4.3 FBG-based online monitoring of HSR

In recent years, optic fibre sensors have been advocated for application to rail infrastructure monitoring. The FBG sensors have merits of being immune to electromagnetic interference (EMI) and no power supply is needed on-site. A monitoring system based on the FBG technology has been developed and installed on an operating rail line in Hong Kong for real-time and continuous detection of rail strain and temperature, rail breaks, axle counting, wheel imperfection assessment and dynamic loading identification [35]. Wang et al. [74] proposed a rail performance monitoring and safety warning system and implemented this system on a rail line by deploying FBG sensors in the rail web and at the expansion joints between supporting concrete slabs. Yoon et al. [75] proposed a distributed fibre sensory system based on Brillouin scattering and a correlation domain analysis technique for longitudinal strain monitoring of rails. Ni et al. developed a deformation monitoring system for an in-service HSR tunnel using an FBG-based monitoring system [76]. An array of FBG bending gauges was deployed at the rail slab of a segment inside the tunnel. Upon occurrences of deformation, there would be relative rotation between two adjacent bending gauges. Phase shift of the FBG sensors caused by the relative rotations was recorded, and the deformation can then be derived, resulting in a profile of the deforming rail slab, and the deformation of the tunnel can be inferred.

FBG sensors for detecting acousto-ultrasonic signals have been studied since the mid-1990s [77]. The conventional interrogation technique for FBGs as sensing elements utilises their spectral encoding and decoding capabilities for the measurand; however, the spectral decoding capability cannot be used to detect high-frequency signals (e.g., acoustic and ultrasonic waves) due to the low wavelength scanning speed. Appropriate demodulation techniques capable of highsensitivity detection of high-frequency waves are necessary to develop acoustoultrasonic FBG sensors. There are two main approaches to detecting acoustic and ultrasonic waves with FBGs: the first one uses a narrowband light source to

Contemporary Inspection and Monitoring for High-Speed Rail System DOI: http://dx.doi.org/10.5772/intechopen.81159

Figure 8. Schematic set-up of the FBG-PZT monitoring system [79].

illuminate the FBGs and demodulate the power intensity variation when the waves impinge on the FBGs, while the second one uses a broadband light source and an optic filter. Minardo et al. [78] conducted a numerical investigation on the response of FBGs subjected to longitudinal ultrasonic waves. Ni et al. [79] have developed a hybrid monitoring system using FBG sensors to interrogate ultrasound signals emitted by PZT sensors (Figure 8). The hybrid system has been verified in lab and a test line in mainland China.
