Detection of Shape and Depth of Defects in Aluminum Structures Using SQUIDS

We have developed a low temperature SQUID measurement system for detection of defects deep under the surface of aluminum structures using eddy current techniques. The system uses an orthogonal planar inducer with two different excitation frequencies. We have also developed a data analysis software that enabled us to distinguish between round objects (fastener holes), straight defects (cracks) and cracks close to holes simulating cracks around fasteners in aluminum structures. We were able to detect defects that are 8mm below the surface. We have also measured the change in phase of the detected signal as a function of depth of the defect. This relationship can be used to determine the depth of hidden flaws. A similar system using High Temperature SQUID with better spatial resolution was also able to detect cracks close to fastener holes in layered aluminum sample.