Nonferromagnetic material inserted magnetostrictive patch bonding technique for torsional modal testing of a ferromagnetic cylinder

Abstract

Torsional vibration testing requires pure torsional mode transduction. However, no reliable technique is available for exciting torsional vibrations in ferromagnetic cylinders for a relatively wide range of frequencies. A magnetostrictive patch bonding excitation method developed for nonferromagnetic cylinders, based on the magnetostrictive principle, may be considered for ferromagnetic cylinders. However, the direct bonding of a magnetostrictive patch onto a ferromagnetic cylinder fails because the applied magnetic field spreads to both the patch and the cylinder and thus generates unwanted modes of vibration such as longitudinal modes. Thus, a modified or new technique must be developed in order to concentrate the applied magnetic field mainly on the patch, while causing the patch deformation to fully transfer to the cylinder. This paper presents a modified magnetostrictive patch bonding excitation and measurement technique suitable for ferromagnetic cylinders. The key idea is to insert a nonferromagnetic material between the test cylinder and the magnetostrictive patch. Because the thickness of the material or the gap size critically affects the torsional vibration test by the magnetostrictive patch bonding method, a series of numerical and experimental investigations to find the optimal gap size are conducted. Other factors affecting the developed modal testing method are also considered. The validity of the developed method is checked by comparing the experimentally obtained eigenfrequencies of a test ferromagnetic cylinder with the theoretical eigenfrequencies.