Study On Propagation Mechanism Of Acoustic Emission Waves For Health Monitoring Of Bogie Structure

It's well-known that the most remarkable features of contemporary railway development are high speed and heavy load.Under this background,bogie,as a combination of multiple functions such as track guidance,weight support and impact bearing of locomotive and vehicle,may cause serious property damages or even casualties if structural failure occurs.At present,non-destructive testing techniques used for regular maintenance of bogie structures cannot yet fully examine or evaluate the development of dynamic cracks and their degree of danger.Health monitoring technology,especially real-time online acoustic emission technology,may become an important way to solve the safety problems of the bogie structure.However,due to the structural complexity and damage diversity of bogie,the mechanism and propagation mechanism of the acoustic emission wave have not been fully clarified,which has become a bottleneck restricting the further application and development of the acoustic emission technology in the field of bogie structural health monitoring.In response to the above problems,a series of studies,such as the acoustic emission properties of bogie frame materials,the propagation characteristics of acoustic emission waves,the attenuation characteristics of acoustic emission waves,have been carried out.The ultimate goal is to lay the theoretical foundation for the sound emission health monitoring of the bogie structure.Research contents and conclusions of this paper are as follows:(1)Acoustic emission generation and performance analysis in bogie frame materialsStarting from theories of dislocation origin,proliferation and seizure,the theoretical roots of acoustic emission are deeply studied in this paper.The relationship between energy,amplitude and grain size of the acoustic emission signal,the relationship between signal amplitude and dislocation shear stress,the relationship between event count rate and dislocation density and experimental parameters are hence derived.Three new parameters more accurately reflecting the acoustic emission characteristics are identified:amplitude statistics rate,energy statistics rate and impact rate.Based on the dislocation density distribution function and the dislocation-free zone theory,the reason for the large number of acoustic emission signals generated by the preset crack specimens in the elastic stage is clarified.Based on the above theories and new parameters,the uniaxial stretching process of the frame specimen of material S355J2 is accurately divided into five stages,namely initial clamping,elasticity,yielding,strengthening,and necking(fracture),and the characteristics of the"saw-toothed platform" of yielding stage and the "three-stage" of strengthening stage were successfully characterized.It is shown that the acoustic emission parameters are more sensitive to the micro-fracture of the material than the mechanical parameters,and the acoustic emission properties of the five stages of unidirectional stretching of the bogie frame S355J2 material are fully grasped.(2)Simulation of acoustic emission wave propagation and wave velocity study in bogie frameThe theories of the propagation characteristics of body waves and Lamb waves of acoustic emission waves are analyzed.The LS-DYNA finite element software was used to simulate the generation mechanism and propagation characteristics of two mode waves such as acoustic emission extended wave and bending wave excited by Hsu-Neilsen pencil-lead breaking,and the key points in the simulation process,such as the lead-excited force value,the grid maximum and setting parameters,are listed.Based on the above,a method for simulating the wave velocity of two modal waves of a flat panel using LS-DYNA finite element software is proposed.The longitudinal vibration velocity and the vertical vibration velocity of the nodes in the simulation model are extracted as important parameters,which are respectively used to calculate the acoustic emission extended wave and the bending wave velocity value.The deviations between these simulated values and the theoretical values calculated from the dispersion characteristics are small.The deviation between the wave velocity simulation value,the broken lead experimental value and the theoretical value is relatively small,which proves that the method is fully applicable to the acoustic emission sound field simulation of complex structures.(3)Study on amplitude attenuation law of acoustic emission wave in bogie frameFrom the different wavefront angles of plane waves and cylindrical waves,the attenuation law of the amplitude of acoustic emission wave of plate-type structure is theoretically deduced.Three types of amplitude attenuation models such as traditional exponential function,modified exponential function and improved power function are obtained respectively.The first peaks of the single-mode extended wave of the pencil-lead breaking test signal are extracted as the attenuation measurement scale to verify the three models.The results prove that the attenuation law of the amplitude of the acoustic emission wave in the whole field is that the amplitude usually decreases in form of a power function as the propagation distance increases.Especially in the near field range,the law is more appropriate.Only in the far field range,the traditional exponential amplitude attenuation model can meet the actual attenuation law of acoustic emission waves.Then,based on the attenuation law of the power function amplitude,a new method of acoustic emission source localization is proposed,and the positioning experiment is carried out on the bogie frame steel plate.The results show that,according to the detected first-wave peaks of the acoustic emission main mode wave,a smaller range of sound source localization can be achieved,which is distinguished from traditional amplitude attenuation zone positioning.(4)Study on acoustic pressure transmitting coefficient of acoustic emission wave in bogie frameThe model of multilayer medium(with a plane wave obliquely incident on a bogie frame steel plate,an oxide skin layer,a coupling liquid layer,and a piezoelectric element layer of acoustic emission sensor)is proposed.The calculation formulas of the reflection coefficient and the transmitting coefficient of the displacement between the 1?n layers are derived,and the transmitting characteristics of the plane longitudinal wave perpendicularly incident into the multilayer medium are also derived.Through the acoustic pressure balance equation and the velocity balance equation,the acoustic pressure transmitting coefficient formulas for three conditions,such as no couplant state,couplant state,and oxide state,are solved.The experimental system of the acoustic pressure transmitting coefficient of the steel plate of the bogie frame was built,and the waveform amplitude detection was carried out for the above three states,which confirmed the correctness of the theoretical method and experimental means.Furthermore,these factors'influence on the acoustic pressure transmitting coefficient is analyzed.These factors include the acoustic impedance value and thickness of the couplant and scale layer,the acoustic impedance value of the sensor ceramic wafer,and the parameter variations of the internal and external locations of the coupling layer medium.In this paper,theoretical analysis,finite element simulation and experimental research methods are adopted,and the life history of the generation,diffusion,attenuation and reception of acoustic emission waves in the bogie structure is regarded as the main line of research.The acoustic emission generation of the framework material,the propagation characteristics and attenuation characteristics of the acoustic emission wave,and the transmission of the interface acoustic pressure are mainly analyzed.The research results in this paper can provide theoretical guidance and technical support for the application of acoustic emission technology to the health monitoring of bogie structures.