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Magnetostrictive Transducer for Torsional Guided Waves in Pipes and Plates

The use of low frequency (up to 300 kHz) guided waves as a screening tool for different kinds of structures has become increasingly popular over the past decade. Three predominant types of probes have typically been used for generating and receiving guided waves: piezoelectric, electromagnetic-acoustic and magnetostrictive. The major benefit of the magnetostrictive type is the relatively low cost of materials, which makes them promising as a cost effective approach for permanent monitoring of components. Generation of torsional mode guided waves using conventional magnetostrictive sensors requires the permanent magnetic bias field to be oriented along the magnetostrictive strip. The residual magnetic field, together with a time varying magnetic field, provides generation and reception of the waves. The typical designs, however, limit the power level of the time varying magnetic field, which also has the effect of reducing the residual magnetic field. A magnetostrictive transducer design developed at our company allows the use of a low cost generator of the bias magnetic field as an embedded part of the transducer. When the bias field is actively sustained, there is no limitation to the time varying magnetic field. As a result, the power output of the transducer as well as the sensitivity can be maintained at a higher level. The magnetostrictive transducer design was successfully tested over a frequency range of 16 to 250 kHz. A number of prototypes have been developed for different applications and components, including permanent monitoring and rapid screening of structures using a dry coupling technique. Applicability and limitations of the magnetostrictive transducer for application to screening and monitoring of piping will be discussed herein and some trial data will be presented.

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