Width and depth gauging of rectangular subsurface defects based on all-optical laser-ultrasonic technology

Abstract

In this study, width and depth of rectangular subsurface defects have been successfully gauged based on all-optical laser-ultrasonic technology. A specific all-optical laser-ultrasonic system has been developed to obtain Rayleigh waves from subsurface defects. Time domain and frequency domain analysis of the obtained Rayleigh waves show that most of the high-frequency Rayleigh waves with small wavelengths transmit the subsurface defect. Only a small part of the low-frequency Rayleigh waves with large wavelengths are reflected by the subsurface defect. This phenomenon is completely different from that of surface defect, because surface defect reflects high-frequency Rayleigh waves and allows low-frequency Rayleigh waves to transmit. In addition, it is found that the transmitted mode-converted Rayleigh wave has the smallest amplitude but the highest center frequency compared with other obtained incident, reflected mode-converted and transmitted Rayleigh waves. Width and depth of subsurface defects are measured by the proposed three-step detection method, which is based on the arrival time of the incident, reflected mode-converted, transmitted and transmitted mode-converted Rayleigh waves, as well as the movement distance of samples. Agreement between experimental results and digital microscope measurement reference-results indicates applicability of the proposed three-step detection method in simultaneous measuring width and depth of rectangular subsurface defects.