Start Date

2016 12:00 AM

Description

Electromagnetic acoustic transducers (EMATs) do not require direct contact with the specimen under test. Moreover, they can be used to measure through non-conductive coatings reducing the need for surface preparation tasks. However, they produce weak signals, which makes the optimization of their design a paramount task. Several Lorentz force SH wave EMAT configurations which are suitable for thickness gauging applications were investigated. It was found that a configuration that consists of several magnets axi-symmetrically surrounding a ferromagnetic core with like poles of the magnets facing the core produced the best results. This paper presents the results of a study that involved extensive simulations. Many different EMATs and specimen geometries were analyzed so as to understand the trade-offs between the overall EMAT volume and signal intensity for a given ultrasonic aperture and EMAT lift-off. The outcome of this study is a set of design criteria that can be used to optimize the proposed configuration. It is shown that Magnetic flux densities in excess of 3T can be obtained at the ultrasonic aperture. For the proposed configuration flux densities are in general two times greater than those for EMATs that use a single magnet. The increase in the flux density implies an increase in the signal intensity. Simulations and experimental validation show that signal intensities with the proposed design can exceed those of a single magnet by 20 dB. Furthermore, the influence of the coil configuration that results in either linearly or radially polarized shear waves was investigated. Simulations showing that higher mode purity and signal intensity can be obtained with the linear polarization are presented.

Language

en

File Format

application/pdf

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Jan 1st, 12:00 AM

Optimization of the Bias Magnetic Field of Shear Wave (SH) EMATs for Thickness Gauging Applications

Electromagnetic acoustic transducers (EMATs) do not require direct contact with the specimen under test. Moreover, they can be used to measure through non-conductive coatings reducing the need for surface preparation tasks. However, they produce weak signals, which makes the optimization of their design a paramount task. Several Lorentz force SH wave EMAT configurations which are suitable for thickness gauging applications were investigated. It was found that a configuration that consists of several magnets axi-symmetrically surrounding a ferromagnetic core with like poles of the magnets facing the core produced the best results. This paper presents the results of a study that involved extensive simulations. Many different EMATs and specimen geometries were analyzed so as to understand the trade-offs between the overall EMAT volume and signal intensity for a given ultrasonic aperture and EMAT lift-off. The outcome of this study is a set of design criteria that can be used to optimize the proposed configuration. It is shown that Magnetic flux densities in excess of 3T can be obtained at the ultrasonic aperture. For the proposed configuration flux densities are in general two times greater than those for EMATs that use a single magnet. The increase in the flux density implies an increase in the signal intensity. Simulations and experimental validation show that signal intensities with the proposed design can exceed those of a single magnet by 20 dB. Furthermore, the influence of the coil configuration that results in either linearly or radially polarized shear waves was investigated. Simulations showing that higher mode purity and signal intensity can be obtained with the linear polarization are presented.