Research On Wavelength Characteristics Of Guided Waves Excited By Emats In Plates

Electromagnetic ultrasonic guided wave technique possesses two kinds of advantages which come from the guided wave testing and nondestructive testing techniques of electromagnetic ultrasound, not only has the capability of long-range testing and high efficiency inspection, but also can generate ultrasound without couplant in flexible way. Electromagnetic acoustic transducer(EMAT) is the key component of electromagnetic ultrasonic guided wave(EUGW) technique. However, the study for wavelength characteristics of guided waves excited by EMATs is weak, and researchers seldom notice the phenomenon of wavelength deviation and harmonic wavelength, and thus the true value of the propagation parameters of EUGW such as frequency and wavelength cannot be accurately obtained, which limits the further development of EUGW technique. Therefore, this paper extensively studies the EUGW excitation method and its wavelength characteristics.The paper establishes the corresponding relationship between the propagation parameters of guided waves and the excitation parameters of EMATs with the meander-line coils. By analyzing the basic ultrasound theory of guided waves and the transduction mechanism of EMATs, it is found that ultrasound’s frequency and EMATs’ excitation frequency equal or deviate from each other, and the ultrasound’s frequency and its bandwidth is limited by the frequency bandwidth of EMATs. Moreover, the ultrasound’s wavelength and EMATs’ spatial period equal to each other or deviate from each other, and the ultrasound’s wavelength and its bandwidth is limited by the bandwidth of EMATs’ spatial period.Aiming at studying the excitation and propagation of electromagnetic ultrasonic guided waves, this paper puts forward a guided wave excitation method by EMATs. According to the corresponding relationship between the propagation parameters of guided waves and the excitation parameters of EMATs, the method analyzes the excitation process of EMATs’ operating zone on the excitation curves by establishing the excitation equation of electromagnetic ultrasonic guided waves. Then the generation of wave packet and its propagation velocity are studied, and the dispersion phenomenon of guided waves is explained, the wavelength deviation and harmonic wavelength phenomenon are analyzed.Aiming at explaining the wavelength deviation phenomenon, this paper proposes an analysis method for wavelength deviation characteristics. According to the ultrasound’s frequency and EMATs’ excitation frequency always equal to each other or not, wavelength deviation characteristics can be divided into two parts: equal-frequency wavelength deviation and unequal-frequency wavelength deviation. Moreover, two wavelength deviation measurement methods are proposed for the two parts: one measurement method is based on the excitation equation and the other is based on the time-frequency analysis. Experiments show that the measurement errors for equal-frequency wavelength deviation are less than 1.2%. The wavelength deviation coefficient of unequal-frequency wavelength deviation is in the range of 0.85-1.15, and the measurement errors are less than 1.3%.Aiming at explaining the harmonic wavelength phenomenon, this paper proposes an analysis method for the harmonic wavelength characteristics. By establishing the spatial transversal filter model for EMATs, this paper analyses the fundamental wavelength and the harmonics on the wavelength spectrum for different kinds of EMATs and their amplitude distribution on the spectrum. Then, the methods for generating odd and even harmonic wavelengths by different kinds of EMATs are proposed, and the method for removing all the harmonics is also proposed. Experiments are set up to verify the validity of the proposed methods by designing three kinds of EMATs.