Development of Magnetostriction-Based Non-Contacting Ultrasonic Guided Wave Testing For Ferromagnetic Pipes Using HTS-SQUID Gradiometer

In this work, a magnetostriction-based non-contacting ultrasonic guided wave testing (UGWT) technique was developed and demonstrated. Magnetization method to transceive T (0, 1) mode ultrasonic guided wave (UGW) on pure nickel thin pipe was studied with both electromagnetic field simulator and experiments. The latter utilized HTS-SQUID gradiometer as a part of receiver of the UGW. In the both studies, a pair of electromagnets sandwiching the Ni pipe was used to magnetize the pipe, while rotating the pipe between the electromagnets. The simulation results indicate the magnetization method can give roughly uniform remnant magnetic field of about 240 mT in the circumference of the Ni pipe with supplying 15 A DC current to the electromagnets. In the experiments, we successfully transceived T (0, 1) mode UGWs on the Ni pipe using the magnetization method and the HTS-SQUID gradiometer. All-round UGWT around the pipe was conducted with lift-off distance of about 10 mm. The experimental result indicated that the magnetization distribution in the circumference of the pipe was roughly uniform as well as the simulation result. Improvement of the uniformity of the magnetization was realized with stepwise magnetization and resulted in better uniformity in the magnetic signal peak distribution. The reason of the better uniformities by the new magnetization method is discussed.