Insights into the Simulation of Failure of Ductile Plastics · 2017. 4. 25. · • Shear...

1 © 2017 carhs.training GmbH

Insights into the Simulation of Failure of Ductile Plastics

M. Lobdell, B. Croop, and H. Lobo DatapointLabs Technical Center for Materials, USA automotive CAE Grand Challenge 2017 April 5–6, 2017


Company Introduction


Mission

We strengthen the materials core of manufacturing enterprises to facilitate their use of new materials, novel manufacturing processes, and simulation-based product development.


Outline

Process • Use best in class test methods for properties • Apply candidate material models • Validate against high reliability complex experiments (DIC)

Presentation

• Identification of factors • Test methods used • Findings & observations • Impact on simulation


Factors

• Non-linear elasticity • Deviatoric plastic strain occurs prior to yield Lobo& Hurtado (2006)

• No increase in volume strain prior to yield Lobo, Croop, &Roy (2013)

• Onset of volumetric straining at yield Lobo, Croop, &Roy (2013)

• Strain localization coincides with onset of volumetric straining • Yield surface is not von Mises • Volumetric and deviatoric behavior in flow region • Failure is accompanied by rapid volumetric expansion

Other Factors not considered here • Visco-elastic (time-based behavior) • Property change over product operational temperature • Property change with environmental exposure


DIC Operation

DIC Principle • Speckle pattern is broken into facets (elements) • Speckle pattern is tracked frame to frame • Calibration of a volume is performed through

measuring calibration panel in various orientations in the test space.

• Strains can be captured on the micro-strain level • Strain field can be mapped over the actual part

image DIC Utilization in this work

• Local y, x, z strains are measured • Surface strains for validation of simulation


Obtaining true stress-strain with localized DIC measurement

Effect of gage length selection

0

20

40

60

80

100

120

140

160

180

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

True Strain (unitless)

True

Stre

ss (M

Pa)

1 mm GL

2 mm GL

4 mm GL

25 mm GL

50 mm GL

y x

z


Non-linear elasticity in ductile plastics

Absence of linear elastic region (unlike metals) Presence of visco-elastic behavior (not considered in this work)

Aluminum

Polycarbonate


Determining onset of plasticity

Incremental increasing load-relaxation cycles: capture unrecovered strain

0

200

400

600

800

0 200 400 600 800 1000

time (sec)

Load

(N)

0

0.5

1

1.5

2

0 200 400 600 800 1000

time (sec)

exte

nsio

n (m

m)

plas10

0

0.5

1

1.5

2

2.5

0 2000 4000 6000 8000

time, sex

tensio

n, mm


The onset of deviatoric straining

Non-linear elastic limit occurs before the yield point

0

10

20

30

40

50

60

0.0 1.0 2.0 3.0 4.0 5.0 6.0True Strain (%)

True

Str

ess

(MPa

)


Elasto-plastic model limitations

Unable to capture non-linear elastic behavior Cannot correctly capture deviatoric strain prior to yield Cannot capture post-yield volumetric strain


Elasto-plastic model validation

Instron 8872 universal testing machine (UTM) 1 mm/min displacement of nose Apply speckle patter to part to allow use of DIC strain capture Two camera DIC to capture

3D strain


Deformed Part

•Loaded past yield •Observed symmetric buckling inwards •Slight indentation of support pins causing stress whitening on the reverse side of the part


Simulation with Abaqus: material model

Polypropylene • Tensile and Density Test • Elastic

- E = 1572 [MPa] - υ = 0.29

• Plastic curve (Right) • Density

- ρ = 7.9 E-06 [tonne/mm3] Measured at QS speeds


Strain field validation

Abaqus DIC


Comparison of Simulation to Experiment

• Force vs. loading nose displacement • Similar response throughout


DIC Comparison of Simulation to Experiment

• Local strain vs. Displacement • Diverges after 2 mm • Corresponds to yield

on stress-strain curve


POST YIELD BEHAVIOR


3D Stress-strain measurements for plastics

Actual measurements in 3D Local y, x, z strains are measured

0.20

10.20

20.20

30.20

40.20

50.20

60.20

70.20

80.20

90.20

100.20

0.0 50.0 100.0 150.0 200.0 250.0 300.0

Time (s)

Stra

in (%

)

EpsX %

EpsY %

EpsZ %

0.20

10.20

20.20

30.20

40.20

50.20

60.20

70.20

80.20

0.0 50.0 100.0 150.0 200.0 250.0 300.0

Time (s)

