbending does not have significant effect on fracture
behavior
• Effective plastic strain is not a good index for fracture under
bending-unbending mode
• Total strain is a more appropriate parameter to characterize fracture
• For punched edges, prestrain effect is significantly reduced by
punch-induced damages
• Manufacturing effects must be considered in AHSS fracture
prediction of crash events
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Future Work Needed
• Testing: develop industrial standard testing procedures to
characterize manufacturing effect
• FEA simulations: re-evaluate and develop simulation
methodologies (e.g., material models and fracture
criteria) for chain simulations to incorporate
manufacturing effects into structure analysis (e.g. crash
simulation)
w w w . a u t o s t e e l . o r g 27
Acknowledgments
• Dr. L. Greve at Volkswagen AG
• Mr. S. Malcolm at Honda R & D Americas
• Drs. B. Yan and Blake Zuidema at ArcelorMittal Global R & D
• Mr. G. Girman and Mr. G. Volk at ArcelorMittal Global R & D
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North American
Light Vehicle Metallic Material Trends
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