Quantitative Detection Of Surface Defects Of Aluminum Plate At High Temperature Based On Laser Ultrasound Surface Wave

In this paper,the finite element method was employed to study the physical process of ultrasound waves generation by laser point source in aluminum plate.Then the finite element model of laser line source induced surface acoustic wave was established,and the interaction between surface acoustic wave and surface and subsurface defects are studied,respectively.The axisymmetric finite element model was established.The propagation velocities of longitudinal,transverse and surface waves generated by laser excitation at different temperatures are studied,and the relationship between the velocities of longitudinal,transverse and surface acoustic waves and temperature was obtained.According to the expression of the sound velocity and temperature,the elastic modulus and Poisson's ratio of aluminum plate at high temperature are derived.Based on the plane strain theory,the finite element model of the interaction between surface acoustic wave and surface notch excited by laser line source was established.The interaction between the front and rear edge of the surface notch and surface acoustic wave are studied,respectively.The influence of the existence of width on the front of the defect and the surface acoustic waves was discussed.In addition,the process of ultrasonic scattering and mode conversion in the interaction between surface acoustic wave and surface notch was studied.The results show that the oscillation signal can quantitatively characterize the surface notch at different temperatures,where the notch width and depth can be characterized by the arrival time and the temporal cycles of the oscillation signal,respectively.Moreover,as the temperature increases,the arrival time of the oscillation signal will be delayed,but the temporal cycles will remain unchanged.The finite element method was employed to research the interaction of subsurface defect with laser-generated surface waves,and then a method to calculate the width of the subsurface defect using surface waves was presented.First,surface waves are first generated on one side of the subsurface defect and then on the other side.Finally,incident and reflected surface waves from subsurface defect are detected on both sides of the defect in the two detections,respectively.When the subsurface defect is within a wavelength range of the surface wave,the subsurface defect width can be calculated based on the arrival time of incident and reflected surface waves.Finite element method was used to simulate the surface wave generated by laser at different positions near the surface notch.The displacement signal will change significantly with the position of laser excitation.The advantages of SLLS in detecting surface micro-defects are verified.Then the characteristic quantity(oscillation signal)which can quantify the depth of the defect is selected from the displacement signal,the influence of the depth variation and width variation on the oscillation signal is further analyzed.Finally,the source of the oscillation signal is explained according to the displacement field of the surface acoustic wave interaction with the rear edge of the defect.The research results of this paper provide theoretical basis for nondestructive testing of surface-breaking defects on metal materials using laser ultrasonic at high temperature,and also provide some reference value for quantitative detection of other types of surface defects by using the arrival time of oscillating signals.