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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
© FAT October 2008 Folie 4
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
© FAT October 2008 Folie 5
Introduction: Typical front crash model in 1995
Foam modeled with spring elements
© FAT October 2008 Folie 6
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
© FAT October 2008 Folie 7
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)
© FAT October 2008 Folie 8
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
© FAT October 2008 Folie 9
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
© FAT October 2008 Folie 10
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
© FAT October 2008 Folie 11
Experiments: EPP (Expanded Polypropylen) foam
EPP core foam structure Foam block
EPP foam skin
© FAT October 2008 Folie 12
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
© FAT October 2008 Folie 13
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
© FAT October 2008 Folie 14
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
© FAT October 2008 Folie 15
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
© FAT October 2008 Folie 16
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
© FAT October 2008 Folie 17
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
© FAT October 2008 Folie 18
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.
© FAT October 2008 Folie 19
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!