Development of Z-shaped patch-type magnetostrictive torsional transducer for performance improvement

Guided waves have been widely used for the long-range non-destructive damage detection of cylindrical waveguides such as pipes. Noting that the non-dispersive torsional wave mode is the most preferred in long-range inspection, this work in concerned with the generation and measurement of the torsional wave by a new magnetostrictive transducer. The magnetostrictive effect represents the coupling phenomena between magnetic field and mechanical deformation of magnetostrictive materials such as nickel. Because earlier magnetostrictive transducers require circumferential pre-magnetization of the magnetostrictive patch before actual experimentation, they are not so desirable for long-term on-line monitoring. To avoid the pre-magnetization process, we have recently developed a new transducer that employs slender rectangular nickel patches bonded at 45 degrees to the pipe/tube axial direction. Though this transducer does not require pre-magnetization, its performance can be substantially improved if patch shape is altered. This paper presents a new patch shape yielding higher output voltage and better signal-to-noise ratio. The key idea was to use yoke to concentrate the magnetic flux density at the slender rectangular patch. When the yokes are attached at both ends of the rectangular patch, the patch looks like an alphabet character Z. Several sets of experiments were conducted to check the transduction efficiency of the proposed transducer. When the Z-shaped patches were employed in actual experiments, the magnitude of the measured wave signal was 13 times larger than that by the yokeless transducer. Other experimental findings are also reported in this work.