The recent work for aging aircraft NDE
has led to the development of phase-sensitive, depth-selective techniques for low frequency SQUID eddy current application, which includes a self-referencing method, that are well-suited for detecting the cracks at the second or third layers adjacent to rivets in aircraft. The inducer is a sheet carrying a uniform current density. The induced eddy currents in the test plate produce a background magnetic field. A small flaw, such as a crack or inclusion, in the conductor disturbs the eddy current distribution and produces an additional magnetic field. Both the magnitude and the phase of the eddy current vary with depth. Consequently, the magnetic signal associated with the eddy current disturbed by the flaw depends upon the location of the flaw and its extension beneath the surface. By choosing an appropriate phase, the signals due to the surface structures may be reduced while the signals due to the subsurface structures may be relative enhanced.

The most of eddy current SQUID NDE studies are performed using the Magnetic Imaging Facility, which includes a four-channel, high resolution MicroSQUID I magnetometer system, that was custom built for non-destructive evaluation for measurements of action currents in biological systems. A 1 m by 1.7 m by 2 m tall magnetic shield with two layers of mumetal and two layers of aluminum provides a 12 fT/Hz-^1/2 envi-ronment for the SQUID. A nonmagnetic, high speed scanning stage in the shield can move samples at 10 cm/s for rapid imaging.

The images for sample #1, an 18 mm thick 7075-T6 aluminum plate. The surface with the open holes faces down. The induced eddy current is in the longitudinal direction. At a frequency of 170 Hz, three holes are detected (2 mm, 5 mm, and 8 mm below surface) at phases of 0° and 25°. The dipolar signal at the upper-left corner of each image is due to the hole at 2 mm below the surface. The large extended feature in the background is due to the inhomogeneous conductivity of the material.

After ten years service, the MicroSQUID I will be replaced by MicroSQUID III with two possible configurations. One of which has a three-axis vector gradiometer with a noise-canceling reference SQUID, and the other of which is a high sensitivity, two-channel magnetometer with noise of only 10 fT/Hz^1/2.

Yu Pei Ma
yu.p.ma@vanderbilt.edu