Research On Rail Defect Detection Technology Based On Specific Mode Excitation Method Of Ultrasonic Guided Wave

The rail status of high-speed railway directly affects the operation safety.It is of great significance to discover the internal defects of rails in time for maintaining the integrity of railway infrastructure structure and ensuring the safety of railway transportation.At present,the two kinds of detection vehicles used in the railway field are running in the maintenance time,generally from 12 a.m.to 4 a.m.,and cannot be used for real-time monitoring of rail internal defects.Ultrasonic guided wave inspection technology has the advantage of small signal energy attenuation during long-distance inspection.Based on the ultrasonic guided wave defects detection of seamless rail,when the guided wave signal propagates in the rail,it will generate the echo signal at the defect.And the on-line monitoring of rail internal defects can be implemented by detecting the echo signals generated at the rail defects during the propagation of the guided waves.The rail has a complicated cross-section and there are many guided wave modes propagating at the same time.And the defect echo signal is complex.Therefore,it is the key point to study the excitation methods of ultrasonic guided wave modes in rail defect detection.Aiming at the problem of target mode excitation in rails,this paper proposes a new method for exciting a specific mode of guided ultrasonic wavein a rail.The method requires the cross-sectional mode shape to determine excitation direction,excitation coefficient,and excitation position of each mode.The cross-sectional mode shape data of all modes areacquired by the semi-analytical finite element(SAFE)method.The optimum excitation direction is obtained by solving the Euclidean distance between different modes.And the direction with the largest average Euclidean distance is selected as the optimal excitation direction of the mode.The crosssection of a rail is divided into three segments,including the rail base,the rail web,and the rail head.The excitation coefficient of different rail sections is determined by calculating the proportion of vibration characteristics of three segments.By establishing the covariance matrix of vibration data,the optimal excitation point is determined by using the covariance.The method of excitation response calculation is used to simulate and verify the mode excitation method of guided wave.The excitation signals with the center frequency of 200 Hz and 35 k Hz are selected in the simulation process.In the optimal excitation node and direction of rail,according to the excitation coefficient,a certain proportion of excitation signal is applied to the rail bottom,rail waist and rail head.And the mode identification is carried out by measuring the simulation group velocity of each mode.The simulation results show that the excitation method proposed in the paper can excite the target mode and the group velocity error is small.However,there are existing interference modes at high frequencies.It is to optimize the mode excitation method of guided wave,phase-controlled delay technology is introduced,and the excitation method is optimized from point excitation to array excitation.Set the transducer installation interval and excitation time sequence based on the target guided wave mode.Adopt ANSYS for three-dimensional simulation.Mode identification is achieved by measuring the guided wave group velocity and measuring the guided wave phase velocity based on the Two-dimensional Fourier transform(2D-FFT)method.And simulation results show that this method can excite a single specific mode and suppress the generation of interference mode.Finally,the method of guided wave mode excitation and identification is applied to rail defect detection.Different guided wave mode has different sensitivity to the defects of rail head,rail web and rail base in the detection of rail defects using ultrasonic guided wave.In order to detect and locate the three parts of rail defects effectively,this paper proposes a novel guided wave mode selection method.The guided wave modes are selected,combining the strain energy distribution diagrams with the phase velocity dispersion curves of modes,which are sensitive to the defects of the rail head,rail web and rail base.ANSYS is used to excite the specific modes and detect defects in different rail parts to validate the proposed methods.During the experiment,the selected three modes are excited to detect and locate the defects in different parts of the 3.5 meters long rail.The error of the simulation and experiment results is small.And the results show that the selected specific modes are sensitive to the defects of different position and the positioning error is small enough for the maintenance staff to accept.