A recoverable composite auxetic absorber optimized for low-energy impacts

This study proposes, validates, and optimizes an innovative recoverable composite energy absorber based on a foam-filled hexachiral auxetic architecture made of a visco-hyperelastic polymer. The absorber combined a 3D-printed hexachiral frame made of thermoplastic polyurethane reinforced by waste tire rubber (TPU-WTR) and a strain-rate-sensitive highly compressible open-cell polyurethane foam. While the combination of frame auxeticity with foam filling enhanced localized energy absorption, the specific materials adopted guaranteed full recoverability after impact and the variation of geometrical parameters in the hexachiral topology provided a significant design flexibility. A comprehensive mechanical characterization was performed, based on static and dynamic tests, which were used to calibrate accurately their numerical models. An absorber prototype was produced and experimentally tested, assessing the full recoverability at different impact velocities. Finite element models of the absorber were developed by using the identified material models and were validated from the quantitative and qualitative standpoint. Leveraging the design flexibility of the concept, fully parametric models were used to optimize a configuration for an impact of a 12 kg mass at 9 m/s, with a force constraint representative of the limit referred to a collision between a vehicle bumper and a pedestrian leg. The exploited Gaussian process regression (GPR) surrogate models, combined with a genetic algorithm, were able to predict the largely variable performance indices of the different configurations and enabled the identification of an optimal solution that significantly minimized the indentation of the impactor, while satisfying the force constraint and preserving full recoverability.