Crack Closure Simulation Research Based On Laser Ultrasonic Technology

Non-destructive testing of cracks appearing on the surface or inside of objects has always been one of the focuses and hot spots in the industrial field.Non-linear laser ultrasonic crack detection technology based on laser excitation / detection has been proposed and widely studied as a high-precision,non-contact crack detection technology.However,the current researches on these technologies are all focusing on real cracks,and the surface morphology of real cracks is complex,and factors such as rough inner walls of cracks can affect the reflection,transmission,and scattering of acoustic signals.In addition,because the distribution of protrusions on the crack wall is random,the research results on a particular crack are not universal.Aiming at the problem of rough inner wall of crack,this paper uses a convex surface of a black glass plane and a convex lens to simulate the two walls of the crack.The influence of the cracks on the inner wall of the crack on the reflection,transmission,and scattering of the acoustic signal is eliminated,and the relative position between the two is simulated by pushing the black glass to simulate the closing process of a real crack when a load is applied.The pump laser is used to excite the ultrasound,and the laser vibrometer is used to detect the ultrasound to obtain the changes in the peak-to-peak value of the modal signals of the black glass and flat convex lens under different contact states.First,a set of experimental devices for detecting ultrasound based on pulsed laserexcited ultrasound,knife-edge-light deflection technology is established in this paper,and the time-of-flight scattering method is used to obtain the propagation speed of each modal signal in the sample,each modal acoustic wave signal in the time-domain waveform diagram is identified.Secondly,a set of experimental devices based on pulsed laser-excited ultrasound and a laser vibrometer to detect ultrasound were set up,fixed excitation light and detection light,one-dimensional line scanning of the contact surface can be achieved by moving the black glass and plano-convex lens attached together.The relationship between the peak-to-peak value of each modal signal as a function of the contact state is obtained through experiments.The experimental results show that the closer the scanning point is to the center of contact,the larger the peak-to-peak value of the direct longitudinal wave signal and the direct shear wave signal,the smaller the peak-to-peak value of the reflected longitudinal wave signal reflected by the black glass.Finally,we performed a two-dimensional surface scan of the contact surface using the same experimental setup.Promote 50 μm of black glass with each scan.The changes of peak-to-peak value of each modal signal and the area of the contact surface between the black glass and the plano-convex lens under different advance distances were obtained through experiments.The experimental results show that as the propulsion distance of the black glass increases,the peak-to-peak value of the shear wave signal and the longitudinal wave signal tend to become larger;When the advance distance of black glass is less than 200 μm,the peak-to-peak value of the mode conversion signal of the longitudinal wave-to-shear wave signal increases first and then decreases,and the peak-to-peak value of the reflected longitudinal wave signal tends to decrease.In addition,when the plano-convex lens is fixed and the black glass is advanced,the two are changed from the initial point contact to the surface contact,we calculated the theoretical contact area of the black glass and convex lens under different propulsion distances through Hertz contact theory,the theoretical value was compared with the actual contact area,and the actual area was found to be consistent with the theoretical value.In this paper,laser ultrasonic crack detection is studied from the perspective of simplified crack walls.The method used not only avoids factors such as crack wall roughness affecting the reflection,transmission,and scattering of acoustic signals,but also provides useful information for laser ultrasonic non-destructive testing technology.It also provides new methods and ideas for the study of cracks by laser ultrasound.