Design and Fabrication of Hybrid Auxetic Structure for Energy Absorption and Dimensional Stability

Abstract

This thesis has detailed the design, analytical and experimental testing of a new Hybrid Hex-S auxetic structure with S-shaped auxetic cells, which are known to have energy absorbing flexibility, integrated into hexagonal honeycomb-like structure because of its inherent stiffness and structural integrity. The mechanical property of auxetic materials that result in a negative Poisson ratio (NPR) is their ability to expand sideways when under tension and contract when under compression. Such an outstanding aspect gives auxetics unique characteristics of energy absorbing capability and high fracture toughness which are highly desired in areas of application where energy absorption and impact resistance are required. Auxetic materials and NPR materials in particular introduce outstanding advantages into the realms of application such as biomedical equipment and personal protection equipment etc. Nevertheless, besides the above benefits of such an elastic lattice structure, also common auxetic substructures, such as re-entrant ones, also experience stress concentration at its sharp corner because of which the premature failure and cyclic fatigue operation under real loadings is limited. Such issues are significant shortcomings to the serviceability, and the serviceability itself of the auxetic materials particularly in real application since in many cases they can be subjected to repetitive loading. To resolve these issues, we created the Hybrid Hex-S structure that utilized S typed auxetic cell to Okamoto hexagonal honeycomb. The uniform S-shaped cells reduce the concentration of the local stress and enhance the uniform deformation to significantly enhance the energy absorption and low cycle fatigue characteristics of the structure. Based on the discussion and analysis of mechanical behavior, Hybrid Hex-S has superior mechanical properties compared to re-entrant and Aux-Hex structures. The re-entrant geometries have highly localized hot spots under loading which have plastic deformation, whilst the Aux Hex designs permanently densify at the beginning due to geometry discontinuities. Conversely, Hybrid Hex-S exhibits the prevalence of curvature-based deformation at enhanced compressive loading and enhanced recoveries in comparison with the reducing and recovering of structures with cyclical compression. In particular, under moderate compressive loading, the Hex-S structure demonstrated a stronger load-bearing capability by exhibiting a peak elastic performance that was approximately 6.7% higher than the Aux-Hex design, while remaining only about 4.4% lower than the Re-entrant configuration, all while maintaining good recoverability after unloading. ASTM standard tests such as compression test, flexural test and tensile test were conducted experimentally on the structures of the Hybrid Hex-S to test the energy absorbency capacity of the material and its fatigue properties. In these experiments, the Hybrid Hex-S is proven to be higher than the conventional auxetic designs in terms of energy dissipation, crack toughness and resilience to cyclic loading. The energy dissipation analysis further indicated that the Hex-S configuration exhibited the lowest dissipation ratio (0.62) compared to the Re-entrant and Aux Hex designs, which recorded higher ratios of 0.73 and 0.77, respectively. A lower dissipation ratio denotes that less strain energy is lost during deformation, implying that the structure is able to recover more of the stored elastic energy rather than dissipating it as permanent damage. Therefore, this reduced energy loss behavior highlights the superior elastic recovery and structural stability of the Hex-S geometry under cyclic or repeated loading. Creation of such a structure is of extremely high demand in the field of additive manufacturing (AM), including Fused Deposition Modeling (FDM) 3D printing. The discussion of scalable and manufacturable hybrid auxetic structures is one of the most important new elements of this thesis. In FDM 3D-printing, the paper addresses some of the most troublesome challenges in the production scale, such as [1 6]: fatigue resistance; mechanical robustness; and the ability to print complex and/or auxetic geometries. This study employs the advantages of FDM and TPU to present a scalable pathway to hybrid auxetic materials that can be produced simply with low costs with no expensive geometry ballooning procession to achieve high accuracy to enable such materials to be used in large-scale manufacturing. Hybrid Hex-S would find very efficient use in impregnable protection gear like helmets and body armor, or even implantable biomedical devices. It is an ideal material in next generation safety products/protection components due to its high energy absorption characteristics and capability to scale-up the manufacturing processes. Lastly, the additively manufactured materials may be designed to fit certain applications and application-specific designs. This work enhances the mechanical metamaterials and develops the auxiliary materials and their uses. The design goal of the Hybrid Hex-S structure is to satisfy the engineering goal in the material scalability, fatigue, and stress homogenization. It demonstrates its ability to have a wide range of uses. Through the application of the latest methods in additive manufacturing and 4d printing, alongside multiscale modeling, this research provides the basis of the further progress of the next generation materials, adjusted to the needs of the modern industries.