A Fully Optical Laser Based System for Damage Detection and Localization in Rail Tracks Using Ultrasonic Rayleigh Waves: A Numerical and Experimental Study

Abstract

The present study focuses on investigating the structural integrity of rail track sections of the high-speed railways using the Rayleigh waves generated and sensed using a fully non-contact optical Laser system. The raw broadband beam of the excitation laser was converted to a narrowband beam using a customized optical system, whereas the propagating Rayleigh waves were sensed using a three-dimensional (3D) scanning laser Doppler vibrometer (3D-SLDV) system. All the experiments were conducted using the pitch-catch method in presence of surface damages on head of the Rail track. Several issues were observed during the experiments, which are noted as follows. First, the noise and unwanted wave packets increase with the increase in the time window and the inspection length. Second, with larger inspection lengths, it is relatively difficult to interpret the response as the reflection of the incident wave packet may arise from any edge of the rail specimen, and shall be difficult to precisely identify the source. Third, as a result of lower Signal-to-Noise ratio (SNR), there may be smaller wave packets that shall be likely deceiving as a reflection of the defect. Fourth, it is observed that a small change in the location of the sensing point may significantly alter the overall signal. Fifth, it is also observed that the actuation and sensing position plays a crucial role in receiving the time-domain data with a sufficient SNR and the one that is easy to analyze and interpret. Based on the numerous experiments, an optimum distance of inspection is estimated which yields damage detection and localization with high accuracy thereby solving all the aforementioned issues. Further, As the quality of received signals differs at different sensing points as a result of the surface conditions of the specimen, the Self Adaptive Smart Algorithm (SASA) method was adopted to filter out the noise and accurately pinpoint the defect reflected wave packet which ultimately aids in better detection and localization. Finally, a 3D Finite Element simulation was conducted to validate the findings and each observation resulting from the experiments. Based on the obtained accuracy of the results, the proposed methodology has been found to be capable of inspecting rail track specimens in a completely non-contact manner with reasonably good accuracy