Progressive Reinforcing of Re-entrant Auxetic Structures by Direct Polyurethane Foaming within Unit Cells

Mechanical metamaterials having a negative Poisson ratio, called auxetic metamaterials, exhibit high specific strength and excellent energy absorption capacity, allowing them to be implemented as potential energy absorbers and vibration dampers in aerospace structures. In addition to their intrinsic and promising features, the structural integrity and energy absorption capability of auxetic structures can be enhanced by polymeric foam reinforcement. Rigid polyurethane (PU) foams, a family of polymeric foams, have been frequently utilized in foam-filling studies of lattice structures, owing to their ease of manufacturing and tailorability via process optimization and micro-/nano- reinforcements. Among the myriad of reinforcement types for PU foams, elastomeric nanoparticles (ENPs) demonstrate a great potential to improve the mechanical and viscoelastic properties of the foams. In addition, achieving direct foaming within the unit cells of auxetic structures offers a more effective procedure as opposed to manual placement of foams into the vacancies since greater adhesion and structural integrity are assured because of the high-temperature foaming process. In this work, PU foams were reinforced with ENPs using various weight fractions and subjected to cyclic compression tests. Following that, re-entrant auxetic structures were filled with neat and ENP-reinforced PU foams and cyclic compression tests were carried out to determine the influence of ENPs on mechanical properties and energy absorption capabilities of auxetic structures.