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.
A recently-developed magnetostrictive transducer consisting of a circular nickel patch and a set of permanent magnets and a figure-of-eight coil has been shown to successfully generate and measure guided waves in non-ferromagnetic plate structures. In this work, various transduction characteristics of the transducer are investigated and the experimental findings are reported. This transducer uses a thin circular nickel patch bonded to a non-ferromagnetic test plate. If alternating current is supplied to the coil, the magnetic field by the coil causes dynamic deformation of a patch mainly in the magnetic field direction; the deformation is due to the magnetostrictive effect of nickel. The patch deformation will then generate elastic waves in the plate. Since the coil and the magnets are enclosed inside a plastic bobbin which is placed on top of the patch, the flux direction can be freely adjusted. Therefore, we can generate and measure waves in any direction by simply changing the flux direction of the transducer without patch detaching and re-bonding. In this study, the directivity and frequency characteristics of generated Lamb and SH (shear-horizontal) waves by the transducer will be investigated. In this report, we first demonstrate that there exist specific magnetic flux directions in which only the Lamb or SH waves can be picked up. These directivity characteristics were experimentally investigated for various alignment angles and the observed results were explained accurately by our theoretical analysis. Additionally, the relation between the frequency characteristics of the transducer and the patch size was also investigated.
The coupling phenomenon between stress and magnetic induction, known as magnetostriction, has been successfully applied to generate and measure elastic waves. Most applications of this phenomenon thus far, however, are rather limited to cylindrical ferromagnetic waveguides. The main objective of this work is to develop a new patch-type, orientation-adjustable magnetostrictive transducer that is applicable for non-cylindrical, non-ferromagnetic waveguides. The existing patch-type transducer consisting of a ferromagnetic patch and a racetrack coil is useful to generate elastic waves only in one specific direction once the patch is bonded to a test specimen. However, the proposed transducer can transmit and receive elastic waves in any direction only with one patch at a given location. The proposed magnetostrictive transducer consists of a circular nickel patch, a figure-of-eight coil, and a couple of bias permanent magnets. Because of the unique configuration of the transducer, the propagating direction of the generated waves can be freely controlled since the set of bias magnets and the coil is not bonded to the magnetostrictive patch. In this work, the characteristics of the proposed transducer were investigated experimentally