Detection of a Surface-breaking Crack by Using the Frequency Attenuation in a Laser Ultrasonic Spectrum

Laser ultrasonic system is a non-contact inspection device with a high spatial resolution and it is applicable to hard-to-access locations like in a nuclear power plant. In this paper, we have attempted to detect the depth of a surface-breaking crack by using the surface wave of a laser ultrasound. We have investigated a variation of the peak-to-valley value in the time domain and the spectrum distribution in the frequency domain according to the crack depth. The peak-to-valley value and high frequency coefficient are decreased in proportion to the crack depth. The high frequency attenuation is proportionally increased and the center-of-mass position of the frequency spectrum is proportionally lowered according to the crack depth. In this paper, we describe the hardware configurations of the laser ultrasonic inspection system and the experimental results to detect a surface-breaking crack. We experimentally confirmed that the centroid variation of the frequency spectrum in the frequency domain provides better information about the crack depth than the amplitude variation of the ultrasonic signal in the time domain.