Study On Nondestructive Evaluation Of Surface Defect And Residual Stress On Metal By Laser Ultrasonic Technology

In the dissertation, the detection mechanics of tiny surface defect on plate sample or cylindrical one by scanning laser line source (SLS) and the propagation characteristics of the acoustic surface waves (SAWs) are studied by the finite element method (FEM) and experimental method. A detection system using laser ultrasonic wave for surface stress distribution, which is an important cause of the surface defect, is set up.By improving the structure of the differential optical beam deflection detection system, the spatial resolution for measuring laser-generated SAW increases. The sensing mechanism and the linearity of the system are discussed by simulating the light propagation in the system using Monte Carlo algorithm. For verifying the detection capability, an experiment, where this system is applied to receive the SAW, is carried out to detect tiny on the surface of medal plate defect by SLS.Based on the classical theory of laser-generated ultrasound thermoelasticly, a finite element model of pulsed laser line source acting on the edge of defects is built. The temperature field and the ultrasonic field induced are simulated. The influence of the defect depth on the waveform of the SAW is also discussed. So the phenomena, that the amplitude of received SAW increases when the laser line source arrives at the edge of the edge, can be explained.The SLS technique is applied to the cylindrical sample. The finite element models are used to simulate the procedure of the pitch-catch method and the SLS detection method and the applicability of SLS technique on cylindrical sample is discussed. The defect information obtained by both methods is analyzed and the defect depth impacts on the validity of both methods are compared with each other. Simultaneously the results on cylindrical structure by SLS are compared to the plate material, and the corresponding experiment to verify that is set up.A detection system according to the acoustoelastic principle is set up to survey surface stress distributions on the material surface. The welding stress is chosen as the measured object and a serial of ultrasonic pulsed signals can be detected by the interferometer due to different distance between fixed detecting point and ultrasonic source point by the scanning of the laser source and sample. The waveform cross-correlation technique was applied to compute the propagation time delay of the adjacent Rayleigh waves, and the propagation velocity of the Rayleigh waves was obtained, by which the stress value was figured out via acoustoelastic relation. The residual stress distribution can be measured around welded joint while the laser source scans around welded joint.The results of this dissertation may provide theoretical and experimental basis for surface defects detection and residual stress measurement on metal using laser ultrasound, and accelerates the development and application of laser ultrasonic nondestructive testing technology.