Structural Health Monitoring in the Railway Field by Fiber-Optic Sensors

Chapter 63 Structural Health Monitoring in the Railway Field by Fiber-Optic Sensors

Aldo Minardo, Agnese Coscetta, Giuseppe Porcaro, Daniele Giannetta, Romeo Bernini and Luigi Zeni

63.1 Introduction

In railway infrastructure monitoring, structural health monitoring (SHM) is a key element of industrial businesses. Standard inspection techniques may fail in reveal- ing defects or unusual features, and some components may not receive close up examination if, for example, access is difficult or operating conditions do not permit it. Distributed optical fiber sensing techniques allow distributed temperature and strain measurements to be captured in real time over lengths of a few meters to tens of kilometers [1]. A permanently installed optical fiber cable provides continuous information about the status of the structure during its whole life cycle, thereby offering a unique opportunity in long-term SHM [2, 3]. Also, distributed sensors are able to acquire in real time the deformation of the rail track induced by train passage [4]. In this work, we report dynamic strain measurements performed by using the Brillouin Optical Time-Domain Analysis (BOTDA) in the Slope-Assisted configu- ration [5]. The tests were performed on the Peschici–San Severo regional railway line, connecting the northern coast of Gargano to the Adriatic railway, and operated

A. Minardo () · A. Coscetta Dept of Industrial and Information Engineering, Seconda Università di Napoli, Via Roma 29, 81031 Aversa, Italy e-mail: [email protected] G. Porcaro Tecnomatica SaS, Corso del Mezzogiorno III trav., 71122 Foggia, Italy D. Giannetta Direzione di Esercizio, Ferrovie del Gargano, San Severo, 71016 Foggia, Italy R. Bernini Istituto per il Rilevamento Elettromagnetico dell’Ambiente, Consiglio Nazionale delle Ricerche, Via Diocleziano, 328, 80124 Napoli, Italy L. Zeni Dept of Industrial and Information Engineering, Seconda Università di Napoli, Via Roma 29, 81031 Aversa, Italy © Springer International Publishing Switzerland 2015 359 D. Compagnone et al. (eds.), Sensors, Lecture Notes in Electrical Engineering 319, DOI 10.1007/978-3-319-09617-9_63 360 A. Minardo et al.

Fig. 63.1 a Instrumented rail sector and b instrumented rail bridge (picture of “sea side”). The red curves indicate the path of the glued optical fiber by Ferrovie del Gargano. In particular, dynamic tests were performed along a 60 m length of rail sector (see Fig. 63.1a) and a stone bridge, (see Fig. 63.1b) located in proximity of the San Menaio Station.

63.2 Results of Dynamic Strain Measurements Along the Rail Track

The acquisition of dynamic strain along the rail track during train passage offers a mechanism to derive a number of useful parameters in the context of railway traffic monitoring, such as axle counting and spacing, speed detection, and dynamic load estimation [4]. Dynamic strain was acquired along a standard telecommunications single-mode optical fiber, glued along the rail foot for a length of 60 m by use of epoxy adhesive. Details on the installation procedure can be found in Ref. [4]. When the train passes over the instrumented rail, the weight of each axle induces a deformation on the rail itself, which is transferred to the optical fiber. If the latter is disposed below the neutral axis of the rail, a localized, tensile strain peak is recorded by the sensor at each axle passage. Therefore, axle counting can be easily achieved by counting the number of strain peaks associated to the passing train, provided that the acquisition rate is sufficiently high [4, 6]. Furthermore, a dynamic strain temporal waveform is retrieved for each sensed position, thereby offering the opportunity to retrieve other pieces of information such as axle spacing and train speed [4]. We show in Fig. 63.2a the map of dynamic strain captured by the Slope-Assisted BOTDA sensor at a spatial resolution of 1 m and an acquisition rate of 31 profiles/s, when the instrumented rail sector is crossed by a diagnostic car. The latter was composed of a two-bogies (four axles) motor car, followed by two trailer cars, each one composed of two bogies. The measurement was performed in a special condition, in which the train, bound for Rodi, was accelerating when passing over the monitored sector. 63 Structural Health Monitoring in the Railway Field by Fiber-Optic Sensors 361

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10 20 20 3HVFKLFL 6WUDLQ> PH@ 30 3HVFKLFL 100 30 40 0 40 -100 50 Position [m] Position 50 [m] Position )LUVWD[OHWUDFH -200 60 ;ĨͿ 60 ;ĂͿ;ďͿ ;ĞͿ ;ĐͿ ;ĚͿ ;ĚͿ 5RGL ;ĞͿ 5RGL ;ĐͿ ;ĨͿ 70 ;ďͿ 70 ;ĂͿ 80 35 40 45 50 55 60 30 35 40 45 50 55 60 65 Time [s] Time [s]

