Interrogating the health condition of rails using the narrowband Rayleigh waves emitted by an innovative design of non-contact laser transduction system

The article reports an innovative optical system that is designed to interrogate the health condition of macroscopically intact rail specimens by measuring its inherent nonlinearity using the narrowband Rayleigh waves. A line-arrayed pattern is developed through the optical system that generates narrowband Rayleigh waves with high power on the surface of the rail. As a result of lattice-anharmonicity, a second harmonic is produced in the wave that is sensed by a scanning laser Doppler vibrometer. The spectral amplitudes of the first and generated second harmonics are used to calculate the inherent nonlinearity using an amplitude-based nonlinearity equation. These measurements are carried out on the head, web, and foot of the rail. The performance of the non-contact experiment is also compared with that of a contact experiment carried out using wedge transducers. The experimentally evaluated nonlinearity of the rail steel is further compared with that obtained using a physics-based nonlinearity equation that relies on the higher-order elastic constants. Agreement of the results shows that the new optical system is effective in generating Rayleigh waves in rails and thereby measuring the inherent nonlinearity of the rail track. The estimation of inherent nonlinearity may help in diagnosing the health status of the macroscopically intact rail specimens in terms of their microstructural consistency and level of dissolved impurities before fixing them on a track.