Polymer foams to optimize passive safety structures in helmets

摘要

Polypropylene foams are used in numerous safety applications (protective helmets, packaging, passive motor vehicle safety …). To improve the mechanical performances of these applications, these structures have to be modeled. Therefore, the foam behaviour must be characterized by high strain rate tests in order to identify the parameters of rheological models commonly implemented in FE codes. This paper presents a study of the performances of two cellular materials in terms of energy absorption under dynamic loading in order to choose the most efficient one. Firstly, a standard impact on a helmet has been studied to determine the type of loading (mainly compression) and the imposed strain rates. From these preliminary results, tests have been carried out on polypropylene and polystyrene foams under high strain rate compression loading. The material behaviour was determined as a function of two parameters, density and strain rate. Samples (at several densities: 70, 80, 90 and 100 kg/m~3) were impacted on a flywheel with a new compression apparatus. From these tests, stress-strain responses of polypropylene foam were defined as a function of density and strain rate. Result curves show three regimes: an elastic behaviour followed by a stress plateau corresponding to plastic yielding. Finally, for high strains, a rising hardening phase occurs due to foam densification. The choice of the cellular material for this application is optimized by an analysis of the energy absorbed by the foam during the impact.