Fig. 63.2 Strain induced on the rail track by diagnostic car passage as a function of time and position. The letters from ( a) to ( f) indicate the six bogies of the train

Note that the positions corresponding to the fiber portion attached to the rail, range from z ∼ 12 m to z ∼ 72 m. Carefully examining the strain map, up to six bogies (12 axles) are recognized: two of them belong to the motor car (e–f), the other four belong to the trailer cars (a–d). We observe that the axle traces are not straight lines, rather they show some curvature. A curvature in the space-time representation is a clear indication of acceleration. In particular, the measurements indicate a positive acceleration of the train during its passage over the monitored sector. Note that, along the very first meters of the instrumented rail (from z ∼ 12 m to z ∼ 20 m), the slopes of the various axle traces are not uniform, instead they increases when moving from bogie (f) to bogie (a). This is due to the fact that, while the motor car is accelerating, the two trailer cars move at lower speed due to inertia, as it is also confirmed by the fact that the axle traces get increasingly closer during the measurement interval. It is also interesting to observe that the last trailer car (boogies (a) and (b)) induces a larger deformation than the other two cars, although all the cars have the same nominal load. This is an obvious consequence of the fact that the train is accelerating, resulting in a load transfer from front to rear axles.

63.3 Results of Dynamic Strain Measurements Along the Rail Bridge

The second test was performed on a railway bridge located close to the instrumented rail sector. The bridge is a 3 m-long, single span, stone arch bridge, showing evident signs of ageing. A piece of single-mode standard fiber, identical to the one employed in the previous test, was glued directly over the bridge following four paths, two of them are highlighted in Fig. 63.1b, while the other two lied on the opposite side. The measurements, carried out at 1 m spatial resolution and 43 profiles/s acquisition rate, were performed during the passage of a two-bogies (four axles) train over the bridge. The results, summarized in Fig. 63.3a, reveal two definite, compressive 362 A. Minardo et al.

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46 -250 15 20 25 30 35 40 10 15 20 25 30 35 40 ab3RVLWLRQ>P@ Position [m]

Fig. 63.3 a Strain induced on the arch bridge by train passage, as a function of time and position. b Strain induced on the arch bridge by train passage as a function of position, at t = 37.3 s ( blue solid line) and t = 38.8 s ( red dashed line) strain peaks acquired at the passage of each bogie. Mapping these two fiber positions on the monitored structure, these two sections correspond to the upper part of the arch keystone. Figure 63.3b reports the strain acquired at the instants in which the two train bogies pass over the bridge.

63.4 Conclusions

A fiber-optic distributed sensor has been employed for integrated monitoring of railway infrastructures. The sensor is based on stimulated Brillouin scattering in an optical fiber. The results indicate that, gluing an optical fiber along the rail track, running conditions of passing trains can be determined. Furthermore, dynamic strain measurements on a rail bridge have been reported, aimed to detect potential structural defects. It is believed that health monitoring systems based on distributed optical fiber sensors may offer valuable information in evaluating structural integ- rity, durability and reliability, and in ensuring optimal maintenance planning and safe operation.

References

1. X. Bao, L. Chen, Recent Progress in Brillouin Scattering Based Fiber Sensors. Sensors 11, 4152–4187 (2011) 2. A. Minardo, R. Bernini, L. Amato, L. Zeni. Bridge Monitoring Using Brillouin Fiber-Optic Sensors. IEEE Sensors J. 12, 145–150 (2012) 3. M. Ko, Y. Q. Ni. Technology developments in structural health monitoring of large-scale bridg- es. Eng. Struct. 27, 1715–1725 (2005) 63 Structural Health Monitoring in the Railway Field by Fiber-Optic Sensors 363

4. A. Minardo, G. Porcaro, D. Giannetta, R. Bernini, L. Zeni. Real-time monitoring of railway traffic using slope-assisted Brillouin distributed sensors. Appl. Opt. 52, 3770–3776 (2013) 5. R. Bernini, A. Minardo, L. Zeni. Dynamic strain measurement in optical fibers by stimulated Brillouin scattering. Opt. Lett. 34, 2613–2615 (2009) 6. M. L. Filograno, P. Corredera Guillen, A. Rodriguez-Barrios, S. Martin-Lopez, M. Rodriguez- Plaza, A. Andres-Alguacil, M. Gonzalez-Herraez. Real-Time Monitoring of Railway Traffic Using Fiber Bragg Grating Sensors. IEEE Sensors J. 12, 85–92 (2012) 本文献由“学霸图书馆-文献云下载”收集自网络，仅供学习交流使用。

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