Complete noncontact laser ultrasonic imaging for automated crack visualization in a plate

This paper presents an automated crack visualization technique using ultrasonic wavefield images obtained by a complete noncontact laser scanning system. First, the complete noncontact laser scanning system is built by integrating and synchronizing a Q-switched Nd:YAG laser for ultrasonic generation, a laser Doppler vibrometer for ultrasonic measurement and galvanometers for scanning. Then, four different laser scanning schemes are compared to find the most effective and practical ultrasonic scanning strategy for the presented application. Second, a novel image processing technique is developed to isolate and visualize crack-induced standing wave energy from the constructed ultrasonic propagation images. Finally, the effectiveness of the proposed laser ultrasonic scanning system and imaging processing technique is experimentally verified using ultrasonic scanning images obtained from an aluminum plate. The test results confirmed that a hidden notch invisible from the scanned surface was successfully detected and visualized, while no false positive alarm was triggered for an intact specimen.