For The Study Of The Metal Surface Defect Detection Laser Saw

This paper studies surface crack detection using the surface acoustic waves (SAW) generated by pulsed laser on metal plates.At first, a brief introduction in development of laser ultrasonic technique is given. A theory of SAW generated by pulsed laser in thermo-elastic mechanism and the property of the propagation of SAW is discussed, which shows the theoretical foundation of using laser line source and SAW in subsequent experiment.The laser ultrasound detection system using two-wave mixing interferometer is built.The experiment utilizing the technology of scanning laser line source(SLLS) can study the action mechanism of the surface defects on the laser-generated SAW in far-field and near-field.The amplitude and frequency of the SAW are studied when the laser source generates in the far-field and near-field of cracks. In far-field of defects,when laser gets close to the surface defects,the existence of surface defects results in that reflected SAW gradually gets close to the directly arriving SAW .The position of the defect can be calculated by the arrival time of the direct arrival SAW and the reflected SAW. In near-field of defects, all kind of surface wave modes overlap for the mode conversion caused by the defect scattering. The amplitude and frequency of the SAW vary obviously as the laser line source scans over the defects.The influence of the defect depth to SAW is analyzed. To extract parameters about mode,amplitude ,frequeny spectrum of signals of different defect depths.The research results will offer the experimental foundation and detection method for the non-destructive evaluation of the surface-breaking defects on metal by laser generated SAW technology.