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Sensing & Measurement

Fiber sensors for structural-health monitoring

Novel serial optical-fiber sensors observe strain in bridges and other civil-engineering structures.
17 March 2009, SPIE Newsroom. DOI: 10.1117/2.1200902.1012

Civil infrastructures are generally large, with long spans between supports, and serve for a long time—decades or even over a hundred years. Over time, they inevitably suffer from environmental corrosion, long-term loading or fatigue, material aging, or a combination of these effects with extreme loading. The damage accumulates gradually, resulting in either degenerating performance, reduced ability of the structure to withstand disasters, or disastrous failure under extreme loading. Therefore, intelligent systems that can monitor structural health are important for studying the damage- or even disaster-evolving characteristics of and laws applicable to these structures, and to ensure their health.1

Fiber-optic sensors have attracted much attention in the context of long-term structural monitoring. However, because of the fragility of bare optical fiber and the practical demands of field applications, we are faced with the difficult problem of how to develop feasible optical-fiber sensors that fully meet the requirements of practical structural-health monitoring (SHM) of our infrastructure. Our research has focused on developing serial optical-fiber sensors for and their applications to civil engineering.

Packaged strain sensors

A reliable packaging technique for bare optical fiber is required to ensure that sensors remain effective for long-term SHM, even when subject to rough construction conditions or a harsh environment. Since fiber-reinforced polymer (FRP) exhibits a high strength-to-modulus ratio, corrosion and fatigue resistance, and high strength, and is nonmagnetic, it has been regarded as a novel packaging material for optical-fiber sensors. FRP also has the attractive quality of a quasi-elastic constitutive relationship that is linear to 15,000μm strain, which is much better than metallic packaging materials for small-strain conditions (<5000μm strain), especially for large strains (>20,000μm strain).2–5 Figure 1 shows parts of our FRP-based optical-fiber sensors.

Figure 1. Rugged optical-fiber sensors packaged using fiber-reinforced polymer (FRP). (left) Optical rebar, and strain sensors based on (center) serial fiber-Bragg grating (FBG) and (right) Brillouin optical time-domain analysis.

To meet the various demands from practical field applications, our group has developed a variety of sensors that provide indirect structural-health measurements, including FBG-based steel rebar transducers and crack sensors (i.e., large-strain sensors), a novel ice-load cell based on dual FBGs, and an FBG-based cable-load cell (which could also be used to measure liquid loading).6,7 Some of the results are shown in Figure 2.

Figure 2. Various FBG-based indirect sensors include FBG-based cable-load (a), soil-load (b), and liquid-load (c) cells.
Smart optical-fiber-sensor-based structures

Combined with FRP, new kinds of smart FRP optical-fiber-Bragg grating (FRP-OFBG) composite rebar, boards, tubes, and sheets have been developed. The FRP-OFBGs can act simultaneously as strain sensors and reinforcing components, and can detect slips and cracks in reinforced concrete structures (see Figure 3).

Figure 3. Smart optical-fiber-sensor-based structures. (a) Rebar, (b) cable, and (c) FRP anchors.

Applications for infrastructure monitoring

With financial support for practical projects, the Harbin Institute of Technology has installed optical-fiber sensors in more than 30 structures as case studies for long-term SHM. These test cases include FBG sensors applied to the Hulan River and Niutou Shan bridges in Heilongjiang province (China), as well as the Yonghe River bridge in Tianjin, Binzhou (see Figure 4). Fiber sensors have also been installed in the Dongying Yellow River bridges in Shangdong province, the Songhua River bridge, the third Nanjing Yangtze River bridge, the Maocaojie Bridge in Hunan province, the Erbian bridge in Sichuan, and the Guangyangdao Bridge in Chongqing. In addition to bridges, SHM systems have been installed in the Olympic swimming center in Beijing, the CB32A offshore platform, and along an oil pipeline. Several optical-fiber sensors have served for more than two years—some even longer than six years—and have provided good data to support long-term SHM systems. One case study is the Binzhou Yellow River bridge. Located in the city of Binzhou, Shandong province, this is a cable-stayed bridge with three towers, with spans measuring 84, 300, 300, and 84m, respectively. In 2002, 138 FBG sensors were installed on the bridge during construction. Since then, they have operated as part of a real-time monitoring system consisting of an FBG interrogator, an optical switch, and a coupler (see Figure 4).

All of our case-study results show that optical-fiber sensors, especially optical-fiber Bragg-grating sensors, are good enough for practical systems to monitor the long-term health of infrastructures, even in harsh environments.

We plan to promote the use of durable sensors, including optical-fiber sensors, for long-term monitoring of both newly built and old structures. We will also study the evolution of damage and evaluate the safety of the infrastructures that currently include sensors.

Figure 4. Monitoring the Binzhou River bridge.

Jinping Ou
School of Civil Engineering and Engineering Mechanics
Dalian University of Technology
Dalian, China
School of Civil Engineering
Harbin Institute of Technology
Harbin, China
Zhi Zhou
School of Civil Engineering
Harbin Institute of Technology
Harbin, China

Zhi Zhou received his BA in materials science and MS in solid mechanics from the Central South University, China, in 1996 and 1999, respectively. In 2003 he was awarded a PhD in engineering mechanics by the Harbin Institute of Technology, where he was appointed as assistant and subsequently associate professor in 2003 and 2005, respectively. His research interests include optical-fiber sensors, smart materials and structures, and local damage analysis based on smart sensors for SHM.