Stra

in (%

)

EpsX %

EpsY %

EpsZ %

PC

ABS

0

50

100

150

200

250

300

350

400

450

0.0 100.0 200.0 300.0 400.0 500.0 600.0 700.0

Time (s)St

rain

(%) EpsX %

EpsY %

EpsZ %

PP


True stress v. volumetric strain and true strain

True stress calculated from x-z strain Y-strain is localized, true Volumetric strain obtained from YXZ strain Fail strain is measured Classical stress-strain does not correlate

PC

ABS

PP

0

5

10

15

20

25

30

35

40

45

50

0 10 20 30 40 50 60 70 80

Strain (%)

True

Stre

ss (M

Pa)

EpsY %

EpsV%

Classical

extruded sheets 0

20

40

60

80

100

120

140

160

180

-10 10 30 50 70 90 110

Strain (%)

True

Stre

ss (M

Pa)

EpsY %

EpsV%

Classical

0

20

40

60

80

100

120

140

160

180

200

0 50 100 150 200 250 300 350 400

Strain (%)Tr

ue S

tress

(MPa

)

EpsY %

EpsV%

Classical


Transverse v. axial strain

X and z strains may not coincide May depend on material and

processing

PC

ABS

PP

0

5

10

15

20

25

30

35

40

45

50

0.0 20.0 40.0 60.0 80.0 100.0

Axial Strain (%)

Tran

sver

se S

train

(%)

EpsX %

EpsZ %

0

2

4

6

8

10

12

14

16

0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0

Axial Strain (%)

Tran

sver

se S

train

(%)

EpsX %

EpsZ %

0

10

20

30

40

50

60

70

0.0 100.0 200.0 300.0 400.0 500.0

Axial Strain (%)

Tran

sver

se S

train

(%)

EpsX %

EpsZ %


Yield behavior in plastics

Yield surface – Available test modes • Tensile • Compressive (CLC) • Shear (classical or Iosipescu) • Biaxial

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14 16 18 20

Eng Strain (%)

Eng

Stre

ss (M

Pa)

Biaxial

Compressive

Shear

Tensile

-1.5

-1

-0.5

0

0.5

1

1.5

-1.5 -1 -0.5 0 0.5 1 1.5

SigX/TensDir1 (MPa/MPa)

SigY

/Ten

sDir1

(MPa

/MPa

)

0

10

20

30

40

50

60

0 10 20 30 40 50 60 70 80 90

Eng Strain (%)

En

g S

tre

ss (

MP

a)

Compressive

Shear

Tensile

PC

PP

PC


Tensile experiment validation with measured FAIL strain

ABS, LS-DYNA MAT_024, 100/s, FAIL at 0.44


Dart impact validation for ABS

LS-DYNA MAT_024 • Failure strain is arbitrary • Much greater than tensile fail strain • Model extrapolation is needed • Model cannot predict softening behavior


Rationale for using larger fail strain

Falling dart is a biaxial experiment Measured biaxial fail strain for ABS is 1.8 (Ericchsen Cupping experiment combined

with DIC, with advice from Dr. H. Gese.)


Incorporating volumetric behavior

SAMP material model Post-yield volumetric behavior converted to plastic Poisson’s ratio


Applying failure criteria to SAMP

Using FAIL = biaxial fail strain approaches test data Softening behavior is predicted Using triaxiality v. fail strain does not work well


LS-DYNA Validation of Dart Impact - PP

MAT_024 material model fitted with Matereality


Falling dart validation for PP

MAT_024 Material Model • Various rate dependency options

VP=1 Fail Strain extrapolated to 1.2 SAMP less important where no softening behavior is seen


Insights

Build a post yield material model containing deviatoric and volumetric terms Decouple two effects

• Allow deviatoric straining prior to yield • Volumetric straining starts at yield

Effect of non von-Mises yield surface should be considered Look at failure strain as a function of stress triaxiality (eg. SAMP)

• Vital for failure prediction • Alternately, arbitrary model extrapolation is required • Not easy to objectively measure fail strain at different stress triaxiality

• Tensile is feasible • Biaxial is feasible • Shear is not feasible; reverts to tensile mode for ductile plastics

Error accumulates due to model infidelities


20 years of DatapointLabs

Focused on “materials in simulation” since 1995 Deep domain expertise

• physics of materials, CAE parameter conversion, software for materials

Insights into the Simulation of Failure of Ductile Plastics · 2017. 4. 25. · • Shear...

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