Synergistic Effects of Foam Reinforcement and Geometric Parameters on the Mechanics of Re-Entrant Auxetic Structures

Background: Enhancing the energy absorption capacity and strength-to-weight ratio of engineering structures under compression and impact loads is crucial. Auxetic lattice structures, which feature a negative Poisson’s ratio, offer enhanced energy absorption through their geometric designs that cause inward clustering rather than outward expansion under compression, yet typically suffer from low stiffness and load-carrying capacity. Objective: Rigid polymeric foam reinforcement within the unit cells can substantially improve their mechanical properties, including compressive stiffness and energy absorption. This study examines how polyurethane (PU) foam reinforcement affects re-entrant auxetic lattice structures, considering variations in cell wall thickness and unit cell numbers. Methods: Utilizing three distinct cell wall thicknesses and three different unit cell numbers while maintaining the overall geometry constant, PU foams are synthesized directly within the unit cells to study the mechanical properties under compression tests. Results: Comprehensive analyses reveal that both cell wall thickness and unit cell numbers significantly enhance mechanical performance, along with the integration of PU foam which dramatically amplifies energy absorption related properties. Additional data-driven modeling revealed that stiffness and strength are predominantly governed by the number of unit cells, while foam reinforcement enhances energy absorption, validating the deformation mechanisms observed during mechanical testing. Among the configurations tested, the sample with the thickest cell walls and the highest number of unit cells, reinforced with directly synthesized polyurethane foam, demonstrated the most significant improvement, achieving a specific energy absorption of 10.211 MJ/kg, which highlights the critical role of optimal foam integration in boosting the mechanical performance of auxetic structures under compressive loads. Conclusions: The proposed method effectively enhances the mechanical performance of auxetic lattice structures by integrating PU foam reinforcement, significantly improving compressive stiffness and energy absorption capacity.