Generating and detecting torsional guided waves using magnetostrictive sensors of crossed coils

In this work, we propose a crossed-coil magnetostrictive sensor consisting of a solenoid coil, a toroidal coil, and a ferromagnetic patch for generating and detecting torsional waves for pipe inspection. We show that the direction of the magnetic field of the ferromagnetic patch can be controlled by adjusting the input current to the toroidal coil. In addition, it is found that the amplitudes of the signals reflected from the pipe end using a crossed-coil sensor are much larger than those using pre-magnetization techniques, and excitation energies are used to generate the torsional waves only. From results on pipes with multiple circumferential notches, any defects larger in diameter than three times the pipe thickness were detected using the crossed-coil magnetostrictive sensor.