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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

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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 .

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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

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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 .

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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 .

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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

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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

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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)

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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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