EXPERIMENTAL INVESTIGATION AND MODELING

OF NONLINEAR VISCOELASTIC PLASTIC

BEHAVIOR OF POLYIMIDE

A. Aniskevich

Institute of Polymer Mechanics, University of Latvia

2

Europe, Latvia, Riga …

3

Outline

• Introduction and objective of the research

• Experimental details

• Our results

• Acknowledgments

4

Institute of Polymer

Mechanics

www.pmi.lu.lv

23 Aizkraukles Str., Riga, LV-1006, Latvia

Since 1963

5

Our research group

• Long-term deformability of polymers and composites;

• Prediction of the durability and deformability;

• Environmental effects.

Prediction of long-term

deformability

Nano-modified composite

matrixes with improved properties

Environmental effects on

deformability

Products made from

recycled plastic

Composite materials with

biomimetic solutions

Technology of fiber-

reinforced polymer-matrix

composites

Development of profiled

grips

Potted anchor with

wedged insert

6

Our research group

• 2 Dr. Habil. Sc.

• 11 Dr. Sc.

• 2 PhD students.

• 2 MSc.

• 2 Master course students.

• 1 Bachelor course student.

• 3 engineers.

Total 23

In 2011:

• 9 journal papers published

or in press.

• 15 conference presentations.

7

Journal Mechanics

of Composite Materials

The bimonthly journal is issued since 1965.

Since the first issue the Journal has been

translated into English and now it is issued by

Springer Science+Business Media, Inc.

8

Conference on Mechanics

of Composite Materials

Since 1965, the Institute has organized 17 conferences.

The number of participants is ca. 150 scientists.

The XVII Conference was held May 28 - June 1, 2012.

May 2012

9

Introduction: Polyimide films

• Tensile strength and initial tear resistance

• Exceptional toughness and resistance to cutthrough and abrasion

• Flame and heat resistanceSpeaker Cones

Pressure-Sensitive Tape

Bar Code Labels Flexible Heater

Flexible display

Hard disk drives

Flexible Circuits

10

Research objective and tasks

• The aim of the present study is to perform a detailed analysis of

strains arising in creep and recovery tests.

– for this purpose elastic and ultimate tensile characteristics of

the material have to be obtained,

– inelastic contribution in deformability of PI should be

investigated,

– deformability of the film should be modelled.

11

Tested material

• Upilex® S UBE industries

Tested specimens had a strip shape of 30 mm width.

Thickness 130 mm.

12

• Standards EN ISO 527-1,

EN ISO 527-2, and ASTM D 638

• Test temperature 202°C,

• 3 - 5 specimens tested

• Testing machine Zwick 2.5

Quasistatic tension tests

13

Quasistatic tension tests

Comparison of Upilex S stress-strain typical diagrams for two strain

rates – slow: e’ 10–5 and fast: 10–2 s–1.

Elastic modulus

Ultimate stress

and strain

Yield point

0

50

100

150

200

250

0 5 10 15 20

σ, M

Pa

ε, %

σ(ε)

Upilex - slow

Upilex - fast

14

Strength and ultimate strain

209

227

100

120

140

160

180

200

220

240

3.3E-05 6.7E-02

σ, M

Pa

e', s-1

Strength

1514

10

11

12

13

14

15

16

17

3.3E-05 6.7E-02

ε, %

e', s-1

Limit strain

15

Elastic modulus

4.865.515.42

5.82

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

3.3E-05 6.7E-02

E, G

Pa

e', s-1

E-Modulus

ε=1-1.5%

ε=0.5-1.0%

16

Yield point determination

• Secant or Offset (0.2%)

• Tangent

σ (ε) + secant

0

20

40

60

80

100

120

140

160

180

200

0.0 1.0 2.0 3.0 4.0 5.0ε, %

σ,

MP

a

Upilex: № 74 @ 800 mm/min

Elastic

Yield (offset)

Yield point

σ (ε)

0

50

100

150

200

250

0 5 10 15ε, %

σ,

MP

a

Upilex: № 74 @ 800 mm/min

Tangents

Yield point

17

Yield stress and strain

2.56 2.73

3.58 3.76

0.00.51.01.52.02.53.03.54.0

3.3E-05 6.7E-02

ε, %

e', s-1

Yield strain ε

Yield ε (offset)

Yield ε (tangent)

115139

178

208

0

50

100

150

200

250

3.3E-05 6.7E-02

s, M

Pa

e', s-1

Yield stress σ

Yield σ (offset)

Yield σ (tangent)

18

Creep tests

• Dead weight loading

• Four loading sections

• Lever with two arms with 1:10

ratio

• Maximal load about 1 kN

• Strip shape specimens

• Grip-to-grip distance was 75 mm

• Gauge length was about 50 mm

for longitudinal strain and 30 mm

for transverse strain

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Strain measurement

Stress

Stress

Thin film

specimenStrainStrain

Strain

Strain

t

e11

e22

Files

PC

Testedspecimen

Photocamera

Strain vs. time graph

20

Strain measurement

Program interface.

