Research On The Scanning Laser Ultrasonic Technology And Application It In The Detection Of Surface Defect

With the rapid development of modern industrial technology,metallic material has a full longterm work in the industrial production and will inevitably have varying degrees of damage.The change of the structure of metallic materials will cause serious damage for our industrial production.In order to rapid detect these metallic materials,a new detection method-laser ultrasonic technique appears.Laser ultrasonic technology has some advantages such as: long-range,non-contact,high sensitivity and wide frequency band,which will get people's wide concern and research.In this paper,we use the finite element numerical calculation and experimental method to study the propagation characteristics of ultrasonic wave laser-generated in the crack,and use the scanning laser ultrasonic inspection technique to visual detect the crack in the austenitic stainless steel and aluminum alloy.The main research and the novel contents of this paper are as followed:According to the thermoelastic equations and heat conduction equations of laser-generated acoustic surface wave,the acoustic surface wave propagation images are obtained by taking finite element solution equations.When the acoustic surface wave propagates through the aluminum plate with the near-surface defects at different depths,which produces an oscillating effect between surface acoustic wave and near-surface defects,and then there is a gradual decrease after gradual decrease of the oscillation wave through the near-surface defect.When the acoustic surface wave propagates through the aluminum plate with the near-surface defect at different depth,there is a certain variation of the center frequency of the oscillation wave.The simulation results show that: when the depth of near-surface defect change from 0.1mm to 0.5mm,the center frequency of the oscillation signal generated by the oscillation effect change from 0.4MHz to 0.76 MHz,the center frequency of the oscillation signal is seriously attenuated with increasing width of near-surface defect.We use finite element method to simulate the scanning laser source to scan the surface of the material with a defect.Due to the existence of defect,the maximum peak time of the longitudinal wave is different when laser source go through the position of defect or the position without defect.According to the difference of the maximum peak time of the longitudinal wave,we can get B scan maps and the longitudinal wave propagation image,which will detection of the surface defect.A scanning laser longitudinal wave testing system is designed to scan the surface defects of different width and depth,different shapes of one-dimensional and two-dimensional scanning,which get B scan maps and the longitudinal wave propagation image.The experimental results show that: the scanning laser longitudinal wave detection technique can quantitatively detect the surface defect of different width and depth.A finite element method is used to simulate the theoretical model of the ultrasonic wave by a scanning laser source excitated in the material with surface defect.The waveform changes of surface acoustic wave at the position of defect are analyzed.There is a first increase and then decrease with the maximum amplitude of the surface acoustic wave when the laser source goes through the position of defect.A scanning laser acoustic surface wave detection system is developed.According to the principle of reciprocity of ultrasonic wave,the ultrasonic wave propagation images are obtained in the material to visual detection of the defect.The position and size of the defect in aluminum alloy material and austenitic stainless steel can not be detected,an algorithm based on Hilbert transform,an algorithm based on energy algorithm and Wigner-Ville analysis is proposed for solving this problem.The experimental results show that these algorithms can not only detect the position of material defects,but also intuitive and efficient initial detect of the defect size and shape.The problem is that the defect of the austenitic stainless steel weld cannot detect.A fast discrete sine transform algorithm is proposed,and we compared with the traditional fast Fourier transform algorithm,which can be used to detect the size of defect.Finally a scanning laser surface acoustic wave detection system based on non-contact is built and gets the ultrasonic wave laser-generated vibration image in the material,which can detect the position and size of the defect in the material.The results of this research may provide the theoretical and experimental basis for the study of laser ultrasonic technique to detect of the defect,and also accelerate the development and application of laser ultrasonic imaging technology in the field of nondestructive testing.