OPTOELECTRONIC APPLICATIONS: FIBEROPTIC SENSING - Fiber sensors lay groundwork for structural health monitoring

In communities across China, fiber Bragg gratings and high-power, swept-laser sensor interrogators are being deployed not just in optical-communications networks but in engineering projects meant to expand and improve the country's physical infrastructure. The Songhuajiang River Bridge in Heilongjiang Province, the Dongying Yellow River Bridge in Shandong Province, the Maocaojie Bridge in Hunan Province are all being outfitted with fiberoptic sensing systems that will monitor physical and chemical changes that can affect structural safety. And these projects are not unique; throughout Europe, the United States, and Asia, fiber-sensor networks are improving the ability to monitor bridges, buildings, oil rigs, power lines, dams, tunnels, even hillsides to monitor stress, performance, and natural degradation over time or following a catastrophic event such as an earthquake or flood (see Fig. 1).

Electrical sensors designed to assess structural integrity have been around for many years, particularly in the aeronautics industry, but optical-sensing technologies have only come into play in civil engineering in the last decade. While not always the best choice for every application, fiber sensors can perform the functions of virtually any conventional sensor—often faster and with greater sensitivity—and they can perform measurement tasks that would be impractical with conventional sensors. For example, the traditional advantages of optical-fiber sensors—immunity to electromagnetic and radio-frequency interference and inability to create sparks in potentially explosive environments—also apply to structural monitoring. In addition, fiberoptic sensors can function under adverse conditions of temperature and pressure, and toxic or corrosive atmospheres that can rapidly erode metals have little effect on optical fibers. They are therefore useful as sensing devices for a wide range of physical and chemical phenomena, including temperature, pressure, acoustic field, position, rotation, electrical current, liquid level, biochemical composition, and chemical concentration.

"There are great technologies that have existed for decades that measure things very well, but where ­conditions are harsh or systems have to last for decades, or the size of the room or structure is small or the distances are long . . . fiber sensors can be the right technology," said Tom Graver, director of the optical-sensing business at Micron Optics (Atlanta, GA). "You can do computer models all day long and still not have a good understanding of the conditions. For example, a bridge designed for a horse and buggy might now be used by semi trucks. How is it handling the increased loads? There are not many tools available to measure this."

One reason for this popularity is that FBG sensors (typically centered around 1550 nm) can measure physical changes such as multiaxis and transverse strain, vibration, and displacement, as well as organic elements such as moisture and ice. In addition, because the temperature and strain states of FBGs directly affect their reflectivity spectrum, they can provide structural engineers with measurements not previously possible, including detecting changes in stress in buildings, bridges, and airplane bodies; depth measurements in streams, rivers, and reservoirs for flood control; and temperature and pressure measurements in deep oil wells. They also offer excellent resolution and range, absolute measurement, and modest cost per channel. Furthermore, because FBG sensors are passive, they can be either time- or wavelength-multiplexed, which allows for distributed sensing—a key advantage for structural health monitoring.

"Because fiber Bragg gratings can be multiplexed and are getting less and less expensive all the time, they are an easy sensor engine to work with," Graver said. "What we are doing now is building transducer packages around the sensor that are easy to use for applications such as strain or temperature measurements—packages that can be poured into concrete or embedded in a polycomposite control surface for an airplane."

Hard at work

The physical principle behind the FBG sensor—that a change in strain, stress, or temperature will alter the center of the wavelength of the light reflected from the FBG—also makes it well suited to these applications. Fiber Bragg gratings are made using an ultraviolet laser to "write" a grating pattern into the core of the fiber, transforming the length of fiber into an optical filter with a specific bandpass. This index grating reflects a narrow spectrum that is directly proportional to the period of the index modulation and the effective index of refraction. The wavelength at which the reflectivity peaks is called the Bragg wavelength. Temperature and strain directly affect the period of the index modulation as well as the effective index of refraction. Thus, any change in temperature and strain directly affects the Bragg wavelength. To measure wavelength shifts that result directly from changes in temperature or tension, FBG sensor systems must include an optical source that continuously interrogates the reflection spectrum, and a detection module that records the shifts in the peak reflectivity versus wavelength.

