Acoustic Emission Signal Monitoring For Rail Damage Based On Fiber Bragg Grating

With the rapid increasing of in-service China's high-speed railway,the reliable,safe and efficient operation is facing great challenge in China's railway transportation.For rail damage,ultrasonic testing,magnetic flux leakage testing,eddy current testing and visual inspection are commonly used.However,these methods require periodic inspection,and acoustic emission monitoring technology can realize real-time monitoring of rail damage process.As a result,it can evaluate the health status of rail.Acoustic emission refers to the process of material releasing strain energy in the form of elastic waves due to structural changes under internal or external forces.The fiber Bragg grating(FBG)has the advantages on good insulation,no power supply,immunity to electromagnetic interference,light weight and small volume.The FBG can be packaged into a sensor to replace a piezoelectric sensor and used for acoustic emission monitoring.Therefore,this thesis focuses on the monitoring of rail damage acoustic emission signals based on fiber Bragg grating,including the basic theory of acoustic emission sensing using fiber Bragg grating,the construction of acoustic emission monitoring system based on fiber Bragg grating,simulation of acoustic emission of rail using finite element,and the analysis of acoustic emission characteristics of the tensile failure of rail materials in experiment,respectively.(1)The modulation process of acoustic emission wave on the center wavelength of fiber Bragg grating is theoretically derived.The effect of two demodulation methods based on the matched filter and the tunable laser are studied through experiments.It is found that the optical power obtained by using the matched filter demodulation method is weak,so that the high sensitivity photodetector is needed.Then,304 stainless steel and plexiglass materials are used to package fiber Bragg gratings respectively,and the responses of two sensors are tested by experiments.It is found that fiber Bragg grating sensors packaged with plexiglass have higher responsivity.(2)The acoustic emission monitoring system based on fiber Bragg grating is proposed,which is tested with standard acoustic emission signal.The system can detect the acoustic emission signal successfully and can estimate the characteristic parameters of acoustic emission signal in real time.(3)In order to monitor the damage state of rail,the propagation of acoustic emission in rail is analyzed.First of all,Lamb wave dispersion curves are drawn based on Rayleigh Lamb wave equation.It is concluded from the dispersion curves that S0 extension wave and A0 bending wave are the main acoustic waves in plate structure.Based on the elastodynamics theory,the characteristics of acoustic emission wave propagation in rail aremodeled and solved by using explicit integration algorithm in ABAQUS/Expicit.The waveform variation and signal attenuation characteristics of acoustic emission waves propagating in the rail are studied in experiment.Finally the wave field snapshots of the simulation model are given,which describes the propagation state of the acoustic emission wave at different times.(4)After the analysis of the propagation law of acoustic emission in rail,it is necessary to judge whether the rail is damaged or not from the characteristics of acoustic emission signals.Therefore,the tensile damage test of rail material is designed,and the acoustic emission signal of the whole tensile process is collected using the acoustic emission monitoring system based on fiber Bragg grating.The stress-time curve is formed to analyze the collected signal.Finally,it is concluded that the material's health condition can be monitored according to the characteristics of acoustic emission signals,proving effective monitoring system for rail damage.