Design and characterisation of acoustic metamaterials in air and water-like media

Acoustic metamaterials (AMMs) have attracted the attention of researchers due to their tuneable properties in controlling and manipulating acoustic waves for specific requirements. Among unique applications of AMMs are superlens, acoustic cloaking, waveguiding, wave filtration, wave amplification and wave enhancement, to name a few. For these exceptional applications, the media's mass density and bulk modulus need to be negative or near-zero, which is not achievable in naturally occurring materials. The acoustic properties of metamaterials are determined by their subwavelength structures, unit cells, other than chemical properties. Therefore, desired effective acoustic properties can be gained by altering the unit cells' structure, dimensions, and materials at their operating frequencies. During the last two decades, acoustic metamaterials have been mainly limited to linear and periodic structures in air medium. The nonlinearity and non-periodicity of the AMMs, however, are rarely investigated due to their complexity. Also, only a tiny fraction of the open literature involves applying acoustic metamaterials in water-like media, including water, blood, and other liquid. Creating acoustic metamaterials that can function in a water-like medium is much more complicated than air due to the existence of shallow contrast between acoustic properties of water and solid. However, such metamaterials can find a wide range of applications, including those related to acoustic waveguides, human tissue imaging, as well as damage detection of composite laminates and offshore gas/oil pipelines. The research community is interested in investigating acoustic metamaterials constituted by nonlinear, non-periodical microstructures that can be operated in water-like media. However, existent design approaches do not cover the development of arbitrary unit cells to achieve the desired acoustic functionality. To develop a design methodology for acoustic metamaterials with certain unit cells, a comprehensive systematic knowledge needs to be created. This research project aims to build a design methodology for developing acoustic metamaterials in air and water-like media possessing specific properties. To this end, using analytical and numerical methods, first, the AMMs design charts based on unit cell parameters have been established. Then, acoustic metamaterials with unique characteristics have been designed by exploiting the material and geometrical nonlinearity of unit cells and their aperiodic arrangement. These AMMs demonstrated significant improvement in enhancing non-destructive evaluation of thick-section composite laminates, blocking sound transmission at low frequencies and acting as an acoustic diode or filter/splitter. Non-destructive evaluation of thick-section composite laminates is a tremendous challenge. Internal reflections and massive sound energy dissipation occur due to the acoustic impedance mismatch between plies when acoustic waves transmit through a laminate thickness. In this thesis, acoustic metamaterials to enhance the ultrasonic inspection of thick-section hybrid composite laminates have been developed by cloaking the virgin composite layers and intensifying acoustic energy to penetrate the thick-section composites. Furthermore, achieving high acoustic transmission loss at low-frequency ranges is not straightforward because the long wavelength at these frequencies can easily pass the subwavelength structures. This study investigates the geometrical nonlinearity and functionally graded material properties of unit cells to build AMMs possessing negative mass density and bulk modulus. Manipulating acoustic waves and their directional filtering using a sound diode has significant practical implications in different industries. In this thesis, an acoustic metamaterial is developed in which the sound waves, entering in the forward direction, pass through the AMMs. In contrast, the sound waves travelling backwards are directly filtered. The design of this AMMs was achieved by coupling geometrical nonlinearity with an aperiodic arrangement of unit cells. The research conducted in this study has been presented in five main chapters. After introductory Chapter 1, a comprehensive literature review, including acoustic principles, acoustic properties of materials, acoustic metamaterials, and related analytical and numerical approaches, are presented in Chapter 2. A new design methodology is proposed in Chapter 3 and the influence of topological and mechanical properties on the effective mass density and bulk modulus of AMMs are investigated. A novel ultrasonic non-destructive inspection for thick-section hybrid composite laminates using acoustic metamaterials is developed in Chapter 4. The effectiveness of this new concept has been numerically demonstrated for hybrid composite laminates with 40mm in thickness in both time- and frequency domains. Aperiodic unit cells are developed for cancelling out layers of the composite laminate with different acoustic properties. Chapter 5 investigated the effects of the geometrical nonlinearity and functionally graded material properties of the AMMs on achieving negative effective mass density and bulk modulus. It has been found that unit cells with curved plates can accomplish a larger frequency band with negative acoustic properties than a flat plate. Chapter 6 demonstrates an AMMs that acts as a sound diode based on nonlinear and non-periodic unit cells. In conclusion, this thesis developed a new systematic design methodology for acoustic metamaterials based on the topology and material properties of the selected unit cells. The new methodology can be generally applied to any unit cell with different structures and materials.