| Metal parts are widely used in many fields,and damages such as cracks will inevitably occur on the surface due to the load during work.If the cracks are not detected in time,it may lead to parts failure and even greater accidents.In order to realize the detection of metal surface cracks,the application of laser ultrasonic technology to detect cracks has attracted more and more attention.This technology has the advantages of non-contact,long distance,high sensitivity,etc.,and can be applied to metal detection in various harsh environments.In this paper,the propagation characteristics and action process of laser surface acoustic waves in metal crack defects are studied by a combination of finite element numerical simulation and experiment.By analyzing the mechanism of different positions and depths of cracks and surface acoustic waves,the crack location is derived And the calculation formula of depth,so as to realize the quantitative detection of cracks;at the same time,a scanning laser ultrasonic inspection system is used to visualize the detection of surface crack defects of metal materials.The main research content and results of this paper are as follows:(1)The finite element method was used to numerically simulate the process of pulsed laser excitation of surface ultrasonic waves in aluminum materials.By analyzing the transmission process of surface waves in V-shaped cracks and groove-shaped cracks at different depths,the mechanism of cracks at different depths was summarized.And deduced the calculation formula of its position and depth,and analyzed the difference between the two types of cracks and the process of surface waves.For V-shaped cracks,this paper divides the depth information into three ranges,and proposes a method to quickly detect the crack depth.The simulation results show that this method can achieve efficient detection of metal V-shaped cracks.(2)A laser ultrasonic testing system was used to test and verify artificial crack specimens.The relationship between the surface acoustic wave reflected echo signal and the crack is analyzed.For the detection of the position of the crack,the correctness of the formula for calculating the position of the crack with the arrival time of the reflected wave is proved.For crack depth detection,the relationship between depth information and surface acoustic wave reflected wave amplitude and transmitted wave amplitude,as well as the relationship between the depth and the converted shear wave when the depth exceeds the center wavelength is verified,and the simulation conclusion is verified.Finally,two different types of cracks are detected.According to the difference between the experimental results and the simulation,the causes are analyzed,and the influence of the crack types on the amplitude of the transmitted wave is analyzed.(3)Using scanning laser source and acoustic reciprocity principle,the visual research of crack detection is carried out.Mainly studied the cumulative total energy algorithm,the cumulative energy algorithm based on wavefield separation and the maximum amplitude imaging algorithm.Experimental results show that the cumulative total energy algorithm is more suitable for detecting the existence of cracks,but is not sensitive to crack position information;on this basis,windowed two-dimensional Fourier transform is used to separate the reflected wave field,and the improved algorithm is effective for cracks.The position detection effect is good;the maximum amplitude imaging algorithm has a good effect on the crack position and width information detection.In order to realize the accurate and intuitive detection of metal surface cracks,this paper systematically studies the detection methods of different depth cracks,and uses the laser scanning system to realize the visual detection of cracks,and proposes a rapid detection method for V-shaped cracks.The research results of this paper Contribute to the further development of laser ultrasonic testing technology. |
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