Improvement of the Accuracy of Eddy-Current Testing of the Electric Conductivity of Materials and the Thickness of Diele
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IMPROVEMENT OF THE ACCURACY OF EDDY-CURRENT TESTING OF THE ELECTRIC CONDUCTIVITY OF MATERIALS AND THE THICKNESS OF DIELECTRIC COATINGS OF THE SHELLS A. Ya. Teterko,1, 2 V. M. Uchanin,1 and V. I. Hutnyk1
UDC 620.179.14
On the basis of construction of a two-parameter nonlinear model of the inverse transformation function of the “eddy-current primary converter–tested object” system, we propose a method for measuring the specific electric conductivity of materials and the thicknesses of dielectric coatings of the shells. This method guarantees a significant decrease in the errors of evaluation of the indicated parameters in the case of their simultaneous variations during the tests and the improvement of the reliability of the results of diagnostics of the state of the tested object. Moreover, this method enables us to significantly simplify the hardware and software realization of the procedure of testing. Keywords: eddy-current testing, nonlinear model of the system, inverse transformation function, monitoring of the characteristics of shells, specific electric conductivity, thickness of the dielectric coating.
The equipment for the nondestructive eddy-current (EC) testing of specific electric conductivity (SEC) regarded as a structure-sensitive characteristic of the metal is extensively used for the estimation of the degree of degradation and corrosion damage of materials as a result of the long-term operation of various structures in power engineering and aircraft industry, as well as of the equipment of chemical, oil, and gas industries, etc. The surfaces of tested objects usually have a layer of protective coating (paint, polymeric materials, etc.) and the changes in its thickness introduce an additional error of the EC testing of SEC (up to 3–7%). Note that the decrease in this error is an important factor of improving the reliability of the results of diagnostics of the state of structures and prediction of their service life.
In the contemporary equipment used for the EC testing of SEC, it is customary to realize the so-called phase method of detuning from the influence of interfering factors [1]. This method is based on the difference between the nonlinear functions of action of separate parameters of the tested object (TO) in the complex plane of the response signal of the eddy-current primary converter (ECPC). According to this method, the error of measuring the SEC of the material caused by the changes in the thickness of the dielectric coating (regarded as an interfering factor) is minimized by the vector addition of a certain compensating signal to the response signal of the ECPC and measuring of the phase of the total signal according to which we determine the SEC of the material of TO. Numerous technical decisions, in particular, described in [2–4], are proposed for the solution of the problem of decreasing the error caused by the action of this interfering factor. However, its decrease is problematic due to the nonlinear and correlated influence of the specific electric conductivity of the material
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