The excitation and detection of lamb waves with planar coil electromagnetic acoustic transducers

Planar coil electromagnetic acoustic transducers (EMATs) are investigated for the excitation and detection of Lamb waves in nonferromagnetic metallic wave-guides. Such EMATs are attractive for certain applications due to their omni-directional sensitivity to wave modes with predominantly in-plane surface displacement, such as the So Lamb wave mode. A model is developed that enables the modal content of the radiated Lamb wave field from a transmitting EMAT to be calculated, and the output voltage from a receiving EMAT to be predicted when a Lamb wave mode is incident on it. The predictions from this model are compared with experimental data obtained from 12 different EMATs tested on a 5-mm thick aluminum plate, and good agreement is obtained. The model then is used to analyze the different effects that contribute to the overall Lamb wave modal sensitivity of an EMAT. The relationship between coil geometry and wavelength is examined.     

Quantitative modeling of the transduction of electromagnetic acoustic transducers operating on ferromagnetic media

The noncontact nature of electromagnetic acoustic transducers (EMATs) offers a series of advantages over traditional piezoelectric transducers, but these features are counter-balanced by their relatively low signal-to-noise ratio and their strong dependence on material properties such as electric conductivity, magnetic permeability, and magnetostriction. The implication is that full exploitation of EMATs needs detailed modeling of their operation. A finite element model, accounting for the main transduction mechanisms, has been developed to allow the optimization of the transducers. Magnetostriction is included and described through an analogy with piezoelectricity. The model is used to predict the performance of a simple EMAT: a single current-carrying wire, parallel to a bias magnetic field generating shear horizontal waves in a nickel plate close to it. The results are validated against experiments. The model is able to successfully predict the wave amplitude dependence on significant parameters: the static bias field, the driving current amplitude, and the excitation frequency. The comparison does not employ any arbitrary adjustable parameter; for the first time an absolute validation of a magnetostrictive EMAT model has been achieved. The results are satisfactory: the discrepancy between the numerical predictions and the measured values of wave amplitude per unit current is less than 20% over a 200 kHz frequency range. The study has also shown that magnetostrictive EMAT sensitivity is not only a function of the magnetostrictive properties, because the magnetic permeability also plays a significant role in the transduction mechanism, partly counterbalancing the magnetostrictive effects.     

Focusing PVDF transducers for acoustic microscopy

A focusing transducer based on a 9-µm thick PVDF foil was fabricated and tested for performance. The transducer operates in the frequency range of 20–160 MHz. For operation at 78 MHz in water, a lateral resolution of 27.5 µm and a vertical resolution of 35 µm have been observed. Acoustic images of a transistor have been obtained with the focusing PVDF transducer.     

Piezoelectric transducers for the measurement of rapidly alternating and acoustic pressures

The characteristics of piezoelectric transducers developed at the Scientific-Research Institute of Physical Measurements (NIIFI) are described. The article is taken from a paper presented at the International Scientific-Practical Conference on Piezoelectric Drives and Sensors in the city of Obninsk in January, 1993.Translated from Izmeritel'naya Tekhnika, No. 6, pp. 52–54, June, 1994.     

Spectral integral formulae for the response of acoustic transducers in cylindrically curved configurations

High-frequency spectral integral formulae are derived for the frequency-domain voltage of reciprocal electro-acoustic ultrasonic transducers interacting with a fluid-immersed cylindrically-layered elastic configuration. The transducers may insonify the cylindrical configuration from either the interior or the exterior and may be operated in either pulse-echo or pitch-catch mode. The formulae are arrived at through the use of a traveling wave spectral representation of all pertinent wave fields along the azimuthal and axial directions and use of Plancherel's theorem to convert the conventional surface integral for the transducer voltage into a spectral one. Within this representation, the transducers are expressed in terms of their radiation and reception spectral amplitudes on a cylindrical surface and the interaction of their fields with the cylindrically layered environment is expressed in terms of a composite spectral reflection coefficient. The integral formulae allow for computation of the transducer voltage and transducer beam interaction with the cylindrical environment in one step. The derivation is carried out for three-dimensional beams and specialized to two-dimensional (sheet) beams that extend to infinity along the axial direction. Furthermore, the formulae are applied to the case where complex transducer points are used to simulate transducers with quasi-Gaussian distribution profiles.     

