<title>Structurally embedded fiber Bragg gratings: civil engineering applications</title>

Abstract

In civil engineering it is of interest to monitor long-term performance of structures made of new lightweight materials like glass or carbon fiber reinforced polymers (GFRP I CFRP). In contrast to surface applied optical fiber sensors, embedded sensors are expected to be better protected against rough handling and harsh environmental conditions. We report on two recently done fiber optical sensor applications in civil engineering. Both include structurally embedded fiber Bragg grating (BG) arrays but have different demands with respect to their operation. For the first application fiber BGs were embedded in GFRP rockbolts of 3-5 m in length either of 3, 8, or 22 mm diameter. The sensor equipped rockbolts are made for distributed measurements of boulder motion during tunnel construction and operation and should withstand strain up to 1 .6 %. Rockbolt sensors were field tested in a tunnel near Sargans in Switzerland. For a second application fiber BGs were embedded in CFRP wires of 5mm diameter used for the pre-stressing cables of a 56 m long bridge near Lucerne in Switzerland. The permanent load on the cable corresponds to 0.8 % strain. Due to the embedded sensors, strain decay inside the cable anchoring heads could be measured for the first time during loading and operation of the cables. For both applications mechanical and thermal loading tests were performed to assess the function of these new elements. Also, temperature and strain sensitivity were calibrated. Reliability studies with respect to stress transfer, fiber mechanical failure, and wavelength shift caused by thermal BG decay as well as monitoring results of both applications are presented.

Optical FBG's sensing properties and practical application in civil infrastructures

“…iK-n;-= ,which is thermal-optical tensor and thermal expansion coefficient respectively , we obtain = (a + )dT = KT dT (8) 1 n Where K7 is thermal sensitivity. If strain not taken into account, (8) is the equation between FBG wavelength shift and temperature.…”

“…iK-n;-= ,which is thermal-optical tensor and thermal expansion coefficient respectively , we obtain = (a + )dT = KT dT (8) 1 n Where K7 is thermal sensitivity. If strain not taken into account, (8) is the equation between FBG wavelength shift and temperature. If the optical fiber core is silicon dioxide,related a + In is about 6.67 x 1 O6Y>k' , the temperature sensitivity coefficient is about 1Opm/'5 for the FBG whose wavelength is also 1550nm.However, for different FBG fabricated process, the strain and temperature sensitivity coefficients ofFBG should be determined through experiments.…”

“…Rao [6] and Tennyson [7] have presented systematic reviews to introduce recent progress in applications ofFBG sensors in large composite and concrete structures, the electrical power industry, medicine, and chemical sensing and so on. As for practical application, Nellen [8,9] installed FBGs on the prestressed cable of Lucerne Bridge and the CFRP of Winterhur Storck Bridge to investigate the deformation of CFRP. The results gotten from FBGs are consistent with that of strain gauge.…”

A Review of Rock Bolt Monitoring Using Smart Sensors

“…The tensile tests indicated that the embedded FBG sensors were able to survive a high strain of 1.5%. The GFRP rock bolts equipped with FBG sensors were installed in a tunnel near Sargans, Switzerland for long-term load monitoring [ 42 , 43 ]. The installation scheme of the FBG-equipped GFRP rock bolts in a tunnel and the one year measurements are shown in Figure 9 .…”

Test and Study of Pipe Pile Penetration in Cohesive Soil Using FBG Sensing Technology

“…The research on the penetration mechanism of jacked pile in viscous soil foundation is relatively small, and while its penetration mechanism is affected by many factors, it is difficult to find an effective research method. In recent years, many domestic and foreign scholars have examined temperature compensation technology, packaging technology, installation technology, and engineering applications of FBG sensors: Nellen et al [9] first measured the strain attenuation in a cable anchorage head during cable loading and operation using embedding sensors, conducted mechanical and thermal load tests, and calibrated the temperature and strain sensitivity. The reliability of wavelength shift caused by stress transfer, optical fiber mechanical failure, and thermal FBG decay was introduced.…”