Auxetic structures exhibiting non-linear deformation are a prevalent research topic in the material sciences due to their negative Poisson's ratio. The auxetic behavior is most efficiently accomplished through buckling or hinging of 3d printed structures created with soft or flexible materials. These structures have been hypothesized to have some unique characteristics and may provide advantages over conventional engineering materials in certain applications. The objective of present study is to gain a better understanding of behavior of auxetic structures subjected to tensile, compressive and impact loads and assess geometric parameters affecting these structures in applications such as impact shielding or biomedicine. Analytical and experimental methods were employed to investigate two different types of auxetic structures which were 3d-printed with TPU (thermoplastic polyurethane). The first was based on symmetric re-entrant angles cells patterned to form sheet-like structure. Rotation of members in opposite directions in a cell induces negative Poisson's ratio when the structure is subjected to tensile loading. The second structure was based on rectangular lattice of circular holes. This structure exhibited auxeticity due to formation of pattern of alternating mutually orthogonal ellipses when subjected to compressive and impact loads. Parameters of interest in this study included hardness of the plastic used in printing the structures, the fill pattern of 3d-printed solid parts, porosity of cylinders in the lattice structure, angles and thickness of members in the re-entrant structure. Preliminary results indicated that per unit weight of material, the re-entrant structure requires less tensile load to strain than a solid structure. This is advantageous in applications where expansion in lateral direction is required. The lattice of circular holes structure exhibited similar trend in impact and compressive loading. The results indicate that geometric parameters influence auxeticity of the structure a great deal. When the porosity of the lattice is too small, positive Poisson's ratio is observed. The length to height ratio of the re-entrant cell has similar effect on the structure's Poisson's ratio. The main advantage gained by employing such structures is their overall ability to resist buckling and withstand impact load without cracking. This study will help to develop 3D-printing techniques in manufacturing better performing structures under similar conditions.
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