numerical simulation of polymeric - altair technology...

Numerical simulation of polymeric

foams under impact loading

presented by

Robert Schilling, Ford-Werke GmbH, Köln

Erwan Mestres, Altair Development France, Antony

Paul Du Bois, CAE Consultant, Offenbach/Main

Hartwig Nahme, Fraunhofer-Institut für Kurzzeitdynamik - Ernst-Mach-Institut, Freiburg

EHTC 2008, Strasbourg, 01. October 2008

© FAT October 2008 Folie 3

Content

� Introduction

� Experiments: Test plan, setup and results

�Numerical simulation of foams

� Validation example: Sphere impact

�Conclusion


Introduction

� In the mid of the 1990th crash simulations were already a standard tool for car development

� The first sophisticated FE dummy models were available

� There were issues regarding the modeling of foams

� In the case of complex loading – not uniaxial compression – the

modeling issues were significant


Introduction: Typical front crash model in 1995

Foam modeled with spring elements


Introduction: FAT working group „Foam“

�Autumn 1996 FAT working group

27, sub group „Foam“ was founded

�Chairman: Christian Stender, Volkswagen AG, Wolfsburg

�Companies involved: AUDI,

Autoliv, Daimler, Ford, Opel, Porsche, Karmann, Keiper, Johnson Controls and

Volkswagen, Bayer, BASF

� Tests: Hartwig Nahme, EMI, Freiburg

� Simulations: Paul Du Bois,

Offenbach/Main

Strasbourg


Introduction: Automotive foam applications

�Phase 1: Study on foam applications in automotive industry

�Classifciation of foams based on type of application and mechanical

behaviour:

� Soft foam: fast elastic springback, small energy absorption,

(e.g. seat foam cushion)

� Padding foam: destructible, no springback, high energy

absorption, (e.g. PUR foam Bayer Bayfill EA 64IF80)

� PUR bumper foam: delayed, partial springback, mediumenergy absorption, (e.g. Bayer Bayfill EA 62IF70)

� EPP foam: delayed, partial springback, medium energyabsorption, (e.g. BASF Neopolen P)


Experiments: Characterization of foams

Phase 2 – Phase 5: Foam tests under different loading conditions forthe 4 different foam families (� enhancement of foam models in

LS-Dyna, Pamcrash and Radioss)

�Quasistatic compression, multi-compression, shear, compression-

shear, tension, hydrostatic compression

�Dynamic compression, multi-compression, shear, compression-

shear, compression relaxation, tension

�Dynamic sphere impactor tests


Experiments: Material database of foams

Phase 6 – Phase 7: Foamtests of foams with different

densities PUR bumper foams, EPP foams (� development

of material cards for foammodels in LS-Dyna, Pamcrash and Radioss)

Cuboid 400*400*40

[mm]

Sphere impact

(big)

DKg

Cuboid 200*200*40 [mm]

Sphere impact(small)

DKk

Cube150Dynamic tensionDZ

Cube0.001Quasistatictension

QZ

Cube50,100,160Dynamic

compression

DD

Cube0.01Quasistatic

compression

QD

Type of specimendε/dt [1/s]Type of test

30, 50, 80, 110, 120, 130, 140, 150 170EPP

50, 70, 90, 110PUR Bumper

50PUR Padding

Density [g/l]Foam

QZ, PUR bumper foams

0

100

200

300

400

500

600

700

0.000 0.010 0.020 0.030 0.040 0.050 0.060 0.070 0.080

Dehnung

Sp

an

nu

ng

/ k

Pa

50-QZ

70-QZ

90-QZ

110-QZ

Strain

Str

ess /

kP

a


Experiments: Test set-up

Dynamic compression test

Impactor

Specimen

Load cell

Loading direction

B/W trigger

Block

Sphere impactor test

Block

Sphere

Specimen

Load cell

Accelerometer

Dynamic tension testImpactorSled

Track

BlockSensor

Specimen

Loading plate


Experiments: EPP (Expanded Polypropylen) foam

EPP core foam structure Foam block

EPP foam skin


Experiments: Results

Dynamic compression test, EPP foam, density 50 g/l, strain rate 130 1/s

0,0

5,0

10,0

15,0

20,0

25,0

30,0

35,0

40,0

45,0

0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0

Compression

Str

ess / M

Pa

DDw4-2

DDw4-3

DDw4-4


Numerical simulation of foams: Aspects

� Foam is a material created by a variety of expansion techniques

�Material laws in commercial FE codes are based on decoupledprincipal stresses and a Poisson ratio close to zero

�Both definition can be applied to foams with low densities(approximately < 200 g/l)

� Tested material curves need to be smoothen and extrapolated to compression values of approximately 98% (numerical stability)

�Strain rate sensitivities need to be considered as well

�High time step required for full vehicle crash applications

�Material model should be efficient and user friendly (no time consuming parameter identification)

� Foam model should represent macroscopic behaviour


Investigated material models of different FE codes:

LS-DYNA PAMCRASH RADIOSS

Soft foam (elastic)

83 45 70

PUR padding foam (visco-elasto-plastic)

75 24 33

PUR bumper foam (visco-elastic)

83 45 70

EPP foam (visco-elastic)

83 45 70

Numerical simulation of foams: Material models


Small sphere model

Model :

Boundary conditions :

Initial velocity is set on the sphere (purple arrow). The sphere mass and initial velocity depend of each case and material density.

The lower plane of the foam bock is clamped (red arrow).

Multi purpose contact interfaces (type 7) are defined between the sphere and the foam block.

Rigid sphere

Foam skin

Foam block

Validation example: Sphere impact model


Small sphere model (material without skin)Material = 50g/l EPP foam, Initial velocity = 4.750 m/s, Sample thickness = 40 mm, mass = 15 kg

Validation example: Sphere impact


Big sphere model (material without skin)Material = 150g/l EPP foam, Initial velocity = 5.050 m/s, Sample thickness = 40 mm, mass = 30 kg

Validation example: Sphere impact


Conclusion

�Different polymer foams have been tested and numerical modelshave been developed to represent the material behaviour under

impact loading.

� In a close and fruitful cooperation between the FAT (Research Association of German Automotive Industry), research institutes and

CAE experts, foam material manufacturers and software vendorsnumerically stable and accurate foam models were developed, improved and tested.

� The Radioss material model 70 was used for visco-elastic polymeric

foams. It has an user-friendly input structure, 10 times higher time-step as the older material model 38 and gives good, realistic resultsunder compressive loads. Enhancements regarding the behaviour

under tension are planned.


Thank you for listening!

We also would like to thank our colleagues from the FAT working group „Foam“ for their support, chairmanChristian Stender (VW) and the FAT organisation.

Excellent team-work!

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