Depth Detection of a Thin Aluminum Plate in Laser Ultrasonic Testing Using a Confocal Fabry-Perot Laser Interferometer

Laser ultrasonic testing is an advanced technique to extract crack information in nondestructive inspections. Though this testing is a fully non-contact detection technique, its application area was not wide until now because of the relatively low signal-to-noise ratio when compared with conventional contacting piezoelectric transducers. In this research, a common laser ultrasound was used to detect crack depth information in a thin aluminum plate. A laser ultrasonic inspection system was fabricated by using a pulse laser, a confocal Fabry-Perot laser interferometer (CFPI) with a CW laser beam, an iris, a high-pass filter, a dynamic stabilizer and a control computer. The laser ultrasonic signal was detected from the opposite epicenter position of an aluminum plate. Crack depth information was extracted from the corresponding wavelength frequency to the roundtrip length in the frequency domain. The results for the crack depth detected in a thin aluminum plate are described in this paper by using experimental data.