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    Opitimization of a Multifrequency Eddy-Current Test System Concerning the Defect Detection Sensibility

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    A multifrequency algorithm is applied to suppress signals caused by disturbing parameters. This procedure (based on the projection principle in a multidimensional vector space) and the electronics employed in prototype form are described. The prototype operates with up to four frequencies in a wide range (500 Hz to 1 MHz). The frequencies are continuously tunable and are fed into the coil one after the other by high-speed multiplexing. The multifrequency approach yields a read-out value affected only by defects. The defect signals then can be assessed by means of reference defects of known depths giving equivalent indication peaks. The elimination of the disturbing signals chiefly increases defect detection sensitivity. In general, however, the suppression is not total, and, additionally, the original defect signals are reduced to a greater or lesser degree by the application of the projection principle. The testing frequencies used have a strong influence on the projection losses and therefore on the defect detection sensitivity. A computer program system that allows the reaction of the impedance of the coil to be calculated as a function of the electric, magnetic, and geometrical properties of the specimens and the dimensions of the coil itself is used to determine an optimum frequency combination for the given test situation. This procedure is applied to the testing of heat-exchanger tubes, claddings, and welded joints. The results obtained with the optimized test system are presented.


    multifrequency algorithm, projection principle, prototype, multiplexing of frequencies, testing frequencies, defect sensitivity, reference defects

    Author Information:

    Becker, R
    Fraunhofer-Institut für zerstörungsfreie Prüfverfahren, Saarbrücken,

    Betzold, K
    Fraunhofer-Institut für zerstörungsfreie Prüfverfahren, Saarbrücken,

    Committee/Subcommittee: E07.07

    DOI: 10.1520/STP27589S