THE EFFECTS OF AHSS SHEAR EDGE CONDITION ON EDGE THE EFFECTS OF AHSS SHEAR EDGE CONDITION ON EDGE THE EFFECTS OF AHSS SHEAR EDGE CONDITION ON EDGE THE EFFECTS OF AHSS SHEAR EDGE CONDITION ON EDGE

FRACTUREFRACTUREFRACTUREFRACTURE

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FRACTUREFRACTUREFRACTUREFRACTURE

Mike ShihMike ShihMike ShihMike Shih

Constantin Chiriac, Ming F. Shi

United States Steel Corporation

Automotive Center

Edge Fracture

Poor Shearing Edge Condition ?

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• Edge Fracture in Stamping

• Conventional FLD fails to predict

Outline

• Background

• Laboratory Shearing Test

– Computer Control Hole Punch/Shear Process

• Evaluation of Punch/Shear Force

• Validation Edge Fracture Test

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• Validation Edge Fracture Test

– Hole Expansion Test

• Evaluation of Shear Edge Condition

• Conclusions

Shear Rake angle Θ3

Front View

Straight Edge Shearing ( Blanking die , Trimming die )

Background

Shear Blade

Clearance

Sheet Metal

Blade Rake angle Θ1

Back Cut angle Θ2

(clearance angle)

Die Θ2

Blank Holder

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Hole Punching ( Hole piercing die)

Die

Clearance

Punch

Blank Holder

Sheet Metal

Θ1 = Θ3

Blank Holder

Clearance

Punch

Bevel Shear

θ

Clearance

Specimen holding fixture

Die

Laboratory Shearing TestComputer control hole punch/shear process

• Computer control

• Bevel angle

• Die clearance

• Shear speed

• Shear load

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Die Spring

Punch Pd

Dd

Punch

Die

Sheet steel

Holder

Tθ Y

X

Pd

tanθYX =

H

H=Pd X tan Θ

(1)

TY

X

cosθFFN ×=

θ

sinθFFf ×=

F

(2)

Schematic of Bevel Shear

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Shearing Test Conditions:

• Bevel Shear Angle: 0, 3, 6, 9, 12, 18, 24 degrees

• Die Clearance: 5, 10, 15, 20, 25, 30 % of test steel thickness

• Low speed

H: height for each bevel angle

Punch force F ~ X ~ 1/ tan ΘΘΘΘ

tanθYX

F: Punch ForceSlug

Y: Punch Tip Penetration

When Y-T=H : penetration near completed

MaterialThickness Yield Tensile Total n

(mm) Strength (MPa) Strength (MPa) Elong. (%) Value

DP780 1 511 876 16.9 0.118

MECHANICAL PROPERTIES OF TESTED STEELS

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DP780 1 511 876 16.9 0.118

DP980 1.2 573 1003 15.1 0.106

GradeThickness

[mm]

Punch diameter

[mm]

Die diameter

[mm]Clearance [%]

DP780 1.0 10.0 10.1 5

DP780 1.0 10.0 10.2 10

DP780 1.0 10.0 10.3 15

DP780 1.0 10.0 10.4 20

DP780 1.0 10.0 10.5 25

Hole Punch/Shear Conditions

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DP780 1.0 10.0 10.5 25

DP780 1.0 10.0 10.6 30

DP980 1.2 10.0 10.2 8

DP980 1.2 10.0 10.3 13

DP980 1.2 10.0 10.4 17

DP980 1.2 10.0 10.5 21

punchdie DtClD +×= 100/2

Transverse Shearing Longitudinal Shearing

Hole Shearing Orientations

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Material Rolling Direction

RR

Punch Load vs. Shear Angle

(DP780 1mm, transverse, die clearance 5%)

500

10001500

20002500

30003500

40004500

5000P

ea

k L

oa

d (

lbf)

5

10

15

20

25

30

35

40

45

Ra

tio

Peak 1Peak 2RatioRatio = 1/ tan Θ

Evaluation of Punch / Shear Force

Group 1 Group 2

Over 55% load reduction

Conventional Empirical

formula :

LTUTS0.7F ×××=

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0500

0 3 6 9 12 18 24

Shear Angle (degree)

0

5

0

500

1000

1500

2000

13 13.5 14 14.5 15 15.5 16 16.5 17

Lo

ad

Time

Peak 1 Peak 2

0

500

1000

1500

2000

13 13.5 14 14.5 15 15.5 16

Load

Time

Peak 1

Die Spring Force

0

500

1000

1500

2000

13 13.5 14 14.5 15 15.5 16 16.5 17

Loa

d

Time

9P-C5

500

1000

1500

200024P-C5

Lo

ad (

lbf)

Increase Shear Angle

From 9 to 24 degree

Reduce Peak 1

increaseT∆

Peak 1

Peak 2

20009P-C30

Increase Die

Clearance

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013 14 15 16 17 18 19 20

Time

0

500

1000

1500

13 13.5 14 14.5 15 15.5 16 16.5 17

Load (

lbf)

Time

Clearance

Reduce Peak 2

Inc. Peak 1

Comparison of Punch Penetration at

Load Peak 1 and Peak 2

4

5

6

Pe

ne

tra

tio

n (

mm

) Y1Y2H

Group 2

Punch Penetration

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0

1

2

3

0 3 6 9 12 18 24

Shear Angle (Degree)

Pe

ne

tra

tio

n (

mm

)

Group 1Group 2

Top View

Bottom View

Curvature

Slug

Photograph of Punch/Shear Samples

3 degrees

Peak 1

6 degrees

Peak 118 degrees

Peak 2

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Bottom View

3 degrees

Peak 16 degrees

Peak 118 degrees

Peak 2

Large

Rollover

Faster shearing

Slower shearing

Group 1 Group 2

Punch Load vs. Die Clearance

( DP780 1 mm, Transverse Shearing Direction )

