Abstract
Mechanical metamaterials are man-made materials dominated by geometry and composition. Unlike bulky materials, their unconventional mechanical properties have attracted strong research interest from researchers in the engineering field. The advent of advanced 3D printing technologies has ushered in a new era for mechanical metamaterials that enabled them with have high manufacturing efficiency and complex structures. However, most of the traditional 3D printing technologies use a single material for printing, resulting in a fixed geometry and monotonous mechanical properties of mechanical metamaterials once fabricated. This is one of the long-standing challenges of mechanical metamaterials in engineering applications. Here, I propose to use the self-developed multi-material 3D printing technology to broaden the application scope of mechanical metamaterials, so that they have more functional design space beyond the structure. By combining the multi-material 3D printing method with a wide range of materials (e.g. brittle and ductile, rigid and soft, rate-dependent and rate-independent) and unique mechanical designs, I have fabricated and characterized a wide range of mechanical metamaterials with excellent mechanical and/or functional properties.I first conduct a comprehensive review and summary of the development and latest progress in the field of mechanical metamaterial. I here provide solutions to address the following three inherent limitations that commonly exist in mechanical metamaterials: foldability and load-bearing capacity, high stiffness and damping, as well as high strength and toughness.
(1) Traditional origami mechanical metamaterials are mainly manually made by single material (thesis or plastic film such as PET), or single material 3D printed, resulting in the inability to achieve both folding and load-bearing of origami metamaterials at the same time. For the first time, I proposed a multi-material FDM 3D printing technology based on the wrapping method. By wrapping rigid materials with soft materials and connecting them to form a hinge, the hinge effectively solved the interface debonding problem between multiple materials. Based on this wrapping-based multi-material 3D printing method, I designed a foldable and load-bearing push-to-pull origami mechanical metamaterial, which can effectively convert out-of-plane loads into in-plane deformation of soft materials and finally dissipate the energy. Impact tests show that the origami-mechanical metamaterial can greatly reduce the initial impact peak at impact energies up to 72J without secondary impacts;
(2) Conventional bulky damping materials have a limited amount of energy dissipated by the polymer chains due to finite compression deformations and easy densification, leading to poor damping performance. To overcome this constraint, we have developed a 3D printable material possessing high damping factor, high rate-dependent, and large elongation. By designing the material with a dual energy dissipation mechanism through hydrogen bonding and dynamic coordination bond, it achieved an ultra-high strain energy dissipation density (26.8 J/cm3) and a modulus increase of up to a hundredfold. By utilizing our in-house multi-material 3D printing technology, we have successfully developed a push-to-pull structure that possesses energy dissipation adaptability by combining the damping material. This push-to-pull structure can be adopted to artificial intervertebral disc and could avoid nerve pain by the zero Poisson ratio design. The push-to-pull design enabled the structure with outstanding energy absorption capability in both compression and torsion settings, as well as remarkable energy dissipation performance even after being used 1000 times. Moreover, I developed a low-frequency and broadband vibration metastructure based on the push-to-pull 3D unit structure. The metastructure allows for an ultra-wide vibration isolation frequency band of ~109 Hz.
(3) Ceramic materials are of great value in various applications due to their excellent properties. However, their inherent brittleness has always been a major limitation in their use. Despite the capability of 3D printing technology to produce intricate ceramic geometries, ceramics' brittle fracture issue still inhibits its potential applications. We propose a method of toughening ceramic structures through DLP 3D printing combined with micro-macro structural design. In-situ observations confirmed that surface micro-defects caused by the step effect during the printing process can effectively induce crack propagation directions, the macroscopic ceramic structure is designed to regulate the rate of crack propagation, resulting in ultra-high compressive strength and fracture strain. Based on the above research, we combined 3D printing and impregnate processes to create bi-continuous ceramic structures containing multiple materials. Specifically, we introduce TPU as a "soft phase" material and incorporate it into the "rigid phase" ceramic structure. The rigid phase exhibits superior load-carrying capacity as the bullet penetrates, whilst the soft phase functions as a mesh bag, wrapping around the bullet and prolonging its penetration time. Results from quasi-static and impact tests demonstrate that the bi-continuous multi-material ceramic structure has ultra-high fracture toughness and impact energy absorption capabilities;
To summarize, the utilization of self-developed high-performing materials, self-built or improved multi-material 3D printing technology, and the integrated mechanical design of materials and structures have facilitated the development of mechanical metamaterials with vast potential for various applications. These mechanical metamaterials possess extraordinary characteristics such as the coexistence of foldability and load-bearing capacity, high strength and toughness, and high stiffness and damping. The implementation of a multi-material approach possesses the capability of generating a noteworthy transformation in the development and production of advanced mechanical metamaterials for both functional and structural purposes.
| Date of Award | 2 Sept 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Jun LIU (Supervisor), Yang Lu (External Co-Supervisor) & Qi Ge (External Supervisor) |