Advanced Material Characterization of Hood Insulator Foams for Pedestrian Head Impact

Hood insulators are widely used in automotive industry to improve noise insulation, pedestrian impact protection and to provide aesthetic appeal. They are attached below the hood panel and are often complex in shape and size. Pedestrian head impacts are highly dynamic events with a compressive strain rate experienced by the insulator exceeding 300/s. The energy generated by the impact is partly absorbed by the hood insulators thus reducing the head injury to the pedestrian. During this process, the insulator experiences multi-axial stress states. The insulators are usually made of soft multi-layered materials, such as polyurethane or fiberglass, and have a thin scrim layer on either side. These materials are foamed to their nominal thickness and are compression molded to take the required shape of the hood. During this process they undergo thickness reduction, thereby increasing their density. Hence, the material properties vary greatly based on the thickness and strain rate experienced by the material. This paper presents a methodology for characterization of hood insulator materials at strain rates ranging from 0.01/s to 300/s for use in finite element (FE) simulations. A rate dependent material model is validated against the results of hemispherical punch tests. Unique tensile response of the hood insulator materials at high strain rates and the challenges it poses for material modeling is discussed in detail. In addition, challenges with specimen extraction and tensile testing of soft multilayer materials are presented, and the improved tensile testing procedure is proposed.