The Study of Eddy Current Sensors Signals over a Group of Cracks in Metal

Abstract

We studied corrosion fatigue cracks of operational origin in a steel gas pipeline and obtained a number of statistical data on their distribution on the metal surface.This information is important for eddy current flaw detection of steel gas pipelines with stress-corrosion cracks. These data were used for making specimens of crack groups in special steel samples. Experimental studies of the signals of eddy current sensors over a group of cracks were carried out using these specimens.Results of experimental studies of the signals of eddy current sensors of various types (absolute, differential) with different diameters of ferrite cores over specimens of crack group in special steel samples that are located at different intervals are presented. The research results show that the waveform of eddy current sensors undergoes significant transformation when changing the parameters of the group of continuity defects in the metal.In the case of an absolute surface eddy current sensor additional local maxima signals, which are located over the center of continuity defects in a group of cracks, appear. In the case of differential surface eddy current sensor additional local extrema (maxima and minima) signals appear. These extrema are associated with the number and location of continuity defects in a group of cracks. It is shown that the waveform of the surface eddy current sensor over a group of cracks is determined by its type and parameters of a group of cracks in the metal. Mathematical conditions that make possible resolution or non-resolution of the signal of the surface eddy current sensor in the space of a group of cracks in the metal are found.It is shown that when the defects in a group of cracks approach each other, a significant increase in the amplitude and half-width of the signal of the surface eddy current sensor occurs. The half-width of the signal of the surface eddy current sensor depends on the interval between the defects of continuity and their number in the group of cracks.