Rail Flaw Detection Based On Ultrasonic Guided Waves

Rail flaw detection is essential to ensure railway safe operation.China Railway Corporation has formulated the Rail Flaw Detection Management Rules and invested a lot of manpower and material resources.At present,rail flaw detection relies on ultrasonic testing technology,but it has blind areas and is easily disturbed by surface damages.Missed detection and erroneous judgement caused by this have become the primary cause of rail breaking in China.Ultrasonic guided waves are suitable for long range inspection of elongate members.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.According to the site investigations and the statistics of damage and broken rails,this thesis mainly focuses on detecting flaws in rail head and rail foot based on ultrasonic guided waves.In order to study the finite element modeling method of guided wave testing,the characteristics of damage echo and the corresponding experimental method,ultrasonic guided wave testing of pipe is conducted as a pilot study in this paper in this paper.The characteristics of guided wave echo of various damage shapes are analyzed by finite element simulation,and it is pointed out that the interference effect of damage echoes should be considered in damage identification.Finally,the verification experiment of guided wave testing of pipe is carried out by using self-developed guided wave detector.The propagation characteristics of guided waves in rail is the premise of rail flaw detection based on guided waves.In this paper,the propagation characteristics of guided waves in rail is studied by three complementary methods,including semi-analytical finite element method for solving dispersion curves,analogy method by solving Lamb waves in plate,and finite element analysis on the propagation of guide waves in rail.On this basis,the influence of excitation frequency on guided wave propagation characteristics in rail is analyzed,and the dominant modes as well as wave structures of guided waves in rail are presented to provide a theoretical guide for rail flaw detection experiments based on guided waves.In order to carry out the experiments of guided wave testing of rails,impedance curves of various piezoelectric ceramic wafers are measured,and a method of selecting impedance characteristics based on “wide-band excitation,narrow-band reception” in guided wave transducer design is proposed in this paper.On this basis,two kinds of piezoelectric guided wave transducers,which are suitable for rail flaw detection,are designed and applied to the experiments of exciting and receiving guided waves in rail.After that,two kinds of excitation and reception schemes for guided wave defect detection of rail are presented.Additionally,a method of optimizing the excitation frequency and signal analysis spectral in guided wave testing based on wavelet transform is proposed.Finally,through the flaw detection experiment on the seriously damaged rails cut off from railroad track in service and artificial damaged rails,the effectiveness of the guided wave testing scheme for rail defective detection is verified in this paper.Guided wave signal processing and damage identification are also important parts of rail flaw detection.In this paper,a damage identification method based on guided wave mode separation and mode conversion is proposed,and the existing problem of detecting weak multi frequency signal using the traditional chaotic oscillator weak signal detection system that excited by monochromatic wave is amended.In addition,the applicability of chaotic oscillator weak signal detection system for guided wave detection is analyzed.This paper also objectively points out the possibility of error detection and the difficulty in assessing the reliability of detection result.