Laser-generated Surface Acoustic Waves Technology For Nondestructive Detection Of Surface-breaking Defects On Metal

The mechanisms of surface acoustic waves (SAW) generated by pulsed laser in thermoelastic region on metal and the application for nondestructive detection of surface-breaking defects are studied in this dissertation in a systematic way. The action mechanism of the surface defects on the laser-generated SAW in far-field and near-field is analyzed.At first the stimulate mechanisms of the SAW by laser point source and laser line source are analyzed on the basis of the analytic model of the laser ultrasonics. And the finite element method is employed to simulate the generation of SAW on metal aluminum plate excited by laser point source in thermoelastic region.A kind of optical difference detection system based on optical beam deflection technique is built. The investigation on the relations of the laser-generated SAW with the type of laser source and laser line source energy is carried out. The results verify rationality of the analytic model for laser generated SAW and demonstrate that the SAW generated by a line source has the advantages of powerful signal and good directivity. The qualitative relation between the amplitude of the SAW and the power density of the incident laser is obtained. When the mechanisms of the excitation change from thermoelasticity to ablation, the amplitude of the SAW vary obviously and the central frequency of the corresponding spectrum shifts to high frequency.The laser ultrasound detection system based on PVDF transducer is also built. Utilizing the scanning laser line source technology (SLLS), the action of the surface-breaking defects and micro cracks in far-field and near-field the laser generated surface wave is studied. The position of the defects can be calculated by the arrival time of the direct arrival SAW and the reflected SAW. For the mode conversion caused by the defect scattering, all kind of surface wave modes overlap in near-field of defects. The amplitude and frequency change of the SAW vary obviously as the laser line source scans over the defects.The laser ultrasound detection system using two-wave mixing interferometer is also built.Ultilizing SLLS, the action mechanism of the surface defects on the laser-generated SAW in far-field and near-field is analyzed. And the influence of the defect depth in near-field SAW is emphasized. The wavelength of the stagnation frequency is equal to the depth when the depth of the defects is smaller than the wavelength for central frequency of the incident SAW. While the influence of the depth on the frequency spectrum of the near-field SAW becomes saturation when the depth of the defects is bigger than the wavelength for the central frequency.The research results will offer the theoretical foundation and detection method for the non-destructive evaluation of the surface-breaking defects on metal by laser generated SAW technology.