The condition of concrete structure can be effectively assessed through crack monitoring. Since the location of cracks in a concrete structure is unknown in a-priori, conventional 'point' sensors (e.g., strain gauges) are not effective in the sensing of cracks. This work focuses on a novel distributed optical fiber sensor that can detect and monitor cracks without any prior knowledge of their locations. The sensing principle is based on crack-induced bending loss of an optical fiber coupled to the concrete structure, oriented at an angle to the potential crack plane. By monitoring the loss in backscattered signal as a function of time, both the crack location and opening size can be determined. With this sensor, a single fiber can be employed to detect and monitor a number of cracks. To...[ Read more ]

The condition of concrete structure can be effectively assessed through crack monitoring. Since the location of cracks in a concrete structure is unknown in a-priori, conventional 'point' sensors (e.g., strain gauges) are not effective in the sensing of cracks. This work focuses on a novel distributed optical fiber sensor that can detect and monitor cracks without any prior knowledge of their locations. The sensing principle is based on crack-induced bending loss of an optical fiber coupled to the concrete structure, oriented at an angle to the potential crack plane. By monitoring the loss in backscattered signal as a function of time, both the crack location and opening size can be determined. With this sensor, a single fiber can be employed to detect and monitor a number of cracks. To provide sufficient sensitivity at small crack openings and to maximize the number of cracks that can be monitored by a single fiber (through limiting the maximum loss at each crack), the optical loss versus crack opening relation needs to be properly 'designed'. In this thesis, a theoretical model considering both mechanical and optical behavior of the fiber sensor has been developed. In the mechanical part, large deformation of the fiber and the effect of possible axial force are considered. In the optical analysis, the photoelastic effect on optical properties is incorporated. To perform experiments, a sensor appropriate for external bonding and internal embedment is designed and fabricated. The optical loss versus crack opening relation for the sensor is found to be in good agreement with model results. Simulations are then performed to illustrate the generation of sensor design guidelines with the theoretical model.

In this work, an extensive experimental program has also been performed to verify the viability of the sensor for multiple crack detection and its applicability under quasi-static loading, cyclic loading as well as cracking under restrained dimensional change. A preliminary study on the feasibility of crack monitoring under mixed-mode condition has also been conducted.

In summary, the present work improves the understanding of the behavior of a novel distributed optical crack sensor, and verifies its sensing capability under various situations. The potential of the sensor for practical applications is hence demonstrated.

KEYWORDS: Cracks, fiber optic sensing, monitoring, non-destructive evaluation