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This paper presents an analytical method for electromagnetic acoustic transducers (EMATs) under voltage excitation and considers the non-uniform distribution of the biased magnetic field. A complete model of EMATs including the non-uniform biased magnetic field, a pulsed eddy current field and the acoustic field is built up. The pulsed voltage excitation is transformed to the frequency domain by fast Fourier transformation (FFT). In terms of the time harmonic field equations of the EMAT system, the impedances of the coils under different frequencies are calculated according to the circuit-field coupling method and Poynting's theorem. Then the currents under different frequencies are calculated according to Ohm's law and the pulsed current excitation is obtained by inverse fast Fourier transformation (IFFT). Lastly, the sequentially coupled finite element method (FEM) is used to calculate the Lorentz force in the EMATs under the current excitation. An actual EMAT with a two-layer two-bundle printed circuit board (PCB) coil, a rectangular permanent magnet and an aluminium specimen is analysed. The coil impedances and the pulsed current are calculated and compared with the experimental results. Their agreement verified the validity of the proposed method. Furthermore, the influences of lift-off distances and the non-uniform static magnetic field on the Lorentz force under pulsed voltage excitation are studied.

A new ultrasonic transducer with multi-belts coil for generating and receiving longitudinal guided wave in ferromagnetic material pipes is proposed. The theory backgrounds and transduction principle of the proposed transducer are presented and analyzed. To verify the performance of the transducer, several experiments are performed. The performance of inspecting crack, frequency-tuned characteristic, effect of bias static magnetic field and dynamic magnetic field, lift-off effect and effect of the period number of the exciting current are investigated. The results show that the proposed coils not only could tune the center frequency but also could improve the amplitude and signal-to-noise (SNR) of the detected signals. Bias static magnetic field and dynamic magnetic field are two important factors influencing the amplitude of the longitudinal guided wave. The amplitude of the longitudinal guided wave is exponentially decreased versus the lift-off distance of the transmitter and receiver. Period number of excitation signal could influence the amplitude and wave width of the ultrasonic wave. The proposed transducer could easily control the wave modes and would be a better choice for pipes' monitoring and inspection compared to traditional single-belt coil transducer. longitudinal guided wave, nondestructive testing, magnetostrictive effect, transducer, frequency tune, lift-off Citation: Hao K S, Huang S L, Zhao W, et al. Multi-belts coil longitudinal guided wave magnetostrictive transducer for ferromagnetic pipes testing .Since the guided waves can travel over several meters along the waveguide axis and be efficient in a long-range and entire volumetric inspection, the guided-wave technology has been receiving more and more attention as a powerful tool for the nondestructive inspection of cylindrical waveguides such as pipes and tubes [1]. The longitudinal guided wave generated by magnetostrictive transducer is an effective choice for ferromagnetic material pipe inspection. It has the advantages of no-couplant and long-range inspection. The single-belt longitudinal guided wave magnetostrictive transducer was first proposed by Kwun [2]. He tested the feasibility of generation and detection for longitudinal guided wave [3], inspection and characterization of defects in pipes [4] and material parameters measurement for pipes [5].Though the single-belt longitudinal wave magnetostrictive transducer is dominant in simple configuration and ability of frequency dispersion characteristics and material parameters measurement for pipes, it is difficult to control the wave mode generated because the coil is so simple. As far as the generation and detection of the longitudinal guided wave are concerned, the best frequency operating point should be determined firstly according to the frequency dispersion curve of pipes. However, there is no effective method to design the coil for the optimum operating point. Therefore, multiple modes of longitudinal guided waves may be generated simultaneously, which will make the waves too compl...

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Facing the problems lack of considering the non-uniform distribution of the static bias magnetic field and computing the particle displacements in the simulation model of electromagnetic acoustic transducer (EMAT), a multi-field coupled model was established and the finite element method (FEM) was presented to calculate the entire transduction process. The multi-field coupled model included the static magnetic field, pulsed eddy current field and mechanical field. The FEM equations of the three fields were derived by Garlerkin FEM method. Thus, the entire transduction process of the EMAT was calculated through sequentially coupling the three fields. The transduction process of a Lamb wave EMAT was calculated according to the present model and method. The results show that, by the present method, it is valid to calculate the particle displacement under the given excitation signal and non-uniformly distributed static magnetic field. Calculation error will be brought about if the non-uniform distribution of the static bias magnetic field is neglected.