The magnetostrictive effect is used to generate ultrasonic waves for a variety of health monitoring applications. Given
the ductile nature of many ferromagnetic materials and the common geometrical configuration of magnetic inductance
coils, magnetostrictive generation of ultrasound is especially suitable for long cylindrical waveguides such as thin wires.
Furthermore, utilizing ultrasonic guided wave modes in such waveguides provides a robust tool for remote inspection of
materials or environments over long distances. Through the use of different guided wave modes, structural health
monitoring sensors could be tailored to suit individual applications. Guided wave modes offer a choice in displacement
profile allowing sensors to be designed to be either sensitive or impervious to surface effects. The dispersivity of the
guided wave velocity can also be optimized for applications involving time-of-flight measurements. Despite the
advantages afforded by guided wave analysis, current magnetostrictive transducers, consisting of coil of wire and a bias
magnet, can not perform at the frequencies necessary to excite higher order guided wave modes. In order to advance the
capability of magnetostrictive transducers for ultrasonic guided waves in wires, the design parameters of inductance
coils are examined. Using a Laser Doppler Vibrometer, ultrasonic displacements are measured over a range of
excitation frequencies for different coil configurations and parameters to determine the feasibility of developing a higher
mode magnetostrictive transducer.
Previous work has led to the design, simulation, and development of a linear phased array transducer. The intention of
the array is to be used as a non-destructive ultrasonic device to monitor and evaluate the health of a given specimen.
The phased array has been manufactured and tested for the detection and characterization of defects on a target. The
array was fabricated with a four-row "stepped" design with four wires to transfer data and one wire for grounding. The
"stepped" design allows for the interrogation of a larger region using time delays and beam sweeping without the use of
additional electrical channels. The array was designed to be utilized in a water immersion environment with about one
inch between the array and the target specimen. An OmniScan MX system was used to operate the phased array and
perform real-time linear and sectorial scans on a set of rectangular plates. S-scans allow for beam sweeping over an
angle range as well as adjustments for time delays and a true-depth display. The array was operated with sixteen active
elements and an angle range of 0 to 30 degrees. The phased array was tested with a variety of targets and was used to
investigate and characterize different types of defects such as cracking, warping, and corrosion. The ability of the
phased array to distinguish between defect types as well as resolve defect size was evaluated.
In this paper we present the results on the design of a unique two-dimensional phased array with low channel applications for imaging defects on a metal surface. First, basic transducer calculations will be shown. Followed by the results of important phased array variables, such as focusing, and angle beam sweeping ability, The final design will be given. Next the computer simulation results will be discussed. These results will indicate the performance of the actual array. The second half of the paper will be devoted to a discussion on the phased array testing results with a demonstration phased array.
A surface wave on a liquid/solid interface is well-known to radiate acoustic energy into the liquid and is therefore rapidly attenuated. In this work, we have been able to show by experiments and calculations that the proximity of another surface (layer 1 to layer 3 and layer 3 to layer 1) sustains the surface wave through long distances for layers of both plates and concentric tubes. In addition, even when the surface wave is reflected from a distant edge, the returning wave is sustained in the multi-layer system and can be easily detected. This is apparently one of the first observations of leaky surface waves traveling over large distances, in this case over a thousand wavelengths. The effect is modeled on the basis of a cooperative phenomenon between two interfaces separated by a water layer. The effect represents a valuable result in the wave propagation of acoustic surface waves and opens the door to many applications.
As per the recent advances in remote in situ monitoring of industrial equipment using long wire waveguides (~10m), novel applications of existing wave generation techniques and new acoustic modeling software have been used to advance waveguide technology. The amount of attainable information from an acoustic signal in such a system is limited by transmission through the waveguide along with frequency content of the generated waves. Magnetostrictive, and Electromagnetic generation techniques were investigated in order to maximize acoustic transmission along the waveguide and broaden the range of usable frequencies. Commercial EMAT, Magnetostrictive and piezoelectric disc transducers (through the innovative use of an acoustic horn) were utilized to generate waves in the wire waveguide. Insertion loss, frequency bandwidth and frequency range were examined for each technique. Electromagnetic techniques are shown to allow for higher frequency wave generation. This increases accessibility of dispersion curves providing further versatility in the selection of guided wave modes, thus increasing the sensitivity to physical characteristics of the specimen. Both electromagnetic and magnetostrictive transducers require the use of a ferromagnetic waveguide, typically coupled to a steel wire when considering long transmission lines (>2m). The interface between these wires introduces an acoustic transmission loss. Coupling designs were examined with acoustic finite element software (Coupled-Acoustic Piezoelectric Analysis). Simulations along with experimental results aided in the design of a novel joint which minimizes transmission loss. These advances result in the increased capability of remote sensing using wire waveguides.