Research On Key Techniques Of Rail Bottom Edge NDT Based On Ultrasonic Guided Waves

Ultrasonic guided wave technique for rail base is a new long-range online NDT technique.The technique solves the problem that the conventional NDT techniques can not detect the loss in rail base,and provides a strong guarantee to the development of the high speed passenger transport and heavy loading freight transport in China.The research is supported by the National Natural Science Fund,Special Major Science and Technology Project of Zhejiang Province,Industry Services Technology Innovation Project of China Academy of Railway Sciences.The research focuses on the key techniques of ultrasonic guided wave NDT.summarizes the status of ultrasonic guided wave technique and its applications in rail,and analyzes the feasibility of rail base detection with ultrasonic guided wave technique.Then dispersion analysis,transducer design,signal processing and detecting system development are carried out.The research of the dissertation lays the foundation for the ultrasonic guided wave NDT application in rail base.The detailed contents of the dissertation are presented as follows:In Chapter One,the background of the research is introduced,and the significance of the research is described.Reviewing the rail NDT techniques and the key techniques of ultrasonic guided wave NDT,the rail base testing technique based on ultrasonic guided wave is proposed.Commenting on the domestic and foreign achievements of ultrasonic guided wave testing for rail,the contents and the frame of the dissertation are determined.In Chapter Two,the SAFEM model for ultrasonic guided waves in rail is established,and the dispersion curves of 60kg/m rail without loads are calculated.As the dissertation studies the ultrasonic guided waves propagating in rail base,the dispersion curves of rail base is calculate to eliminate the rail ultrasonic guided waves not propagating in rail base.As the axial loads is much larger than the excitation of ultrasonic guided waves,the nonlinear factors of mechanical parameters are considered.The SAFEM model considering axial loads is established,and the influences of different axial loads on the dispersion characteristics of rail base are researched.In Chapter Three,the excitation characteristic of ultrasonic guided wave transducer is studied,the principle of shear family waves excitation by Wiedemann effect is analyzed.A simple style transducer and a miniaturized transducer are designed.The arrangement of transducers in rail and the propagation of different frequencies guided waves in rail are studied.A comb guided wave transducer array is designed and its frequency characteristic is analyzed.Combine frequency characteristic of comb transducer array with dispersion characteristic of rail base,the principle of exciting guided waves at a specific operating point by the comb transducer array is presented.In Chapter Four,the rail base ultrasonic guided wave signals are analyzed by short-time Fourier transform,wavelet transform and Wigner-Ville distribution.The influences of time-frequency parameters are analyzed.The time-frequency distribution is studied combining with group velocity dispersion curves.As the frequencies of noise is similar with ultrasonic guided waves,the wavelet threshold noise reduction algorithm and the noise reduction algorithm based on matching pursuit are adopted,the noise reduction results by above algorithms are compared.In Chapter Five,the rail base ultrasonic guided wave testing system is developed.The operating point of the comb guided wave transducer array acquired by theoretical calculation is verified by experiment.The testing results of a simple style transducer and a miniaturized transducer are compared.The detectability of ultrasonic guided wave to the losses of different positions and types is studied,the similar research is done on weld testing.The influences of transverse and axial loads on rail base ultrasonic guided wave testing is studied by experimental methods.The capacity of rail base ultrasonic guided wave testinsg system under field conditions are examined.In Chapter Six,the research results of the dissertation are summarized and the outlooks of the research are also discussed.