Aluminum Sheet StretchBend Limits and Fracture Criterion IABC

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

Outline 1. Background Draw-Bend Fracture Test Wrapping Angle Fixture 2. Results 3. Empirical Criterion 4. Conclusions 2

1. Background 3

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

1. Background: Draw-Bend Fracture Test 5

Draw-Bend Fracture Test (DBF) 6

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

Fixtures For Adjustable Wrapping Angles Design Concept Wrapping angle : 90 o, 75 o, 60 o R uf V 1 Fixture for wrapping angle adjustments 8

Adjustable Wrapping Angles 9

Phenomenological Fracture Types V 2 Type III Type II 65 o Type I: Tensile fracture (unbent region) Type II: Shear fracture (not Type I or III) Type III: Shear fracture (fracture at the roller) Type I 65 o V 1 The maximum load carrying capacity of the sheet sample is limited by Type III (fracture at the roller) fracture 10

2. Results 11

Aluminum Sheet Samples – 90 o Fractures at Tool radii

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

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

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

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: 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). 17

Material Rolling Direction Effect for AL 6011 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. 18

Stress Ratio at Fracture: AL 6022 Maximum Fracture Stress of AL 6022 RD: 275. 4 MPa Maximum Fracture Stress of AL 6022 TD: 272. 6 MPa 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. 19

Material Rolling Direction Effect 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. 20

3. Empirical Criterion 21

Stretch-Bend Fracture Stress Limit – AL 6016 Remarks: 1) Stretch-Bend Fracture Stress Limit is set to the lower bound of Stress Ratio. 22

Stretch-Bend Fracture Stress Limit – AL 6022 Remarks: 1) 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