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Published in Michael Talbot-Smith, Audio Engineer's Reference Book, 2012
The electromagnetic acoustic transducer (EMAT) has proved to be a useful transducer for both detection and generation of ultrasound in electrically conducting samples. A constant magnetic field, usually produced by a strong permanent magnet, applied to the sample surface causes the generation of eddy currents on and just under the surface when the surface moves due to ultrasonic waves. The eddy currents can then be detected by a pick-up coil. The reverse process applies when generating ultrasound using EMATS. A systematic approach to the processes of detection and generation by an EMAT is presented below (see Figures 7.14 and 7.15 ). Detection The process is as follows. (1) A constant magnetic field is applied to the sample penetrating the sample surface. The pick-up coil is in the `middle' of this field (as shown in Figure 7.14 ), wound in a plane perpendicular to the sample surface. (2) An ultrasonic wave arrives at the sample surface and consequently in-plane or out-of-plane motion of the surface occurs. (3) As the surface moves so do the electrons within it, dragged by the electrostatic force, because the velocity of the ultrasonic wave is much
An Acentric Archimedes Spiral Coil For Inspection of an Aluminium Plate with an Inhomogeneous Thickness
Published in Nondestructive Testing and Evaluation, 2023
Zhichao Cai, Zhengshi Lu, Yibo Li, Yihu Sun, Qixiang Zhao
Electromagnetic ultrasonic non-destructive testing technology [8–10] uses an electromagnetic acoustic transducer (EMAT) to excite ultrasonic waves within the skin depth of the surface of a metal specimen without contact; thus, EMAT does not require a couplant. This technology can perform better in high temperatures, high pressure, and other harsh conditions. However, because EMAT uses electromagnetic coupling to excite ultrasonic waves, problems such as a low energy exchange efficiency, low signal-to-noise ratio (SNR) and easy interference by noise [11,12] occur. Electromagnetic acoustic resonance (EMAR) technology [13,14] is used to excite long-period signals and receive interfered sound waves inside the specimen by EMAT to obtain the resonant spectrum of the ultrasonic signal. It can effectively reduce the interference of noise signals on the detection results and improve the SNR [15] to achieve accurate measurements of carbon steel thicknesses [16] and tube wall thinning [17–19].
Study on the Change Law of Transverse Ultrasonic Velocity in a High Temperature Material
Published in Research in Nondestructive Evaluation, 2021
Yang Zheng, Zheng Li, Jinjie Zhou, Zongjian Zhang
Techniques based on the pulse-echo method for measuring the transverse ultrasonic velocity in high-temperature materials include piezoelectric ultrasound, laser ultrasound, and electromagnetic ultrasound. The limitation of piezoelectric ultrasound at high temperatures is mainly the influence of piezoelectric materials properties, such as depolarization, lower piezoelectric coefficient, resistivity characteristics [5–8]. Although current studies have shown that there are materials used at 1000°C, such as single-crystal lithium niobate [9], the most significant disadvantage concerning use of piezoelectric transducers corresponds to their requirement of couplant for acoustic-impedance matching, which becomes problematic at high temperatures. Cause couplant are difficult to apply and easily vaporize. In order to use piezoelectric transducers at higher temperatures, transducers can be installed on a waveguide bar [10,11]. However, effective installation of the waveguide bar requires it to be welded or clamped to the transducer, thereby causing nonuniform propagation of ultrasonic waves within the welded/clamped part. In addition, the transverse ultrasonic velocity within the waveguide comprises two parts – acoustic velocities in the material and that in the waveguide structure – which may lead to the complexity of signal processing. Laser ultrasound is widely used. This method can be used to measure thicknesses of high-temperature materials, complex shapes imaging [12], and tiny specimens [13–15]. But the laser-ultrasound system is expensive and complex. Comparing with the above two techniques, electromagnetic acoustic transducer (EMAT) has the advantages of non-contact and couplant-free operation along with low demand for the specimen surface area [16–18]. More recently, this method has been widely employed in high-temperature-thickness measurement and flaw detection applications [19–21].