1. Aluminum Sheet Stretch-Bend Limits and Fracture Criterion
IABC 2012, Frankfurt, Germany,
May, 2012
Robert H. Wagoner1, Changqing Du2, Dajun Zhou2
1The Ohio State University, Department of Material Science and Engineering, Columbus, OH, USA
2Chrysler Group LLC, Advance Stamping Manufacturing Engineering, Auburn Hills, Michigan

2. Outline Background Results Empirical Criterion Conclusions
Draw-Bend Fracture Test
Wrapping Angle Fixture
Results
Empirical Criterion
Conclusions 2

3. Background 3 3

4. Shear Fracture of Sheet Materials
Fractures along small tool radii observed during stamping trials
AHSS Rail
Aluminum Panel
Focus of this study

5. Draw-Bend Fracture Test
Background:
Draw-Bend Fracture Test 5 5

6. Draw-Bend Fracture Test (DBF)6 6

7. Draw-Bend Fracture Test
Draw-Bend Test, 25mm Strip Width 7

8. Fixtures For Adjustable Wrapping Angles
Design Concept
Wrapping angle : 90o, 75o, 60o R V1 uf
Fixture for wrapping angle adjustments 8 8

9. Adjustable Wrapping Angles9 9

10. Phenomenological Fracture Types
Type I
Type II
Type III
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)
The maximum load carrying capacity of the sheet sample is limited by Type III (fracture at the roller) fracture 10 10

11. 2. Results 11 11

12. Aluminum Sheet Samples – 90o
Fractures at Tool radii

13. Aluminum Sheet (6016) Samples – 75o
Fractures at Tool radii

14. Aluminum Sheet (6016) Samples – 60o
Fractures at Tool radii

15. Shear Fractured Section
Local Fracture: not able to be predict using traditional Forming Limit Curve (FLC)

16. 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) 16

17. Stress Ratio at Fracture: AL 6011
Maximum Fracture Stress of AL 6016 RD: 243 MPa
Maximum Fracture Stress of AL 6016 TD: 244 MPa
Remarks:
Wrapping angle has no effect on the stress ratio.
The Stress ratio at R/T near 1.0 is about less than 0.8 (80%) of the sheet maximum stress.
The Stress Ratio increases as R/T Ratio increases.
The stress Ratio reaches 1.0 when R/T Ratio >4.5. (Tensile fracture observed). 17

18. Material Rolling Direction Effect for AL 6011
Remarks:
In general, insignificant difference was found between RD and TD in stress-ratio.
No Shear fracture RD sample at R/T > 4.
Shear fracture samples at R/T=4 in TD sample only. 18

19. Stress Ratio at Fracture: AL 6022
Maximum Fracture Stress of AL 6022RD: MPa
Maximum Fracture Stress of AL 6022 TD: MPa
Remarks:
Wrapping angle has no effect on the stress ratio.
The Stress ratio at R/T near 1.0 is about 0.8 (80%) of the sheet maximum stress.
The Stress Ratio increases as R/T Ratio increases. 19

20. Material Rolling Direction Effect
Remarks:
In general, R/T < 3, RD samples have higher stress-ratio than TD.
Shear fracture samples at R/T>3.5 in TD sample only. 20

21. 3. Empirical Criterion 21 21

22. Stretch-Bend Fracture Stress Limit – AL 6016
Remarks:
Stretch-Bend Fracture Stress Limit is set to the lower bound of Stress Ratio. 22

23. Stretch-Bend Fracture Stress Limit – AL 6022
Remarks:
Stretch-Bend Fracture Stress Limit is set to the lower bound of Stress Ratio. 23

24. Stretch-Bend Fracture Stress Limit – AL 6016& 6022
A Stretch-Bend Limit Curve may be applied for both AL 6016 and 6022 :
(Stress Ratio) = (R/T) (R/T) 24

25. 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) = (R/T) (R/T) 25

26. Q&A 26 26