Signal Processing Techniques to Detect a Surface-Breaking Crack by Using a Laser Surface Wave

다국어 초록 (Multilingual Abstract)

A laser ultrasonic system is a non-contact inspection device with a
high spatial resolution and it is applicable to hard-to-access
locations as in a nuclear power plant. In this research, we have
attempted to detect the depth of a surface-breaking crack by using
the surface wave of laser ultrasound. We have investigated the
variation of the peak-to-valley value in the time domain and the
spectrum distribution in the frequency domain according to the crack
depth. We developed several signal processing techniques to detect
crack-depth information by using a normalized peak frequency
variation and the attenuation of the high-frequency part of a
normalized transfer function spectrum with a half-period window.
Here, we describe the hardware configurations of the laser
ultrasonic inspection system and the experimental results to detect
a surface-breaking crack. Though the signal variation of the laser
surface wave is considerably high, we experimentally confirmed that
the attenuation of the high-frequency part of the spectral signal in
the frequency domain provides better information about the crack
depth than the amplitude variation of the ultrasonic signal in the
time domain.