Non-destructive Testing Of Small-diameter Thin-walled Pipes Based On Laser Ultrasonics And Circumferential Guided Waves

Small-diameter thin-walled pipes play an extremely important role in economic construction and military development,they are widely used in aerospace,warships,missiles,atomic energy,and other fields.However,due to the unavoidable defects such as cracks and inclusions in the production and use of the pipe,strict quality inspection are required in the actual production and use process.For large-diameter pipes,traditional contact ultrasonic methods are often used.When the diameter of thin-walled pipes is getting smaller and smaller,the reliability of the traditional contact ultrasonic detection methods cannot meet the requirements for some reasons,such as the coupling becomes more and more difficult.For small-diameter thin-wall pipes,the ultrasonic water immersion detection method is usually used.This method has the disadvantages that it cannot detect flaws in situ and the detection speed is slow.The non-contact characteristic of laser ultrasonic and the adjustability of detection and excitation spot,combined with circumferential guided waves,provide a possible new solution of detecting the defect of small-diameter thin-walled pipes.In this paper,based on laser ultrasonic inspection technology and circumferential guided waves,small-diameter thin-walled pipes are studied theoretically and experimentally.In the theoretical part of this paper,by solving the elastic wave equation under free boundary conditions,the wave equation of the circumferential guided wave of the single-layer ring structure is derived,and it is further extended to the multi-layer ring structure.Using the wave equation obtained by theoretical analysis,the influence of elastic parameters and structural parameters of small-diameter thin-walled pipes on the circumferential guided waves are analyzed numerically.The dispersion curves of the phase velocity and group velocity of the pipes corresponding to different parameters are solved and drawn.The relationship between the dispersion curve and the wall thickness,pipe diameter,density and other parameters of the pipes is obtained.In the experimental part of this paper,a laser ultrasonic system was constructed and used to analyze the welding quality of stainless steel pipes with a diameter of 4.20 mm and a wall thickness of 0.30 mm.The circumferential guided wave experiments from multiple angles were carried out from both high frequency and low frequency by using two kinds of laser interferometers.Experiments show that both high and low frequencies can detect defects effectively,and low-frequency detection can get a better signal-to-noise ratio.The experimental results are in agreement with the theoretical analysis,which demonstrates the reliability and practicability of the laser ultrasonic method in the weld quality inspection of small-diameter thin-walled pipes.