ELECTRONIC GRADIOMETRY FOR NDE IN AN UNSHIELDED ENVIRONMENT WITH STATIONARY AND MOVING HTS SQUIDS

Difficulties in the fabrication of multilayer high-temperature superco nductor (HTS) devices have led to recent interest in the use of simple r HTS SQUID magnetometers in electronic gradiometers. One application of such systems is electromagnetic nondestructive evaluation. We have developed a prototype two-SQUID system and we present recent results i n this paper. We first demonstrate the level of interference suppressi on by comparing magnetometer and gradiometer signals. Then we present several results taken conventionally with the HTS SOUIDs stationary ab ove moving specimens and, for the first time, with the SOUIDs unshield ed in motion above stationary specimens. The specimens comprise a pair of wires in a return current loop as a calibration source, and an air craft-grade aluminium plate with fine slits mimicking fatigue cracks, first exposed and then covered with an additional aluminium sheet to s imulate internal flaws. These results are an important, though by no m eans final, step towards practical non-destructive evaluation of real test subjects with HTS SQUIDs. (C) 1996 Elsevier Science Limited