The simplest FBG sensor system combines a broadband light source, such as an amplified-spontaneous-emission (ASE) white light, with a tunable filter and a detector. Because detectors are wavelength-insensitive, the tunable wavelength filter is required to scan the wavelength range of the FBG sensors (typically 40 nm) to determine the sensors' Bragg wavelength. However, because the output power from an ASE white-light source is low, only a limited number of in-line gratings can be measured and with a limited dynamic range. Moreover, an external wavelength filter is required, which limits the accuracy and scan frequency of these systems. Thus, some sensing systems use fiber lasers, diode lasers, or other tunable lasers as the optical source.

How and where a fiber-sensing system is installed, and how many sensors it will require, depends on the structure or material being monitored and what the system is intended to measure. Micron Optics, for example, has been working with international engineering firms such as Smartec (Manno, Switzerland) and Systems Planning and Analysis (SPA; Alexandria, VA) on several bridge projects worldwide, from historic structures in Russia, Greece, and Australia, to new installations in the United States and China.

Other organizations are acting as their own integrators. In Des Moines, IA, for instance, a highway bridge constructed using high-performance steel girders is the first such bridge of its kind in the state, posing a challenge for the Iowa Department of Transportation (IDT) in terms of how to judge wear and tear on the bridge. As part of this project, the Bridge Engineering Center at Iowa State University collaborated with the IDT to install a structural health monitoring system comprising a Micron Optics Interrogator (which utilizes a low-noise swept-laser source and a Fabry-Perot tunable filter), 30 FBG sensors (embedded in thin composite patches that are attached to the structure with an epoxy resin adhesive), video equipment, networking components, and three computers for Web service, data collection, and data storage. The system collects strain information at critical bridge locations, uploads the strain data to the Internet where it can be viewed from anywhere in the world in real time, and automatically transfers the data to the Bridge Engineering Center at Iowa State University for analysis.¹

In similar studies in California, researchers at the University of California at Los Angeles (UCLA) are working with the California Department of Transportation (CALTRANS) to monitor how lateral loads are transferred from soils to reinforced concrete structures in an earthquake. Soils are traditionally modeled as a network of individual noncoupled springs. The UCLA researchers decided to use fiberoptic sensors because they combine accuracy with a large strain range (6%). Although they initially found FBG sensors that were strong enough to withstand the huge elongation, they were unable to devise a way to secure the sensors well enough to survive through the entire range. Later they switched from acrylate-coated FBGs to polyimide-coated FBGs to enable firmer attachment to anchors and provide a range of 2% strain. The FBG sensors and linear variable displacement transducers were embedded in the concrete (independent of the reinforcing bar) to measure the strain in the concrete even after the rebar begins to slip. The sensors were placed in a special package designed by Smartec and suspended in position while the concrete was poured around them.

Using fiber sensors to assess changes in historic structures can pose unique challenges in terms of how the sensors are attached or embedded. In a long-term structural health monitoring project in Australia, for instance, multiplexed fiber sensors were surface-mounted along the span of the Hampden Suspension Bridge (Kangaroo Valley, New South Wales), originally built in 1898, to monitor strain from increased traffic loads. But a project just getting under way in England to assess moisture levels in 200-year-old limestone walls around Worcester College (Oxford) requires that small holes be drilled into the wall and the sensors placed into those holes.

The goal of the Worcester College project, which began in January, is to monitor how limestone is affected by traffic pollution, road salt, temperature, humidity, and wetness and to detect subtle changes in the stone caused by changing moisture levels and salt movement. The ­fiberoptic-based humidity-sensing system uses FBGs coated with moisture-sensitive polymer; the sensing concept exploits the inherent characteristics of the FBG and is based on the strain effect induced in the FBG through the swelling of the polymer coating (see Fig. 2). The same minimally invasive approach can be used to monitor moisture and related effects in other kinds of structures as well, according to Grattan.

"We have also looked at corrosion monitoring in buildings where water gets in and creates rust, which weakens the reinforcements," he said. "What we are trying to do with this project is stretch the style and capability of these sensors to measure not just moisture or temperature but also some of the chemical constituents driven in by the water, such as pH and sulfites, which also contribute to the degradation of the limestone."

REFERENCE

1. www.ctre.iastate.edu/bec/structural_health/hps/index.htm