Aluminum Sheet Stretch-Bend Limits and Fracture Criterion Robert H. Wagoner 1, Changqing Du 2, Dajun...
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Transcript of Aluminum Sheet Stretch-Bend Limits and Fracture Criterion Robert H. Wagoner 1, Changqing Du 2, Dajun...
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
11
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
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)
Stress Ratio at Fracture: AL 6011
17
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
18
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
19
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
20
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
21
3. Empirical Criterion
Stretch-Bend Fracture Stress Limit – AL 6016
22
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
23
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
24
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)
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) = - 0.0079 (R/T)2 + 0.1019 (R/T) + 0.6693
26
Q&A