Laser-Based System for Remote Non-Destructive Inspection of Transportation Tunnels

摘要

A laser system for remote non-destructive inspection of concrete structures has been developed. The system is intended for the location of inner flaws in concrete structures, mainly in concrete transportation tunnels. The technique is based on the initiation and detection of Lamb waves and ultrasonic vibration in concrete with the use of a laser photorefractive interferometer. Vibration in concrete is initiated by laser impact provided by a pulsed Nd:YAG laser operating at wavelength of 1064 nm. A CW Nd:YAG laser radiation of 532 nm wavelength is used for detection of initiated vibration. The laser interferometer uses principles of dynamic holography via two-wave mixing in photorefractive crystal and homodyne detection. A photorefractive crystal - Bismuth Silicon Oxide (BSO) - is used as a nonlinear medium for the recording of the dynamic hologram. Concrete samples used in the laboratory experiments reproduce real concrete material used in transportation tunnels. Various types of inner defects like voids, cracks and honeycombs have been tested. Standing Lamb waves initiated by laser impact are detected by the interferometer from 1.5 m distance. Analysis of Lamb wave spectra provides information of defect location. Accuracy of location is about 2 cm.