3D design of mechanical metamaterial with negative Poisson's ratio

Recently, mechanical metamaterials have garnered extensive interests due to their unique properties. By combining hierarchical structures with the concept of size effect, mechanical metamaterials have enabled the creation of a sequence of novel designs. Those new materials allow us to explore new regions in the materials space, including ultrahigh resilience, high strength-to-volume, strong energy absorption, negative thermal expansion and so on. One of the most interesting properties of mechanical metamaterials is the negative Poisson's ratio.Poisson's ratio is the ratio of transverse contraction strain to longitudinal extension strain in the direction of stretching force. In most cases, Poisson's ratio is positive, which means that as a material is stretched along the lateral direction, it becomes thinner along the transverse direction. In contrast, the thickness of a material with a negative Poisson's ratio increases when it is stretched. A range of natural and artificial materials, including certain polymers, metals, and ceramics, exhibit this so-called auxetic characteristic. Materials with negative Poisson's ratio offer special mechanical and thermal characteristics that are advantageous for a variety of applications, such as energy absorption, sensing, and medical implants. Auxetic materials also exhibit mechanical properties that are unique compared to conventional materials including indentation resistance, shear resistance and fracture resistance. In my thesis work, I have explored four kinds of auxetic designs with interesting properties. The designs in my study include re-entrant ribs and a star shape, both of which exhibit good auxetic behaviors. Two of the designs also demonstrate auxetic behaviors in both directions. I have used stereolithography to fabricate the samples and applied tension test to measure their mechanical properties. The experimental results confirm that all four structures exhibit auxetic characteristics, and the re-entrant ribs and star-shape structures can enhance the auxetic behavior. --Author's abstract