Multipoint measurement of T (0, 1) mode guided wave using HTS-SQUID gradiometer

In this paper, multipoint measurements of ultrasonic guided waves on an aluminum sample pipe were performed using a high-temperature superconductor superconducting quantum interference device (HTS-SQUID) gradiometer. Two magnetized thin nickel plates were glued on the pipe with circumferential defects. One of them was used as a magnetostrictive transmitter by winding a coil around it to generate T (0, 1) mode guided waves. The HTS-SQUID gradiometer was set to measure dB x /dz above the other nickel plate with a lift-off distance of about 3 mm. The multipoint magnetic flux measurements of the guided waves generated on the pipe without and with the defects were conducted while changing position of the HTS-SQUID gradiometer around the pipe. Contour maps of the magnetic signals were made on the plane of time and angle of the SQUID. The magnetic signals due to the defects were successfully measured in the contour maps. Intensity of the magnetic signal due to the defect increased in proportion to width of the defect. The intensities of the magnetic signals slightly varied depending on the angle of the SQUID. We discussed this phenomenon and concluded that magnetization inside the nickel plate of the receiver was no uniform.