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OVERVIEW OF FIBER OPTIC SENSORS FOR NDE APPLICATIONS Thomas Esselman1, Paul Bruck1, Alan Turner2, James Wall3, Sontra Yim1, Bahaa Elaidi1

1Lucius Pitkin Inc., 304 Hudson Street, New York, NY 10013-1015, tesselman@luciuspitkin.com, pbruck@luciuspitkin.com, syim@luciuspitkin.com,

belaidi@luciuspitkin.com, 2Micron Optics Inc., 1852 Century Place, Atlanta, GA 30345, aturner@micronoptics.com,

3Electric Power Research Institute, 1300 West W. T. Harris Blvd., Charlotte, NC 28262, jwall@epri.com

ABSTRACT Over the last few years, optical fiber sensors have seen an increased acceptance as well as a widespread use for structural sensing and monitoring in civil engineering, aerospace, marine, oil & gas, composites and smart structure applications. Optical fiber sensor operation and instrumentation have become well understood and developed. Fiber sensors are attractive sensing devices for non-destructive examination (NDE) applications given their small size, lightweight and dielectric glass construction that renders them immune to electrical noise and Electro Magnetic interference—unlike many conventional electronic sensing systems.

To date, fiber sensors have been widely adopted in the area of NDE in civil structures, especially bridges and buildings. This adoption has allowed for the ruggedization of the sensors for survival in hostile environments and the rigors of installation. Surface mounted and concrete embedded strain and temperature sensors allow for the on-line monitoring, in real time, of changes due to daily usage or accidents due to collisions or fire in or on the structure.

In this paper, we will review the operating principles, sensor types, benefits, and applications of optical fiber sensors for NDE as it relates to the structural integrity monitoring of post-tensioned tendons in a nuclear containment structure. Data will be presented that shows the changes in strains measured during a Structural Integrity Test that was performed during a shut down. The gages will be used for long-term monitoring of the containment. Optical Fiber Bragg Grating Sensors Fiber Bragg Gratings (FBGs) have—over the last few years—been used extensively in the telecommunication industry for dense wavelength division de-multiplexing, dispersion compensation, laser stabilization, and erbium amplifier gain flattening, all at 1550 nanometer (nm). In addition, FBGs have been studied for a wide variety of mechanical sensing applications [2-7] including monitoring of civil structures (highways, bridges, buildings, dams, etc.), smart manufacturing and non-destructive testing (composites, laminates, etc.), remote sensing (oil wells, power cables, pipelines, space stations, etc.), smart structures (airplane wings, ship hulls, buildings, sports equipment, etc.), as well as traditional strain, pressure and temperature sensing. The main advantage of FBGs for mechanical sensing is that these devices perform a direct transformation of the sensed parameter to optical wavelength, independent of light levels, connector or fiber losses, or other FBGs at different wavelengths.

The advantages of FBGs over resistive foil strain gages include: • Totally passive (no resistive heating) • Small size (can be embedded or laminated) • Narrowband with wide wavelength operating range (can be highly multiplexed) • Non-conductive (immune to electromagnetic interference) • Environmentally more stable (glass compared to copper) • Low fiber loss at 1550 nm (for remote sensing) • Potential for very low cost due to device simplicity and high volume telecommunication usage

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Optical Fiber Structure and Characteristics

At the heart of this technology is the optical fiber itself. A hair-thin cylindrical filament made of glass that

is able to guide light through itself by confining it within regions having different optical indices of

refraction. A typical fiber structure is depicted in Fig. 1. The central portion of the fiber, where most of

the light travels, is called the core. Surrounding the core there is a region having a lower index of

refraction, called the cladding. From a simple point of view, light trapped inside the core travels along the

fiber by bouncing off the interfaces with the cladding, due to the effect of the total internal reflection

occurring at these boundaries. The optical energy propagates along the fiber in the form of waveguide

modes that satisfy Maxwell’s equations as well as the boundary conditions and the external perturbations

present at the fiber.

Figure 1. Schematic of a Single Mode Optical Fiber (125 µ is 125 micrometers, approximately 0.005

inches) Fiber Bragg Grating Operating Principles

A Fiber Bragg Grating is a wavelength-dependent filter/reflector formed by introducing a periodic

refractive index structure, with spacing on the order of a wavelength of light, within the core of an optical

fiber. Whenever a broad-spectrum light beam impinges on the grating, a portion of its energy is

transmitted through, and another reflected off as depicted in Fig. 2.

Figure 2. Transmission and Reflection Spectra of a Fiber Bragg Grating.

