Research On Laser Ultrasonic Detection Technology For Surface Defects On Metals

In the process of production and using,it will breed tiny cracks on the surface of metals because of the imposed uneven external force,and the tiny cracks will grow and expand further in virtue of its long-term service,which will pose a huge threat to production safety if it is not timely detected.Detection and evaluation for surface defects of aluminum alloy materials can be realized because that laser ultrasonic detection technology has the advantages of wide frequency band,non-contact detection,high detection sensitivity,detecting the moving components,and so on.Therefore,the laser ultrasonic detection technology is used to study the propagation characteristics of surface waves at metal surface defects and the quantitative measurement of different surface defects.It starts with an analytical model of that pulsed laser generated ultrasound,generation mechanism of laser line source and the change of temperature field and stress field on the surface of the sample during the process of generating ultrasound by laser are analyzed,the propagation direction of the transverse wave,the longitudinal wave and the surface wave generated by laser in the thermal elastic mechanism is analyzed.Then,the basic principle of detecting surface defects by the method of laser ultrasonic scanning is introduced,and the detailed experimental scheme is designed based on that.Finally,the experimental platform of laser ultrasonic detection is built,the detection is done for vertical cracks at different depths,slant cracks at different angles and double cracks on surface of metal material,the propagation process of ultrasonic signal generated by laser at the surface defect is studied according to the collected B-Scan images,the relation between transmission surface wave and defect parameter is analyzed,the method for quantitative analysis of depth and angle of surface defects by transmission coefficient is used,that is the amplitude radio of transmitted surface wave and direct surface wave.The results show that: the temperature increases first and then decreases within the range of that distance is less than one laser spot radius from laser incident point in the process of laser-generated ultrasonic,the stress also focuses on which,the radial distribution range is larger than the axial distribution range.The ability of reflection,transmission and scattering of surface wave generated by laser at different defects is different,the parameter of surface defect can be judged according to the transmission phenomenon of surface wave,the location of surface defects can be realized based on the amplitude change of surface wave,the transmission coefficient has a certain change rule with the depth and the angle of surface defects showed by the fitting curves between transmission coefficient and depths and angles,the detection of the depth and angle of the defect can be realized according to the transmission coefficient.The research results will promote the study of detecting the surface defects using laser ultrasonic technology from laboratory to industrial application and provide a theoretical basis and experimental method for further research on the application of laser ultrasound to surface defect detection.