Research On Optimizing Of Railway Axle Fatigue Crack Ultrasonic Detecting

In the 21st century, the development of the globe railway is booming. China is now planning a grand railway transit. As one of the key parts of the railway vehicle, the axle tends to generate fatigue crack at high stress concentration area under long-term fatigue load which may cause axle broken. Periodic NDT for in-service axles can effectively prevent axle broken accidents caused by fatigue crack. To ensure the axle operation safety,the purpose of this paper is to study the axle fatigue crack ultrasonic testing method and improve the axle press-fit part defect detecting ability.In this paper, the ultrasonic testing technology for solid and hollow axles in different repair technology and working conditions has been studied. The ultrasonic testing technology based on phased array technology is designed and optimized for coupling from axle body and axle end face. Through simulating and testing, a self-focusing phased array ultrasonic probe with concave elements is designed and applied to improve axle defect detecting ability. The press-fit interface reflection echo during axle inspection is found,which causes the SNR reduction greatly and affects defect identification. By analyzing the interface reflection echo, the ultrasonic image enhancing and specific image denoising are conducted in digital image processing way. The morphological wavelet threshold denoising method and the morphological anisotropic diffusion denoising algorithm are proposed also.The optimized filter combination scheme is formed through experiment and study to ensure retaining defect information effectively while removing the interface echoes as many as possible. In the end, the tests on reference axles with artificial cracks and in-service wheel-sets prove that the method can effectively keep the crack information, weaken the press-fit interface echoes, and improve the SNR of defect detection with good applicability.The main contribution of the paper is shown as follows.(1) The influence of railway axle structure, stress and fatigue features etc. on axle damage has been studied and several measures to prevent axle failure have been given.Based on the study of the ultrasonic testing technology in different repair technologies and working conditions, the research on axle ultrasonic testing of Chinese HX series of locomotive, passenger and cargo vehicle have been carried out. The research is focused on the ultrasonic testing technology of phased array probe coupling from axle body and axle end face. The method coupling at axle end face is proposed with phased array sector scan,secondary reflection and transmission models to realize the axle full coverage. It is a new solution for solid axle ultrasonic inspection without dismounting wheel-set from the train or with coating on axle body.(2) For the axle press-fit part, the most important and critical area, a self-focusing phased array probe with concave elements is proposed to improve the ultrasonic beam focusing performance. Through the simulating and testing, comparing with linear phased array ultrasonic probe with flat elements, better performance for transverse crack detection is achieved. The average inspection gain is reduced 14.7dB and the SNR of defect detection is improved 8.3dB. The influence of the press-fit interface echo to defect detection is tested and studied. Comparing with the bare axle crack detection, 18dB SNR is decreased after press-fit. The causes of interface reflection echo are analyzed.(3) By analyzing time-frequency characteristics and imaging features of ultrasonic testing, the digital image processing technology is used to get higher SNR and improve the performance of ultrasonic image. The 'power normalization', 'S-curve', 'homomorphic filtering', 'medium filter' and 'wiener filtering' methods are focused on suppressing press-fit interface echo by calculating single point or points within an image window. Studies have shown that these methods can suppress press-fit interface echo in certain situations or can remove certain kinds of image noise.(4) Based on the difference between defect echoes and interface reflection echoes in axle press-fit area ultrasonic testing image, a morphological filter is used to remove press-fit interface echoes. The study of wavelet threshold denoising method and anisotropic diffusion denoising method are carried out, and both of them can ensure the premise of retaining crack defect information while removing press-fit interface echoes. The experiment shows that the two methods have obvious effects and can realize safe filtering.(5) At last, based on the above research results, an optimal filter algorithm combination scheme is formed through experiment and study. By testing on reference axles with artificial cracks, the algorithm combination can retain defect information effectively.The defect value has only 0.4 1dB weaker in average while the press-fit interface echo is removed effectively with 6.07dB weaker in average. The SNR of whole image is obviously improved. The verification of 108 in-service wheel-sets testing data indicates that 3 manually confirmed natural defects can be all retained effectively while the alerting number is decrease from 63 to 19. It means that the press-fit interface echoes are well removed and the re-check work is much less than befor.In brief, for the axle fatigue crack detection, ultrasonic testing technology has been studied systematically. Aiming at axle press-fit part, a critical area, studies of improving phased array probe ability and ultrasonic image optimization with denoising algorithm have been carried out. After testing with real data, the research result shows that the axle fatigue crack detection ability is proved.