Time-of-Flight measurement on a rail by laser-ultrasound

Stress states of thermal origin on the Continuously Welded Rail (CWR) are a source of possible inconvenience due to winter breakage or dangerous summer buckling. To date, alongside procedures based on the measurement of rail displacements or rail cutting, several methods for measuring the stress state in the rail have been proposed, based on X-ray diffraction, Barkhausen noise, and the acoustoelastic effect; all these approaches have some critical issues for extensive use in the field. The acoustoelastic method is based on the dependence of the ultrasonic wave speed on the stress state of the material and involves the measurement of the Time of Flight (ToF). Because of the modest acoustoelastic effect, the speed variations caused by the stress states are very limited, which is reflected in the need for ToF measurements with an accuracy of the order of a few nanoseconds. In the past, the research group successfully used the acoustoelastic method to monitor a 3 km portion of a rail between Florence and Pontassieve, with fixed measurement stations, where the ultrasonic probes were permanently positioned on the neutral axis of the rail. Today, the need is to have a mobile measuring system, which can measure in several points or even continuously as the system moves along the rail. This appears to be possible through the use of, for example, non-contact systems such as those based on laser and air-coupled probes or lasers alone. In the first case, however, the acoustic wave partly propagates in the air before reaching the piezoelectric probe, and this path is subjected to variations in temperature and thus ToF. In the second case, the ultrasonic waves are generated and detected by optical methods with lasers. For the generation of acoustic waves, the use of pulsed lasers has become a widespread, reliable practice, which allows researchers to obtain signals of high amplitude and wide bandwidth. For the reception of the acoustic signal, the precision required in the measurement involves careful analysis and experimentation to identify the most appropriate laser technique, depending on the quality of the surface on which the measurement is performed, the sensitivity of the method, and the possibility of taking measurements while moving. In the present work, a technique for laser detection of ultrasonic waves, Optical Beam Deflection (OBD), is analysed. The technique is based on the deflection of an optical beam: the beam irradiates the surface of the rail and is reflected at angles that depend on the orientation of the surface at the point of measurement, which is perturbed by the passage of the acoustic wave. The technique, developed in the 1990s, has limited applications in non-destructive testing, where interferometer-based techniques are more widespread; still, they require more sophisticated and expensive equipment. Different experimental arrangements and initial experimental results are analysed and presented, to optimise their sensitivity and verify their suitability for the purpose.