Novel Magnetostrictive Guided Wave Testing for STPG370 Pipes Using HTS-SQUID

We have been developing a noncontact nondestructive testing technique for STPG370 pipes without removing insulation using magnetostrictive ultrasonic guided wave transceiver technology and a high-temperature superconducting (HTS)-superconducting quantum interference device (SQUID) gradiometer. In this study, a method to generate longitudinal guided waves by applying an excitation field parallel to the magnetization in a pipe was examined based on the magnetostrictive effect. For the magnetization, the entire circumference of the pipe is axially magnetized using two short cylindrical electromagnets. The magnetization conditions were computationally analyzed. The analysis results showed that an STPG370 pipe can be magnetized to a saturation flux density of approximately 1.7 T, even assuming practical insulation with a thickness of 20 mm. In experiments, the magnetic signals derived from the longitudinal guided waves generated by our method on an STPG370 pipe were measured by a differential pickup coil, which was set on the receiver part on the pipe, based on the inverse magnetostrictive effect. The signal measured by the coil was read by the HTS-SQUID gradiometer, which was inductively coupled with the coil. Consequently, a slit on the pipe was detected.