Compressible plastic foams are used throughout the interior and bumper systems of modern automobiles for safety enhancement and damage prevention. Consequently, modeling of foams has become very important for automobile engineers. To date, most work has focused on predicting foam performance up to approximately 80% compression. However, in certain 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 a pedestrian leg-form at 11.1 m/s-to investigate the 'bottoming-out' phenomenon. Multiple material models in three different explicit Finite Element Method (FEM) packages (RADIOSS, FCRASH, and LS-DYNA) were used to predict the performance. The finite element models consisted of a foam covered leg-form impacting a fixed bumper beam with a foam energy absorber. The predicted leg-form acceleration over time was then compared to the 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 foam were evaluated as to their accuracy in simulating ConforTM foam on the pedestrian leg-form and polyurethane energy-absorbing foam on a bumper beam under extreme compression or deformation conditions. Extreme deformation which occurs after 80% compression can cause excessive hourglassing of certain types of elements. During this extreme event many solid element material types will not exhibit the correct foam behavior, consequently the results lead to an incorrect prediction. This study attempts to determine the best material type to use during this type of large deformation impact.


Mechanical Engineering

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URL: http://digitalcommons.calpoly.edu/meng_fac/94