Surface-detached V-shaped yoke of obliquely bonded magnetostrictive strips for high transduction of ultrasonic torsional waves

When slender magnetostrictive strips that are obliquely bonded on the surface of a pipe are subjected to a time-varying magnetic field, the strips deform mechanically because of the magnetostrictive effect and in turn, ultrasonic torsional waves are generated in the pipe. If the magnetic flux density in the strips can be increased by simply modifying the magnetic path, the sensitivity of the generated wave signal with respect to the applied magnetic field strength can be considerably enhanced even without increasing the magnetic field strength. Sensitivity has been one of the most important factors in sensors. To achieve this objective, yokes can be used, but improper yoke configurations can generate unwanted additional wave modes with no sensitivity improvement. Consequently, the problem of an optimal yoke configuration to increase the sensitivity was formulated as a topology optimization problem, and an interesting V-shaped yoke was found to be the optimal configuration. When the designed yokes were employed in actual experiments, the sensitivity of the measured wave signal was increased by as much as 7.5 times without needing to increase the magnetic strength. (c) 2005 American Institute of Physics.