Mechanism Of Laser Ultrasonic Detection Of Surface Defects

In this paper, the relationships between the excitation source and the laser ultrasonicsignal characteristics have been studied theoretically and experimentally, focusing on themechanism of SAW interaction with surface-breaking detects.Firstly, we introduced several methods and mechanism of laser ultrasonic surface defectdetection. Analysis the mechanism of laser ultrasonic detection of surface defects detectionsystem, introduced the principle of excitation source systems, laser ultrasonic testing systems,data acquisition systems, and the presence of surface defects of the standard sample. On thebasis, build a laser-based ultrasonic pulse laser and ultrasonic receiver surface defectdetection probe experimental system.Introduced the basic theory of laser ultrasonic excitation, laser line source heat-structure coupling model is established, analyses the result of numerical simulation andverified the accuracy of the results. A model of laser line source heat-structure coupling ispresented by using the finite element software. The ultrasonic frequency characteristics havebeen numerically simulated, that is induced by a line source of different widths, and linesource array. The results show that the ultrasound-induced bandwidth can be improved bynarrowing laser line width in bandwidth limitations. The ultrasound-induced signal has thenarrower bandwidth and the better SNR by the line source array. It is shown that theblackened surface of the sample can effectively improve the efficiency of the laser ultrasonicexcitation. The experimental results verify that the ultrasound-induced bandwidth can beimproved by the smaller laser spot size.It is theoretically analyzed that the characteristics of acoustic wave propagation alongthe solid surface, experimental measured probe direction and the Rayleigh wave velocity. Theposition of surface-breaking detects is measured using SAW. It is the characteristic parameterthat is extracted for characterizing the crack depth. The preliminary results indicate that the extracted characteristic quantity can characterize effectively the depth of crack in the limits ofdetection range. Numerical simulation analyses frequency-domain characteristics of theacoustic wave signal with different crack depths. The results showed that, according to themonotonically increasing amount of quantitative characterization of reflected wave andtransmitted wave the size of the defect depth can be distinguished. It is provide a theoreticalbasis study of laser ultrasonic detection of surface defects. The preliminary results indicatethat the extracted characteristic quantity can characterize effectively the depth of crack in thelimits of detection range.