David G. Bellemore

Determination of cantilever plate shapes using wavelength division multiplexed fiber Bragg grating sensors and a least-squares strain-fitting algorithm

An algorithm has been developed to determine the full deformation field of a cantilever honeycomb plate under arbitrary loading conditions. The algorithm utilizes strain information from a set of sixteen fiber Bragg grating sensors mounted on the plate so that all sensors measure strains along the clamped-free direction. The sensors were interrogated using a wavelength division multiplexing scheme. A two-dimensional polynomial function which represents the strain field was created using a least-squares surface-fitting algorithm. This function was integrated twice with the known boundary conditions applied to yield the deformation field for the plate. Finite-element comparisons were performed to test the accuracy of the strain-displacement algorithm. Single-point loading tests were also experimentally performed to further verify the accuracy of the algorithm.

Hybrid time- and wavelength-division multiplexed fiber Bragg grating sensor array

A nine element fiber Bragg grating sensor array is demonstrated utilizing a combination of wavelength and time division addressing techniques. This system utilized a pulsed broad-band source and a scanning Fabry-Perot element to recover sensor strain information from each of the FBG elements. By combining two multiplexing techniques, the potential number of FBG sensor elements that can be addressed is significantly increased.

High-sensor-count Bragg grating instrumentation system for large-scale structural monitoring applications

We describe an instrumentation system which provides the capability to monitor a large number of Bragg gratings using a common source and a scanning narrowband filter. The system described has the capability to monitor 12 FBG sensors along each of 5 fibers for a total of 60 sensor elements. We demonstrate the use of this system to address multiple sensors embedded in and attached to a quarter scale bridge span.

High-strain monitoring in composite-wrapped concrete cylinders using embedded fiber Bragg grating arrays

We describe the use of first Bragg gratings for monitoring the tensile and compressive strains in a composite wrapped concrete cylinder subjected to load testing. Compressive strains of up to approximately 3.8% were measured using the system.

Design and performance of a fiber Bragg grating distributed strain sensor system

We describe the design and performance of a prototype fiber Bragg grating demodulation system based on the use of a scanning fiber Fabry-Perot filter. The computer driven system is capable of demodulating several arrays of wavelength division multiplexed gratings at various scanning rates for real-time strain display and data logging. The instrument represents a new measurement tool which should be useful in a variety of structural health monitoring applications. Results obtained by the system in several applications are presented and system performance limitations are discussed.

Structural strain mapping using a wavelength/time division addressed fiber Bragg grating array

We describe a multiplexed nine element Bragg grating array used for strain mapping over a plate which is subjected to deflection forces. The system comprises two sub arrays of gratings which are sequentially addressed, and read via a single interrogation system into a PC. Real time strain distributions of the plate are determined and displayed.

Cantilever-plate-deformation monitoring using wavelength-division multiplexed fiber Bragg grating sensors

An algorithm has been developed to determine deformations of a cantilever honeycomb plate under arbitrary loading conditions. The algorithm utilizes strain information from a set of sixteen fiber Bragg grating sensors, mounted on the plate so that all sensors measure strains along the clamped-free direction. The sensors are interrogated using a wavelength-division multiplexing scheme. A two-dimensional bi-polynomial function which represents the strain field is created using a least-squares algorithm. This function is integrated twice with the known boundary conditions applied to yield the deformation field for the plate. Maximum differences between finite-element solutions and least-squares estimates did not exceed 29.0 percent for any of the 16 investigated load scenarios. However, when considering areas of maximum deflection, the least-squares estimates did not exceed 13.3 percent difference. The algorithms used to interrogate the sensors, perform the strain-displacement calculations, and generate a real-time (approximately 4 Hz) mesh of displacement are encoded in a C program.

Damage assessment of a full-scale bridge using an optical fiber monitoring system

A network of distributed optical Bragg grating sensors is used for monitoring of a full scale laboratory bridge in its pristine and damaged state. Damages consist of a series of cuts that are introduced in an external girder to simulate fracture or fatigue crack of a main load carrying bridge component. The after fracture behavior is described in terms of load path redistribution and strain level changes in the structure.

Fiber Optic Sensor System for Bridge Monitoring with Both Static Load and Dynamic Modal Sensing Capabilities

We describe a fiber optic Bragg grating distributed strain sensor system for large scale structural monitoring applications, such as bridge monitoring. The system is capable of assessing both long term static structural loading changes and dynamic/modal behavior of the structure using two different optical interrogation schemes to address the same sensor array. The system has been used to monitor over 45 sensors attached to or embedded in a single-lane bridge span, for damage assessment.

Subcarrier-based path-integrating strain sensor array utilizing fiber Bragg gratings

A fiber optic sensor system capable of detecting the strain distribution in a long length of optical fiber containing a series of fiber Bragg gratings is described. The technique utilizes RF sub-carrier based determination of the optical propagation time between the grating elements, and wavelength division-based addressing of the individual FBGs in the system. The approach will have applications in structural strain monitoring where long gage length sensing is required at multiple locations.