A. Tino Alavie

Bragg grating structural sensing system for bridge monitoring

Corrosion of steel within bridges and other large concrete structures has become a serious problem and consequently there is growing interest in replacing the steel within such structures with carbon fiber based composite materials. The first highway bridge in the world to use carbon fiber composite prestressing tendons was constructed and opened to the public in the fall of 1993. This two span bridge was also unique in another respect, it is the first highway bridge in the world to have been built with a set of fiber optic Bragg grating sensors structurally integrated into several of its precast concrete deck support girders. A four-channel fiber laser sensor demodulation system that was rugged, compact and transportable was developed for this project. This demodulation system monitored the changes in the internal strain on all three types (steel and two types of carbon fiber composite) of prestressing tendons over several months. The same structurally integrated fiber optic sensing system has also been used to measure the change in the internal strain within the deck girders arising from both static and dynamic loading of the bridge with a large truck.

Fiber optic Bragg grating sensor network installed in a concrete road bridge

The installation of a fiber optic Bragg grating strain sensor network in a new road bridge is described. These sensors are attached to prestressing tendons embedded in prefabricated concrete girders. Three types of prestressing tendons are being monitored: conventional steel strand and two types of carbon fibers reinforced plastic tendons. Sensor durability issues are reviewed and the installation is described. Initial measurements indicate that the sensors are operational and provide some early comparison of tendon performance.

Fiber Bragg grating laser sensor

An erbium-doped fiber laser utilizing a broadband mirror as the end reflector and an intracore Bragg grating as the output coupler is designed and developed. This arrangement is used as a laser sensor to improve interrogation efficiency of intracore Bragg gratings over broadband sensor interrogation methods. Wavelength tuning of the fiber laser has been achieved by varying the temperature and strain on the Bragg grating, demonstrating an improved SNR with respect to the previous techniques that use broadband interrogation of the Bragg grating sensor.

Characteristics of fiber grating sensors and their relation to manufacturing techniques

This work examines the effects of different manufacturing techniques of fiber optic intracore Bragg gratings on their sensing and system parameters. Specifically, we have looked at tolerance with respect to center wavelength, strain, and temperature sensitivity. Our test results indicate a slight variation in the strain and temperature sensitivity of different gratings. In an attempt to quantify the effect of transverse loading on fiber gratings, three differently manufactured sensors were diametrically loaded and their spectrums studied. Our experimental results indicate a splitting of the grating spectrum with small loads for an optical fiber grating manufactured in either boron doped fiber or hydrogen loaded fibers. On the other hand, very small transverse sensitivity was observed for a fiber optic grating formed in a bend insensitive fiber suggesting some degree of control on the transverse sensitivity of these sensors.

Bragg grating fiber optic sensing for bridges and other structures

We have demonstrated that fiber optic intracore Bragg grating sensors are able to measure the strain relief experienced over an extended period of time by both steel and carbon composite tendons within the concrete deck support girders of a recently constructed two span highway bridge. This is the first bridge in the world to test the prospects of using carbon fiber composite tendons to replace steel tendons. This unique set of measurements was accomplished with an array of 15 Bragg grating fiber optic sensors that were embedded within the precast concrete girders during their construction. We have also demonstrated that these same sensors can measure the change in the internal strain within the girders associated with both static and dynamic loading of the bridge with a truck. We are now studying the ability of Bragg grating fiber optic sensors to measure strong strain gradients and thereby provide a warning of debonding of any Bragg grating sensor from its host structure...one of the most important failure modes for any fiber optic strain sensor.

Application and characterization of intracore grating sensors in a CFRP prestressed concrete girder

Intracore Bragg grating sensors have been bonded on to the CFRP tendons of a prestressed concrete girder. The sensors survived both the installation procedure and casting of the concrete beam. Sensor performance is characterized in terms of maximum tensile strength and fatigue behavior. The fiber sensors survived strains of greater than 8000 (mu) (epsilon) and showed no change in either center wavelength or spectral content for 2000 (mu) (epsilon) over 320,000 cycles. The intracore grating sensor was used in a static loading test of the girder to failure and showed excellent stability and durability in comparison with the conventional technology.

Multiplexed Bragg grating laser sensors for civil engineering

A portable, rugged, and compact 4-channel Bragg grating fiber laser sensor demodulation system was developed for interrogating an array of 20-intracore Bragg grating sensors that we embedded within several of the concrete girders used to support the deck of a new two span road in Calgary, Alberta. Another unique feature of this bridge is that two types of Carbon fiber based composite prestressing tendons are being tested in a number of the concrete girders. We have instrumented five of the approximately 60 ft long concrete girders with fiber optic Bragg grating strain and temperature sensors in order to monitor the loads on these girders and their response during the construction of the bridge and subsequently over an extended period of time.

Structural sensing using a fiber laser strain sensor

The development of a fiber laser sensor which permits efficient interrogation of Bragg grating sensors is reported. The fiber laser is linewidth-narrowed and tuned by a remotely located, sensing Bragg grating that is surface adhered to a structure under test. The Bragg grating- tuned fiber laser is used in conjunction with a passive wavelength demodulation system (WDS) to form a fiber laser strain sensor system (FLS3), which was used to track both static and dynamic strains on an aluminum beam. The FLS3 could measure strains with a resolution of approximately 4 (mu) (epsilon) and a bandwidth of 13.0 kHz. The viability of the laser strain sensor concept lends itself to the development of a compact, potentially embeddable smart sensor that would output demodulated sensing data directly to the user.

Practical considerations for a four-channel multiplexed Bragg grating fiber sensor system

A multichannel Bragg grating fiber laser demodulation system capable of interrogating four or more Bragg grating sensors for strain and temperature monitoring has been designed and developed. System configuration and various practical considerations for a field deployable system are discussed. Preliminary data indicates a dynamic strain range in excess of 5000 (mu) (epsilon) at a resolution of 1 (mu) (epsilon) . Both the strain dynamic range and resolution are easily modified.

Applicability of fiber optic sensors to the detection of surface acoustic waves on metals

This paper examines the detection of laser generated surface acoustic waves (SAWs) by fiber optic sensors. A Nd:YAG laser is used to illuminate the surface of an aluminum sample in various ways. Elastic waves are launched into the bulk and along the surface of the sample. The emphasis of the work presented is the performance of the fiber optic sensor as compared to theoretical predictions and that of a conventional sensor. By such analysis, the efficiency and limits of the fiber optic sensors for the interrogation of surface acoustic waves are investigated.