Eddy-current nondestructive material evaluation by high-temperature SQUID gradiometer using rotating magnetic fields

To exploit the sensitivity offered by HTS SQUID systems for detection of deep faults inside conductive samples a new approach was taken to avoid additional noise sources typical of scanning measurements. The excitation fields are rotated by electronic control which permits one to keep the sample and the system stationary during measurements. Flaws of 10 mm and 40 mm length were detected under 12 mm thick covers of aluminum. The orientation of the flaws was mapped by taking advantage of the anisotropic excitation of eddy currents by the chosen differential coils. This approach is particularly promising for ferromagnetically contaminated samples and radially oriented defects, e. g. originating from rivets.