8.1 DETECTION OF RAIL DEFECTS FROM THE DEGREE OF ITS DEVELOPMENT
Abstract
The article deals with the detection of rail defects in high-speed sections of railroad track.
Key words: Rail, trackless track, rail defects, rail diagnostics.
To determine the number of serial defectoscopic diagnostics in the cycle of defectoscopic diagnostics of rails condition in the track according to [1] it is necessary to know the dependence of the probability of defect detection on the degree of its development, and the smallest interval between the outputs of rails by defects associated with internal damage to the metal, which can be detected by means of defectoscopic diagnostics. The same defect is the cause of more than 50 % of rail fractures not detected at their non-destructive defectoscopic diagnostics. Therefore, the speed of transverse crack development and its limiting dimensions are used as criteria of periodicity of defectoscopic diagnostics.
According to [1], the defect formation is conditionally divided into three stages. At the first stage, internal longitudinal microcracks are formed in the rail head, which under the action of train load develop into a transverse crack. At the second stage, the transverse crack reaches the size at which it can be detected by defectoscopic diagnostics. For modern typical diagnostics this size is 8-10 % of the cross section of the rail head. At the third stage, the transverse crack continues to develop and reaches a critical size at which brittle fracture of the rail can occur. During the third stage the crack should be detected by means of defectoscopic diagnostics before reaching the critical size.
The probability of transverse crack detection also depends on the applied means of defectoscopic diagnostics.
For new means of defectoscopic diagnostics in this case we used curves of probability of defect detection depending on the degree of their development used on Union Pacific railroad (USA), where defectoscopic diagnostics means are close in their characteristics in the range of transverse crack detection at the level of its development 10-35 % of the rail head cross-section.
For the old means of defectoscopic diagnostics the curve is constructed taking into account the 3-fold increase in the accumulated probability of missing a defect in the range of its development 10-35 % compared to the new means, which according to [1] was confirmed by repeated comparative tests.
