Principles of acoustic devices

When a sensor is located within a metallic housing, conveying data back to a controller or data logger is not trivial. To maintain structural integrity, or simply for ease of installation, it is often not desirable to break the wall of the vessel, preventing any hard-wired solution. The conducting nature of the structure prevents the effective use of radio communications due to skin effect. Applications involving sealed containers also have a requirement for power delivery, as the periodic changing of batteries is impractical. In this paper, a number of systems are presented for through-metal communications and power delivery. A novel noncontact electromagnetic acoustic transducer communication system is demonstrated, which is capable of data rates in excess of 1 Mb/s. Furthermore, a power transfer efficiency of approximately 4% is shown to be achievable through 20-mm-thick stainless steel using inductive coupling. These novel solutions are critically compared to previous systems based on piezoelectric transducers.

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Investigation of Methods for Data Communication and Power Delivery Through Metals

Non-contact inspection is the main advantage of the electromagnetic acoustic transducer (EMAT). However, it lowers the transduction efficiency between ultrasonic energy and electromagnetic energy. The lift-off of the transducer has a close relationship with transmitting and receiving ultrasonic signals, because it reduces the magnitude of the alternating magnetic field in the specimen. The distribution and intensity of the alternating magnetic field at different lift-off values are simulated in ANSOFT. The relationship between the lift-off and received signals is analyzed through experiments. Results show that the suitable lift-off value for this specific EMAT system should be around 2 mm, considering both sensitivity and stability of the transducer.

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Study on the Lift-off Effect of EMAT

The amplitude of an ultrasonic signal generated by electromagnetic acoustic transducers (EMATs) is typically low when compared to those generated by contacting transducers, which restricts the application of EMATs in the fields of nondestructive testing and nondestructive evaluation. The transmission process of a surface wave EMAT is studied, based on a previously established 3-D model, with the aim of enhancing the amplitude of ultrasonic waves generated by the EMAT. The effect of changing various EMAT parameters on the surface wave is investigated, by utilizing the orthogonal test method. Results indicate that after optimization, the signal amplitude of the EMAT has increased by 25.2%.

Enhancement of signal amplitude of surface wave EMATs based on 3-D simulation analysis and orthogonal test method

Shear horizontal wave transducers for structural health monitoring and nondestructive testing: A review.

This paper presents an enhanced in-plane electromagnetic acoustic transducer (EMAT) for detecting laser-generated ultrasound. Instead of using post-processing of numerical data to enhance signal-to-noise, the paper describes a new EMAT design to increase the magnetic flux density applied to the sample. The EMAT's magnetic flux density was modelled in 3D using a finite element software to predict new magnetic spatial field distributions. A configuration was found to increase the flux density by a factor of 1.8 ± 0.2, compared to magnetic configurations previously used in conventional designs. Field predictions were implemented and confirmed experimentally. As a consequence, laser-ultrasound Rayleigh waves have been used to validate the performance of this sensor system. It was found that the EMAT's in-plane sensitivity increased by a factor of 2.0 ± 0.2.

A new magnetic configuration for a small in-plane electromagnetic acoustic transducer applied to laser-ultrasound measurements: Modelling and validation

Modelling of magnetic fields to enhance the performance of an in-plane EMAT for laser-generated ultrasound

One of the contributing factors to graphite degradation is oxidation. In order to detect this type of degradation, samples with different levels of oxidation have been studied. Longitudinal velocity measurements were carried out on these samples, using laser ultrasound techniques. These techniques have the advantage that they are non-contact, and therefore do not perturb the sample under investigation. From these measurements, the Young's modulus was estimated, based on a typical Poisson's ratio of 0.20. This was performed for several cylindrical samples; some samples had parallel microstructure orientation in the through-thickness measurement direction, while others had perpendicular orientation. Both orientations had very different longitudinal velocities, with corresponding variations in Young's modulus. Degradation was assessed for both cases of parallel and perpendicular oriented samples. Furthermore, in these perpendicular oriented samples, velocities were dependent on sample rotation about the cylindrical axis. Additional measurements were taken at 90° orientations to confirm this feature. The data positively show polarization of longitudinal velocity components. This may be the first time that acoustic birefringence has been observed in high porosity graphite.

Graphite anisotropy measurements using laser-generated ultrasound

Rayleigh waves have been used to measure anisotropy in metal alloys using a transient Rayleigh pulse and an eight-element electromagnetic acoustic transducer array. The array spacing determined sensor separation, so that velocity measurements were made independent of the source-to-detector distance. Elimination of this distance, which would normally lead to systematic errors, has resulted in a measurement system capable of measuring velocities in metal to an uncertainty of 0.1%. As an example, variations in the Rayleigh velocity have characterized the anisotropy in rolled bars of aluminum.

Anisotropy measurements in metal alloys using a laser/electromagnetic acoustic transducer array system

This paper describes two applications of laser/ electromagnetic acoustic transducer (EMAT) systems. The first application uses laser-generated ultrasound for the characterisation of rear surface artificial defects (vertical slots) in metal samples. An EMAT sensitive to in-plane motion was used to detect these ultrasonic waves. B-scan images were used to visualise any changes of interrogating waves due to defects. These images were generated as the sensor head was moved along the material's surface, forming a 2D intensity profile that revealed changes in the presence of a defect. The defects produced distinctive parabolic features in the images from the time-of-flight diffraction by shear waves. The paper presents a new method based on a generalized Radon transform (GRT) to automatically determine the depth and location of back-surface artificial defects from B-scan images. Experimental evidence validated predicted results for the case of 1.5-3.5 mm-deep defects. The second application uses Rayleigh waves to quantify inhomogeneities in metal alloys using a transient Rayleigh pulse detected with an eight-element EMAT-array. The array spacing established sensor separation, so that velocity measurements were performed independent of the source-to-detector separation. Eliminating this distance, which would normally lead to systematic errors, has produced a measurement system capable of measuring velocities in metal with a precision of 99.9 %. As an example, variations in the Rayleigh wave velocity have characterised the inhomogeneities in rolled bars of aluminium.

http://iopscience.iop.org/article/10.1088/1742-6596/76/1/012013/pdf

B. Dutton

Papers

A new magnetic configuration for a small in-plane electromagnetic acoustic transducer applied to laser-ultrasound measurements: Modelling and validation

Modelling of magnetic fields to enhance the performance of an in-plane EMAT for laser-generated ultrasound

Graphite anisotropy measurements using laser-generated ultrasound

Anisotropy measurements in metal alloys using a laser/electromagnetic acoustic transducer array system

Laser/EMAT measurement systems for materials evaluation