Propagation properties of interface waves at fluid-coated solid interface
Authors: Ma Qi  , Hu Wen-Xiang  , Xu Yan-Feng  , Wang Hao  ,   Institute of Acoustics, Tongji University, Shanghai 200092, China
Published: December 2020

The interface waves propagating along liquid-solid interface are widely studied and used in a lot of fields, especially in ocean acoustics, ocean engineering, and ocean geophysics. The dispersion characteristics of this kind of interface wave are closely related to the seafloor medium parameters, which is an effective means for the inversion of the seafloor sediments. However, the interface wave is difficult to use for ultrasonic nondestructive material characterization, especially for stiff and dense solid materials, owing to the mode shape or wave structure of the liquid-solid interface waves.The fraction of the total wave energy that travels in the fluid compared with the solid depends on the properties of the solid material. Usually, for a stiff and dense solid compared with the fluid, most of the energy travels in the fluid, while for a soft solid more energy travels in the solid. Therefore, it is difficult to use this kind interface wave for stiff solid material characterization. However, in the case of liquid-coated solid interface, the behaviors and properties of interface waves are quite different.In this paper, we use pulsed laser to generate the interface waves at the water-coated solid interfaces. The theoretical analysis of the laser-induced excitation of acoustic waves propagating along a plane interface between liquid and layered elastic solid is perforemd first. The general solution for the interface motion is derived. The analytic expression of the transient response is then obtained. Based on this expression, the dispersion characteristics of the interface waves, which propagate along the fluid-coated solid interface for the cases of slow coating on fast substrate and fast coating on slow substrate, are calculated and analyzed. The transient response signals are further calculated. In the case of slow coating on fast substrate, the interface wave shows an evident dispersion, in which its phase velocity is larger than its group velocity. In the case of fast coating on slow substrate, the interface wave also shows a remarkable dispersion within a smaller frequency-thickness product range, in which its phase velocity is less than its group velocity. The theoretical transient signals show the same properties.In order to verify the theoretical results, an experimental system is set up, and the interface waves are generated and measured. The experimental system mainly consists of pulsed laser, hydrophone, oscilloscope, and movable translation stage. The pulsed laser is used to excite the interface waves, and the hydrophone mounted on the movable translation stage is placed near the interface to receive the signals. Two kinds of samples of slow coating on fast substrate and fast coating on slow substrate are made and measured. The recorded testing signals are then processed and analyzed.The theoretical results and the experimental ones are in good agreement. The research results presented in this paper can provide theoretical basis for ultrasonic nondestructive characterization of coating and film material in immersion testing mode, and also for seafloor sediment parameter inversion.