Research On Surface Defect Detection Of Metallic Materials Based On Laser Ultrasonic

Sheet metal is widely used in daily life,aerospace,power industry,shipbuilding,automobile and other industrial fields.However,in the process of production and use,there are often micro cracks or other types of surface defects.The use will further enlarge these defects,and if the detection cannot be performed in time,it will threaten the life safety of equipment and personnel.The application of ultrasonic to detect defects in thin metal plates has always been a research hotspot in the field of non-destructive testing,and it is also a difficult point.Ultrasonic Lamb wave is a guided wave form of ultrasound.Compared with traditional ultrasound,it has the characteristics of large area propagation,long propagation distance,sensitivity to defects,and slow attenuation speed.However,laser ultrasound technology is due to its non-contact excitation and frequency bandwidth.It is not affected by the environment,and it is often combined with the ultrasonic Lamb wave to detect surface defects of metal materials.Therefore,this thesis adopts laser ultrasonic Lamb wave detection technology,and uses laser ultrasonic visual detector to detect the surface defects of metal material aluminum plate,so as to complete the quantitative characterization of surface defects of different sizes.First of all,this thesis introduces the propagation characteristics of ultrasonic Lamb waves in metallic aluminum plates,studies the thermo-elasticity theory and thermo-elastic coupling theory in the metal material aluminum plate,solves the ultrasonic Lamb wave dispersion equations of metallic aluminum plates,and plots them Dispersion curve of phase velocity and group velocity of ultrasonic Lamb wave in aluminum plate.Secondly,use the ABAQUS finite element numerical simulation software to establish the aluminum plate model,simulate the propagation velocity cloud diagram of the ultrasonic Lamb wave in the aluminum plate,and intuitively see the propagation process of the ultrasonic Lamb wave in the aluminum plate,at the same time,according to the intact aluminum plate and the aluminum plate with defects,the displacement signal diagrams of the grid cell nodes are compared with each other,so that the difference between the two can be understood more clearly,and the defect echo time can be better judged,so as to complete the defect identification and positioning analysis.Finally,the laser ultrasonic visual inspection instrument is used to experimentally detect the surface defects of aluminum plates of different sizes and depths,so as to complete the quantitative characterization of the position,size and depth of the defects.The ultrasonic dynamic propagation diagram realizes the visualization of the ultrasonic propagation process.According to the ultrasonic maximum amplitude diagram,the defect size can be seen intuitively.The defect size is measured by the LUVI software,and the error is within 5%.The defect is located through theoretical analysis and defect echo time.The error is within 4%;based on the ultrasonic Lamb wave B scan,determine the group velocity of different modes of Lamb wave,use the relationship between the group velocity and the depth of the defect to infer the depth of the defect,the error is larger when the depth of the defect is small,and the largest It is 8%,and when the depth is large,the error is within 4%.The research in this thesis provides theoretical and experimental basis for the quantitative characterization of aluminum plate surface defects.The results show that the laser ultrasonic Lamb wave is applicable to the detection of metal material aluminum plate surface defects.The error is within the allowable range,and the use of Lamb wave group velocity is verified.Deduced the effectiveness of the defect depth method.