Nondestructive Detection of Small Internal Defects in Carbon Steel by Laser Ultrasonics

A small internal defect 100 µm in diameter in a carbon steel sample was successfully detected by the laser ultrasonic technique. Irradiation of a Q-switched Nd:YAG laser was used for ultrasonic generation, and a frequency-doubled CW Nd:YAG laser combined with a Fabry-Perot etalon was used for detection of ultrasonic vibration on the carbon steel surface. The ratio of the internal defect diameter to the ultrasonic wavelength ( d /λ) was estimated to be about 0.07. The dependence of the normalized amplitude of the defect signal on the defect diameter was measured, and the ultrasonic reflectance by the cylindrical internal defect was roughly evaluated using the experimental results. It is considered that the laser ultrasonic technique has the ability to detect internal defects in industrial components and engineering structures, which is comparable to that of the conventional ultrasonic testing technique.