Research On Laser Technique Based Vibration And Ultrasonic Testing

Nondestructive testing(NDT)technique for aircraft structures plays an important role in preventing fatal accidents,improving economic performance and reducing maintenance cost.However,the traditional NDT methods usually use the contact way to scan the structures.Therefore,for the aircraft structure with curved surface,the traditional NDT methods are very difficult to achieve high automation and efficiency.Aimed at this problem,this thesis focuses on the laser technique based non-contact NDT methods.The basic principle of the proposed method is extracting the damage-induced changes in structural dynamic responses to estimate the damage position and extent,by using laser generation or laser measurement.During the detection process,the laser point can be easily moved through the principle of reflection.By controlling the reflection angle of the scan mirrors,the laser measurement can achieve a wide range of auto scanning.Thus,this method is suitable for large scale detection in aircraft structures.This thesis can be divided into two parts:laser vibration based NDT method and laser ultrasonic based NDT method.First,laser vibration based NDT method uses the dynamic response with low frequency to detect the damage position.Second,laser ultrasonic based NDT method generates the Lamb wave in the structure and can image the damage size and shape.From theoretical derivation,numerical simulation,system design to experimental validation,the proposed methods have been well studied.The main contributions and innovations of this thesis include:1.A virtual element boundary measurement based pseudo excitation approach is established.?The framework of "weak" formulation based pseudo excitation approach is proposed.? The damage index of the "weak" formulation is derived,which is only related with vibration information at the boundaries of virtual element.? The selection of the excitation frequency and the weight function is proposed,by which the numerical difference approach for calculating the high order derivatives of the vibration displacement can be avoided.? By using numerical simulation,the effectiveness of the proposed method is verified.This part mainly solves two problems of the "strong"formulation based pseudo excitation approach:low noise immunity capacity and dense measurement points.2.Laser vibration based NDT system is constructed.? Based on the virtual element boundary measurement based pseudo excitation approach,a NDT system is built by combining two different measurement systems:laser Doppler vibrometer and metal-core piezoelectric fiber based smart layer.? A method is developed for packaging the metal-core piezoelectric fiber based smart layer,as well as its calibration.? By using a beam structural component,the damage position can be detected,whist showing the effectiveness of the proposed method.This part mainly solves the problem on measuring the distributed strains under high frequency vibration.By using multi-type measurement system,the virtual element boundary measurement can be achieved.3.The damage imaging method based on wavefield analysis is developed.? Based on the wavefield data obtained by laser ultrasonic system,the fundamental model of the damage in Lamb wave propagation is built.? An enhanced damage imaging method based on the interference wave energy using wavelet transform is proposed.? The anomalous incident wave energy is extracted as a damage index to image the damage.This part solves the problems on measuring the wave propagation data and calculating the damage index.The results lay the foundation of the laser ultrasonic system and damage imaging method.4.Laser ultrasonic based NDT system is constructed.? By controlling the 2D-scan mirror,the system can achieve non-contact and movable laser generation.Through the reciprocity principle,the Lamb wave propagation can be visualized.? Aimed at practical engineering application,a management software including hardware driver,scan control,signal processing and data analysis is developed.? The experiments validate that the proposed system can measure the Lamb wave propagation on metal and composite structure,as well as the aircraft wing structure.5.The applications of the laser ultrasonic system are validated.? Aimed at the metal and composite structures,the proposed method can image the damage on both of the structures,whist showing the improvement on damage resolution.? The relation between the damage resolution and the damage depth or extension direction is studied through experimental comparison.? Laser ultrasonic system and fatigue test system are used to obtain the Lamb wave data with different load cycles experimentally.Several signal processing methods are implemented to extract the parameters,such as phase velocity,amplitude attenuation and mode energy ratio.The relation between these parameters and the load cycles are studied.This part solves the problem on laser ultrasonic system applications,which promotes the development of its practicability in damage evaluation and composite structural residual life prediction.