Finding the Level of Useful Signals on Interpretation of Magnetic and Eddy-Current Defectograms

Abstract

Nondestructive testing of rails using various approaches and methods, including methods of magnetic and eddy-current flaw detection, is regularly carried out to ensure traffic safety on railway transport. This article is devoted to the problem of automatic determination of the threshold level of the amplitudes of useful signals (from the flaws and structural elements of the track) when interpreting defectograms of magnetic and eddy-current flaw detectors. A signal is considered useful (and is subject to further analysis), if the deviation of its value from the average value of all signals is, at least, twice more than the threshold noise level of rails. The probability of appearance of a signal with some amplitude in flawless rails in a section without structural elements, i.e. being a rail noise, is characterized by the normal distribution law. Thus, the three-sigma rule can be used to calculate the threshold noise level, and doubling the noise threshold gives a level that, if it exceeds the amplitude deviation from the sample mean, means that the signal is useful. This article proposes an algorithm to find the threshold noise level of rails and gives its theoretical justification, as well as examines examples of its operation in several fragments of real magnetic and eddy-current defectograms.