Theory And Technique Of Laser Ultrasonic Testing For Surface Defect Of Metal Plate Component

Metal plate components are widely used in aerospace industries,long-span bridges,highrise buildings,heavy equipment manufactures.They may prone to surface crack defects in the process of casting and forging.Additionally,they will case invalidation in the risks of strong radiation,high temperature and high pressure,which seriously affect the quality of components and the safety of engineering structures.Therefore,the effective testing techniques for decimating the surface defects are the crucial measure to guarantee the safety,integrity and durability of metal plates.Recently,laser ultrasonic(LU)-fiber Bragg grating(FBG)technique has been developed for testing surface detects.It possesses the characteristics of non-contact,high resolution,light weight,small size,immune to electromagnetic noise and distributed sensing.In the dissertation,based on the interaction mechanism between laser ultrasonic and metal,focusing on the testing of metal plate surface cracks and cavities,the laser ultrasonic testing technique of metal plate component surface defects based on Fiber Bragg grating sensing was studied to break through the challenges between laser ultrasonic and fiber Bragg grating.It has important theoretical significance and practical value for promoting the surface defect testing technique of metal plate components.The main content and innovation of the dissertation are as follows:(1)Excitation mechanism and propagation characteristics of laser ultrasonic in metal plate.Firstly,the basic theory of the interaction between laser and metal is illustrated according to fundamental laser physics.Two typical laser excitation mechanisms of thermoelasticity and ablation are analyzed in the process of laser acting on the metal plate.On the one hand,taking T2 copper plate and copper alloy plate with rust covering as the research object,the plasma spectrum of high-energy laser pulse interacting with them and the relationship of laser pulse intensity with respect with plasma spectrum are investigated based on laser ablation mechanism.The influence of plate surface state on the strength and properties of excitation source(thermoelastic source or ablation source)is analyzed.On the other hand,the effect laws of laser radiation spot’s diameter and sensor to detection’s distance are studied by analyzing the timedomain and frequency-domain of laser-excited ultrasonic surface wave.By optimizing the diameter of the laser spot and the detection distance of the sensor,the incident power density of the laser pulse is increased,and the laser ultrasonic is controlled to be dominated by the ablation mechanism,so that the laser beam is separated from the plasma shielding effect,thereby improving the photoacoustic conversion efficiency and generating a high-intensity and wide-radiation ultrasonic field.It provides a basis for improving the excitation efficiency of laser ultrasonic.(2)Laser ultrasonic quantitative testing technique for surface defect of metal plate.A laser ultrasonic quantitative detection system is established for metal plate surface defect.Two defect feature extraction methods are proposed based on power spectral density(PSD)algorithm and wavelet packet energy(WPE)algorithm.The laser ultrasonics response signal obtained by the piezoelectric ultrasonic probe is analyzed with combination of laser ultrasonics propagation characteristics.The linear relationships between the defect and the size of crack and cavity surface defects in T2 copper plate are established through extracting the characteristic quantity based on PSD and WPE algorithm,and their linear correlations are greater than 0.98.The experimental results and theoretical analysis show that the low-frequency components powers extracted based on PSD algorithm are more suitable for the quantitative characterization of defects in laser ultrasonics.(3)Miniaturization of laser ultrasonic FBG sensing system based on DFB laser demodulation.Firstly,the homemade PCB control main board is used to drive the laser output with stable frequency and power,and the wavelength drift of the FBG sensor is converted into the modulation signal of the laser power.A miniaturized FBG sensor is developed based on the DFB laser demodulation method.Its temperature and strain sensitivity coefficients are 6.09 pm/℃and 0.637 pm/με,respectively.Then,the FBG sensor is applied to the tensile and fatigue testing experiments of the standard center crack tensile M(T)specimen of 2524-T3 aluminum alloy.The results show that the FBG sensor has the capacity of monitoring load and in-situ strain,and it has the ability of long-term monitoring of crack propagation under complex load conditions.Finally,combined with the ultrasonic strain sensing mechanism of FBG,the πPS-FBG is proposed to replace the conventional FBG to meet the detection requirements of high sensitivity,high resolution and high frequency response of laser ultrasonic.(4)Development of laser ultrasonic-πPS-FBG balanced differential system for surface defect of metal plate.Firstly,the piezoelectric ceramic tube is taken as the actuator to use its inverse piezoelectric effect.The Bragg wavelength of the bare FBG attached to it is indirectly affected by DC driving voltage,and the corresponding displacement is associated with the measurement of resistive strain gauge.The comparison of strain sensitivity between the bare πPS-FBG and FBG and their sensing systems under two demodulation methods(matched grating demodulation method and DFB laser demodulation method)are performed,respectively.The results present that the system strain sensitivity of the πPS-FBG sensor is 125.655 m V/με,which is about 10 times higher than that of the conventional FBG sensor.The static characteristics of high sensitivity are verified in experiment.Secondly,the dynamic response performance to high-frequency ultrasonic is investigated by using the πPS-FBG sensing system to detect high-frequency ultrasonic excited by the ceramic piezoelectric patch.The results show that the minimum ultrasonic strain that the πPS-FBG sensor can detect is 79.582 pε.Its response sensitivity(signal-to-noise ratio)to 1 MHz to 5 MHz ultrasonic frequency can reach more than 105 d B,and the fluctuation range with the variation of ultrasonic frequency is less than 19 d B.Finally,a set of laser ultrasonic-πPS-FBG balanced differential detection system is developed based on the ablation mechanism.The detection system is used to measure the ultrasonic surface wave velocity in 6061 aluminum alloy plate specimen,and the measurement error is 4.13 %.The elastic modulus of material and locate the surface defect can be potentially evaluated by the ultrasonic surface wave velocity measured by the detection system.