The Interaction Between The Acoustic Waves And The Micro-structures In Metal Materials

In industrial society, ultrasonic testing method and acoustic metamaterials have been widely used. This sets higher requirements for the study of the interaction between the acoustic waves and material structures. This thesis establishes two-dimensional aluminum plate models based on finite difference time domain method(FDTD), and studies the acoustic metamaterial structure impacts on acoustic transmission properties through numerical simulations.The relationship among the slow-wave, the echo pulse broadening effects in reflected acoustic wave and the thickness of the interface layer for the acoustic metamaterials has been theoretically investigated. It has been observed that not only the slow time for reflected acoustic wave but also the echo pulse broadening saturates with increasing size of the acoustic metamaterials. From the viewpoint of acoustic wave, there is an interface layer in the metamaterials which determines the slow-wave and the echo pulse broadening effects. The largest slow time, which is the time needed for transmitting 0.08 periods of acoustic metamaterials, occurs when the thickness of the interface layer is 1.89λ. And the thickness of the echo pulse is broadened no more than 0.13 periods when the interface layer thickness is about 2.69λ. A new method to measure the distance among scatters, the density of scatters, and the scatters’ diameter in artificial composite materials has been proposed. This method is based on detecting the reflection amplitude change(amp) of the echo signal reflected from scatters. Simulation results show that such a method is valid for the distance less than four times of the acoustic wavelength, because the coupling between the scatters can be neglected for the distance larger than four times of the acoustic wavelength. Therefore, this new measure method can be always valid by choosing appropriate frequency according to the scaling rule discussed in this paper. At the same time, it is found that the diameter of scatters is half of the wavelength where the curve peak of the amp vs frequency occurs. It implies that such a new method can also be used to measure the diameter of scatters in solids and liquids.For sub-wavelength scatters, whether single unit or multiple units, the reflection and transmission waves both contain abundant time domain information. By comparing these waveforms, we can distinguish the existence of the microstructures and confirm whether the microstructure is located in the center line according to the significant change trend. We can even detect the number of the scatters. These time domain waveforms contain abundant detailed information. Follow-up studies can lay a good foundation for multiple sub-wavelength ultrasonic testing.