Pipeline Surface Defect Detection With Laser Ultrasound

Today,major energy sources such as petroleum products in all walks of life are transported through safe long-distance pipelines.Most of these pipelines are buried,so it is very important to ensure their safe service.All kinds of defects may cause pipeline failure and damage,which will lead to irreparable consequences.Therefore,prediction of the health status of industrial pipelines are of great significance for improving the safety and reliability of pipeline operation.In this paper,the surface wave excited by laser ultrasound is used for non-contact detection,which has certain research significance in the field of pipeline quality evaluation and nondestructive testing.The content of the paper includes the following aspects:(1)Laser ultrasound generation mechanism and theoretical analysis.The principle of laser-excited ultrasound is expounded from thermal bomb and ablation.Based on the consideration of not causing damage to the surface of the test object,the thermoelastic mechanism is selected.Based on the thermoelastic mechanism,the laser ultrasonic excitation model is established by using the heat conduction equation and control equation in the finite element software.According to the finite element simulation results,laser ultrasound can excite a variety of modes during the propagation of metal workpiece,and simulate the scattering process of defects on ultrasound,which establishes a useful data basis for the application of laser ultrasound and detecting surface defects by laser ultrasonic.(2)The finite element analysis of laser ultrasonic is carried out on the inspected workpiece with artificial defects.The laser excitation of ultrasonic longitudinal wave,ultrasonic transverse wave and ultrasonic surface wave in materials is simulated by finite element numerical simulation method.Among them,the ultrasonic surface wave has a higher amplitude,which proves the advantages of the laser in exciting the ultrasonic surface wave.By establishing a laser ultrasonic detection surface defect model and comparing with the results of the defect-free model,It is found that when there is a defect,the location of the defect can be determined according to the arrival time of the ultrasonic surface wave and the arrival time of the defect echo.For defects with the same depth,the time interval of echo signal is the same,and the time interval is roughly proportional to the defect depth.(3)A laser ultrasonic testing system is designed.Design and manufacture of system hardware and software,which is composed of excitation generator,scanning detection part,data acquisition part,result imaging and analysis part,etc.The hardware design mainly realizes the laser excitation and ultrasonic receiving system and the whole detection structure.The software can set the parameter information and detection parameters of the inspected workpiece,and display the detection results in a/b/c imaging mode.(4)Carry out the experimental research on laser ultrasonic testing of pipelinesurface defects,analyze the interference factors affecting the laser ultrasonic testing signal and optimize the testing process parameters.It mainly includes metal plate surface defect detection experiment and metal pipe surface defect detection experiment,and inspection of the upper surface defects of pipeline welds that are common in engineering.Using laser ultrasonic surface wave detection technology,the artificial grooves with different depths on the metal plate surface can be clearly identified;For surface crack defects of different depths and lengths on metal pipes,the laser ultrasonic surface wave shows good detection sensitivity;Surface φp2mm semi-through hole and through hole defects,with good detection effect.