Coating

Cladding

Core

125µ

n2

n1

n = index of refraction

n1> n

2

Refracted Light

Reflected

Light

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The reflected light signal will be very narrow (few nm) and will be centered at the Bragg

wavelength (�b) which corresponds to twice the periodic unit spacing Λ. This is the so-called Bragg

condition and is expressed as:

λλλλb = 2ΛΛΛΛnm [1]

Where Λ is the grating’s period and nm is the average index of refraction seen by the propagating

light wave inside the fiber’s core. Any change in the modal index or grating pitch of the fiber caused by

strain, temperature or polarization changes, will result in a Bragg wavelength shift. In general, the

temperature sensitivity of a grating occurs principally as a result of the temperature dependence of the

refractive index in the fiber material and, to a lesser extent, the thermal expansion in the material which

changes the grating period spacing. Typically, the fractional wavelength change in the peak Bragg

wavelength is of the order of 10pm/°C at 1550nm. It is important to note that for accurate strain

measurements the temperature effects must be subtracted from the strain measurement. This is known as

temperature compensation and is achieved by locating a temperature compensation gage in close

proximity to the strain gage.

Figure 3 illustrates the ability to put multiple FBGs along a single fiber for many sensors on one

fiber optic sensor array. This allows for monitoring multiple parameters at multiple locations, such as

temperature and strain, along one fiber. A sensor interrogator measures the FBG sensors’ response,

specifically detecting FBG wavelength shifts with high resolution, accuracy, and speed. The versatile

instruments can interrogate simultaneously, hundreds of sensors at kHz speeds, with 2 picometer (pm)

wavelength stability and sub-pm resolution and repeatability.

Figure 3. Each grating sensor is at a different fiber location and allocated a separate central

wavelength and spectral operating window band.

Nuclear Application

An augmented inspection of a containment building at the R. E. Ginna Nuclear Plant (Ginna) was

sponsored by the Department of Energy (DOE), the Electric Power Research Institute (EPRI), and

Constellation Energy Nuclear Group (CENG). The objective of the augmented inspection was to

supplement the industry’s understanding of the potential for long term operation of the concrete

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containment structure.

The containment structure at Ginna has cylindrical concrete walls that are post-tensioned with 160

vertical tendons within the wall of the containment. A spherical dome caps the cylindrical wall to form

the containment. The tendons are equally spaced circumferentially around the containment. These

tendons are incorporated into the containment design in order to keep the cylindrical containment walls in

the vertical axis in compression during internal pressurization. At Ginna, unlike some other post-

tensioned containment designs, the spherical dome is not post-tensioned, and the cylindrical wall relies on

traditional concrete steel reinforcement bars for hoop direction tensile strength. Each tendon consists of a

bundle of ninety (90) ¼” diameter wires housed in a grease filled sleeve. Each tendon is coupled to a rock

anchor at the bottom of the tendon and is capped at the top with an anchor head which engages a

buttonhead on each wire. The tendons are thus accessible only from the top of the containment.

The topmost end of each tendon (approximately 1.5 feet) protrudes from the top of the containment

wall and is protected from the environment by a tendon vent can (see Figure 4a). A photograph with the

can removed, showing the top tendon anchor is provided in Figure 4b. The anchor head, shown in Figure

4b, provides the bearing surface for the buttonheads of all the wires and enables the tendon to be stressed.

The tendons are tensioned to a load of over 650,000 pounds to allow for time dependent losses.

Tensioning or stressing of the tendons is accomplished by engaging and lifting the anchor head using a

hydraulic tendon pulling device. Once the proper tension is obtained, one large main shim and a series of

smaller shims (as necessary) are placed under the anchor head to retain the compressive load on the

containment wall.

Each tendon main shim is then under constant compression. If that compressive load changes, it

would indicate that the tendon is losing load which could be due to tendon relaxation, wire breakage or

local concrete failure under the anchorage. Fiber optic strain gages were placed on the shim in order to

monitor for changes in tendon load.

To supplement the strain gages on the tendon shims, an exposed hoop direction reinforcing bar was

also monitored together with an adjacent section of the cylindrical wall concrete, also in the hoop

direction.

Figure 4 (a) and (b): Top of Containment Wall and Tendon Top with Can Removed

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Sensor Types Utilized on the R. E. Ginna Pilot Project

The illustrations below show the sensors utilized on the post-tension tendon shims, on the rebar in the

concrete oriented in the hoop direction and on the concrete surface of the containment structure wall.

Figure 5a illustrates the application of novel commercial FBG strain sensor design which mimics the

characteristics of conventional foil strain gages, but based on the use of optical fibers. The FBG sensing

element is pre-stretched and mounted on a protective metallic carrier flexure. The strain sensor (Fig. 5a)

can be surface mounted to test specimens of interest using epoxy bonding or spot welding techniques.

This, in practice, becomes the optical fiber equivalent of a conventional foil strain gage sensor. Figure 5b

illustrates a temperature compensation sensor packaged to isolate the FBG from the strain field, exposing

it only to the temperature effects. Figure 6a illustrates a long gage strain gage, surface mounted to the

concrete next to the rebar strain gage. Figure 6b illustrates a temperature sensor with a calibrated output

for measuring absolute temperature of the environment.

