Foam-filled energy absorbers with auxetic behaviour for localized impacts

The paper proposes an innovative concept for energy absorption in case of localized impacts. The concept is based on the contraction of cellular structures with negative Poisson's ratio under impact, which is combined with the energy absorption capability of a foam filler. An experimental assessment was accomplished considering a 3D-printed polymeric hexa-chiral frame filled with open-cell soft polyurethane foam inserts, subjected to quasi-static and dynamic indentation. The energy absorbed by the combination of the auxetic frame and foam is significantly higher than the sum of the energies absorbed by constituent elements tested separately. Based on the experiments, a numerical approach for foam-filled absorbers is developed and validated. Then non-linear models of metallic chiral units are considered in a genetic optimization process, set up to optimize the maximum frame contraction under compression before its failure. Finally, the previously validated numerical approach is applied to the optimized geometries, and it is shown that the foam filler can provide significant enhancement of specific absorbed energy and load uniformity ratio also for frames made of ductile metallic material