An electromagnetic microscope for eddy current evaluation of materials

The concept for a novel instrument to be used in nondestructive evaluation (NDE) is presented. It is called an electromagnetic microscope, formed by superconductive microprobes arrayed in parallel rows. When moved over a test piece, the array generates a scanned image of flaws, stress variations, or changes in composition. Each microprobe comprises drive coils a few millimeters in radius that encircle pickup loops forming a concentric superconducting quantum interference device (SQUID). Drive coils transmit an oscillating magnetic field that induces eddy or magnetization currents in conductive or ferromagnetic materials, respectively. The gradiometer senses distortions in paths of induced currents. The extreme sensitivity of SQUIDS increases sensitivity, penetration depth, and spatial resolution over existing eddy current and magnetic NDE systems. Estimates of performance predict that a current of 1 A oscillating at 1 kHz in the drive coils allows detection of a flaw 0.1-mm in diameter to a depth of several millimeters in aluminum, with a horizontal resolution of about 1 mm and a vertical resolution of 0.3 mm or so