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Polyurethane material models for simulation of Leg Impact
  4-1 Polyurethane Material Models for Simulating Leg-Form Impactin Explicit Transient Dynamics Joe Hassan DaimlerChrysler Peter Schuster and G. Frederick Ford Motor Company  4-2 ABSTRACT Modeling of foams has become a very important task for automobile engineers due to the factthat compressible plastic foams are used throughout the interior and bumper systems of modern automobiles for safety enhancement and damage prevention. To date, most work hasfocused on predicting foam performance up to approximately 80% compression. However, incertain cases, it is important to predict the foam under maximum compression, or 'bottoming-out.' This paper uses one such case—a thin low-density bumper foam impacted by apedestrian leg-form at 11.1 m/s—to investigate the 'bottoming-out' phenomenon. Multiplematerial models in three different explicit Finite Element Method (FEM) packages(RADIOSS, FCRASH, and LS-DYNA) were used to predict the performance. The finiteelement models consisted of a foam covered leg-form impacting a fixed bumper beam with afoam energy absorber. The predicted leg-form acceleration over time was then compared tothe leg-form acceleration observed during a physical test.Within the finite element models solid elements using material types such as honeycomb,advanced foam curvilinear recoverable, strain rate foam recoverable, and low density foamwere evaluated as to their accuracy in simulating Confor™ foam on the pedestrian leg-formand polyurethane energy-absorbing foam on a bumper beam under extreme compression ordeformation conditions. Extreme deformation which occurs after 80% compression can causeexcessive hourglassing of certain types of elements. During this extreme event many solidelement material types will not exhibit the correct foam behavior, consequently the resultslead to an incorrect prediction. This study attempts to determine the best material type to useduring this type of large deformation impact. INTRODUCTION The European Commission is proposing legislation aimed at reducing the severity of injuriessustained by pedestrians in the event of an impact with the front-end of a motor vehicle [1].One aspect of this proposed legislation is reducing the pedestrian’s lower limb injuries due tocontact with the bumper and frontal surfaces of a vehicle, assessed using a ‘pedestrian legimpact device,’ or ‘leg-form’ impactor.As the first vehicle component contacted by the leg-form impactor, the bumper system playsthe most important role in the vehicle’s performance. In order to understand in more detailhow the bumper system affects the leg impact, a variable buck was built and tested in adesigned experiment with different geometry and stiffness levels. Details of the test setup andthe results of this investigation are reported in an earlier paper [2].  4-3The next stage of this research was to correlate a leg-form and generic vehicle front-end CAEconcept model with the test results. While in general, the CAE concept model results werefound to be similar to the test results, in one case the physical leg-form acceleration wassignificantly higher than that predicted by the CAE model. This particular case included botha minimal bumper energy absorber package depth (70 mm) and a low density (95 kPa stressat 40% compression) polyurethane foam energy absorber. A comparison of the accelerationplot and the high-speed video revealed that the peak acceleration occurred at the time of maximum intrusion of the leg-form into the bumper foam. This was the same time as theCAE model predicted, but in the CAE model it was easy to observe that the foam had‘bottomed-out’ at that time. This paper presents the results of an investigation to identify thebest finite element material model for predicting the acceleration of an object impacting afoam which ‘bottoms-out.’  4-4 TEST SETUP Pedestrian leg impact performance is assessed through the use of a 'leg-form' impactor—twosteel tubular structures connected by deformable 'ligaments' and wrapped in Confor™ foam.Although the EEVC has proposed measuring tibia acceleration, knee bend angle, and kneeshear displacement, no current leg-form impactor can repeatably measure shear. Because of this, the impactor used in this test series did not include any shear measurement device. Theacceleration is measured by a uniaxial accelerometer—oriented in the impact direction—inthe lower structure (the 'tibia') 66 mm below the knee joint. The knee bend angle is measuredusing angular transducers at the knee joint.The test setup consisted of a Front-End Buck [2] rigidly mounted to a steel bed-plate placed infront of a Bendix Impactor. There was a carriage attached to the impactor to support thepedestrian leg-form during the initial acceleration of the cylinder. The carriage was stoppedafter the initial acceleration was complete, allowing the leg to travel the last 0.6 m to theVariable Front-End Buck in free flight at 11.1 m/s.The CAE models are compared to the results of a single experimental impact. Because of this, we can expect some level of experimental error in the physical test results. A recentFigure 2b. Typical foam response
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