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 ν21 ranging from −1 to 0 in the 1–2 plane and a tangent modulus Et2 ranging from 0.2 MPa to 2 MPa. X-ray μ-CT and 3D skeletonization enabled the extraction of the topological parameters of the pristine and the auxetic foams. The auxetic foam is transverse isotropic with cell structures exhibiting a re-entrant shape along the thermoforming 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 thermoforming compression ratios rc. Auxeticity is present in the in 1–2 plane only for compression ratios between 40% and 80%. Finite Element models for the pristine and auxetic foams have also been built from the μ-CT scanned 3D models. The numerical results show a good agreement with the experimental data and help explaining the deformation mechanisms of these auxetic foams.