Dispersion Engineering in Elastic Guided Waves by Phononic Crystals and Metamaterials

Abstract

an inputted wave is distorted due to an excitation of multiple modes and their strongly dispersive nature. Here, a method to control the dispersion relations of multiple guided modes by phononic crystals and elastic metamaterials is developed. In this thesis, dispersion relations of guided waves are engineered in frequency and wavenumber domains. The main contributions are to separate multiple guided modes, to suppress an undesired mode, and to reduce group velocity dispersion. Multiple shear-horizontal waves in a plate are separated by engineered phononic crystal plates. The dispersion relation of each guided mode is manipulated in the wavenumber domain to obtain different propagation directions. Also, the excitation of undesired wave modes is suppressed by opening the forbidden band gap of phononic crystals over a target frequency range. A wave distortion due to the dispersion effect is prevented by employing phononic crystals and anisotropic metamaterials

the dispersion relation in the frequency domain is tailored to exhibit a constant group velocity. Realization of phononic crystals and elastic metamaterials is a challenging task to achieve the desired wave properties for multiple wave modes. Here, systematic engineering methods including size, shape, and topology optimization methods are proposed to obtain proper structures. The lattice parameters of phononic crystals and a topology of a unit structure in anisotropic metamaterials can be designed through the suggested methods. To confirm wave properties in the engineered waveguides, numerical simulations and ultrasonic wave experiments are conducted. In experiments, ultrasonic transducers are properly designed to selectively excite a target wave mode and adjust its beam pattern in an elastic waveguide. Through this research, it is demonstrated that phononic crystals and elastic metamaterials can be effectively exploited to engineer wave properties in elastic waveguides.

This dissertation presents manipulation of elastic guided waves by artificial structured materials: phononic crystals and metamaterials. Recently, due to their ability to control waves, artificial materials for electromagnetic waves have been extensively studied, leading to interesting results such as sub-diffraction lenses and cloaking devices. This study demonstrates that phononic crystals and elastic metamaterials can provide the promising functionalities in elastic guided waves. The main focus of this investigation is on tailoring multiple guided modes in an elastic waveguide. In a waveguide, multiple guided waves exhibit complex behavior which causes problematic issues in practical applications