Abstract
Superconducting QUantum Interference Devices (SQUIDs) are extremety sensitive detectors for the measurement of magnetic flux. In particular the high critical temperature SQUIDs, which can be operated at liquid nitrogen temperature with easy cryogenic requirement, are well suited for nondestructive evaluation (NDE) of cracks and corrosion which result from the machining process or aging of electrically conductive structures. The main characteristics of SQUIDs as eddy-current NDE sensors are: high sensitivity even in unshielded environments, a large frequency range of operation which allows the detection of surface and subsurface flaws and high spatial resolution. We performed magnetic field measurements on the eddy-current double-D inducing coil. used in many NDE experimental systems. and on 200 x 100 x 4 mm 3 Al plates with a 6mm surface-breaking hole using high critical temperature SQUIDs in unshielded environments. The sensitivity to the anomalous magnetic field related to the defect along the three spatial directions is discussed in terms of unperturbed magnetic field distributions due to the eddy-current source.
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