Study on high-performance acoustic metamaterials (AMMs) based on smart optimization algorithm and 3D printing fabrication.
Creators: name: Yin, Chengzhen
Publisher: name: The Australian National University
Year: 2024

description: The increasing demand for novel materials with tailored physical properties has driven significant research in the field of metamaterials, which offer exceptional attributes absent in natural materials. Among these, a representative and paramount type of metamaterials is the acoustic metamaterials (AMMs) or specifically the phononic crystals (PCs), which are artificial periodic microstructures composed of one or more materials with peculiar mechanical and acoustic properties. They are designed to manipulate the propagation of acoustic waves withing the media, showcasing promise in revolutionizing various engineering applications, including noise control, vibration attenuation, and imaging technologies, etc. The distinctive characteristics of AMMs arise from the spatial distribution of materials and their mutual interactions, thereby it is imperative to design and optimize the unit cells - the fundamental elements of AMMs - to attain superior performance. This study aims at advancing the understanding and application of AMMs through the exploration of innovative design and optimization techniques, wherein it employs the compression-twist coupling structures (C-TCSs), genetic algorithm (GA), and discrete topology optimization strategies. Novel PCs based on C-TCSs and lattice configurations are obtained that exhibit enhanced bandgap properties, alongside the establishment of universal optimization frameworks for exploring new meta-structures. Firstly, one objective of this research is to enhance the performance of 1D PCs with C-TCSs. To achieve this, a novel diatomic model is established, providing a platform for optimizing geometric parameters and thereby improving the acoustic performance of the proposed unit cell. The final optimized PCs present low starting frequency and large width of the bandgap. These findings hold considerable engineering application prospects, providing solutions for improved noise control and vibration mitigation. Expanding upon the exploration of 1D PCs with C-TCSs, the study incorporates curved beams, departing from the conventional straight rods. A GA-based optimization framework is introduced to optimize the PCs with diverse objectives, including lower starting frequency, larger bandgap width, and the prescribed central frequency. Lastly, an acoustic structure with extremely large stopband is proposed using gradient parameters, which possesses working frequency ranging from 200 Hz to 25 kHz. The significance of this method lies in the potential expansion of the design domain for such PCs, offering engineers greater flexibility in tailoring their acoustic properties. This thesis further exploits discrete topology optimization method combined with smart algorithm to search for novel lattice PCs with both excellent mechanical and acoustic properties. The ground structure method is selected as the modelling and optimizing strategy of the spatial lattice, followed by the establishment of a multi-objective GA-based framework to explore the desired configurations. The resulted various groups of lattice PCs exhibit improved mechanical (equivalent bulk modulus, shear modulus and elastic modulus) and bandgap performance, validating the effectiveness and practicality of the proposed method. This contributes to a comprehensive understanding of designing diverse metamaterials with desired multi-physical properties. In conclusion, this thesis not only contributes to the theoretical understanding of mechanism of bandgap generation for C-TCSs based PCs, but also provides practical engineering solutions for design and optimization of unit cells with desired properties. The novel models and optimization frameworks presented in this study open new avenues for tailoring the acoustic and mechanical properties of AMMs, addressing current research gap, and paving the way for future advancements in studying and devising various metamaterials.