Research On Laser Ultrasonic Defect Reconstruction Method Based On Line Scan Excitation

As a new type of non-destructive testing technology,laser ultrasound has the unique advantages of rapid detection,non-contact and high sensitivity,and is suitable for detecting defects in components in complex environments.Feature extraction and image reconstruction of ultrasonic signals carrying defect information using suitable imaging methods are of great significance for the visualization and industrial application of laser ultrasound technology.In this paper,a combination of numerical simulation and experimental testing is used to carry out quantitative detection and image reconstruction of different types of defects in flat and tubular components,and the imaging method is applied to the defect detection of plastic products.The main work and conclusions are as follows:(1)Based on the basic theory of laser ultrasound and the wave equation of sound pressure in solids,the velocity of each mode of ultrasonic wave in metal aluminum was calculated according to the material parameters;MATLAB software was used to complete the programming of the scanning imaging algorithm and the synthetic aperture focusing imaging algorithm.The peak of the echo signal is extracted in the domain for C-scan imaging to enhance the image clarity.On the basis of the traditional synthetic aperture method,the shear wave inversion and image difference technology are added to invert the position and size of the defect.Complete bandpass filter designs and apply them to signal processing and noise reduction.(2)Based on the finite element analysis software COMSOL,a laser ultrasonic simulation model with different types of defective plates and tubes is established.The propagation law of surface acoustic wave R,transverse wave S,and longitudinal wave L is obtained by analyzing the changes of the displacement field at different times after the laser is incident.Combined with the time-frequency analysis results of the signals extracted at the receiving position,the reflection and transformation effects of defects on the ultrasonic waves of each mode are studied.: The reflected longitudinal wave LL is suitable for detecting the defects inside the thick plate;the reflected transverse wave SS can detect the bottom defects of the thick plate from the opposite side.Using different imaging methods to post-process the simulation data,the results show that: B-scan imaging can achieve rough judgment of the internal defect position of the plate,and the positioning accuracy of the synthetic aperture method is higher,and the error is within 5%.The annular B-scan imaging results It can identify the defects inside the thin-walled circular tube and obtain the width information of the defects.(3)Experimental study on laser ultrasonic nondestructive testing of different types of defects in metal products.A set of laser ultrasonic testing experimental equipment was built independently,and the defect structure inversion imaging method established in this paper was written into a software program in combination with the relevant parameters of the experiment,which was used for experimental data processing and realized visual inspection.The test is carried out under the conditions of moving scanning excitation and fixed point receiving to study the influence of different ultrasonic transducers on the signal acquisition quality.It is more conducive to the acquisition of longitudinal wave signals.Under the conditions of sampling frequency 100 MHz,average times 64/128 times,and signal length 50 us,the ultrasonic signal excited by laser in aluminum products was extracted,and the measured data was reconstructed by synthetic aperture method,and the aluminum plate with thickness of 10 mm was measured.For groove defects of different sizes at the bottom,the quantitative error of the imaging results to the width is within 5%;the circular scanning method of excitation and reception synchronous movement is used to detect the inner wall defects in the metal tube with a wall thickness of 5 mm,and the B-scan results can distinguish the defects The degree of bulge,depression and width.(4)Research on the application of laser ultrasound visualization imaging technology in plastic inspection.The energy of the online laser source is less than 1.8 m J,and the non-destructive testing of plastic products can be realized under the condition that the low-frequency probe of 100 k Hz is used as the receiving device.Under the condition of two-dimensional scanning,the ultrasonic C-scan imaging technology was used to successfully detect the scratch defects on the surface of polypropylene splines.The imaging results can accurately determine the position and shape of scratches and crack defects.Two different high-density polyethylene plastics prepared by injection molding were tested,and the effects of laser energy,material properties and defects on ultrasonic attenuation were studied by extracting the ultrasonic sound pressure attenuation curve and calculating the attenuation coefficient.Combined with the scanning imaging results,the position of the forming defects in the sample was analyzed,and then the quality of the sample was evaluated.In this study,the imaging method under the excitation condition of laser line source scanning was explored.The probes were arranged to receive signals from the same side and the opposite side of the defect respectively,to simulate the detection of defects in complex profiles,and provide the visualization and industrial application of laser ultrasound.