| 摘要 |
PCT No. PCT/SE84/00175 Sec. 371 Date Nov. 13, 1984 Sec. 102(e) Date Nov. 13, 1984 PCT Filed May 10, 1984 PCT Pub. No. WO84/04596 PCT Pub. Date Nov. 22, 1984.During so-called eddy current testing by way of several frequencies, it is often difficult efficiently to separate and detect superficially located defects from the disturbances caused by a varying distance from the transducer to the object under test. This is due to the fact that the fault direction and the lift-off direction normally differ from each other to too small an extent and the fact that the signal processing, which is necessary for the suppression of the lift-off dependence, also suppresses the fault signal. This problem can be eliminated completely or partially by means of the present invention, which comprises selecting the used frequencies in such a way that, in relation to the depths of cracks occurring, they fulfill certain conditions, one of which being that at least one frequency is relatively insensitive to the defect in question. This effect is achieved by selecting such a high frequency that its depth of current penetration is limited seen in relation to the depth of the defect.
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