The transducer

The instrumentation for fabrication of the ET normally employs a thermistor as a temperature transducer. Thermistors are resistors with a very high negative temperature coefficient of resistance. These resistors are ceramic semiconductors, made by sintering mixtures of metal oxides from manganese, nickel, cobalt, copper, iron and uranium. They can be obtained from the manufacturers in many different configurations, sizes (down to 0.1-0.3 mm beads) and with varying resistance values The best empirical expression to date describing the resistance-temperature relationship is the Steinhart-Hart equation  

When the analyte internets with the recognition element of a sensor, there is a change in one or more physieochemical parameters associated with the interaction. The transducer converts these parameters into an electrical output signal that can be amplified, processed and displayed in a suitable form. 

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The X coordinate (coordinate along the transducer alignment) of the sources that have overcome the screening test is calculated. 

Fig.5 Example of histogram presentatioa The lower picture is a schematic representation of the monitored component with Uie position of the transducers (1-4). The higher window is the total AE counts vs linear location representation of the located AE sources...

The technique presented above has been extensively evaluated experimentally using ultrasonic data acquired from a test block made of cast stainless steel with cotirse material structure. Here we briefly present selected results obtained using two pressure wave transducers, with refraction angles of 45° and 0°. The -lOdB frequency ranges of the transducers were 1.4-2.8 MHz and 0.7-1.4 MHz, respectively. The ultrasonic response signals were sampled at a rate of 40 MHz, with a resolution of 8 bits, prior to computer processing. 

Simulations of that kind result in a wide variety of A-scans and wavefront snapshots. The first screening of this material reveals, that the simulations in which the transducer is coupling partly to the V-butt weld and partly to the steel exhibit quite a number of pulses in the A-scans because the coupling at the interface of the weld results — due to the anisotropic behavior of the weld — in a complicated splitting of the transmitted wavefront. The different parts of the splitted wavefront are reflected and diffracted by the backwall, the interface, and — if present — by the notch and, therefore, many small signals are received by the transducer, which can only be separated and interpreted with great difficultie.s. 

Only the simulations in which the transducer is coupling either to the V-butt weld or to the surrounding steel can be analyzed in a simple and intuitive way, which means that the different pulses in the A-scan signals can be related uniquely to the reflection or diffraction of the wavefront at the weld, the backwall, and/or the notch. 

Second corner reflection The first corner reflection appears as usual when the transducer is coupled to the probe at a certain distance from the V-butt weld. The second corner reflection appears if the transducer is positioned well above the V-hutt weld. If the weld is made of isotropic material the wavefront will miss (pass) the notch without causing any reflection or diffraction (see Fig. 3(a)) for this particular transducer position. In the anisotropic case, the direction of the phase velocity vector will differ from the 45° direction in the isotropic case. Moreover, the direction of the group velocity vector will no longer be the same as the direction of the phase velocity vector (see Fig. 3(b), 3(c)). This can be explained by comparing the corresponding slowness and group velocity diagrams. 

Splitting into two quasi shear waves If the transducer is coupling to the isotropic steel the incident shear wave may split into two independent quasi shear vertical wave-... 

There are different possibilities to address the above set of equations which can be solved provided 2in > 3i, and provided the measured ToF information varies between measurement points. For the purpose of the present work we have taken two simplifying assumptions (a) one virtual source predominates at each measurement point, m and (b) each virtual source predominates at more than one measurement point. Note that assumption (b) ensures the condition 2m > 3i that is necessary to obtain solutions for Equations (2) and (3). These assumptions are justified by considering the defect surface as an acoustic secondary field source. At each measurement point the transducer predominantly receives signals from an... 

More recently, the circular array was proposed to assess the reflectivity of cylindrical specimens [3]. First, a circular C-scan image was obtained. The total scan time was about 25 min., which does not include a relatively time consuming alignment of the specimen. From the circular C-scan image, circular B-scan profiles were chosen in specific planes. The transducer was a focused high frequency transducer with a center frequency of 25 MHz of the transducer bandwidth. This frequency corresponds to a wavelength of 0.11 mm and 0.25 mm in the Plexiglas specimen and the AlSi-alloy, respectively. Additional experimental parameters are presented in Table 1. 

Under ideal conditions (e.g., point sources producing spherical waves and no multiple reflections) a rectified backscattered signal represents line integrals of the ultrasonic reflectivity over concentric arcs centered at the transducer position. To reconstruct the reflection tomo-... 

The transducer with orthogonal coils was presented in [1], [2], [3], It includes two coils, one parallel to the examined surface and the other perpendicular to the first one (see fig. 1)... 

By using the method of the dyadic Green s function [4] and the adequate boundary conditions [5], the expressions of the electric field in the zone where the transducer is placed can be written as [2]... 

For an effective control operation, the transducer with orthogonal coils has been coupled to a SFT 6000N control equipment, the results being sampled and quantized on 12 bits, stored and post-processed. 

Setting specific conditions for the examined material, discontinuity position and transducer construction, and using relation (6) one can calculate the transducer response to different discontinuities. These data can be used to determine the model matrix if one wishes to determine the discontinuity location by solving the inverse problem [10]. 

Figure 3 presents the dependence of the modulus of the e.m.f induced in the pick-up coil, on the distance between the transducer and the discontinuity, obtained both theoretically using the Rel. (6) and experimentally. The working fi equency is 5 kHz. The transducer has been balanced for a material zone far from the discontinuity. The modulus of the output e.m.f of the utilized control equipment was devided by the overall gain of the measuring chain, to directly obtain the transducer output voltage. 

The transducer has the two coils with a width of 2.2 mm, 20 turns each and the wire diameter is 0.03 mm. The material under test is a block of 7075-T6 aluminum alloy, with the conductivity of 1.89x10 S/m. 

The electromagnetic field created by the transducer propagates through the examined material as a wave with the length... 

Let u(x,y,z) be the complex voltage delivered by the control equipment with the transducer placed in a point with the coordinates x, y, z. 

One notes by V(x, y, z) the bi-dimensional Fourier transform of the voltage u x,y,z) when the transducer is scanning the inspected surface at a height z=const. 

The properties of the piezocomposite material mentioned above offer special benefits when the transducer is coupled to a material of low acoustic impedance. This especially applies to probes having plastic delay lines or wedges and to immersion and medical probes. These probes with piezocomposite elements can be designed to have not only a high sensitivity but also at the same time an excellent resolution and, in addition, the effort required for the probe s mechanical damping can be reduced. 

The transducers discussed above were designed to propagate waves in both directions normal to the direction of the fingers. It has been shown [17] that they produce a roughly collimated beam so they can be used to inspect a band of structure whose width is the transducer finger length the maximum distance away from the transducer covered by the beam is dependent on the attenuation of the wave and the signal-noise ratio, but is typically around 1-2 m in a... 

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