No abstract

Angled shear vertical (SV) waves have been successfully employed in the non-destructive testing of welds, pipes, and railways. Non-contact meander-line coil electromagnetic acoustic transducers (EMAT) have many benefits in generating angled SV waves. The most important benefit is that the incidence angle of an SV wave can be controlled by the excitation frequency. However, the incidence angle of a traditional SV-wave EMAT is reported to be under 45 degrees in many cases. In this work, such cases are tested, and the problems of the received signal at large incidence angles are found to be due to wave interference and small signal amplitudes. An equivalent finite element (FE) model is established to analyze the problem, and the main reason is found to be the head wave. An alternative configuration of angled SV-wave EMAT with horizontal magnetization is proposed to reduce the influence of the head wave. Finally, the results from simulations and experiments show that the proposed EMAT has a larger signal amplitude and significantly reduced interference in large-incidence angle scenarios. Moreover, an incidence angle of an SV wave of up to 60 degrees can be achieved, which will help improve the performance and capability of nondestructive testing.

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 Fe3O4 coating to the test specimen, and quantitatively studying the enhancement effect of Fe3O4 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 Fe3O4 demonstrate the feasibility of EMATs generating and receiving ultrasonic waves through the coating, by a magnetoelastic mechanism. The transduction efficiency of EMATs on an Fe3O4 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 Fe3O4 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 Fe3O4 coating can be optimized, showing considerable potential to expand the application range of EMATs.