Eddy-current flaw detection in structural elements
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EDDY-CURRENT FLAW DETECTION IN STRUCTURAL ELEMENTS V. M. Uchanin
UDC 629.179.14
We describe promising directions in the development of the eddy-current method for the flaw detection of structural elements. We show the possibilities of subsurface flaw location based on the use of low working frequencies and quantitative control with the identification and evaluation of parameters of the found defects. The possibilities of applying high working frequencies for the control of thin surface layers of an article and low-conducting materials are described. We present also the possibilities of new eddy-current converters and flaw detectors as well as testing technologies developed on the basis of their application. Finally, we describe new methods of information processing, in particular, the two-dimensional visualization of testing results.
At present, the eddy-current method (ECM) of nondestructive testing (NDT) is taking on greater and greater significance for providing the reliability of critical objects of long-term operation, in particular, in aviation, power engineering, and oil, gas, and chemical industry [1 – 5]. It is important that eddy-current control can be realized without contact with the controlled object (CO), which guarantees its immediacy and possibility of automation. One can move the eddy-current converter (ECC) with respect to the surface of the CO with high velocity, which results in a high productivity of this method. It is important that surface contamination, moisture, and radioactive emission do not affect the signal of the ECC. In general, it is multiparametric: it depends on the electrophysical and geometrical characteristics of the material of the CO as well as on its discontinuities (defects). This fact enables one to solve the problems of flaw detection, to evaluate changes in the structure of materials (connected, e.g., with the quality of thermal treatment or the degradation of the material), and to measure the geometrical parameters of the CO (the thickness of sheets and pipes, the diameter of wires and bars, gaps between shells, the thickness of protective coatings of various types, etc.). For a long time, the positive properties of the ECM were not known completely, which hindered its practical application. This method of NDT has come a long way in its development, in the course of which numerous restrictions declared at the beginning of its formation have been eliminated. In particular, it was not recommended for controlling ferromagnetic materials due to the possible influence of the inhomogeneity of magnetic permeability. However, at present, there are many examples of successive application of the ECM for the flaw detection of ferromagnetic materials. Even in the case of high inhomogeneity of the magnetic properties of the CO, the influence of this feature can be reduced by biasing with a constant field or by using multifrequency methods of eddycurrent control or differential ECC [3 – 5]. As an example of serious understanding of the role of the ECM, we should mention Germany. He
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