In this study, the feasibility of continuous, online monitoring of power lines using ultrasonic waves is considered. Local and global wave-based approaches for wire break detection in overhead transmission lines are presented. Both methods use a sending/receiving transducer to generate an ultrasonic, longitudinal, elastic wave in the cable. Defects in the cable cause a portion of the incident ultrasonic wave to be reflected back to the transducer, which when received, can be used to identify the presence of the defect. Although the transducers can only be attached to the surface of the cable, subsurface wires can also be interrogated since elastic energy spreads to these wires through friction contact. This study also explores how the elastic energy of a propagating wave becomes distributed among contacting rods via friction contact. This work focuses specifically on a two-rod system in which the wave energy from an excited "active" rod is transmitted to a neighboring "passive" rod through friction contact. An energy-based model is used to approximate the time average elastic wave power in the two rods as a function of propagation distance. Power predictions from the energy-based model compare well with experimental measurements and finite element simulations.
A modulator assembly that excites longitudinal vibrations in a short (19 cm) silica fiber segment is described. A physical model of the modulator assembly is used to theoretically predict the behavior of longitudinal vibrations in the fiber segment. The longitudinal vibrations are experimentally characterized using an intrinsic fiber Bragg grating. Experimental results are compared to theoretical predictions.
A fiber optic interferometric sensor, designed to detect tangential surface strains in the frequency range ∼200 kHz to 1 MHz, is described. The sensor is configured with a large number of segments of the fiber bonded to a thin plastic or metal base plate. The base plate can then be attached with a shear wave couplant to the solid sample of interest and the fiber sensor can be used to detect, in this sample, ultrasonic waves with a tangential surface displacement component. The sensor forms one arm of a Mach–Zehnder interferometer and the sensor sensitivity is proportional to the number of fiber segments bonded to the base plate. The fiber segments bonded to the base plate are of equal length and the maximum response of the sensor is at the frequency for which the bonded segment length is one-half of the tangential component of ultrasonic wavelength. The sensor is directional, with maximum response in the direction broadside to the bonded segment array. Measurements of the sensor performance are presented and compared with a theoretical model. [This work is supported by the ONR M-URI program.]
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