Center forAdvancedMaterials andManufacturing ofAutomotiveComponents

Aluminum Sheet Stretch-Bend Limits and Fracture Criterion

Robert H. Wagoner1, Changqing Du2, Dajun Zhou2

1The Ohio State University, Department of Material Science and Engineering, Columbus, OH, USA2Chrysler Group LLC, Advance Stamping Manufacturing Engineering, Auburn Hills, Michigan

IABC 2012, Frankfurt, Germany, May, 2012

Outline

1. BackgroundDraw-Bend Fracture TestWrapping Angle Fixture

2. Results

3. Empirical Criterion

4. Conclusions

2

3

1. Background

Shear Fracture of Sheet Materials

4

Fractures along small tool radii observed during stamping trials

AHSS Rail Aluminum PanelFocus of this study

5

1. Background:

Draw-Bend Fracture Test

6

Draw-Bend Fracture Test (DBF)

Aluminum Alloys: 6016, 6022Sheet thickness: 0.8mm to 0.9mm

Specimen width: 25mm

Tool radius choices (mm): 1.0, 1.5, 2.0, 3.2, 4.8

a = V2/V1: 0 and 0.3

Draw-Bend Fracture Test

Draw-Bend Test, 25mm Strip Width

7

8

Fixtures For Adjustable Wrapping Angles

V 1

R

u f

Design Concept

Fixture for wrapping angle adjustments

Wrapping angle : 90o, 75o, 60o

9

Adjustable Wrapping Angles

Phenomenological Fracture Types

Type I

Type II

Type III

65o

65o

V2

V1

Type I: Tensile fracture (unbent region)

Type II: Shear fracture (not Type I or III)

Type III: Shear fracture (fracture at the roller)

10

The maximum load carrying capacity of the sheet sample is limited by Type III (fracture at the roller) fracture

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2. Results

Aluminum Sheet Samples – 90o

Fractures at Tool radii

Aluminum Sheet (6016) Samples – 75o

Fractures at Tool radii

Aluminum Sheet (6016) Samples – 60o

Fractures at Tool radii

Shear Fractured Section

15

Local Fracture: not able to be predict using traditional Forming Limit Curve (FLC)

Stress Ratio at Fracture

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Stress Ratio at Fracture

Stress Ratio definition: Sample_MPa/Group_Max (Sample Maximum Stress at Fracture )/ (Maximum Fracture Stress of Material Group)

To minimize the material aging effect : material tensile testing conducted at different time as DB testing.

Notations:

RD: Rolling Direction // stretching direction TD: Transversal Direction // stretching direction

R/T Ratio: Ratio of (Radius of tool)/(thickness of sample)

Stress Ratio at Fracture: AL 6011

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Maximum Fracture Stress of AL 6016 RD: 243 MPa Maximum Fracture Stress of AL 6016 TD: 244 MPa

0

0.2

0.4

0.6

0.8

1

1.2

1 2 3 4 5 6 7

Stre

ss R

atio

(MPa

/Max

)

R/T Ratio

AL 6016 RD

60 Deg

75 Deg

90 Deg

90 Deg (Shear)

Remarks: 1) Wrapping angle has no effect on the stress ratio.2) The Stress ratio at R/T near 1.0 is about less than 0.8 (80%) of the sheet maximum

stress.3) The Stress Ratio increases as R/T Ratio increases.4) The stress Ratio reaches 1.0 when R/T Ratio >4.5. (Tensile fracture observed).

0

0.2

0.4

0.6

0.8

1

1.2

1 2 3 4 5 6 7St

ress

Rati

o (M

Pa/M

ax)

R/T Ratio

AL 6016 TD

60 Deg

75 Deg

90 Deg

90 Deg (Shear)

Material Rolling Direction Effect for AL 6011

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Remarks: 1) In general, insignificant difference was found between RD and TD in stress-ratio.2) No Shear fracture RD sample at R/T > 4.3) Shear fracture samples at R/T=4 in TD sample only.

