Inspection of aircraft parts with high remanent magnetization by eddy current SQUID NDE

We have developed an eddy current NDE system based on HTS rf SQUIDs as magnetic field sensors. Due to their high field sensitivity even at very low frequencies, SQUIDs are especially suitable for applications where a large eddy current penetration depth is required, We have used 3 GHz rf SQUIDs in our system made from YBCO thin films with a field resolution of about 1 pT/root Hz and a high dynamic range, more than 140 dB/root Hz in unshielded environment. With this system we could detect deep lying (up to 26 mm) cracks in test samples and original aircraft parts. Measurements made on the same sample showed an improvement in the signal to noise ratio of up to 3 orders of magnitude for cracks deeper than 13 mm, when comparing a conventional eddy current NDE unit with our SQUID system. Test objects containing ferromagnetic structures with a high remanent magnetization (often more than 1 mT), like aircraft wheels or steel bolts in part of the aircraft wing, very often cause instabilities of the flux locked loop operation of the SQUID. To prevent unlocking, we have developed a new background field compensation scheme, Special compensation electronics takes care of slowly varying magnetic fields of up to 1 mT/s and enabled us to perform eddy current measurements in presence of slow (< 30 Hz) background field variations of up to 5 mT.