Research On Rail Base Nondestructive Testing Technology Based On Ultrasonic Guided Waves

With the development of railway system and urban mass transit,safety must be the top priority,rail flaw detection is essential to ensure the safe operation of the train.At present,rail flaw detection relies on the traditional ultrasonic detection technology in China.But it has blind area at the bottom of rail,which is easy to cause missed detecting and erroneous judgment.Nondestructive testing(NDT)technology based on ultrasonic guided waves has advantages of long detection distance,full cross-section detection and high detection efficiency,and is especially suitable for the slender component.If guided wave testing can be applied to the rail flaw detection,it will contribute much to reduce missed detection and erroneous judgement and improve detection efficiency,and has important theoretical and practical significance to ensure the safe operation of rail system and enhance the infrastructure detection level.The relatively complete process of the excitation,propagation,reception and signal processing of guided wave NDT for the rail base area is studied by combining theoretical research,numerical simulation and experimental research.The research on the propagation characteristics of ultrasonic guided wave in rail is the premise of rail flaw detection based on guided waves.The dispersion curve of 60kg/m rail is solved by semi-analytical finite element method.The rails in different regions are compared to plates of corresponding thickness,and the propagation characteristics of Lamb waves in the plates are analyzed.It is concluded that the guided wave mode under the specific excitation mode of the rail base is highly similar to the anti-symmetric mode of Lamb waves in 12 mm plate.The propagation of the guided wave at the rail base is simulated by the finite element method.The simulation directly shows the propagation process of guided wave on the rail bottom.The frequency range and mode of guided wave suitable for rail bottom guided wave inspection are optimized through simulation.Considering the restraint of the fastener on the rail bottom,the fastener is equivalent to a spring element.The results show that the fastener has a certain influence on the guided wave propagation at the rail base area.Through the finite element simulation,the interaction principle of different damage and guided wave of rail base area is studied,which is used to guide the experiment of rail base flaw detection.In the simulation,the guided waves are respectively excited in different regions of the rail and in different directions at the best position,so as to optimize the excitation position and direction suitable for rail base flaw detection.The sensitivity of guided waves to cracks in different directions at the rail base is studied by setting cracks with gradient growth in different directions,the results show that the guided wave mode is sensitive to transverse cracks,and the actual rail bottom cracks are mostly in the form of transverse cracks.The simulation shows the interference phenomenon of guided waves with multiple damage on the rail bottom at a specific location,and it shows the interference phenomenon of guided waves with multiple damage on the rail base at a specific location.In addition,the nuclear damage is simulated by element stiffness reduction,and the effectiveness of guided wave detection for fatigue damage is analyzed.Based on the theoretical and simulation research,the experimental scheme of rail base detection based on guided waves is designed.Different sizes of artificial cracks are made at the rail flange.Through the detection results of different receiving transducers under different working conditions,the most suitable guided wave receiving transducer is selected,and the recognition and location of 3mm cracks on the rail bottom are realized.The validity of rail base detection based on guided wave in this study is verified by experiments.Guided wave signal processing and damage identification are also important parts of rail base flaw detection.This thesis studies the signal denoising scheme of the Fourier transform filter,and applies the short-time Fourier transform and wavelet transform to the signal processing,which can effectively identify the 3mm rail flange crack,but the damage judgment has certain subjectivity,and it is difficult to build quantitative judgment indicators.In addition,due to the attenuation of guided waves,the long-distance damage detection in the weak signal recognition is also one of the difficulties.Based on those,the research of weak signal guided wave detection method based on Duffing chaotic oscillator is carried out.Due to the defects of the traditional chaotic oscillator detection system in the guided wave detection in rail detection,the multi-frequency excitation Duffing chaotic oscillator weak signal detection system is developed,and the time-moving window function is constructed and the Lyapunov exponent and Lyapunov dimension are used as the damage judgment factors to realize the quantitative identification and location of the damage.The graphic user interface of Duffing oscillator chaotic detection system for guided waves is developed,which is helpful to promote the practical application for the detection system.