The Design of a Non-Contact Inspection System Integrated With the Time of Flight-Based Flaw Detection (TOFFD) Criterion to Investigate the Structural Integrity of the Rail Track

Rail flaws can be initiated either from manufacturing or service-related anomalies. These flaws may lead to train derailment while in service if not detected timely. Traditional non-destructive testing (NDT) techniques are mainly contact-type and have low inspection speeds. Laser ultrasonic testing is an advanced NDT that provides non-contact, broadband, and highly sensitive inspection. Meanwhile, Rayleigh waves suffer little attenuation during propagation and travel long distances on curved surfaces like rail. Therefore, this research proposes a design of a fully non-contact laser-based rail inspection system to detect surface and subsurface defects at different parts (head, web, and foot) of the inspected rail track. The laser-based thermoelastic generation of narrowband Rayleigh waves was investigated using the finite element method (FEM). The experiments were performed on defective rail samples with artificial surface and circular subsurface defects in the inspected rail track. The time and frequency analyses of both simulation and experimental results show that the line arrayed pattern (LAP) type of laser illumination is promising in generating narrowband Rayleigh waves that have great potential in detecting rail defects. In field measurements, finding the defect's location is usually challenging because unwanted wave packets mostly surround defect echoes in a laser-generated ultrasonic signal. In this regard, a criterion named time-of-flight-based flaw detection (TOFFD) was proposed to identify defect echoes surrounded by high noise peaks automatically. TOFFD was successfully applied to laser-generated ultrasonic signals captured at the rail track to find the location of the flaw by identifying the defect echo.