电磁超声换能器激发超声兰姆波的模式展开分析

提出一种用于研究电磁超声换能器(EM.AT)激发超声兰姆波的模式展开分析方法。从EMAT曲折线圈的电流分布出发,推导出铝板表面洛伦兹切向应力的形式解;对其进行空间傅里叶变换,得到激发超声兰姆波的表面驱动应力之波数谱密度。在此基础上采用导波激发的模式展开分析方法,推导出与EMAT线圈的几何参数密切相关的兰姆波模式展开系数的数学表达式,为严格分析EMAT的兰姆波激发问题奠定了理论基础。数值分析结果表明,通过改变EMAT线圈的几何参数,可对兰姆波的模式展开系数进行适当调整,从而实现有效抑制不需要兰姆波模式的目的,这一结果为采用EMAT激发纯度高的单一兰姆波模式提供了理论依据。

Modal expansion analyses of ultrasonic Lamb waves generated by electromagnetic ultrasonic transducers

A modal expansion approach has been proposed for investigating generation of ultrasonic Lamb waves with electromagnetic acoustic transducers (EMATs). Based on the current distribution of the EMAT's meandering coil, the formal solution for Lorentz shear stress applied on the aluminum plate surface has been derived. Then the function of the wave-number spectral density of surface driving stress for generating ultrasonic Lamb waves can be obtained using the spatial Fourier transform. On this basis, with the modal expansion approach for waveguide excitation, the mathematical expression of Lamb wave's mode expansion coefficient has been deduced, which is closely related with the geometrical parameters of the EMAT's meandering coil. The mathematical relationship between Lamb wave's mode expansion coefficient and the EMAT's geometrical parameters, obtained in this paper, lays a theoretical foundation for exactly analyzing generation of Lamb waves with EMATs. Further, the numerical analyses performed indicate that the Lamb wave's mode expansion coefficient can be appropriately adjusted by changing the geometrical parameters of the EMAT's coil, and thus the unwanted Lamb wave modes can be effectively restrained. This result provides a theoretical basis for generating a single and pure Lamb wave mode with EMATs.