Eddy current aircraft testing with mobile HTS-SQUID gradiometer system

摘要

Using HTS SQUID systems for industrial applications imposes special requirements on system reliability, stability, and handling. For the detection of deep-lying flaws in aircraft structures, a mobile eddy current system in conjunction with an THS SQUID gradiometer is being developed. The challenge is to operate the SQUID sensor during movement in strong ambient fields, independent of orientation. A planar double-hole rf gradiometer with a gradient sensitivity of 500 fT/ (cm Hz sq root) was designed for that purpose. Two cooling concepts were successfully used: an orientation-independent nitrogen cryostat and a commercial Joule-Thomson-cryocooler, providing liquid nitrogen-free SQUID cooling. A SQUID integration scheme using a sapphire cold finger eliminates motion-related additional noise almost completely. In conjunction with differential eddy current excitation, the cryostat system was used to perform two-dimensional scans of aircraft structures. The system was successfully operated in the Lufthansa maintenance hall at Frankfurt airport. Scans of aircraft fuselage and sheels are presented, showing the advantages of SQUIDs compared to conventional NDE techniques.