Optimization Design And Experiment Of Rail Surface Crack With Electromagnetic Ultrasonic Surface Wave Transducer

With the rapid development of high-speed railway in China,the interaction of the rail wheel and the rail tread causes the rail track rolling contact fatigue(RCF)crack to present a new development state.This kind of rolling contact fatigue damage extends from the rail edge to the driving direction in a certain angle,bending down to a certain extent,resulting in rail fracture,which seriously threatens the safe operation of the railway.At present,our rail flaw detection car mainly adopts ultrasonic longitudinal and shear wave combined probe technology to inspect the rail.Due to the blind area of ultrasonic probe and the trend and appearance characteristics of rolling fatigue crack on the rail tread,it is difficult to identify and detect such defects.Ultrasonic surface wave can effectively detect the surface crack of 1~2 wavelengths,which is an effective method to identify RCF cracks on the rail tread.In this paper,using the electromagnetic ultrasonic method,the RCF crack of the rail tread is detected by the optimized design transducer excited surface wave,which has the advantages of non contact,rapid and easy to realize automatic detection.Compared with the piezoelectric ultrasonic method,the efficiency of electromagnetic ultrasonic energy exchange is not high,and the signal to noise ratio is low.How to optimize the design of electromagnetic ultrasonic surface wave transducer is very important to improve the performance of the probe's excitation and reception.In this paper,finite element method was applied to establish two dimensional multi physical field finite element model for electromagnetic ultrasonic surface wave excitation and reception.The magnetic field intensity and magnetic force line distribution of the bias magnetic field were given by numerical analysis.According to the propagation of surface waves in rail tread,the model parameters can be adjusted to optimize the model.In order to improve the efficiency of transducer design,orthogonal test table was established by mathematical statistics analysis software.And the influence of the combined parameters of the excitation and receiving transducer on the performance of the transducer was analyzed,and the best combination parameters of excitation and reception were obtained.The simulation result of the best combination parameter was basically the same as that of the mathematical statistics analysis software,with little error.It was proved that the orthogonal test table established by mathematical statistics analysis software was correct and effective.On the basis of this,the influence of the meandering coils with different root numbers on the excitation and reception performance was analyzed.The influence of three different types of permanent magnets on the amplitude of the received voltage was studied,and these simulation rules finally were tested to get the combination parameters of the transducer,which is suitable for measuring the rail tread.In the long running operation of rail rails,the structure of rail tread surface material will change,and the hardened layer will form on tread.The hardened layer and the substrate with different acoustic impedance of rail head.The influence of the thickness of the hardened layer and the acoustic impedance of the transverse wave on the surface wave propagation was studied by establishing the numerical model of the surface wave propagation of the rail tread on the layered medium.The theory and method were provided for the successor test.Based on the above numerical simulation and transducer optimization design,the experimental verification of rail tread crack detection was carried out.The surface waves of electromagnetic ultrasonic transducer with excitation frequency of 0.9MHz,0.5MHz and 0.3MHz were used to detect the surface waves of the straight and oblique cracks on the edge of the rail tread by B scanning.The experimental results showed that the design of electromagnetic ultrasonic surface wave transducer can accurately identify and measure the single straight crack and oblique crack on rail tread.The shape and opening direction of the slanting crack have great interference to the surface wave detection.It is easy to form the waveform conversion and coupling,which makes the oblique crack reflection back wave packet wider,the defect location was not accurate,and the signal processing should be further enhanced.For dense oblique cracks,due to the interference between cracks,it is difficult to identify and detect them one by one.