Laser Saw Metal Cylindrical Surface Defects Detection

In this paper, laser-generated surface acoustic waves(SAWs) detecting surface-breaking defects on metal solid cylinders have been studied theoretically and experimentally.A theoretical model for laser-generated surface waves on elastic cylinder material is built according to the thermoelastic theory. The finite element method is used to calculate. By changing the relative distance between the laser source and the surface notch in the computation, the scanning procedure is simulated. The corresponding varieties of amplitudes,polarity and spectrums of the surface waves, which propagate circumferentially on the cylinder material, are presented and the physical mechanisms are analyzed. The influence of the depth of the surface notch is also discussed. The results demonstrate the scanning laser source (SLS) technique can be applied to detect tiny crack which is smaller than the wavelength of the SAW. The changing trend of peak-to-peak value of SAW propagating on a cylinder is similar with a plate. But the maximum value occurs when the laser line source generated on the crack edge for a plate, and for a cylinder it happens near the crack. The same trend occurs for the different depths of the defects.The laser ultrasound optical detection system using laser interferometer is also built. Utilizing the SLS technique, the action mechanism of the surface defects on the laser-generated SAW is analyzed. The varieties of amplitudes,polarity and spectrums of experimental signals are in good agreement with simulations. The results also show that the overlap of the incident wave and the reflected wave causes the reversal of the acoustic pulse observed in the experiment. But the waveform of the reflected waves are changed due to the different transmission distances, so the dispersion effect is also need to be taken into account.