Nature-inspired lightweight auxetic structures for enhanced stiffness and absorption capacity

• Main Author: Zhang, Ee Teng
• Other Authors: Ng Bing Feng
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Engineering; Energy absorption; Bio-inspired; Auxetic structure; Plateau stress
• Online Access: https://hdl.handle.net/10356/179816
• Institution: Nanyang Technological University

Bio-organisms often exhibit curved-like, graded and hierarchical structures which significantly contribute to their exceptional mechanical adaptability and performances in diverse range of environmental conditions. These structures offer lightweight properties alongside enhancements to crashworthiness and impact resistance. In this thesis, we delve into the development of innovative materials by mimicking the structural designs from nature, emphasizing on the exploration of lightweight solutions for applications across various domains. Specifically, through the introduction of curved ligaments, gradient variations and hierarchical arrangements, this thesis will demonstrate improvements to the mechanical properties and energy absorption (EA) capabilities of honeycomb structures. Firstly, the use of arc-shaped ligaments in re-entrant anti-trichiral (REAT) structures introduces unique deformation patterns through the rotation of cylinders, contact interactions, and petal-like cell interactions. Experimental and Finite Element Analysis (FEA) demonstrate the positive influence of curved ligaments on elastic modulus, Poisson's ratio (PR), plateau stress, and SEA capabilities. The findings reveal significant improvements in normalised Young's modulus and SEA with the introduction of combined curved ligaments (Dc + Uc) design. Further parametric studies highlight the influence of ligament curvature and cylinder diameter on mechanical properties and deformation patterns. The gradient-based approach in the design of REAT honeycombs represents an innovative method to enhance certain mechanical properties. Traditionally, gradients in these structures have been introduced through variations in thickness. However, this thesis explores an alternative by varying the cylindrical diameter (chiral) and the height of the unit cell. This shift from the conventional thickness-based gradient approach has shown to offer promising improvements in the mechanical properties of REAT structures, including elastic stiffness, SEA (SEA), and densification strain. In contrast to the thickness-based gradient approach, which often results in layer-by-layer collapse due to significant stiffness differences between layers, the geometrical gradient approach utilizes variations in Poisson’s ratio between layers as the primary mechanism for deformation. In comparison to their uniform counterparts, chiral-based gradient REAT structures exhibited a sustained increase in the quasi-plateau stage and a relative constant EA efficiency at 25% more efficient than the base REAT structure. While the results for chiral-graded REAT structures were promising, height-based graded REAT did not reveal any dramatic improvements. Here, different variations in curvature and layer arrangements are investigated, which include conventional REAT structures, those with arc-shaped ligaments, and graded REAT designs. Negligible differences in mechanical performance are observed among the structures due to similar elastic and plastic deformation behaviors. In addition, improvements in plateau stress and elastic stiffness are observed in REAT structures with smaller cylindrical sizes and graded curvature. However, a consistent curvature with varied radius only showed marginally enhanced stiffness. This suggests that exploring simultaneous variations in cylindrical size and curvature could unveil more complex deformation behaviors, indicating a direction for future studies using advanced optimization methods like particle swarm optimization. Expanding on the idea of functional grading approach, a series of novel fractal honeycombs are proposed by combining functionally graded and fractal self-similarity features. These structures are generated by varying the fractal parameter in each layer of the traditional self-similar honeycombs. The proposed structures exhibit notable improvements in EA and MCF as compared to traditional honeycombs. Additionally, the introduction of different gradient distributions allows for controlled deformation patterns during dynamic crushing. Building on the foundation of hierarchical architectures in nature, a novel graded hierarchical honeycomb is further proposed. By replacing cell walls of regular honeycombs with triangular and hexagonal sub-structures, and varying the hierarchical length ratio in each layer, the structure exhibits enhanced crashworthiness behaviors. The graded hierarchical honeycombs demonstrate a progressive deformation model under different impact velocities, revealing three distinct plateau stages during in-plane crushing. The triangular sub-structure exhibits superior EA compared to the hexagonal sub-structure, resulting in an increased SEA of up to 32.2% compared to uniform hierarchical honeycombs. Finally, a new hybrid 3D auxetic structure along with its variant are investigated for both quasi-static and dynamic compression properties. Drawing inspiration from biological structures with hierarchical configurations, the investigation seeks to develop an auxetic structure with superior or equivalent properties to those derived from 2D designs, optimizing for dynamic loading conditions. The designs are based on existing literature and tested to evaluate their performance in different planes and printing directions. In the design of 3D structures, it's crucial to optimize the cell design to effectively manage EA in dynamic loading situations. Although maximizing EA in these scenarios is challenging due to the limited impact energy, leveraging the auxetic effect—which requires less compression, can be highly beneficial. For dynamic impacts, it is recommended to prioritize the ease of deformation in the auxetic design. In contrast, when dealing with quasi-static loading scenarios, the focus should remain on enhancing the auxetic effect itself. This approach ensures that the 3D structures are designed to efficiently manage different types of loading conditions. In essence, this thesis presents a comprehensive exploration into design and optimization of honeycomb structures, providing novel insights into improve mechanical properties, responses and EA capabilities. The combinations of approaches inspired from natural bio-organisms and their mechanisms paves the way for a future of lightweight EA systems for enhanced safety and protection performance.