【作者】 孙莉；

【导师】 毛凌锋；

【作者基本信息】 苏州大学 ， 测试计量技术及仪器， 2016， 硕士

【摘要】 超声检测和声学超材料在工业社会中的广泛应用,对超声与材料结构之间关系的研究提出了更高的要求。本文基于时域有限差分法(FDTD)建立了含有不同微结构的二维铝板模型,系统研究了声学超材料中微结构对超声波传播特性的影响。文章从理论上研究了超声反射波的慢波效应、脉冲展宽效应与声学超材料中微结构界面层之间的关系。研究结果表明,慢波效应和脉冲展宽效应都随着微结构中散射体厚度的增加逐渐达到饱和状态。从超声波的角度来看,在声学超材料中存在一个有效微结构界面层厚度,它决定了慢波效应和脉冲展宽效应的程度。当微结构厚度为1.89λ时,可得到最佳慢波效应,此时声波大约慢了0.08个入射波周期。当微结构厚度为2.69λ时,脉冲展宽效应最明显,脉冲展宽大约0.13个周期。文中还提出了一种测量声学超材料中散射体圆心距以及散射体直径的新方法,该方法主要是基于检测从微结构反射回来的回波的幅值(amp)。计算结果表明,该方法适用于散射体圆心距小于测量声波波长的4倍的情况,因为当散射体之间的间距大于4倍波长时,散射体之间的耦合效应基本可以忽略。因此,通过选择合适的测量声波频率,该方法总能有效地测量出散射体的圆心距。同时,扫频方法检测下,散射体的直径等于amp曲线峰值处对应的声源波长的一半。这就意味着,该方法可以用于测量散射体的直径大小。对亚波长尺寸的散射体组成的微结构进行研究,无论是单个还是多个散射体单元,其反射与透射波形均具有丰富的时域信息,通过对比分析可以从波形的变化趋势判断铝板中是否存在微结构,以及在声源传播的中心线上是否有微结构的存在。再具体一点还可以得出微结构是由单个散射体还是多个散射体组成。这些时域波形包含了大量可研究信息,后续更为全面系统的分析将会给超声无损检测提供相应的理论指导。更多还原

【Abstract】 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.更多还原