Author(s)

P. Viot & D. Bernard

Abstract

The mechanical behaviour of polymeric foams depends on several parameters such as temperature, material density and strain rate. The identification of the parameters characterizing this behaviour under dynamic loading requires the design of special apparatus like a fly wheel, drop tower or Hopkinson bars, allowing high compression speeds. The foams studied here are multi-scale materials; the agglomerated beads (mesoscopic scale, millimetric diameters) are composed of microscopic closed cells (a few tens of microns). Constitutive materials of these foams are polypropylene, polystyrene and cork. The response of the material to a dynamic loading consists of three regions: an elastic phase, a plastic phase and densification. The first part of this work deals with the identification of the behaviour of these multi-scale foams as a function of density and strain rate. In the second part, original observations of the physical phenomena initiated during the yield plateau are presented and analysed. Buckling of bead and cell wall and strong localisation of damage were studied using several devices and techniques such as high speed cameras, SEM, and micro tomography. Keywords: multi-scale foam, cellular material, dynamic loading. 1 Introduction Polymer foams are used in many applications of passive safety for consumer goods (packaging for electronic equipment…) or for consumers themselves (helmets, knee pads…). It is usual to classify cellular materials in closed or opened cell foams [1], but it is also important to distinguish between foams which are constituted only of micro cells (commonly used in large plates for thermal or sound isolation applications in civil engineering) and those composed of fused beads (their size is millimetric and constitute the mesostructure) which are themselves made of micro cells (figures 1 and 2).

Keywords

multi-scale foam, cellular material, dynamic loading.