21

Experimental determination of the

viscoelastic properties

Strain = Elastic + Viscoelastic (non) linear + Plastic

σ(ε)

0

50

100

150

200

250

0 10 20 30 40 50ε, %

σ, M

Pa

Kapton HN - slow

Kapton HN - fast

Upilex - slow

Upilex - fast

Creep testsQuasistatic tests

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Creep tests

0

1

2

3

4

5

6

0 5000 10000t, s

e,

%

12 MPa 25 MPa 50 MPa75 MPa 100 MPa 125 MPa150 MPa

Creep curves for Upilex S at different stress levels, 3 h tests.

23

Creep isochrones

Deviates from linear!

0

20

40

60

80

100

120

140

160

0 1 2 3 4 5 6

σ, M

Pa

ԑ, %

Isochrones

t=10s

t=1200s

t=10800s

t=10s

t=1200s

t=72000s

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Quasi elastic modulus

In creep loading (10 sec) and recovery unloading (10 sec).

Loading and unloading coincide.

Upilex is linear elastic up to 75 MPa.

0

1

2

3

4

5

6

7

8

0 50 100 150 200

E, G

Pa

s, MPa

Upilex(load)

Upilex(unload)

25

Creep tests

Creep (3 h) and long-term recovery

(one week) test for stress 150 MPa.

Upilex creep 150 MPa

0

1

2

3

4

5

6

7

0 5 10 15ln(t)

e,

%

foto U35

(unload-week)

26

Non recoverable strain

Non recoverable strain 20 h (active : passive = 1:7).

Upilex reveals viscoelastic behavior (no residual strain) up to s = 75 MPa.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 50 100 150 200

enre

c,

%

s, MPa

Non recoverable strain 20 h af ter 3 h creep

Upilex

27

Creep modeling

• Elastic

• Viscoelastic

non linear

• Plastic

( , ) ( ) ( , ) ( , ),crel ve plt t te s e s e s e s

( ) ,el Ee s s

( )1 exp ,ve

n

iii

a tA

E

s se

1 2( ) exp( ),a s s

( , ) .pl

mj b

jj

t c te s s

28

Creep modeling

10 100 1 103

1 104

1 105

0

1

2

3

4

5

6

Time, s

Str

ain

, %

Experimental and

calculated creep curves for

Upilex S for 7 stress levels

from 12 till 150 MPa.

29

Creep modeling

Construction of the generalized curve.

Dots — experiment.

103

105

107

109

1011

0.01

t, s

J.10

0, M

Pa

5.10-3

30

Creep modeling

Calculated curves of creep (a) and creep recovery (b) (solid lines). Dots —

experiment, dashed lines — reversible strains (elastic and viscoelastic

components). σ = 150 (1), 125 (2), 100 (3), 75 (4), 50 (5), and 25 (6) MPa.

10 100 10 103 4

6

5

4

3

2

1

ecr, %

1

2

3

a

t , s

4

5,6

b

10 100 10 103 4 5

10

2

1

1

2

erec, %

31

Conclusions

• Elastic, viscoelastic, and plastic components of material

deformation were experimentally separated and determined.

• In creep tests, the threshold load σ = 75 MPa was determined on

exceeding which irreversible strains arose in the material.

• Creep behavior of the films was described successful. The

calculations were carried out on the basis of the STS principle.

• The nonlinear viscoelastic component was separated from the

general strain of the material by approximating the experimental

data on the accumulation of irreversible strains during loading.

ECCM 15, June 24-28, 2012, Venice, Italy

Acknowledgment

European Regional Development Fund Project:

• ―Support to international cooperation projects on the

physicomechanical investigation of polymer composite

materials‖

Agreement No. 2010/0201/2DP/2.1.1.2.0/10/APIA/VIAA/005

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