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

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

Background 3 3

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

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

Draw-Bend Fracture Test (DBF) 6 6

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

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

Adjustable Wrapping Angles 9 9

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

2. Results 11 11

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

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

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

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

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

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

3. Empirical Criterion 21 21

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

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

Stretch-Bend Fracture Stress Limit – AL 6016& 6022 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 24

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 25

Q&A 26 26