4000

5000

6000

Pe

ak

Lo

ad

(L

bf)

0

3

6

9

12

18

24

Punch Force – DP780 1mm

Over 50 % load reduction

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0

1000

2000

3000

5 10 15 20 25 30

Die Clearance (%)

Pe

ak

Lo

ad

(L

bf)

Over 50 % load reduction

Effects of Hole Shearing Orientations

1500

2000

2500

3000

Peak L

oad

(L

bf)

3-Trans

3-Long

6-Trans

6-Long

9-Trans

9-Long

Punch Force – DP780 1mm

Difference is larger at 3 degrees

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0

500

1000

1500

10 15 20

Die Clearance (%)

Peak L

oad

(L

bf)

9-Long

Punch Load vs. Die Clearance

(DP980 1.2 mm, Longitudinal and Transverse)

4000

5000

6000

7000

Pe

ak

Lo

ad

(L

bf)

0

3-L

3-T

6-L

6-T

9-L

9-T

Punch Force – DP980 1.2 mm

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0

1000

2000

3000

4000

8% 13% 17% 21%

Die Clearance

Pe

ak

Lo

ad

(L

bf)

9-T

Validation Edge Fracture Test Hole Expansion Test

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• Hole expansion test

• Specimen orientation : Burr Up

• Conical punch

• 1 inch/minTinus-Olsen formability machine

DP780 , 1mm, Transverse

40%

50%

60%

Ho

le E

xp

an

sio

n R

ati

o

flat3 degree6 degree9 degree12 degree18 degree24 degree

Effects of Bevel Shear Angle and Die clearance

Over 51 % improvement

3 or 9 degrees ?

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0%

10%

20%

30%

5% 10% 15% 20% 25% 30%

Die Clearance

Ho

le E

xp

an

sio

n R

ati

o

Dp780, 1mm

40%

50%

60%

Ho

le E

xp

an

sio

n R

ati

o

flat 3 degree-L

3 degree-T 6 degree-L

6 degree-T 9 degree-L

9 degree-T

Effects of Material Shearing Orientations

3 or 6 degrees

15% < CL < 20%

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0%

10%

20%

30%

10% 15% 20%

Die Clearance

Ho

le E

xp

an

sio

n R

ati

o

DP980, 1.2 mm

25%

30%

35%

40%

45%

50%

Ho

le E

xp

an

sio

n R

ati

o

flat

980-3-L

980-3-T

980-6-L

980-6-T

9 degree-L

9 degree-T

Effects of Material Shearing Orientations

Over 60 % improvement3, 6 and 9

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0%

5%

10%

15%

20%

25%

8% 13% 17% 21%

Die Clearance

Ho

le E

xp

an

sio

n R

ati

o

Evaluation of Shear Edge Condition

Rollover depth

Burnish surface

Fracture angle

Penetration depth

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Burr height

Rough fracture surface

Fracture depth

Rough fracture surface Flat Punch

Smooth fracture surfaceAngle Punch

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No Visible Burr With Visible Burr

50 X 50 X

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Die Clearance < = 20 % Die Clearance >=25 %

~ 0.138 mm~ 0.006 mm

Material Rolling Direction

Transverse Shearing

R

Longitudinal Shearing

R

HET Fracture Locations / Surface

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R

HET fracture locations

along rolling direction

R

1

2

3

R1

2

3

Longitudinal Shearing (9 degrees, 10% CL)

No Fracture

Shear direction

Shear direction

Fracture

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R 1

3

HET fracture locations

2

Location 1 3D FormationLocation 2

No Fracture

Shear direction

Location 3

Large Plastic

Deformation

Transverse Shearing

(12 degrees, 15% CL)

Location 1

3D Formation

Fracture location

at 1 or 3

Fracture

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R

1

2

3

Location 3

Large Rollover

On trailing edge of the bevel shear punch

(Bevel Shear Angle > 9 degrees)

Tensile mixed type failure

Fracture

12 degree, 15 % clearance- Loc. 3

Large rollover

Location 3

Cause for relative larger load

peak 2 - tensile /shear mixed

type failure

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18 degree, 15 % clearance- Loc. 1

Back Cut

Negative rollover

0

Burnish Depth

Burnish Depth vs. Die clearance

(Dp780, Location 1, Transverse)

25

30

35

Bu

rnis

h D

ep

th

(% o

f m

ate

rial th

ickn

ess)

F-B

3T-B

6T-B

9T-B

12T-B

18T-B

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0

5

10

15

20

10 15 20

Die Clearance (%)

Bu

rnis

h D

ep

th

(% o

f m

ate

rial th

ickn

ess)

24T-B

Rollover Depth

Rollover Depth vs. Die Clearance

(DP780, Location 1, Transverse)

15

20

25

Ro

llo

ve

r D

ep

th

(% o

f T

hic

kn

es

s)

F-Roll

3T-R

6T-R

9T-R

12T-R

18T-R

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-5

0

5

10

10 15 20

Die Clearance (%)

Ro

llo

ve

r D

ep

th

(% o

f T

hic

kn

es

s)

18T-R

24T-R

� Beveling shearing/piercing process has been developed at a lab base

hydraulic press and has been successfully used for piercing holes for

AHSS.

� The maximum shearing force can be significantly reduced more than

50% when a beveling angle is used during shearing. The shearing

force depends also upon the die clearance during shearing.

Conclusions

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� The optimal shearing condition results in more than 60% improvement

in the hole expansion ratio when compared to conventional flat head

punching process.

� The optimized shearing condition for piercing and shearing AHSS is

the combination of a beveling angle between 3 and 6 degrees and a

17% die clearance and the shearing direction parallel to the material

rolling direction.

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