OS13-2-2 A new Apparatus of Dynamic Compression to Identify Cellular Material Behavior

Abstract

Polypropylene foams are widely used in many impact-absorbing applications (head protective helmet, packaging, passive safety application of vehicle). To improve the mechanical performance of these applications, these structures have to be modeled. This approach requires the characterisation of the foam behavior with high strain rate tests in order to identify parameters of rheological models commonly implemented in FE code. Tests were achieved on polypropylene foams under compression at high strain rate. The material behavior has been determined as a function of two parameters, density and strain rate. Samples (at several values of density 70, 80, 90 and 100 kg/m3) were impacted on a dynamic apparatus - a fly wheel-. This test consists in compressing the sample by an uniaxial compression module mounted on a metallic wheel (diameter 1 m, mass 617kg) turning at high speed. With this testing machine, the dynamic compressive behavior has been evaluated in the strain rate range of 40-400 s^<-1>.These classical tests allow to apply uniaxial compression on cellular material but these rheological tests were not sufficient to describe the foam behavior under complex state of stress. New rheological experiments have to be developed. For that, a complementary module was designed to apply hydrostatic compression at different strain rates. From these tests, stress-strain responses of polypropylene foam were defined as a function of density, strain rate and type of solicitation (uniaxial and hydrostatic compressions). Result curves show three regimes : an elastic behavior followed in a second step by a stress plateau corresponding to plastic yielding. Finally, for high strains, a rising hardening phase occurs due to foam densification.