Context: Materials for transport applications

14-4-2016 2

• 3 credits

• Evaluation: o Oral exam

o Presentation

o Executive summary report

o Active contribution in seminars

• Compulsory readingo Davies, Materials for automotive bodies, Elsevier. 2003

o Yamagata, The science and technology of materials in automotiveengines, Woodhead Publishing. 2005

• Additional literatureo Banabic, Sheet Metal Forming Processes, Springer, 2010

o Illig, Thermoforming: a practical guide, Hanser Verlag, 2001

o Matthews & Rawling, Composite Materials, Elsevier, 1999

1 general aspects of

plasticity

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Strength and stiffness

• Stiffness

o Determined by composition and atom binding energy

• Strength

o Determined by

• Composition

• Microstructure

• Point defects (intersitial or substitutional atoms, vacancies)

• Dislocations

o Affected by

• Mechanical treatment (cold forming)

• Heat treatment (annealing, hardening, tempering …)

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Ductility

• Importance

o Forming

o Crash behavior

• Determined by

o Composition

o Microstructure

o Point defects (interstitial or substitutional atoms, vacancies)

o Dislocations

• Affected by

o Mechanical treatment (cold forming)

o Heat treatment (annealing, hardening, tempering …)

Plasticity: strain definitions

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• Engineering strain / technical strain / Cauchy strain

• True strain / logarithmic strain / Hencky strain

• Green strain

• Euler-Almansi strain

10

L

Le

lnln0

1

L

L

12

1

2

1 2

20

20

21

L

LLG

221

20

21 1

12

1

2

1

L

LLE

Stress definitions

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• Cauchy stress tensor

• Principal stresses

Plane stress condition

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• Principal stresses in plane stress condition

• Maximum and minimum shear stress

o In principal stress terms:

Tresca yield criterion (1864)

Hydrostatic and distortional stress

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• Hydrostatic:

o deforms the body (changes the volume)

o Does not cause plastic deformation

• Distortional stress component:

o distorts the body

o Plastic deformation

ijijij s

ij

Von Mises stress / octahedral stress

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• Invariants of the distortional stress tensor

• Von Mises stress

• Octahedral stress: 2222

3

2

3

1JIIIIIIIIIIIIoct

2222

32

1JIIIIIIIIIIIIVM

Huber-Mises-Hencky yield criterion

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• Yielding starts when J2 reaches a critical value

General

Principal stress

Plane stress

general

Plane stress

principal

Pure shear

uniaxial

22222 IIIIIIIIIIIIy

222IIIIIIy

231

223

212

21133

23322

22211

2 62 y

212

2222211

211

2 3 y

123 y

1 y

Graphic representation

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I

Compressiontest

Tensile test

Tensile testSphericalpressurevessel

Sphericalbathosphere

Torsion test

Torsion test

Compressiontest

Von Mises

Tresca

Uniaxial loading

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2 Forming processes

Shaping processes

Stretch forming – deep drawing

Defects in deep

drawing

Parameters influencing

formability

Example

3 formability of metals

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Formability definitions

• Work-hardening factor ‘n’

o Strengthening during plastic deformation

o Related to stretching

o Important for energy absorption and impact

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Importance of work hardening n

• Importance:

o Stretch forming

o Energy absorption

• Determined by microstructure;

dislocations, point defects :

o composition

o Heat treatment

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Formability definitions

• Anisotropy factor r

o Related to part thinning during deformation

lw

w

t

wr

Directional dependence of r

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Coefficient of normal anisotropy

• Depends on strain

4

2 90450

rrrrm

• Variation of normal anisotropy

• Responsible for earing

4

2 90450

rrrr

Planar anisotropy

Biaxial anisotropy coefficient

• Barlat Pöhland

o Flatwise compression ° Biaxial tensionRD

TDbr

Consequences of anisotropy

• Tresca and Von Mises criteria no longer valid

• More yield stresses needed

• Yield criterion for anisotropic materials: Hill1948

1222 212

213

223

221

213

232 NMLHGF

2

2

2

222

222

222

12

12

12

1112

1112

1112

TN

SM

RL

ZYXH

YZXG

XZYF

Hill 1990

b: yield stress in biaxial tension

: yield stress in pure shear

Further developments

• New criteria for the complex steel alloys for automotive

Experimental correlation

• Further reading: D. Banabic, Sheet Metal Forming Processes,

Springer, 2010

4 Determination of

formability

• Tensile tests

o n

o r

o Plasticity during necking

• Forming limit tests

o Punch stretching methods: Erichsen test, Hecker test

o Deep drawing methods: Swift test

o Forming limit methods: Nakazima test

Punch stretching methods

• Erichsen test

o Indentation depth (mm)

• Hecker test

Deep drawing methods

• Swift test

o Limit drawing ratio

LDR=𝐷𝑚𝑎𝑥

𝑑

Forming limit strain diagram

Nakazima test

• Forming test with varying boundary conditions

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Determination of deformability limits:

