Because of their unparallelled sensitivity, bandwidth, and spatial resolution, Superconducting Quantum Interference Device (SQUID) magnetometers are unequalled in their ability to image the temporal and spatial variation of weak, low frequency magnetic field . For eddy current NDE, SQUIDs can be used with a low frequency excitation current to image cracks or material loss deep in aluminum structures. To detect the subsurface cracks adjacent to a fastener in multilayer aircraft structures, SQUID NDE measurements have utilized a sheet inducer [1] combined with depth-selective techniques [2, 3], a gridiron coil inducer combined with pulsed eddy current techniques [4], and a double-D inducer [5].

Our approach is to reduce the signal from the rivet and enhance the signal from the crack. Because the phase of the eddy current is a well defined function of depth, the component of the magnetic signal from the eddy current at any desired phase relative to the excitation current provides information about the current distribution at a certain depth below the surface [2, 3]. Since the strength of the signal also depends upon the cross section of the crack or corrosion in the direction of the current, the sensitivity of detection may be optimized by changing the orientation of the excitation current. In addition, the dependence of the amplitude on the current orientation provides information about the geometry of the flaw.

We have used an orthogonal sheet inducer, which can apply an ac magnetic field parallel to the test surface, to induce a large extended eddy current in a desired orientation. We have devised a self-referencing method to determine the geometry of the flaw.