Experimental techniques to evaluate the mechanical response of polymeric foams under different stress states

VALERIA PETTARIN; IGNACIO DURRUTY; LAURA FASCE; PATRICIA FRONTINI

Los Cocos, Argentina

Simposio; IV Simposio Chileno-Argentino de Polímeros ARCHIPOL 2009; 2009

Cellular polymer foams are widely used in applications where low density, thermal insulation, cushion properties and/or impact damping are required. Rigid polymer foams are currently being used as core materials in sandwich structures for aerospace, marine, automobiles, and petroleum industry (Schramm, 1994). A detailed characterization of their mechanical behavior is essential for their efficient use in engineering applications. Mechanical properties of foams depend on structure and orientation of cells, foam density and direction of the deformation force, i.e. tension or compression. In engineering applications foams are subjected to multiaxial stresses. The combination of stresses causing failure, which defines the failure surface when it is plotted in stress space, is therefore the important data to the designer. It has been stated that for porous materials such as foams, yield depends on both the deviatoric stress, se, and the mean stress, sm (Gibson et al., 1989; Miller, 2000; Deshpande and Fleck, 2000). However, mechanical testing of polymer foams is experimentally difficult. Although tensile tests are simple tests for other materials, gripping foams to apply tensile loading is complex. If true stress is intended to be determined, knowledge of the lateral strains is required. Strains must therefore be measured not only in the axial direction but also in one or both lateral directions. However, attachment of a mechanical extensometer may influence load-displacement curve and rupture point.  Through this work several uniaxial and biaxial experimental techniques as applied to polymer foams are depicted. Two isotropic polyurethane rigid foams (PUR) differing in density are tested, and a universal failure surface is determined.