The electromagnetic acoustic transducer (EMAT), which uses a flexible dual-coil configuration can be easily applied for non-contact ultrasonic testing of curved structures and objects with narrow inspection space. In this paper, the high-temperature performance of a flexible dual-coil EMAT on ferromagnetic steel measurement is analyzed. The flexible dual-coil EMAT consists of an electromagnetic coil, an eddy-current coil, and a test specimen. Firstly, the working principle and transduction mechanism in ferromagnetic steel by the designed dual-coil EMAT are analyzed. Then, a finite element model of the designed dual-coil EMAT is established, and the magnetic field and acoustic field distribution of the electromagnetic coil are simulated. Finally, a high-temperature experimental platform of flexible dual-coil EMAT is built, and the relationship between the designed EMAT’s received signal waveform and the temperature is analyzed. The results show that the designed EMAT allows the measurement of ferromagnetic steels between 25°C to 770°C without physical coupling or active cooling.

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.

Electromagnetic acoustic transducers (EMATs) are non-contact in nature and eliminate the need for couplants, offering extensive potential for applications in high-temperature measurements. However, a main disadvantage of the EMAT is its low transduction efficiency, particularly in the Lorentz force mechanism, where the efficiency further decreases at elevated temperatures, often resulting in ultrasonic signals being obscured by noise; this is a challenge for the high-temperature application of the Lorentz force mechanism EMAT, but the reasons behind the temperature-induced efficiency loss have not been fully solved in existing research. Therefore, this paper proposes an analysis method to investigate the transduction efficiency of the Lorentz force mechanism EMAT at high temperatures, with the aim of identifying effective ways to increase its performance. Analytical expressions for ultrasonic wave generation and reception of the Lorentz force mechanism EMAT are derived, followed by validation through a finite element model. By combining analytical calculation solutions with experimental results, the impact of temperature-dependent parameters on the transduction efficiency is investigated and improvement methods are proposed. The research results provide a foundation for the optimal design of high-temperature Lorentz force mechanism EMATs.

Stainless steel is widely used in petrochemical, marine, and electrical engineering applications owing to its excellent corrosion resistance and superior mechanical properties. Electromagnetic acoustic transducers (EMATs), which require no acoustic couplant, are an attractive non-contact ultrasonic technique for the inspection of such materials. However, the amplitude of ultrasonic signals generated and received by EMATs is generally low, which is particularly true for stainless steel test pieces. To overcome this challenge, a novel magnetostrictive-based EMAT incorporating a Terfenol-D coating is proposed in this paper to significantly enhance the ultrasonic signal amplitude. A polymer prepared by mixing Terfenol-D powder with epoxy resin adhesive is applied to the surface of the specimen, serving as an integral part of the EMAT transduction mechanism. Finite element simulation models are established for specimens with and without the Terfenol-D coating using COMSOL Multiphysics 6.0. The distributions of the electromagnetic field and ultrasonic velocity field generated by the proposed EMAT are simulated and analyzed. Experiments are subsequently carried out on the Terfenol-D-coated specimens and the uncoated reference specimens. The effects of the polishing of the coating and the Terfenol-D-to-epoxy weight ratio on the signal enhancement are also experimentally investigated. The results indicate that the proposed EMAT with the Terfenol-D coating can efficiently generate and receive ultrasonic signals. Compared with the conventional EMAT on an uncoated stainless-steel specimen, the received signal amplitude is increased by approximately 6.7 × 10³ times. These results suggest that the proposed EMAT is a promising candidate for non-destructive testing and evaluation (NDT&E) applications of stainless-steel specimens.