LURI emits three important wave types with their own directivity patterns giving enhanced flaw identification. The wave types are sent and received at once without any changes in transmitter/receiver configurations .
LURI gives a high level of flexibility of the laser beam in terms of distance and side positioning.
In this paper, we describe the LURI system briefly and report results of its field-testing on a railroad line containing man-made structural defects.
The basic principles of LURI: The basic principle of LURI system is shown in Fig. 1. Generation of ultrasound in the rail is performed in the ablation regime by a sufficiently powerful laser. In order to inspect rails during movement with appropriate spatial resolution, the laser operates at sufficiently high repetition rates. Bulk longitudinal waves as well as their surface-skimming mode, bulk shear waves, and surface Rayleigh waves are generated in the rail body as a result of the recoil effect leading into material ablation. The ultrasonic modes are reflected and scattered by the free surface of the rail body and possible flaws i.e. discontinuities in the rail body or breaks of the surface, reach the predetermined receiving point where they are detected by means of a second laser. The detection laser is very stable in frequency and intensity. When the ultrasonic wave reaches the detection laser point it causes a small surface motion in the nanometer range. The motion produces a Doppler frequency shift on the scattered light of the detection laser. The scattered light is coupled into an interferometer, which demodulates the Doppler shifted light into time variation of light intensity. The registered intensity signal represents surface displacement or velocity history and enables rail defect detection in much the same manner as conventional contact or electromagnetic ultrasonic rail flaw detectors. Since the running surface of the rail is optically rough, the scattered light has a speckle pattern that continuously changes during movement of the detection laser and causes speckle noise. A confocal Fabry–Pérot interferometer (CFPI) was used in this version of LURI to reduce the speckle noise. The CFPI allows significant suppression of a speckle noise pattern by means of active high-pass filtering and implementation of a weighted conjugation optical detection scheme. With this system, stable and reproducible ultrasonic measurements were demonstrated in our laboratory on fast rotating discs at more than 100 km/h (62 mph).
Figure 1. Basic principle of LURI.