The uneven surface of the weld seam makes eddy current testing more susceptible to the lift-off effect of the probe. Therefore, the defect of carbon steel plate welds has always been a difficult problem in eddy current testing. This study aimed to design a new type of eddy current orthogonal axial probe and establish the finite element simulation model of the probe. The effect of the probe structure, coil turns, and coil size on the detection sensitivity was simulated. Further, a designed orthogonal axial probe was used to conduct a systematic experiment on the weld of carbon steel specimens, and the 0.2 mm width and 1 mm depth of weld defects of carbon steel plates were effectively detected. The experimental results showed that the new orthogonal axial eddy current probe effectively suppressed the unevenness effect of the weld surface on the lift-off effect during the detection process.

To further simplify the bolt stress measurement process, the feasibility of using a dual-mode EMAT that excites both longitudinal and shear waves is investigated to achieve rapid and accurate measurement. The propagation characteristics of cylindrical guided waves in a rod are studied by a Semi-analytical Finite Element model. Through the combination of the theoretical formula and the finite element simulation, the non-uniform distribution of axial stress is directly used to calculate the time of flight of cylindrical guided waves without building any approximate model. The effect of stress distribution on guided wave modes with different energy concentration regions is analyzed. The propagation characteristics of guided waves and the theoretical values of group velocity of these two modes are studied by cylindrical guided wave dispersion curve and mode shape analysis. Then, by combining the static stress distribution inside the bolt with the theoretical formula, the influence of the different distributions of guided wave energy in the cross section of the two modes on the velocity ratio was analyzed. Finally, the feasibility of the theory is verified by the experiment. The result of the experiment in the aluminum bolt verified the effectiveness of the dual-mode EMAT on stress measurement and the novel ultrasonic testing models proposed in this paper.

Periodic permanent magnet(PPM) electromagnetic acoustic transducers (EMATs) are commonly employed for axial defect inspection in pipelines. However, the lowest-order shear horizontal waves (SH0) guided waves have difficulties in distinctly differentiating internal and external defects. To enhance the signal-to-noise ratio and resolution, a unidirectional electromagnetic acoustic transducer (EMAT) based on Circumferential Lamb waves (CLamb waves) is developed. Through structural parameter optimization and excitation frequency adjustment, high-amplitude and low-dispersion CLamb waves are successfully generated in the high-frequency-thickness product region of the dispersion curve. Finite element simulations and experimental validation confirm the capability of this EMAT in exciting CLamb waves for the detection of crack-like defects. Experimental results demonstrate that the excitation efficiency of the CLamb EMAT exceeds that of the periodic permanent magnet electromagnetic acoustic transducer by more than tenfold. The defect reflection signal of the CLamb EMAT exhibits higher resolution and more significant amplitude compared to the PPM EMAT. The integration of this method with SH0 mode detection allows for the inspection of both internal and external defects in pipelines, offering a new avenue for EMAT applications in pipeline inspection.

An electromagnetic acoustic transducer (EMAT) is suitable for measuring the propagation time more accurately without causing abrasion to the transducer during testing due to the principle of its excitation. This work designs a flux-concentrating EMAT with a radial-flux-focusing permanent magnet to significantly enhance static magnetic field strength. Through theoretical analysis and finite element simulation, two kinds of coils are designed according to the concentration areas of the horizontal and vertical components of the magnetic field. One is used to generate pure longitudinal waves, and the other is used to generate both longitudinal waves and shear waves. The experimental comparison shows that the amplitudes of the pure longitudinal wave and the dual-mode wave excited by the two kinds of coils with the radial-flux-focusing magnet are more than two times higher than those with the ordinary magnet. Therefore, the flux-concentrating EMAT with the appropriate coil provides an insight into realizing more accurate detection where longitudinal wave detection is required.

Electromagnetic acoustic transducers (EMATs) are non-contact ultrasonic transducers. The transduction efficiency of a particular EMAT on a given specimen is dependent on the lift-off distance, which is the distance between the EMAT coil and the specimen surface. The transduction efficiency drops dramatically with increased lift-off distance, requiring EMATs to be in close proximity to the specimen, usually within a few millimetres. This paper proposes a new EMAT method of applying an Fe ₃ O ₄ coating to the test specimen, and quantitatively studying the enhancement effect of Fe ₃ O ₄ coating on lift-off distance. To eliminate the interference of the electrical and magnetic properties of the tested specimen, a non-magnetic and non-conductive glass specimen is selected. The experimental results on a glass substrate coated with Fe ₃ O ₄ demonstrate the feasibility of EMATs generating and receiving ultrasonic waves through the coating, by a magneto-elastic mechanism. The transduction efficiency of EMATs on an Fe ₃ O ₄ coating does not increase linearly with the bias static magnetic field, and the maximum measured signal amplitude value occurs at a relatively low flux density of ~0.12 T. More specifically, it has been shown the Fe ₃ O ₄ coating can significantly enhance the lift-off distance of EMATs operating at 4 MHz to 8 mm on coated stainless steel. The performance of the Fe ₃ O ₄ coating can be optimized, showing considerable potential to expand the application range of EMATs.