paperhtc simtech.pdf0p. p. p. p

8/10/2019 PaperHTC SimTech.pdf0P. P. P. P

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A STUDY OF SPRINGBACK

USING RADIOSS INCREMENTAL What is springback ?

Relevant material properties Mechanics of the bending/unbending

Test case description Model description

Results


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SPRINGBACK MECHANICSThe most important thing

about springback is thatflexure deformation givesimportant springback, butmembrane does not

The rest is about findingwhere the flexure comesfrom

R

zf

fm

+=


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MATERIAL PROPERTIESRELEVANT TO METAL FORMING

Ductility strain hardening (behavior undermonotonic loading)

Hardening type (behavior under cyclicloading)

Anisotropy Transverse Planar

Strain rate sensitivity Friction


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DUCTILITYA basic engineering notion is that material behavior in the first stages of deformation is

approximately elastic, i.e. the material returns to its initial state after the external cause (force)is removed.

Further deformation will be at least partially permanent. For metals, this pattern of permanentdeformation is called plasticity.

After the onset of plastic deformation (yield point) the stress generated in the material continues

to grow (even though at a slower pace) as deformation increases. This phenomenon is called strainhardening. The ability of the material to deform plastically before failure is called ductility.

Engineering strain e

Engineering stress s

Rm

y A%

yielding

necking

ruptureTE

NSILETE

ST


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RADIOSS MATERIAL LAWS

( )nHpK 0+=

nJ

py B+=

Hill / Krupkovsky-Swift(also available for one-step)

Johnson-Cook

Engineering strain e

Engineering stress s

Rm

yA%

log

log

nJ

nH

nnym

mHJ

=


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BEHAVIORUNDER

CYCLICLOADS

Material resistance (yield and ultimate strength) may be significantly different after aprior deformation.

Two idealized models are used:

Isotropic hardening. If loading isreversed after a first monotonic

loading (up to 1), the second yieldingpoint is symmetrical with respect tothe maximum stress in monotonic

loading (-1).

Kinematic hardening. If loading isreversed after a first monotonic

loading (up to 1), the material showsalways the same apparent resistanceto yielding, so that the yielding point

for the reverse load is 01 2 y=

yo

1

1

2

2

monotonicload

cyclicload

E EE

isotropic hardening

monotonic

loading

cyclic loading

E E

1

2 10

kinematic hardening


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1/y

anisotropic

Hill

isotropicvon Mises

2/y

1/y

Initial VM

2/y

isotropic hardening

kinematic hardening

Anisotropy changes theshape of the initial yieldsurface

Hardening type changethe shape of yield surface

during loading


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i t h f


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R

t

Ft

Upper and lower fibers end up with

the same deformation but throughdifferent histories

Plasticdeformation may thereforebe different

-0.05

0

0.05

0.1

0.15

0.2

-4 -2 0 2 4 6 8 10 12curv coord s

eps

eps_sup

epsP_sup

eps_inf

epsP_inf

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-400

-300

-200

-100

0

100

200

300

400

500

0 0.05 0.1 0.15 0.2

sigmaVM_sup

sigmaVM_inf

-400

-300

-200

-100

0

100

200

300

400

500

0 0.05 0.1 0.15 0.2

sigmaVM_sup

sigmaVM_inf

R

t Different deformation cycles meanthat the final stress is different

The section will spring back with afinal curvature even if the shapeafter forming does not have any

ISOTROPIC HARDENING KINEMATIC HARDENING

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FE MODEL

675 elements in blank (2 mm mesh)

Punch (3 mm radius), speed control

Bhl, counter punch, force control(parameter)

Die, 6 mm radius, fixed

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Design Of Experiment

Parameters Krupkowsky hardening coefficient

0.08 0.15 Kinematic hardening coefficient

0 (purely isotropic) 1 (purely kinematic)

BHL restraining stress 15 30 MPa

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Design Of Experiment

Responses Residual stress on wall

Springback shape(wall curvature)

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Residual stress analysis

Springback analysis Correlation

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outer wallinner wall

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Residual stresses vs. curvature/shape

Shape vs. Material characteristics


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erved INWARD SPRINGBACK POINTS

HAVE ALL KINEMATIC HARDENING

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LARGE SPRINGBACK POINTS HAVE


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LARGE SPRINGBACK POINTS HAVEALL ISOTROPIC HARDENING

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The evolution of the hardeningsurface is key to understandingspringback

Can we control springback choosing a

right material ?

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