Guided wave transduction in non-ferromagnetic plates using a circular magnetostrictive patch

Abstract

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.