Abstract
The aim of this paper is to investigate the ability to measure simultaneously the axial and radial strain with a single optical fiber sensor. The discussion begins with the analytical study of a cylinder subjected to pure tensile strain and thermal load. We emphasize the necessity of measuring the radial strain in order to determine the axial strain of an embedded sensor with accuracy. Then, we describe a few sensors able to measure axial and radial strains and study their efficiency. The conclusion is that the best structure is made of the juxtaposition of a short and a long period grating.
Highlights
Fiber Bragg gratings (FBGs) are written in optical fibers by UV irradiation
Conclusion about the shift in Bragg wavelength analysis in response to a mechanical load. These few examples show the necessity of considering a model of coupled strains between the FBG and the host material in order to measure the strain in the case of embedded sensors
It is essential to make two measurements, which is not possible using classical FBG sensors and their associated interrogation devices. Does this mean that the employment of the FBG strain sensor has to be restricted to surface measurement? We do not think so
Summary
Fiber Bragg gratings (FBGs) are written in optical fibers by UV irradiation. An interferometric device creates fringes inside the core of the fiber. Any external constraint that modifies the effective index or the period of modulation induces a shift in the Bragg wavelength: λB neff λB = neff + The measurement of this shift can lead to the applied load if the sensor has been previously calibrated or if an appropriate model is available. The analysis of data obtained with such surface sensors is well known and provides reliable information on the strain of the studied structure following the direction of the sensor axis (see [6]) Because of their small size, the fiber-optic sensors are embedded in mechanical components, such as resin parts (polyester or epoxy) to perform structural health monitoring or production process monitoring [2, 7, 8].
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