We investigate theoretically the properties of phononic crystal plates with hollow pillars. Such crystals can exhibit confined whispering gallery modes around the hollow parts of the pillars whose localization can be increased by separating the pillar from the plate by a full cylinder. We discuss the behaviors of these modes and their potential applications in guiding and filtering. Filling the hollow parts with a fluid gives rise to new localized modes, which depend on the physical properties and height of the fluid. Thus, these modes can be actively controlled for the purpose of multichannel multiplexing. In particular, one can obtain localized modes associated with the compressional vibrations of the fluid along its height. They can be used for the purpose of sensing the acoustic properties of the fluid or their variations with temperature.

The underwater broadband anechoic coating is highly demanded in acoustic stealth for underwater vehicles. In this work, we propose a gradient index metamaterial for broadband underwater sound absorption, which is further experimentally validated under high pressure conditions. The gradient index metamaterial consisting of radius-varied cavities in rubber allows the sound waves to follow the objective trajectory in a broad band with impedance matching at the interface with water. The viscosity of the materials and the coupling effect among the gradient cavities contribute to broadband sound absorption. The performance of broadband sound absorption from 1 to 10 kHz is validated experimentally, which agrees well with the theoretical and numerical results. We further experimentally demonstrate the sound absorption performance with hydrostatic pressure up to 3 MPa and analyze the mechanism of the sound absorption deviation caused by high pressures. The proposed gradient index metamaterial provides a simple and efficient way to develop underwater broadband acoustic coatings.

A model is proposed to predict acoustic waves generated in a transversely isotropic cylinder by a laser line pulse extended in beamwidth and time duration, and an application to elastic constants measurement is presented. Documented good agreements are observed in the comparison of experimental and theoretical normal displacements for aluminum cylinders under either ablation or thermoelastic generation. Bulk waves are identified and processed for the elastic constants measurement. The effects of source beamwidth and time duration on wave forms and on the elastic constants measurement are predicted by numerical simulations. For nondestructive evaluation applications using bulk waves, a radius of 0.3 mm appears as a minimum limit for the sample size using a laser source of 0.1 mm beamwidth and 20 ns time duration. Elastic constants of aluminum rods are experimentally measured with very good accuracy.

Quantification of damage sizes in cylindrical structures such as pipes and rods is of paramount importance in various industries. This work proposes an efficient damage quantification method by using a dry-point-contact (DPC) transducer based on the non-dispersive torsional waves in the low-frequency range. Theoretical analyses are first carried out to investigate the torsional wave interaction with different sizes of defects in cylindrical structures. A damage quantification algorithm is designed based on the wave reflections from the defect and end. Capitalizing on multiple excitations at different frequencies, the proposed algorithm constructs a damage image that identifies the geometric parameters of the defects. Numerical simulations are conducted to validate the characteristics of the theoretically-predicted wave-damage interaction analyses as well as the feasibility of the designed damage quantification method. Using the DPC transducer, experiments are efficiently carried out with a simple physical system. The captured responses are first assessed to confirm the capability of the DPC transducer for generating and sensing torsional waves. The sizes of the defects in two representative steel rods are then quantified with the proposed method. Both numerical and experimental results demonstrate the efficacy of the proposed damage quantification method. The understandings of the wave-damage interaction and the concept of the damage quantification algorithm lay out the foundation for engineering applications.