Standard method for detection of magnetic defects in aircraft engine discs using a HTS SQUID gradiometer

We have developed a new high sensitive magnetic method for non-destructive testing (NDT) of small ferrous inclusions in aircraft engine discs. The measuring method is based on a second order gradiometer using three thin film Superconducting Quantum Interferometer Devices (SQUIDs) as magnetic sensors. The high balance of the gradiometer setup with a base length of 60 mm insures that the measuring system is extremely stable against external disturbances without having a magnetic shield. At the same time the resolution of the local magnetic field is high, about 130 fT/Hz/sup 1/2/. The system is suitable for NDT of technical components, especially for tests into large depth of parts with large size. Using this new method, the location, depth and size of magnetic inclusions in non-magnetic alloys can be determined. The method is used as a standard testing procedure in production quality control of aircraft engine discs.