Rail Fatigue Crack Depth Quantitative Detection Based On Infrared Thermal Wave Technology

The contact fatigue crack of an oblique-line shape is a new form of rail tread failure that appears in the speed-up section. Under repeated fatigue load, an oblique crack will gradually extend into center of the rail, and ultimately lead to its fatigue fracture. Therefore, a timely detection of the fatigue microcracks on rail subsurface and an accurate measurement of the expansion depths of these oblique cracks have some practical significances for the engineering department to reasonably draw up preventive maintenance programs that can avoid the irretrievable defects of rails. Since the depth of an oblique crack cannot be measured out by traditional detection method of ultrasonic external excitation, an active infrared thermal wave non-destructive testing technique was proposed in this thesis for a quantitative measurement of the depths of oblique cracks on rail subsurface. Based on intensive studies the key techniques, including finite element simulation, the thermal wave testing, and infrared image processing, etc., the theoretical and the actual experimental values of the oblique crack depths were calculated.Firstly, the mechanism of initiation, extension and further expansion of oblique cracks on the rail tread, as well as the basic structures of subsurface cracks were analyzed; and an one-dimensional heat conduction model of infrared radiation in the oblique-crack depth direction was set up. In ANSYS simulation environment, a two dimensional model of finite element of the rail subsurface cracks was constructed, and the process of infrared thermal wave testing was simulated using the method of loading heat flow to get a curve of the temperature variation. The T-t curve obtained from the above simulation would be processed using the peak of second-order derivative temperature-time(PSDT) method, and then the theoretical values of the depths at each reference points of the cracks were calculated.Secondly, with the low energy of rail thermal radiation taken into account, two different types of external incentive heat sources were designed to make a comparative study, thereby setting up an experimental platform of infrared thermal wave detection system and collecting the infrared image sequences that could indicate oblique cracks with a thermal imager controlled by an on-line software.Thirdly, the image texture and edge of oblique crack were enhanced using adaptive multiscale products threshold algorithm based on non-subsampled contourlet transform(NSCT), the inner minutia areas of the crack images were segmented using region growing method, and information of the characteristics of minutia pixels were extracted from the segmentation-processed images, the temperature data of infrared image sequences at characteristic points could be derived from the off-line analysis software of the thermal imager, and based on which, as well as some mathematical calculation methods, a quantitative measurement of the expansion depths of oblique cracks were completed in the experiment.