A nondestructive inspection (NDI) system using an HTS SQUID for copper heat-exchanger tubes has been constructed. An eddy-current-based NDI method using an HTS SQUID gradiometer cooled using a cryocooler was employed for the detection of micro flaws in thin copper tubes 6.35 mm in diameter and 0.8 mm in thickness. With an excitation field of 1.6 μT at 5 kHz, a flaw in the tube, measuring 30 μm deep and 15 mm long, was successfully detected by the system with a high signal-noise ratio of at least 20. Numerical simulations were also conducted to determine how many sensors would be required for inspection around the circumference of an entire tube.
A practical robotic three-dimensional (3D) SQUID-NDE system was developed for the first time. This system is based on a previously developed traveling SQUID-NDE (i.e., one that is not stationary, but the SQUID sensor itself moves or scans along the surface of the object), which incorporates SQUID sensor travel technologies such as the `pseudo' third-order SQUID gradiometer. The system developed is composed of a traveling SQUID gradiometer, an articulated-type robot used as a SQUID sensor manipulator, a laser CCD displacement sensor, and a numerical interpolation procedure to reconstruct the surface of the evaluated object without the need for CAD data. To demonstrate the capability of this imaging system, it was used to detect artificial damage in a stainless steel (type-304) tube. The system successfully detected the artificial damage. It is anticipated that the robotic 3D SQUID-NDE imaging system will be in practical use in the near future.