Directional transducers

The transmission times can be used to determine the depth to the defects. As yet the use of this kind of testing in concrete is based on the rektilinear propagation of compression waves from the surface. No directional transducers for use on concrete are known to exist. 

If measurements of the shock velocity in water are available at various distances from the charge, one can compute shock pressures at these distances. This approach can be used close to the charge where direct transducer measurements present formidable problems. An exptl arrangement for measuring shock velocity is shown in Fig 16 (Ref 1). Measurements thus obtained are compared with theoretical predictions in Fig 17 (closed circles). Also shown are direct pressure measurements (open circles). 

Biosensors that directly transduce binding events into measurable optical signals are valuable tools in the areas of therapeutics and diagnostics.1,2 Especially, the fluorescent biosensors have been the most successful, and still have the most promising future for practical application. Macromolecular receptors, such as... 

These chemosensors represent the simplest of gas sensors in that their chemical reactivity is directly transduced into an electrical signal. Changes in resistance, capacitance, voltage, or current indicate the presence of a particular analyte. These types of sensors are widely used for gas and odor measurements in commercial e-nose sensor arrays. 

Another model-free method for active damage detection is based on directional transducers. For passive sensing. Section 16.9 has already discussed the use of directional strain rosettes (both piezo and optical) to effectively detect an acoustic source without the need for a structural model. For active sensing, directional transmission as well as reception of guided wave can be achieved with phased-array transducers. 

Directional transducers, such as MFC, can be placed along the desired directions for ultrasonic interrogation. In addition, MFC transducers can be arranged in rosette configurations and then be excited with phased signals such that a beam steering effect is achieved. An extensive theoretical and experimental smdy of the capabilities of directional MFC transducers and MFC rosettes is given in Collet et al. [61]. 

Another method of constructing transducers with intrinsic beam-steering capabilities for large-area SHM imaging is described in Salas and Cesnik [62]. A steerable directional transducer is constructed with eight independently addressable sectors arranged around the circle at 45° pitch (Figure 16.28(a)). Each sector has an actuator... 

In the last few decades this need for new actuators has drawn considerable effort towards the development of materials that can directly transduce an input energy into mechanical work. Much of this attention has been focused, and is increasingly being focused today, on electroactive polymers (EAPs). This is a large family of materials that includes many different chemical structures, actuation mechanisms and electromechanical performances. 

A different renewable solid surface is this employed in membrane-based assays where the membrane allows the covalent immobilization of the reagents, giving rise to a more stable and reproducible assay. The membrane is brought into contact with a SPE so that the immunoassay can be directly transduced [30]. 

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. 

Some of the problems often encountered during ultrasonic inspection of plane specimens are also found on cylindrical specimens. For example, problems associated with the directional characteristic of the ultrasonic transducer. Furthermore, the discontinuity influences the shape and propagation direction of a reflected pulse, causing wave mode transformation. In addition, the specimen influences the shape and amplitude of the reflected pulse by sound absorption. 

Some discontinuities may be identified by a conventional two-dimensional ultrasonic technique, from which the well-known C-scan image is the most popular. The C-scan technique is relatively easy to implement and the results from several NDE studies have been very encouraging [1]. In the case of cylindrical specimens, a circular C-scan image is convenient to show discontinuity information. The circular C-scan image shows the peak amplitude of a back-scattered pulse received in the circular array. The axial scan direction is shown as a function of transducer position in the circular array. The circular C-scan image serves also as an initial step for choosing circular B-scan profiles. The latter provides a mapping between distance to the discontinuity and transducer position in the circular array. 

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 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... 

Moving the transducer along the concrete surface one is able to directly compare the data from different positions simultaneously reeieving information on intensity and phases of the signal and their relative changes. A Pentium-PC, included in tlie testing-system,... 

Based on the results from the initial experiments ultrasonic equipment and transducers for the scanning system were selected. Also a measuring chamber with transducer fixtures was constructed and manufactured for measurement on the tubes directly on the drawing bench. 

The echo directivity was influenced by the test frequency, the angle of refraction (the velocity in the wedge and the incident angle), and the height of the transducer. 

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