0

0.2

0.4

0.6

0.8

1

1.2

1 2 3 4 5 6 7

Stre

ss R

atio

(MPa

/Max

)

R/T Ratio

AL 6016 - All Samples

Shear RD

Tensile RD

Shear TD

Tensile TD

Stress Ratio at Fracture: AL 6022

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Maximum Fracture Stress of AL 6022RD: 275.4 MPa Maximum Fracture Stress of AL 6022 TD: 272.6 MPa

0

0.2

0.4

0.6

0.8

1

1.2

1 2 3 4 5 6 7

Stre

ss R

atio

(MPa

/Max

)

R/T Ratio

AL 6022 RD

60 Deg

75 Deg

90 Deg

90 Deg (Shear)

Remarks: 1) Wrapping angle has no effect on the stress ratio.2) The Stress ratio at R/T near 1.0 is about 0.8 (80%) of the sheet maximum stress.3) The Stress Ratio increases as R/T Ratio increases.

0

0.2

0.4

0.6

0.8

1

1.2

1.00 2.00 3.00 4.00 5.00 6.00 7.00

Stre

ss R

atio

(MPa

/Max

)

R/T Ratio

AL 6022 TD

60 Deg

75 Deg

90 Deg

90 Deg (Shear)

Material Rolling Direction Effect

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Remarks: 1) In general, R/T < 3, RD samples have higher stress-ratio than TD. 2) Shear fracture samples at R/T>3.5 in TD sample only.

0

0.2

0.4

0.6

0.8

1

1.2

1 2 3 4 5 6 7

Stre

ss R

atio

(MPa

/Max

)

R/T Ratio

AL 6022 - All Samples

Shear RD

Tensile RD

Shear TD

Tensile TD

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3. Empirical Criterion

Stretch-Bend Fracture Stress Limit – AL 6016

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Remarks: 1) Stretch-Bend Fracture Stress Limit is set to the lower bound of Stress Ratio.

y = -0.0073x2 + 0.0958x + 0.6837R² = 0.9973

0

0.2

0.4

0.6

0.8

1

1.2

1 2 3 4 5 6 7

Stre

ss R

atio

(MPa

/Max

)

R/T Ratio

AL 6016 - All Samples

Shear RD

Tensile RD

Shear TD

Tensile TD

Stretch-Bend Limit

Poly. (Stretch-Bend Limit)

Stretch-Bend Fracture Stress Limit – AL 6022

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Remarks: 1) Stretch-Bend Fracture Stress Limit is set to the lower bound of Stress Ratio.

y = -0.0093x2 + 0.1142x + 0.6549R² = 0.9973

0

0.2

0.4

0.6

0.8

1

1.2

1 2 3 4 5 6 7

Stre

ss R

atio

(MPa

/Max

)

R/T Ratio

AL 6022 - All Samples

Shear RD

Tensile RD

Shear TD

Tensile TD

Stretch-Bend Limit

Poly. (Stretch-Bend Limit)

Stretch-Bend Fracture Stress Limit – AL 6016& 6022

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A Stretch-Bend Limit Curve may be applied for both AL 6016 and 6022 : (Stress Ratio) = - 0.0079 (R/T)2 + 0.1019 (R/T) + 0.6693

y = -0.0079x2 + 0.1019x + 0.6693R² = 0.9977

0

0.2

0.4

0.6

0.8

1

1.2

1 2 3 4 5 6 7

Stre

ss R

atio

(MPa

/Max

)

R/T Ratio

ALuminum Sheet (6016 and 6022) Stretch-Bend Limit

AL 6022 Stretch-Bend Limit

AL 6016 Stretch-Bend Limit

Lower Bound

Poly. (Lower Bound)

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Conclusions

• Aluminum sheets, 6016 and 6022, Stretch-Bend Fracture phenomenon has been recreated in laboratory using Draw-Bend testing system with three wrapping angles and five radius to sheet thickness (R/T) ratios .

• Stress Ratio is chosen to quantify the fracture stress – (Sample Maximum Stress at Fracture )/ (Maximum Fracture

Stress of Material Group)• Wrapping angle has no effect on the fracture stress ratio.• AL 6022 RD samples have higher stress-ratio than TD.• The Stretch-Bend Limit curves have been developed for AL 6016

and AL 6022. • Since these two curves are very close to each other, one Stretch-

Bend Limit curve for both AL 6016 and 6022 has been developed for stamping application: – (Stress Ratio) = - 0.0079 (R/T)2 + 0.1019 (R/T) + 0.6693

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Q&A