Simulation Study On The Mechanism Of Laser Ultrasound And Structural Defects

On the premise of not destroying the structure of the component and not changing the property of the component,the product is accurately detected.Generally,surface cracks are detected by acoustic surface waves and internal cracks by longitudinal waves.Based on the action mechanism of laser ultrasound and structural defects,the defects were quantitatively evaluated,and the damage caused by defects was more accurately estimated.In this paper,aiming at the mechanism of laser ultrasound and structural defects,the laser excitation source model and defect model are established by using finite element method,and the interaction between surface signal and surface crack,longitudinal wave and internal crack is simulated.Appropriate excitation source parameters,including laser pulse width,laser energy,laser spot size and surface constraints,are selected for different defects to improve the detection accuracy.The influences of the width and depth of the surface crack defects on the ultrasonic surface wave and the size and depth of the internal defects on the ultrasonic longitudinal wave were investigated respectively in the time and frequency domains with the optimized excitation source parameters.Combining with the different arrival points of the same waveform in the reflection echo,the longitudinal and transverse dimensions of the surface crack are quantitatively derived,and the arrival time of the reflected longitudinal wave is quantitatively derived for the depth and width of the internal defect.The relationship between the depth of surface crack defects and the main frequency of surface reflected wave and the influence of hole size and buried depth on the spectrum characteristics of reflected longitudinal wave were studied.Finally,the conclusion of the interaction between acoustic surface wave and defects and the quantitative formula of surface defects were verified by taking aluminum plate defects of different depths with defect depth of 100um-800 um and increment of 100 um respectively.Circular hole defects with diameter of 1mm,2mm and 3mm were selected to verify the conclusion of the interaction between longitudinal wave and defects and the quantitative formula of circular hole defects.The results show that the experimental results are consistent with the simulation results and the error of the quantitative formula is within the range.