| With the increase of transport volume,improving rail inspection efficiency becomes more and more important.For rail inspection,ultrasonic technology is mainly adopted to cover both bulk and subsurface defects.There are several kinds of defects,such as transverse defect,bolt-hole cracks,base corrosion etc.which are typically existing in rail.Due to the limitation of conventional UT and PA technique,it is very hard to get high Probability of Detection(POD)while the inspection vehicle operates in relatively high speed,such as 80-100km/h.Now mainstream inspection vehicle can reach maximum speed of less than 80km/h.In this thesis,FAAST(Fast Automated Angle Scan Technique)is introduced for rail inspection,which is very different from conventional PA technique,multi-beam can be transmitted at the same time and parallel signal process can be done,which provide capability to increase the inspection speed and coverage.In our work,firstly,typical rail defects are introduced with corresponding inspection methods.Especially for transverse defect which is common in Chinese heavy haul rail but with low POD.Calculation and comparison has been given between direct scan technique and double traverse technique.From the comparison,the traverse wave is more suitable for transverse defect with large inclination.Based on the analysis,we can find that,for transverse defect,it is better for us to use FAAST-PA to transmit 8 beams in one shot with second wave inspection rather than conventional method that only use one beam to interact with defect directly.Then CIVA simulations have been done for choose right probe's position and also for angles that FAAST-PA need to transmit.Finally,the experiments results comfirmed the correctness of this configuration.And the principle of linear phased array is summarized for achieving beam steering and focus.In order to get the performance of the probe,sound field calculation methods are combed out.Then,rectangular radiator method and Stepanishen's model have been used for sound field simulation with MATLAB.From the simulation results we can get the sound pressure distribution and evaluate the rationality of probe parameters,which is important for us to choose or custom probe in practical application.Furthermore,in order to optimize the sound field,delay-and-sum beamforming method is used.And different window functions are used for comparison.Based on the MATLAB simulations,we can see that adding window can suppress the undesirable sidelobes while enlarge the mainlobe which means it can increase the SNR while decrease the resolution.And there are slightly differences between these window functions.In the high speed case,it is better to add window for we are more inclined to get high SNR rather than resolution.In addition,for high speed rail inspection,we have to make compromise when we choose pulse repetition frequency(PRF).Because with the increase of PRF,the inspection range will reduce.However,for keeping high beam-cover-ratio,it is necessary for us to keep relatively high PRF.Thesis analyzes the factors which influence the PRF,and recommends 5kHz PRF for high speed rail inspection.In experiment part,calibration block is chosen according to China rail standard'TBT2340-2014'.And USPC devices are used for data acquisition.The experiment mainly verified the probe arrangement's ability to detect typical detects.And SNR as an indicator for results evaluation.With analyzed experiment results,it can be seen that arrangement proposed is capable of detecting typical defects in reference rail.Comparing to the conventional method,the coverage ratio of rail head has been improved.Especially the utilization of multi-beam FAAST-PA gives the arrangement more ability to detect the transverse defects in rail head.Finally,the whole inspection system structure based on phased array technique and parallel processing unit is concluded.And the advantages of the system is that it has ability to achieve automatic defect discrimination and automatic location,providing clues for further research. |
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