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AHSS FORMING SIMULATION FOR SHEAR
FRACTURE AND EDGE CRACKING
Ming Chen
United States Steel Corporation
D.J. ZhouChrysler LLC
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Outline
AHSS characteristics
Conventional formability evaluation
Edge cracking
Stretch bending at tight radius
Solutions
Tool/Process modifications
Optimized material selection
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Automotive Sheet Steels
SF590
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AHSS Characteristics
(Advantages)
High tensile strength
High crash energy absorption High dent resistance
Good combination of strength and formability High stretch limit
and durability (Stretch
flange high strength steel)
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AHSS Characteristics
(Disadvantages)
Edge splitting
Stretch bending in tight radius Springback
Tool wear & deflection High press and binder force
Weldability
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Simulation
and Prediction
Conventional formability evaluation
Edge cracking
Stretch bending at tight radius
Case studies
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Conventional Simulation
Prediction
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DP600 Panel Forming
Draw split
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DP780 Forming
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DP980 B-pillar Reinf
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Suspension Member
Edge Stretch Cracking
Stretch flanging out of plane
Edge Stretch in draw in plane
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Poor Correlation in
Predicting Edge Cracking
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Forming Limit Curve and
Hole Expansion Limit
Deformation Mode atstretch edge is Uniaxial
tension
Point A: Limit of idealedge condition
Point C: Limit in worse
condition
Forming Limit Diagram
0
20
40
6080
100
120
140160
-50 -40 -30 -20 -10 0 10 20 30 40 50
Minor Strain %
MajorS
train
FLD
Tension
A
B
C
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Simulation of Conical
Punch Hole Expansion
8
9
10
1112
13
14
15
16
0 1 2 3 4 5 6 7 8 9 10
Punch (mm)
Expan
D(mm)
Inner D
Test
Plot of strain mode along edge
Strain path along uniaxial tension
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Hole Expansion Test
1000
0 XD
DDf
=(HER)
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Lowest HER
590Y - Conical
0.0%
20.0%40.0%
60.0%
80.0%
100.0%
Ream Laser 1.1% 6.4% 13.6% 20.7%
Clearance
HoleExpansionRatio
Burr Up Burr Down
Lowest HER for limit in production environment
Burr inBurr out
DP600
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Case Study 1
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Major Strain Exceeds
Edge Stretch Limit
Principal Strain 24.5 %
DP600
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Case Study 2
DP780 Steel Flanging
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Max stretch is 22% which exceeds the edge
stretch limit
Scallop Pre-form
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Modify Pre-form
deform locations in pre-form stage
undeform locations for
stretch in flanging stage
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Modified Pre-form
(Major strain =10.5%)
No edge stretch
in pre-form
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No Pre-form
(Major strain =15.5%)
Comparison of
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p
Different Pre-forms
Major strain = 22%
No pre-form
Major strain = 15%
Modified pre-form
Major strain = 10%
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Stretch Comparison of
Different Pre-forms
Edge Stretch Comparison
0
0.04
0.08
0.12
0.16
0.2
0.24
0 0.01 0.02 0.03 0.04
Time (ms)
Major
Strain
Scallop Pre-form
No Preform
Modi Preform
Optimal Material Designe
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for High Edge Stretc
Applicatio
SF590 for stretch
greater than 100%
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Stretch Bending Fracture
DP980 Coil#: GE656163010
Severe crackingDP980
DP780
Poor Prediction
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Correlation
DP780
Stretch Bending Limit
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Study for AHSS
Stretch Bending Limit
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Stretch Bending Limit
Material Improvement
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Steel n YS
(MPa)
UTS
(MPa)
Total
Elongation (%)
Yield
Ratio
(5)
High Yield 0.086 744 1023 12.2 0.73 25
Low Yield 0.098 632 1011 14.2 0.63 18
Material Improvement
Stress-Strain curves
0
200
400
600
800
1000
0.00 0.10 0.20 0.30
True strain
Trues
tressinMPa
DP
HY
LY
LY
Improved HER
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AHSS Hole Expansion
0
5
10
15
20
25
30
35
0.0 1.6 4.4
Pre-s train (%)
HoleExp
ansion(%)
Regular
Process2
LY
HY
Improved Stretch
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Bending
DP780LY
DP780HY
Improved Local
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Formability
DP980LY
DP980HY
Summary
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Conventional FLD is suitable for general
deformation mode only
Utilize appropriate hole expansion limit in thesimulation for
edge stretch
Check major strain in stretching edges Check R/T ratio in
stretch bending
Optimal material selection based on specificapplications
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References
M. F. Shi, X.M. Chen, Stretch Flange-ability Limits of
Advanced High Strength Steels, SAE Technical paper #
2007-01-1693
A. Konieczny and T. Henderson, On Formability
Limitations in Stamping Involving Sheared Edge
Stretching, SAE Technical paper # 2007-01-0340
M.S. Walp, A. Wurm, J.F. Siekirk, and A.K. Desai,Shear Fracture
in Advanced High Strength Steels,
SAE Technical paper # 2006-01-1433
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Questions Comments
?
