Development of Bragg grating sensors at CEFET-PR

Existing technology used to measure stress in granular materials is susceptible to water ingress and resulting damage to the electrical components, which limits this technology's use in long-term monitoring of soil structures. The connections of these instruments to readout or recording devices are also fragile and easily damaged. In order to explore the efficacy of fibre optic technology not just as a replacement component, but rather as a material of robust elastic properties, a series of experiments was devised to test the possibility of applying a transverse lateral stress to the fibre and measuring its transformed longitudinal strain. The fibre was inscribed with a 5 mm long Bragg grating and encapsulated in a softer material of high Poisson's ratio in an attempt to enhance the longitudinal strain developed in the fibre. This encapsulated device was then subjected to one-dimensional stress in a standard sand, and the strain in the fibre measured. The experiment showed that satisfactory correlation exists between the measured strain output when converted to an applied stress and the mathematically (and numerically) derived stresses. The fibre/silicone bonding was also modelled and results showed that slippage on that interface could be considered insignificant. In both theoretical and practical applications, this experiment can be regarded as successfully validating the principle of deriving stress from a longitudinal elastic strain measured normal to the applied stress. Hence, development can move towards both miniaturization (for research) and more robust construction (to withstand field conditions). Further research will encompass investigating the response of the cell to water and saturated soil conditions, particularly the device sensitivity to transient stress conditions. In addition, development of the sensor to read the complete three-dimensional state of stress in a soil remains the ultimate goal.

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“…Two general texts have been listed in the bibliography: DaSilva et al (2003) andOthonos and Kalli (1999) yielded excellent overview information.…”

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confidence: 99%

A fibre Bragg grating stress cell for geotechnical engineering applications

Legge

Swart

Zyl

et al. 2006

Meas. Sci. Technol.

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“…1. Gratings 1 and 2 were fabricated at CEFET-PR [5] and have Bragg wavelengths at 1536.88 nm and 1536.48 nm, respectively. Both gratings show 12 dB rejection and 0.5 dB bandwidth.…”

Section: A Experimentsmentioning

confidence: 99%