Rail defect monitoring based on laser Doppler frequency shift theory

The flaw detection equipment applied on railway in China can only be inspected at the time of maintenance-skylight and cannot be on-line monitored at the present stage. A detection method of rail internal defects based on ultrasonic guided waves and laser Doppler frequency shift theory is proposed.First, the semi-analytical finite element method is improved by adding the environment temperature as a variable. The method is used to obtain the dispersion curve of the CHN60 rail in China at a specific temperature. Then, the modes which are suitable for the detection of defects and incentive methods have been selected through combining the analysis of mode shape with stimulation and response algorithm. Then, the laser is divided into reference light and measuring light by semi-transparent mirror. The measurement light is irradiated on the rail surface through Bragg Cell, and the change curve of the light intensity is obtained by the Doppler shift of reflected light and the interference of the reference light. The echo velocity signal of the internal defect of the rail is measured by signal processing and calibration. While this mode is stimulated and propagating in the railroad, the position of the defect can be detected after digital signal processing. Finally, the research group has conducted field experiments on the Beijing Ring Railway Experimental Base and verified the feasibility of the method. It shows that the error in defect location is less than 0.5 m. Using laser Doppler frequency shift method for guided wave signals to locate defects can effectively avoid the error caused by transducer contact measurement. It not only guarantees the normal operation of the train, but also realizes the all-weather on-line monitoring without interruption, which improves the detection efficiency.