Elasto-Plastic Bonding of Embedded Optical Fiber Sensors in Concrete

Fiberoptic sensors are increasingly employed for sensing and measurement of strains in structural materials. The glass core of the optical fiber senses the strain through intensity fluctuations, interference, or frequency modulation. Brittleness of the glass core limits practical usage, and therefore, the glass core of optical fibers is coated with low modulus softer protective coatings. The protective coating alters the strain transduction capabilities of the sensor. It absorbs a portion of the strain, and hence only a segment of structural strain is sensed. The study reported here corrects for this error through development of a theoretical model to account for the loss of strain in the protective coating of the optical fiber. The model considers the coating as an elasto-plastic material and formulates strain transfer coefficients for elastic, elasto-plastic, and plastic phases of coating deformation. The theoretical findings were verified through laboratory experimentation. The experimental program involved fabrication of interferometric optical fiber sensors, embedment within mortar samples, and tensile tests in a closed-loop servo-hydraulic testing machine. The elasto-plastic strain transfer coefficients developed in this study were employed for correction of optical fiber sensor data and results were compared with conventional extensometers.