Reciprocity calibration of impulse responses of acoustic emission transducers

By means of reciprocity calibration in Rayleigh-wave and longitudinal-wave sound fields, frequency characteristics of amplitude and phase of absolute sensitivity of acoustic emission transducers were measured on the basis of the newly derived complex reciprocity parameters, and the impulse responses were obtained through inverse Fourier transform. Calibration results were confirmed with supplemental experiments in which the fracturing of a pencil lead was utilized for the source of elastic waves. Impulse responses of acoustic emission transducers to both the Rayleigh-wave and longitudinal-wave displacement velocities were determined by means of purely electrical measurements without the use of mechanical sound sources or reference transducers.     

宽带纵振Tonpilz型水声换能器的优化设计

文章研究了拥有喇叭型前辐射头的纵振Tonpilz型压电水声换能器的参数优化问题。希望可以通过合理的调整Tonpilz型换能器各部分的几何参数,得到满意的频带宽度。研究结果表明,在没有采用其它频带展宽方法的前提下,通过优化几何参数,可以降低换能器的机械品质因素Qm,从而增大带宽。     

Optical imaging of transient acoustic fields generated bypiezocomposite transducers

Transient surface displacements of 1-3 piezocomposite transducers are measured and their correlative effects on the diffraction of the transmitted pulses are studied. The displacements of the surface of a 210 μm pitch piezocomposite are probed with a heterodyne, optical interferometer. An animate sequence is then built from the stored transient displacements so that each frame shows the instantaneous state of the surface of the piezocomposite. Such an animation proves to be a powerful analysing tool which provides a real insight into the piezocomposite's impulse behavior. Finally, the ultrasonic field transmitted in water and measured by the same optical probe is compared with simulations based on the diffraction impulse theory. The results of that comparison clearly show that, as far as diffraction is concerned, a piezocomposite transducer behaves like a homogeneous material     

An evaluation of acoustic emission for the detection of defects produced during fusion welding of mild and stainless steels

Acoustic emission is being evaluated for the detection of defects during fusion welding. In these trials, both the US-GARD and UKAEA-RNL equipments were used to monitor deliberately introduced defects during welding of mild steel and stainless steel plates. The plates were then examined by conventional non-destructive testing methods to establish the presence of the intentional and some non-intentional defects, and the results compared with the indications given by acoustic emission. It was found that one or other of the equipments detected 15 out of the established 20 major defects and that each separately detected 10 of them. Some indications given by acoustic emission were not related to known defects and are attributed to noises produced by the welding process.     

Evaluation of the accuracy of production of transducers for ultrasonic flowmeters with several acoustic channels

Translated from Izmeritel'naya Tekhnika, No. 2, pp. 31–32, February, 1990.     

Tonpilz piezoelectric transducers with acoustic matching plates for underwater color image transmission

Tonpilz piezoelectric transducers with multiple acoustic matching plates are suitable for color image acoustic transmission, to achieve wideband low-ripple characteristics as well as high-efficiency high-power transmitting capability. The design method for the transducers was investigated on the basis of multiple-mode filter synthesis theory. For transducers with single, double, and triple matching plates, optimum specific acoustic impedances and lengths were calculated. Moreover, based on this design method, a 24 kHz array comprising nine identical transducers with single matching plates was built and evaluated. As a result, this array showed high-efficiency, low-ripple, and wideband characteristics. Excellent agreement between theoretical values and experimental results was obtained. A field test was carried out on color image transmission from a 3500 m sea depth, using the fabricated array, during which good color images were received.