During electromagnetic ultrasonic testing, it is difficult to recognize small-size bottom cracks by time of flight (ToF), and the lift-off fluctuation of the probe affects the accuracy and consistency of the inspection results. In order to overcome the difficulty, a novel composite sensor of an electromagnetic acoustic transducer (EMAT) and pulse eddy current (PEC) is designed. We use the amplitude of a bottom echo recorded by EMAT to identify the tiny bottom crack as well as the amplitude of PEC signals picked up by the integrated symmetric coils to measure the average lift-off of the probe in real time. Firstly, the effects of lift-off and bottom cracks on the amplitude of bottom echo are distinguished by combining the theoretical analysis and finite element method (FEM). And then an amplitude correction method based on the fusion of EMAT and PEC signals is proposed to reduce the impact of lift-off on the defect signal. The experimental results demonstrate that the designed composite sensor can effectively detect a bottom crack as small as 0.1 mm × 0.3 mm. The signal fusion method can accurately correct the amplitude of defect signals and the relative error is less than ±8%.

Eddy current testing is a widely used technique for electrical conductivity measurement due to its advantage of contactless measurement. However, the results are easily influenced by liftoff change. In this study, a new solution to compensate for the liftoff effect in eddy current measurements of conductivity is proposed. By measuring the inductance of coils with different frequencies, the liftoff–conductivity solution sets at each frequency are obtained from an analytical model. By finding a common solution for all frequencies, we can obtain the liftoff of the probe and the conductivity of the specimen simultaneously, thus improving the accuracy and reliability of conductivity measurements. For a liftoff variation of 1.14 mm, the introduced measurement error is up to 70.46% for aluminum alloys without liftoff compensation. By finding the common solution for multiple frequencies, the error is reduced to less than 5.57%. The selection of the frequency and tolerance limit is also discussed for the proposed method.

In electromagnetic ultrasonic testing, it is difficult to recognize small-size bottom cracks by time of flight (ToF), and the lift-off fluctuation of the probe affects the accuracy and consistency of the inspection results. In order to overcome the difficulty, a novel EMAT (electromagnetic acoustic transducer)-PEC (pulse eddy current) composite sensor is designed. We use amplitude of bottom echo recorded by EMAT to identify the tiny bottom crack as well as the amplitude of PEC signals picked up by the integrated symmetric coils to measure the average lift-off of the probe in real time. Firstly, the effects of lift-off and bottom cracks on amplitude of bottom echo are distinguished combining the theoretical analysis and finite element method (FEM). And then an amplitude correction method based on the fusion of EMAT and PEC signals is proposed to reduce the impact of lift-off on the defect signal. The experimental results demonstrate that the designed composite sensor can effectively detect the bottom crack as small as 0.1mm* 0.3mm. The signal fusion method can accurately correct the amplitude of defect signal and the relative error is less than ±8%.

The conventional electromagnetic acoustic transducer (EMAT) testing method has inherent limitations, particularly in its detection range. Combining EMAT with other nondestructive testing (NDT) methods can expand the detection range and improve accuracy. Most EMAT-based dual testing methods can detect surface and backside defects, but cannot be applied in high-speed inspection conditions. Therefore, this article proposes a dual testing method that combines EMAT with motion-induced eddy current (MIEC), enabling the detection of defects on both the surface and backside of the specimen under high-speed conditions. The probe consists of a permanent magnet and a butterfly coil. First, the principles of EMAT and MIEC are analyzed, along with the interactions and interferences between the two methods. A signal separation technique is proposed based on spectral analysis of the individual EMAT and MIEC signals. A 2-D simulation model was developed to assess the detection range of both methods and evaluate the impact of the MIEC effect on EMAT detection. Experimental verification confirmed the effectiveness of the method, and the sensitivity and stability of the probe were also evaluated. The results show that the dual EMAT–MIEC testing method effectively detects millimeter-scale cracks on both surfaces and backsides at a speed of up to 21 m/s.

Electromagnetic acoustic transducer (EMAT) typically employ rigid permanent magnets (PMs) or flexible magnetizing coils (FMCs) energized by high pulse currents to provide a bias magnetic field. However, the PMs results in a bulkier transducer, whereas the FMCs requires accurate timing control of the magnetizing current. Based on traditional static bias magnetostrictive shear horizontal (SH) wave theory, this paper designs a novel dual alternating-current (AC) magnetostrictive SH wave transducer and investigates the design parameters influencing its operational efficiency. The working mechanism of the dual AC magnetostrictive SH wave transducer is derived, followed by the development of a finite element model for the SH wave transducer under AC magnetization. The effects of parameters such as magnetizing current amplitude, frequency, excitation current amplitude, and wire length on the excitation wave amplitude are examined. It is observed that with increasing magnetization frequency, the wave amplitude first increases rapidly, then slightly decreases, and finally stabilizes, approaching the direct current (DC) magnetization state. An experimental system is subsequently built, and the experimental results regarding the effect of magnetization frequency on the excitation wave amplitude are found to be in good agreement with the simulation results, validating the feasibility of the dual AC magnetostrictive method for exciting SH waves. Further defect detection experiments are conducted on steel plates with hole and crack defects of varying thickness. The results show that when the magnetization frequency and current are set to 10 kHz and 30 A, respectively, only the proposed transducer can effectively detect a 3 mm hole defect. For a 20 mm × 1 mm × 2 mm crack defect, the signal-to-noise ratio (SNR) of the new transducer increases by more than three times. When the plate thickness is increased to 3 mm, the SNR for both transducers decreases, but the SNR of the new transducer increases by more than 2.5 times compared to the permanent magnetized transducer.

This study presents a novel, coil-only magnetostrictive ultrasonic detection method that operates effectively without permanent magnets, introducing a simpler alternative to conventional designs. The system configuration is streamlined, consisting of a single meander coil, an excitation source, and a nickel sheet, with both the bias magnetic field and ultrasonic excitation achieved by a composite excitation containing both DC and AC components. This design offers significant advantages, enabling high-frequency Lamb wave generation in nickel sheets for ultrasonic detection while reducing device complexity. Experimental validation demonstrated that an S0-mode Lamb wave at a frequency of 2.625 MHz could be effectively excited in a 0.2 mm nickel sheet using a double-layer meander coil. The experimentally measured wave velocity was 4.9946 m/s, with a deviation of only 0.4985% from the theoretical value, confirming the accuracy of the method. Additionally, this work provides a theoretical basis for future development of flexible MEMS-based magnetostrictive ultrasonic transducers, expanding the potential for miniaturized magnetostrictive patch transducers.