Characterization of embedded optical fiber Bragg grating sensors

Fiber reinforced structural elements equipped with optical fiber sensors are of potential interest for long term surveillance of bridges, dams, and other civil engineering applications. We report herein on the embedding of optical fiber Bragg grating sensors into continuously pultruded glass fiber reinforced epoxy material (GFRP) rods used as structural elements. Two fibers, one with acrylic, one with polyimide coating, and fiber Bragg gratings were extensively characterized. The residual axial strain after embedding was measured. Temperature and strain coefficients of Bragg gratings during quasi-static cycling and temperature cycling tests were monitored and compared to extensometric and resistance strain gauge measurements. The effect of fiber embedding was also studied with push-out tests giving information about adhesion properties like surface bond energy, lateral stress, and coefficient of friction. For sensor purposes long term characteristics of the conversion factor between the measured Bragg grating wavelength and strain or temperature in the specimen have to be known to identify best-suited coatings for embedding and protecting fiber sensors and for obtaining reliability data on sensor performance.