Surface acoustic waves, such as Rayleigh or Lamb waves, can be used to characterise the geometry of surface-breaking defects. For accurate measurement of defect length and depth, the in-plane (IP) and out-of-plane (OP) components of the displacement or velocity can be measured and compared. Electromagnetic acoustic transducers (EMATs) offer a simple, non-contact method of doing this, as it is possible to design detection EMATs such that they are sensitive predominantly to either the IP or the OP velocity component. Here we present a new design of differential coil EMAT which measures both components simultaneously, and through simple processing gives the IP and OP velocities. This is compared to previous designs, showing that these components are obtained accurately. The system is used to characterise three artificial surface-breaking defects in aluminium and one thermal fatigue crack in 10 mm thick steel plate under thin coating.

Differential Coil EMAT for Simultaneous Detection of In-Plane and Out-of-Plane Components of Surface Acoustic Waves

The performance degradation of 12CrMoV steel generally occurs under a long-term working period in a high-temperature environment. In order to improve the reliability of a single nondestructive testing method for the performance degradation analysis of 12CrMoV steel, an advanced magneto–acoustic compound inspection method, in which pulsed eddy current (PEC) and ultrasonic waves were obtained from a single electromagnetic acoustic transducer (EMAT) testing signal, was adopted. The magneto–acoustic compound inspection method was carried out on 12CrMoV steel plates heat-treated at different temperatures and times. New parameters, including the slope and peak value of PEC, and the amplitude and attenuation of ultrasonic waves, were introduced to evaluate the performance degradation of 12CrMoV steel. It was found that ultrasonic waves manifested higher sensitivity than eddy current when the microstructure changed significantly. Therefore, the magneto–acoustic compound inspection method is advantageous for evaluating the performance degradation of 12CrMoV steel.

Performance degradation detection of 12CrMoV steel by magneto-acoustic compound inspection method

In this study, an advanced magnetoacoustic compound inspection method for a high-order Lamb waves mode electromagnetic acoustic transducer (EMAT) was proposed. The proposed EMAT can achieve the excitation and reception of the A 1 mode which is the first-order Lamb waves mode. According to the characteristics of the A 1 mode EMAT testing signal, a signal processing method for obtaining the pulsed eddy current signal by curve fitting was developed to extract the information for the pulsed eddy current. The voltage of the ultrasonic waves was carried on the pulsed eddy current voltage, and it was obtained by removing the pulsed eddy current voltage and extracting the time-domain testing signal of the A 1 mode EMAT. Through A 1 mode EMAT single detection that was used to obtain the pulsed eddy current and A 1 mode, the multi-parameter from the magnetoacoustic compound testing signal of the A 1 mode EMAT could be used to characterize the aluminum plate performance degradation. The detection accuracy of aluminum plate performance degradation was improved by the magnetoacoustic compound inspection compared with a single detection method.

An Advanced Magnetoacoustic Compound Inspection Method of High-Order Mode EMAT

Quantitative sizing of compound location defects based on PECT-EMAT hybrid testing methods

Nondestructive testing (NDT) is an essential method for assessing structural integrity in the oil and gas industry. Electromagnetic acoustic transducers (EMATs) have been extensively used to detect the wall-thickness reduction of plate-like structures, because they do not require direct contact. The pulse intervals of echoes are used to calculate the remnant thickness of structures. If the width of a single pulse is too large, multiple pulses will be superimposed, making it more difficult to extract the pulse interval. Thus, the width of a single pulse affects the resolution of measurements. This paper investigates the impacts of the backplate position on the pulse width and amplitude of thickness-measurement signals, using EMATs. By means of impedance modeling and measurement, it can be shown that the output impedance of the receiving coil is strongly influenced by the coil-backplate gap. With the increment in the coil-backplate gap, the signal amplitude and damping coefficient increase, while the self-resonant frequency decreases. By means of signal measurements on the specimen, it is shown that the pulse width and the signal amplitude can be significantly influenced by the backplate position. By reducing the coil–backplate gap, the pulse width can be reduced by over 80%, and by increasing the gap, the signal amplitude can be increased by over 300%. These research results can be used to optimize EMAT design, thereby suppressing the superposition of pulse echoes.

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Study of the Influence of the Backplate Position on EMAT Thickness-Measurement Signals

To improve the efficiency and robustness of electromagnetic ultrasonic testing, magneto-acoustic compound inspection for ferromagnetic materials is proposed. Both ultrasonic waves and pulsed eddy current (PEC) could be obtained by the single detection of an electromagnetic acoustic transducer (EMAT), where ultrasonic waves were generated based on the coupling of the magnetostriction effect and the Lorentz force mechanism, when the EMAT was used to detect ferromagnetic materials. In this study, three simulation models of the EMAT were built by coupling different physics fields while keeping the geometry of the models unchanged and the other parameters of the EMAT constant. Transverse wave could be generated by magnetostriction force, and longitudinal wave could be generated by Lorentz force. Based on time-frequency analysis of the simulation results, the energies of ultrasonic waves and PEC were concentrated in different frequency ranges. Through the filtering strategy and by separating the time-domain signal, magnetostriction force ultrasonic wave (MFUW), Lorentz force ultrasonic wave (LFUW), and PEC could be decoupled from the EMAT testing signal. Multiple parameters can be extracted by a single detection of the EMAT, and the ability of the electromagnetic ultrasonic testing can be improved.

Zhengyu Chen

Papers

Performance degradation detection of 12CrMoV steel by magneto-acoustic compound inspection method

Numerical Decoupling Study of EMAT Testing Signal for Ferromagnetic Materials