Specific features of the space distribution of the signal of an eddy-current converter caused by cracks of different len
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SPECIFIC FEATURES OF THE SPACE DISTRIBUTION OF THE SIGNAL OF AN EDDY-CURRENT CONVERTER CAUSED BY CRACKS OF DIFFERENT LENGTHS V. M. Uchanin
UDC 620.179.14
We calculate and analyze the space distribution of the signal of an eddy current converter for crack-like defects of different lengths. We have established the differences in eddy current signals between short and long cracks. We also present recommendations for the optimal selection of the parameters of an eddy current converter depending on the crack length.
Formulation of the Problem Establishing the specific features of space distribution of the signal from cracks for different ratios between their parameters and the sizes of the eddy current converter (ECC) is important for the selection of its optimal geometry and the scheme of scanning of the controlled object (CO). An inappropriate choice of these parameters can lead to the omission of critical defects or to an unwarranted increase in the time expenditures for testing. In addition, information on the space distribution of such a signal helps to interpret the results of testing and to substantiate the technologies of quantitative control with determining the parameters of recorded defects. The specific features of space distribution of the signal from defects was studied earlier [1 – 8]. In [1, 2], the electromagnetic fields of a defect in a CO under the action of a uniform primary field, in particular, in the form of a plane wave, were considered. Using such models, one can determine the general laws of formation of the electromagnetic field of defects, its dependence on the parameters of defects, working frequency, and the specific conductance of the material (especially of transformer ECC with spaced windings [3]). However, numerous important problems are solved with the help of parametric ECC of the local type, where, for reaching the necessary sensitivity and resolution, the sizes of windings are chosen commensurable with the parameters of the minimum defect [4, 5]. In the course of testing, one moves the ECC along the CO surface, i.e., the local source of primary field changes its position with respect to the defect. In such ECC, the primary electromagnetic field, interacting with the defect, is substantially nonuniform, and, hence, models with a uniform field are unsuitable here. Parametric or transformer ECC with the axial arrangement of windings were also investigated, but the results obtained do not cover the entire range of ratios between the parameters of defects and the sizes of ECC and are sometimes contradictory [5 – 8]. Consider a cylindrical parametric ECC interacting with a current-conducting nonmagnetic half-space ( Z < 0 ) with a surface crack-like defect (Fig. 1). We model this defect by a narrow rectangular discontinuity flaw of the material (notch) of length a, depth b, and width (opening) c. We interpret here a crack as a defect for which a >> b >> c. The coordinate axes are situated as shown in Fig. 1. The average diameter of the winding is Dc , its thickness (the difference betw
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