Study On Detection Method Of Thermal Stress In Continuous Welded Rails Based On Nonlinear Ultrasonic Waves

When the temperature of continuous welded rail(CWR)changes,the longitudinal thermal stress will be generated inside rail,and in serious cases,it will cause buckling rail,broken rail and other accidents,endangering the safety of train operation.In view of the current situation of the lack of thermal stress detection means of CWR,this paper deeply studies the basic theory and method of thermal stress detection of CWR based on nonlinear ultrasonic waves,and systematically analyzes the response laws of nonlinear ultrasonic bulk waves and nonlinear ultrasonic guided waves under the action of stress and temperature.The feasibility of applying nonlinear ultrasonic bulk waves and guided waves to detect the thermal stress in CWR is explored.Firstly,the influence mechanism of stress and temperature on nonlinear ultrasonic bulk waves is studied.The propagation model of ultrasonic longitudinal waves in free and constrained directions in solid medium is established from the atomic microscopic point of view.The variation law of nonlinear parameter of ultrasonic bulk waves under the action of stress and temperature is deduced theoretically.The results show that stress and temperature have significant influence on nonlinear ultrasonic bulk waves.The relative ultrasonic nonlinear parameter of nonlinear ultrasonic bulk waves increases with the increase of tensile stress and decreases with the increase of compressive stress,and increases with the increase of temperature and decreases with the decrease of temperature.The research results prove that the nonlinear ultrasonic bulk waves are not suitable for detecting the absolute value of thermal stress in CWR,and improve the basic theory of nonlinear ultrasonic bulk waves.Due to its multi-modal characteristics,the response law of ultrasonic guided waves to stress and temperature is more complex than that of bulk waves.However,ultrasonic guided waves have the potential to detect thermal stress.There is still a lack of a general excitation method of nonlinear ultrasonic guided waves for waveguide medium,such as rail,which has complex cross-section.In order to solve this problem,a method based on the semi-analytical finite element model is proposed to analyze the coupling law of fundamental and multiple frequency modes of guided waves.By using the modal energy transfer theory and introducing the second harmonic modal amplitude coefficient to characterize the intensity of the second harmonic,the optimal combination of fundamental and second harmonic modes in arbitrary complex cross-section waveguide medium can be analyzed.This method solves the problem of modal analysis and excitation of nonlinear ultrasonic guided waves in rail,and lays the foundation for studying the propagation law of nonlinear ultrasonic guided waves in rail.In order to study the response law of nonlinear ultrasonic guided waves under the action of stress and temperature,this paper takes the plate structure as the preliminary research object.According to the selection method of mode combination of nonlinear ultrasonic guided waves proposed in this paper,the estimated values of second harmonic modal amplitude coefficients under different stress and temperature states are obtained.The response laws of different second harmonic modes of nonlinear ultrasonic guided waves under the action of stress and temperature are obtained.The results show that both stress and temperature have obvious effects on the nonlinear ultrasonic guided waves.When the stress in the waveguide medium changes from compressive stress to tensile stress,the amplitudes of different second harmonic modes can be monotonically increasing,decreasing,U-shaped or n-shaped;when the temperature of the waveguide medium increases,the amplitudes of different second harmonic modes can be monotonically increasing or monotonically increasing.Because the waveguide mediums with different cross-section types have different guided waves modes,the response law of nonlinear guided waves needs to be analyzed specifically.Finally,taking CHN60 rail as the research object,the response laws of different second harmonic modal amplitudes of nonlinear ultrasonic guided waves in rail under the influence of single stress,single temperature and the joint influence of temperature and stress are studied.The theoretical results show that the neutral temperature of CWR can be accurately determined by measuring the change of relative ultrasonic nonlinear parameter,that is,the rail temperature when the stress inside CWR is zero.Then the longitudinal thermal stress of the rail at any temperature can be calculated.Through the establishment of rail temperature control experimental platform,the relevant verification experiments are carried out,and the experimental results are consistent with the simulation results.The results show that the nonlinear ultrasonic guided waves can be used to detect the thermal stress in CWR.The research conclusions and methods proposed in this paper can be applied to the researches of nonlinear ultrasonic testing of metal solid media,and have important theoretical reference value for the researches of nondestructive testing technology based on nonlinear ultrasonic waves.