Topological characteristics and mechanical properties of uniaxially thermoformed auxetic foam

We present here a simplified procedure to manufacture auxetic PU foam via a single direction thermoforming compression process applied to conventional (pristine) open cell foam samples. The auxetic foams produced here have a Poisson’s ratio ν_21 ranging from -1 to 0 in the 1-2 plane and a tangent modulus Et2 ranging from 0.2MPa to 2MPa. X-ray μ-CT and 3D skeletonization enabled extraction of the topological parameters of pristine and auxetic foams. The auxetic foam is transverse isotropic with cell structures exhibiting a re-entrant shape along the thermoform compression direction 1. More ribs are also oriented within the transverse plane 2-3, in which the stiffness is larger. Tensile and compression quasi-static tests have been carried out on samples cut along different directions and having various thermoform compression ratios rc. Auxeticity is present in the in 1-2 plane only for compression ratios between 40% to 80%. Finite Element models for the pristine and auxetic foams have been also built from the μ-CT scanned 3D models. The numerical results show good agreement with the experimental data and help explaining the deformation mechanisms of these auxetic foams.