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

Mechanical metamaterials have been attracting researchers for their outstanding mechanical properties including high strength, impact resistance, and energy absorption capacity. Auxetic structures, a special family of mechanical metamaterials, exhibit negative Poisson’s ratio which can typically be achieved by lattice designs. As a result of the negative Poisson’s ratio, auxetic structures offer high indentation resistance since as deformation occurs, materials in the adjacent region try to fill the deformation area under compression. Filling the vacancies of auxetic structures with polymeric foams allows to achieve of higher energy performance. The hybrid structures offer superior energy absorption capability and compressive response owing to the synergistic effect between foam and lattice structures. Considering their low cost, ease of manufacture, and tailorability, polyurethane (PU) foams are a good candidate for the filling material to auxetic structures. However, instead of filling every unit cell making up the auxetic structure with PU foams, determining the cells that are prone to deformation the most and filling them with PU foams can provide a guideline for improving the specific mechanical properties of auxetic structures. In this study, both experimental and computational approaches were adopted to determine the cells that deform the most at the early stages of quasi-static compression. Then, these cells were progressively filled with PU foam, and the mechanical characteristics of the final samples were compared.