Defects Detection Of Rail Surface And Subsurface With Laser Ultrasonic Technology

Ultrasonic technology is widely used in non-destructive testing of rail defects,but there is a blind area of sound field in conventional ultrasound technology,so defects in the rail surface and subsurface(5mm below the surface)can not be detected.Laser ultrasound technology can stimulate a variety of modes of ultrasonic wave in the solid surface by non-contact,and there is no blind area in the sound field,so the application of laser ultrasound in rail defect detection is of great significance.The aim of this thesis is to study on the application of surface and subsurface defects detection for rail with laser ultrasonic technology.Firstly,the excitation theory of laser ultrasound was introduced,and the finite element form of thermal elastic coupling equation was obtained,the center difference method,an explicit integration algorithm,was used to solve this equation.Then,a numerical model of rail defect detection with laser ultrasonic was established on a simulation softwave.In the process of grid division,the convergence analysis method was used to determine the size of the grid,and the correctness of the model is verified by physical analysis and experimental method.The optimal excitation problem of laser ultrasonic detecting rail surface and subsurface defects was studied by finite element method(FEM)and experimental method.In the study of laser related parameters,the energy,pulse width and spot size of laser will affect the amplitude of the ultrasonic signal,however the pulse width and transverse width of laser spot are the main factors affecting the time delay of ultrasonic reception,but Only the transverse width of laser spot will affect the spectrum.Based on the study of the optimal excitation,four defects of 60 kg rail surface and the subsurface in the transverse and longitudinal direction were analyzed experimentally.In the experiment,a transverse wave probe is used to receive ultrasonic signals,so there are other modes in addition to the surface wave.In order to analyze the time domain signal comprehensively,combined with the finite element method,we got the conclusion that the laser ultrasonic can effectively detect the surface and subsurface defects of the rail.In addition,it was found that P-wave and S-wave were converted to each other,and the scattering phenomenon after the sound wave encountered the defect was verified.Finally,Because of the problem that laser ultrasonic is insensitive to the width of defects in actual detection,and unable to make quantitative detection of defects,defect visualization detection of staggered scanning laser ultrasound was studied.Ultrasonic was excited at the two sides at the same time of a rail head with two staggered beams of laser separately to detect irregular scratch defects on rail surface.Then the whole profile of defect was extracted through imaging analysis technology,the approach offered a new method for the effective and quantitative detection of rail surface-connected defects through laser ultrasonic technology.