Hecker method• Deformation of a printed circular grid

o Largest strain 1

• Always positive

o Smallest strain 2

• Positive or negative

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Forming limit curve

n important

r important

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Influencing parameters

Other parameters

• Temperature

• Strain rate

• Punch curvature

• Pressure

• Dimensions of the grid

• …

Alternative measurement

method:

Digital Image Correlation

B. Van Mieghem et al. Digital image correlation for on-line wall

thickness measurements in thick gauge thermoforming. Key

Engineering Materials (2013)

In e

ver

y s

tep o

f th

e

pro

cess

Introduction – Digital Image Correlation

47

Time t

UndeformedTime t’

Deformed

Def

orm

atio

n

mat

rix

Corr

elat

ion

coef

fici

ent

DIC in forming - Challenges

48

• Strain > 100% is not exceptional when judging strain,

choice of strain definition is important

• Strain rate 0,01/s up to 10/s

• Speckle technique on sheet (low surface energy)

o Custom paint

o Spray/Print

• Forming temperature 100-300°C

• Big displacements in 3D

o Field of View

o Depth of Field

o Lighting

DIC in thermoforming

49

366 mm

59

mm

16

9 m

m

15

3 m

m

63

mm

197 mm

147 mm

Top view

Side view

75

mm

160 mm

13

4 m

m

Top view

Side view

Stress relaxation

Bubble inflation

DIC in thermoforming - Applications

50

• Full field in-situ thickness maps

• Influence of extrusion anisotropy

• Simulation optimisation

• Process optimisation

• ...

Applications – Full field thickness maps

51

• Symmetric products are not always symmetric in thickness

• Find cause by measuring in-situ during the process

Simulation

Thickness symmetry

DIC

Shape

symmetry

DIC

Thickness

asymmetry

Applications – Influence of extrusion

anisotropy

52

B. Van Mieghem et al. Impact assessment of extrusion anisotropy on quality of thermoformed products.

Composites Week @ Leuven and TEXCOMP-11 Conference (2013)

Final thickness distribution

Asymmetric thinning due to sag

Stress relaxation

53

Applications – Simulation optimisation

• Cross sectional thickness measurement

• What influences the thickness distribution

• Sheet mould contact

o Temperature dependent friction

o Heat transfer coefficient air/sheet

o Heat transfer coefficient mould

Applications – Simulation optimisation

54

Use of DIC to determine FLC

Example: steel

Effect of history and

crystallographic texture

on anisotropy

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Steel production

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Steel production 2

• Improvements for automotive:

o AK (Al-killed)

• Binding of Al with N: improved ageing resistance

• Pancake grains

o Vacuum degassing:

• Removal of all inclusions

• Low level of impurities

• Addition of Ti and Nb

- Binds C and N

• IF-steel (interstitial free)

• Ultradeep drawable steel

- Improved r-factor

• Complete ferritic matrix

- C content very low C < 0.0002%

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Production of steel slabs

• Continuous casting

• Ingots

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Sheet steel manufacturing

• Hot rolling to thickness of 3mm-1.6mm at 900-1200°C

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Sheet steel manufacturing

• Pickling with hydrochloric acido Removal of oxide skin

• Cold rolling to 0.5mm

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Effect of cold work on anisotropy

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Annealing

• batch

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Continuous Annealing (CAPL)

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Annealing differences

batch CAPL

Slow heating: 30 hours Rapid heating: 90 s

Slow cooling: 25 hours Fast cooling: 10 min

Coarse grain Finer grain

Stronger

Higher ductility

High r: 1.6-2.1 Low r 1.0-1.4

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skin pass

• 1% deformation after annealing

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Lüdersbanden

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Surface topography

• Importance of the surface topography

o Forming process

• Lubrication characteristics

o Paint quality

• Adhesion

• gloss

• Determined by the skin pass

High strength steels

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IF HSS

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• Vacuum degassedo Removal of C, N, O

o Reduced dent resistence

• Complete ferritic

• Alloyed with Mn, Si, P

• Further strengthening via bake hardening

Second generation AHSS

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TWIP

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• TWIP: twinning induced plasticity

• High Mn (17-24%)

• Fully austenitic at room temperature

• Cold forming causes twinning

o Fine sized austenite grains increased strength

• Expensive

TRIP > < TWIP

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Consequences

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Steel versus

aluminium

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Al alloys used in automotive

Comparison 5xxx and 6xxx

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5xxx 6xxx

Formability +++ ++

Corrosion resistance +++ ++

Heat resistance > 65°C Not good for >

% Mg

+++

Crash performance +++ +++

Surface quality Stretcher strain

markings

+++

Strength + +++

Effect heat treatment on strength decrease increase

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Deformability: tensile stress-strain curve

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Deformability: forming limit diagram

Спасибо