Reliability modeling and testing of optical fiber Bragg sensors for strain measurements

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Abstract
Surveillance of facilities and critical structures by monitoring mechanical strength and integrity is necessary for safety in use. Fiber-optic sensors still have poor industrial acceptance due to their lack of demonstrated reliability and long-term stability. Reliability testing for objects with expected life times of 10 to 100 years has to rely on accelerated aging procedures. We report on a series of aging tests of optical fibers and Bragg gratings at elevated temperature, humidity and mechanical stress performed in regard to field applications. Tensile and climate tests were performed with optical fibers embedded in glass fiber reinforced polymers (GFRP) and surface attached to carbon fiber reinforced polymers (CFRP). For surface attached fiber sensors relative humidity was found to be a critical parameter with strong influence on life time. A cable-stayed bridge (Storchenbrucke in Winterthur, CH, still under construction) where for the first time two steel cables are replaced by CFRP cables was equipped with optical-fiber Bragg gratings and standard resistance strain sensors.

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