Abstract
The main advantage of electromagnetic acoustic transducers (EMATs) over piezoelectric transducers is that no direct contact with the specimen under test is required. Therefore, EMATs can be used to test through coating layers. However, they produce weaker signals, and hence, their design has to be optimized. This paper focuses on the design of a Lorentz force shear wave EMAT and its application in thickness gaging; special emphasis is placed on the optimization of the design elements that correspond to the bias magnetic field of the EMAT. A configuration that consists of several magnets axisymmetrically arranged around a ferromagnetic core with like poles facing the core was found to give the best results. By using this configuration, magnetic flux densities in excess of 3 T were obtained in the surface of a specimen; the maximum value achieved by a single magnet under similar conditions is roughly 1.2 T. If the diameter of an EMAT ultrasonic aperture is 10 mm, the proposed configuration produces signals roughly 20 dB greater than a single magnet, while for a given overall EMAT volume, signals were greater than 3–6 dB. Linear and radial shear wave polarizations were also compared; a higher mode purity and signal intensity were obtained with the linear polarization.
Highlights
E LECTROMAGNETIC acoustic transducers (EMATs) do not require direct contact with the specimen under test, and they outperform piezoelectric transducers in applications where couplant cannot be used, where there are unfavorable coating layers, or when noncontact transduction is required [1]–[4]
EMATs may rely on various transduction mechanisms, but this paper focuses on the Lorentz force as the dominant transduction mechanism in mild steel and ignores the influence of other mechanisms, such as magnetostriction; this assumption is sound based on [1] and [14]–[17]
This paper presented an in-depth analysis of the bias magnetic field strength and resulting signal amplitude of different magnet configurations for shear wave EMATs on mild steel
Summary
E LECTROMAGNETIC acoustic transducers (EMATs) do not require direct contact with the specimen under test, and they outperform piezoelectric transducers in applications where couplant cannot be used, where there are unfavorable coating layers, or when noncontact transduction is required [1]–[4]. The coil of the EMAT induces eddy currents in a conductive specimen, whose path tends to mimic that of the coil These eddy currents with density Je interact with the bias magnetic field, whose flux density is B, and the resulting Lorentz force density on the charged particles (electrons) is given by produce ultrasonic waves with high mode purity are still unknown. Despite all these new configurations, there remains a need for more quantitative studies of the optimal configurations and dimensions of EMATs, such as that conducted in [25]. It should be noted that for an EMAT in pulse-echo configuration, the resulting signal is proportional to the square of the magnetic flux density B, since it contributes twice on transmission and reception
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