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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。
A new method for analyzing the mode of Launched ultrasonic Lamb waves using EMAT is proposed. Based on the current distribution of the EMAT winding coil, a formal solution of the Lorentzian tangential stress on the aluminum plate surface is deduced. The Fourier transform of the Lorentz plate is used to obtain the wavenumber spectral density of the surface-driven stress in the excited Lamb waves. Based on this, the analytical method of guided wave excitation is used to deduce the mathematical expression of the expansion coefficient of Lamb wave mode which is closely related to the geometrical parameters of EMAT coil, which lays a theoretical foundation for the rigorous analysis of EMAT Lamb wave excitation problem . The results of numerical analysis show that by changing the geometrical parameters of the EMAT coil, the mode expansion coefficients of the Lamb wave can be properly adjusted so as to effectively suppress the unwanted Lamb wave mode. The result is a single Lamb wave mode provides the theoretical basis.