The problem of an arbitrarily varying axial strain field's
transfer from material host to embedded fiber-optic sensor is
studied. A derivation is proposed by which the axial in-fiber
strain field is predicted given an arbitrary, axially varying,
strain field at some distance in the material host. The
spatial fields are considered in terms of spatial wavelength content
and a spatial wavelength-dependent transfer function is outlined, to
assist in the solution of the transfer problem. Results from the
proposed approach are plotted against conventional finite-element
results for the same physical problem.
For an optical fiber sensor capable of distributed measurements, the elastic transfer between a substrate and the sensor itself is investigated. The degree to which a strain gradient in the optical fiber sensor reflects an existing strain gradient in the substrate is analysed for both the embedded and surface bonded optical fiber installations. The material and geometric parameters on which the strain transfer depends are described and a method by which the quality of strain transfer can be qualitatively assessed is proposed. Results include charts which may be used to predict the expected quality of gradient transfer.
This paper presents an overview of the development and application of ISIS fibre optic sensor (FOS) technology by the University of Toronto Institute for Aerospace Studies and Department of Electrical and Computer Engineering. The primary focus of this technology has involved the use of fibre Bragg gratings (FBGs) to measure strain and temperature in concrete structures and fibre reinforced plastic (FRP) overwraps applied to concrete structures. A brief review of existing fibre optic sensor configurations and the advantages of using FOS compared to other strain sensors is first presented. Subsequently, the development of new sensor concepts such as a long gauge of arbitrary length, a distributed gauge for measuring local strain gradients, and multiple FBGs on a single fibre optic cable are discussed, with examples of their application to civil engineering structures. In addition, the specialized instruments under development that are essential for obtaining strain information from these sensors are also described. Finally, the issue of wireless remote monitoring of FOS systems is addressed.
A new method of distributed in-fiber Bragg grating sensing is proposed. A method is outlined by which the strain field imposed upon a Bragg grating sensor is obtained by measurement of the sensor's reflectivity and delay characteristics. The proposal is demonstrated by interrogation of two loaded samples. Strain distributions from these experiments are compared against a theoretical estimate. The data treatment is also discussed. Strain resolution is found to be +/-24 microepsilon, with a spatial resolution defined by a minimum spatial wavelength component of the coupling distribution of 1.65 mm.
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