The authors have developed eddy curent NDE systems which make use of High Temperatur (HT) Superconducting Quantum Interference Devices (SQUID's) as magnetic field sensors. Due to their high field sensitivity even at low frequencies, SQUIDs are especially suitable for applications, where a large penetration depth is required. It was used 3 GHz rf SQUIDs made from YBCO with a field resolution of about 1 pt/?Hz in the systems. In order to obtain a high dynamic range, gradiometric excitation coils shaped as a 'double-D', and in some cases also circular field coils with local compensation of the excitation field at the SQUID by a second coil were used to excite eddy currents. The authors have set up two different SQUID based eddy current NDE systems, one of which is optimised for testing aircraft wheels. It consists of a fixed cryostat, which contains liquid nitrogen as coolant for the SQUID, and is mounted close to the outer surface of the rotating wheel. The second system can test planar objects. A motorised x-y-stage moves the cryostat containing the SQUID above planar samples. Here the authors could detect in aluminium plates flaws with lengths of 40 mm, having a height of 1.2 mm in a depth of 26 mm. A comparison between conventional eddy current systems and the SQUID based system shows an improvement in the signal to noise ratio of up to three order of magnitude for cracks deeper than 13 mm, when using the SQUID system.


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    Title :

    Detection of deep lying cracks by eddy current SQUID NDE


    Additional title:

    Nachweis tieferliegender Risse durch zerstörungsfreie Prüfung mit Wirbelstrom-SQUID


    Contributors:
    Kreutzbruck, M. von (author) / Baby, U. (author) / Mück, M. (author) / Heiden, C. (author)


    Publication date :

    1998


    Size :

    7 Seiten, 5 Bilder, 11 Quellen



    Type of media :

    Conference paper


    Type of material :

    Print


    Language :

    English




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