Light Deflection Differential Laser Ultrasonic Detection System Design And Defect Detection

Laser ultrasonic techniques for non destructive testing (NDT) have attracted considerable attentions. This dissertation has studied the principle of laser generated ultrasonic, the propagation of surface acoustic wave (SAW), detection techniques and its application in NDT theoretically and experimentally. 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 rationality of use of line resource mode and SAW in subsequent experiment.After contrasted with mangy techniques, reflected optical difference technique is introduced. The characteristic of optical difference technique is taken into account. A new detection method—incidence plane is perpendicular to the direction of the propagation of SAW—is discussed. The analysis of the principle of concludes that this method advances the spatial resolution of the detective beam, and lessens the signal elements which make processing of data easier. Moreover, by appending filter and interference light filter, the signal noise ratio of the system is enhanced greatly.The application of reflected optical difference system in surface crack of aluminum detection is also discussed. By analyzing the experimental data, we found that the reflected pulsed wave techniques could be used to measure the position of crack precisely and depth imprecisely. Transmitted wave contain much size information of the crack, despite without position information. The spectrum of transmitted is sensitive to the depth of crack. The deeper the crack is, the narrower the spectrum bandwidth is. It is proved that laser ultrasonic technique is potential in NDT.