Laser Excitation Of Saw For The Experimental Study Of The Surface Defect Detection

In this paper, the principles of laser-generated ultrasound, the propagation of surface acoustic waves and the applications of surface-breaking defects have been theoretically and experimentally studied.The laser-generated ultrasound theory in thermo-elastic mechanism is discussed first. Then laser-generated ultrasonic in the plate material is simulated numerically by using the finite element method. According to acoustic wave equation, the property and propagation of SAW is expounded.Comparing with some laser-generated ultrasound detection techniques, a kind of optical difference detection system based on optical beam deflection technique is introduced, it is shown that the setup is simple and easy to adjust. Furthermore the system has advantages such as high efficiency of photo-electricity conversion, high frequency responding, the ability of ant-intervene and broadband detection etc. It can be used not only to receive SAW, but also to detect surface-breaking defects in sample.Both point-source mode and line-source mode are analyzed. The experimental investigation on the relations of the surface acoustic waves with source types, the line source energy, detection's position is carried out. The experimental results verify the rationality of the theoretical model for laser generated surface acoustic waves and demonstrate that the surface acoustic waves generated by a line source has the advantages of powerful signal and good directivity.Surface acoustic waves have been excited on sample aluminum with artificial surface-breaking defects. From the experiment, we attain the characters of reflected wave and transmitted wave by SAW interacted with the surface-breaking defects. Furthermore, we use FFT to analyze the spectrogram of these signals.