Detection of a Surface-Breaking Crack Depth by Using the Surface Waves of Multiple Laser Beams

A laser ultrasonic system is a non-contact inspection device with a long stand-off distance and it is applicable to hard-to-access locations such as in a nuclear power plant. In this paper, multiple surface waves generated by line-shaped multiple pulse laser beams are adopted to precisely detect a surface-breaking crack with its depth information. When a surface crack is positioned at the center of multiple pulse laser beams, acquired surface waves are composed of two parts, a reference part and a data part. The reference part is the front part of the acquired multiple surface waves and the data part is the rear part of the surface waves. Detail crack depth information can be extracted from the normalized multiple surface waves by using the reference part. The crack depth information is extracted by acquiring the difference between two peak frequencies of the reference and the data signal. Also, during a scanning test, we can elucidate a crack advent by observing the amplitude variation and frequency spectrum variation of the multiple surface waves when the crack is positioned in the range of the multiple pulse laser beams.