Research On Nondestructive Testing For Metal Surface Defect Detection And Rail Tread Residual Stress Determination Using A Laser Ultrasonic Technique

Nondestructive testing is a kind of technique used in science and industry to evaluate the properties of a material,or system without causing damage.Laser ultrasonic which uses lasers to generate and detect ultrasonic waves has several advantages over the traditional ultrasonic method.It is receiving extensive attention to nondestructive testing field.With the development of China Railway,quality inspection requirements of railway increase,the influence of residual stress on the fatigue performance of the rail is obvious,which is an important reason for the crack at workpiece.In this paper,we use a double-line laser source to generate ultrasonics,the mechanism of surface defect detection by scanning double-line laser source is studied by the finite element method and experimental method.Meanwhile,a detection system uses laser-generated ultrasonics for measuring the residual stress on rail treads is set up.The main content is listed as follows:Firstly,a double-line laser source as an irradiation pattern of the generation of ultrasonic waves is proposed.The shear waves generated by the double-line laser will converge on symmetry axes of the plate under thermoelastic regime,which effectively improves the efficiency of excitation of ultrasonic waves.We use the finite element method and experimental methods to study the converging effect and the influence of spacing on converging effect generated by the double-line source,experimental results are consistent with numerical results.Secondly,A method of surface detection using scanning double-line source is proposed.The FEM was used to simulate the interaction between surface defects and the focused shear waves.Simulation results show that the method can improve the signal to noise ratio and the detection of the defect length and direction is more accurate.Thirdly,A surface detection system using scanning double-line source is set up.In the experiment,the double-line source and laser ultrasonic detector synchronously scanned and receive ultrasonic signals.The experimental results show that the scanning double-line source method has advantages of non-contact,non-destruction and high signal to noise ratio.Finally,A non-destructive method of measuring the residual stress on rail treads that uses laser generated surface acoustic waves is proposed.The residual stress distribution of different parts of both the new rail and used rail were examined.A digital correlation method is used for calculating the propagation time delay of adjacent surface acoustic waves,and the velocity of the surface acoustic waves is obtained.Based on the acoustoelastic relation we can get residual stress distribution of rail treads.The effects of ultrasonic propagation distance and surface roughness on the determination of residual stress were analyzed and simulated.Results from the study demonstrate that the stress distribution results are accordant with the practical situation.