Figure 5 (a) and (b). Photograph of a commercial fiber Bragg grating strain sensor model os3210

and a temperature compensation sensor model os4100. Fiber is pre-mounted on a metallic weldable

carrier and is mounted to the shim and rebar (photo courtesy of Micron Optics Inc.)

Figure 6 (a) and (b). Photograph of a commercial fiber Bragg grating long gage (24 cm) strain

sensor model number os3600 and absolute temperature sensor model number os4310 (photo courtesy of

Micron Optics Inc.).

Of the 160 tendons, a group of 20 tendons were selected to be instrumented. This was determined

to be a statistically significant sample. A five sensor array was installed on each shim. One strain and one

temperature compensation is on each half of the shim. These were OS3110 and OS4100 gages and were

spot welded to the shim via a DC spot welder specifically designed for strain gage welding. An absolute

temperature gage is attached to the shim with epoxy. The gage array is protected by a layer of non-

adhesive Kapton sheet, Kapton adhesive tape, aluminum foil tape and an epoxy cover. An installed strain

gage array is shown in Figure 7.

After installation of the fiber optic strain gages, each location was calibrated to the load measured

during liftoff testing. Tendon liftoff testing was accomplished by engaging the tendon anchor head with a

hydraulic ram and lifting the anchor head off the shim.

Strain gages were also applied to a section of previously exposed hoop rebar and to the concrete

near the rebar.

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Figure 7: Strain Gage Array Installed and Protected

Calibration of the Tendon Gages

Subsequent to the installation of the tendon gages and the data acquisition system, load lifting of the

instrumented tendons was performed. This allowed for accurate measurement of the tendon forces and

resetting of the fiber strain gages.

Containment Structural Integrity Test

Data is being recorded starting from the completion of installation in April 2011 and continuing to the

present. This included a Structural Integrity Test (SIT) when the containment was pressurized to 59.8 psig.

The data for one tendon (tendon 83) is presented in Figure 8 for the period of time just prior to, during,

and following the SIT.

The data presented in Figure 8 (and other data that has been reviewed) shows that the tendon load is

not greatly affected by outside ambient and shim temperature fluctuations, as the lines representing tendon

load are relatively flat (with exception of during the SIT). The ambient temperature varied from a low of

approximately 54°F to a high of approximately 88°F. The tendon tension during the SIT increased which

is inferred from a change in compression in the shim. The tendon tension increased from approximately

687 kips to approximately 698 kips. This change was consistent with finite element analysis that was

performed on the containment.

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Figure 8: Tendon 83 Data during the Structural Integrity Test, Shown on this figure is the temperature and

pressure inside the containment (“Temp-Cont” and “Pressure-Cont”), the temperature on tendon 83

(“T83B_ATG-8313037”), the ambient temperature taken at a location away from the tendons and out of

the sunlight (HRC_ATG), and the shim compression in tendon 83 (strain converted into load in kips on

the right hand axis (“T83(+)”). This is plotted versus days on the x-axis.

SUMMARY

Fiber optic gages were selected for the application for the augmented inspection due to their expected

stable performance over long periods of time, small size, accuracy, and environmental stability. It is

intended that they will be monitored continuously for years to determine the real-time performance of the

tendons during weather extremes and over time.

This monitoring provides a “real-time” view of the performance of the tendons in the containment.

This will provide a future ability to detect multiple wire breaks, wire relaxation, local concrete failure

under tendon anchorage or loss of a rock anchor.

REFERENCES

1. Jose Miguel Lopez-Higuera, Handbook of Optical Fibre Sensing Technology, John Wiley & Sons

Inc., 2002. ISBN 0-47182-053-9. 828 pp.

2. Raymond M. Measures, Structural Monitoring with Fiber Optic Technology, Academic Press, 2001.

ISBN 0-12-487430-4. 716 pp.

3. Eric Udd, Fiber Optic Smart Structures, John Wiley & Sons Inc., 1995. ISBN 0-471-55448-0 671

4. Farhad Ansari and Stein Sture, Nondestructive Testing of Concrete Elements and Structures,

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American Society of Civil Engineers, 1992.

5. Farhad Ansari, Applications of Fiber Optic Sensors in Engineering Mechanics, American Society of

Civil Engineers, 1993. ISBN 0-87262-895-7. 230 pp.

6. Farhad Ansari, Fiber Optic Sensors for Construction Materials and Bridges, Technomic Publishing

Co., 1998. ISBN 1-56676-671-0. 267 pp.

7. Brain Culshaw, Smart Structures and Materials, Artech House, 1996. ISBN 0-89006-681-7. 207 pp.