Study On Photoacoustic Defect Detection Technology Based On Laser Vibration Measurement

With the rapid development of the industrial era,various metal pipe fittings have been widely used in various industrial fields and various mechanical systems.During use,various types of defects can occur due to many factors such as high temperature,high pressure,corrosion and radiation,etc.in the operating environment.In order to avoid causing serious safety accidents,it is necessary to promptly detect the defects of metal pipe fittings.Non-destructive testing technology is commonly used to detect structural defects in metal components that are unevenly distributed,complex,and randomly occurring.Photoacoustic defect testing technology is a unique non-destructive testing technology that can overcome the shortcomings of optical testing in imaging depth and ultrasonic testing in imaging resolution,combining the advantages of optical imaging and ultrasonic detection.This dissertation analyzes the relevant theoretical mechanisms of photoacoustic defect detection technology,and studies the propagation laws of photoacoustic signals in defect-free and defective pipe fittings through simulation experiments.On this basis,combined with the laser Doppler effect and lock-in amplifier,a photoacoustic defect detection system based on laser vibration measurement is designed and built.A set of photoacoustic detection control software is developed using Lab VIEW programming language,which realized the accurate control of the scanning motion and data visualization of the detection system.The surface and internal defects of pipe fittings were detected,and the surface defect detection error was statistically analyzed through multiple experiments.The influence of the excitation frequency and initial phase of the modulated signal on the internal defect detection was also studied.The experimental results show that the constructed detection system can effectively detect surface defects,with detection error of 0.5%-6% and detection resolution of up to60μm.And it can also effectively detect internal defects,with detection error of 2.5%-7%,detection depth of up to 900μm and vertical resolution of up to 200μm.In addition,it has been shown that the system can obtain the depth distribution information of internal defects by changing the initial phase,and at different excitation frequencies,the system can detect internal defects by amplitude,but as the excitation frequency increases,the depth of detection by phase decreases.All detection experiments have shown that both amplitude and phase can be used for the detection of surface defects,while phase is more advantageous for the detection of internal defects.Therefore,this dissertation proposes a photoacoustic defect detection technology based on laser vibration measurement,which is non-contact and all-optical detection based,and can achieve detection and real-time imaging of metal pipe defects via both amplitude and phase signal.