Direct Foaming Approach for Reinforcing Re-entrant Auxetic Structures for Superior Compressive Properties and Energy Absorption

Mechanical metamaterials with negative Poisson's ratio, called auxetic structures, typically consist of specially-designed lattice geometries with repetitive units. Auxetics offer high indentation and impact resistance and expected to be used as potential lightweight materials in air and space industry by increasing the energy absorption and impact resistance with almost no weight increase by filling the unit cells with polymeric foams. This study addresses the reinforcement of re-entrant auxetic structures with polyurethane foams, via manual placement of as-produced foams into unit cells and direct foaming within unit cells. Compression tests were applied to assess compressive properties and energy absorption. Regardless of the foam-filling technique, both compressive properties and energy absorption were promoted. Furthermore, direct foaming within unit cells enabled better adhesion, yielding greater compressive strength and energy absorption compared to manual foam placement.