Methods for Structural Health Monitoring based on Distributed Fiber Optic Sensor

Many countries around the world are facing serious problems with the aging of infrastructures. Besides the degradation process that is usually associated with aging, it is also important to consider the design and construction standards that may have led to deterioration of the built environment. Sensor-based Structural Health Monitoring (SHM) has been identified as one of the most important approaches to ensure structural safety, and the resilience of civil constructions. Brillouin scattering-based distributed fiber optic sensors have been distinguished as a great tool for SHM. Their success in sensor application is due to many advantages when compared with their electrical counterparts, including immunity to electrical and magnetic interference, geometrical adaptability, serving both as the sensor and the lead line, and others. Accurate interpretation of substrate strains from the optical measurements may require direct determination of elastic properties of optical fibers from tension tests. Therefore, stress-strain response of optical fibers in direct tension has been investigated in research presented herein. The study involved direct tension tests on two types of optical fibers, namely ribbon and standard single mode fiber (SMF-28). By employing the experimental data and by using the generalized Ramberg-Osgood law, the theoretical stress-strain responses of the optical fibers were established. The impetus for the study was the need for the elastic properties of the optical fibers, such as the modulus of elasticity and the elastic limit for accurate interpretation of strains measured by optical fiber sensors. The mechanical characteristics, and the stress-strain responses of the optical fibers serve as important parameters in numerical modeling, theoretical analysis, and accurate interpretation of sensed strains. For example, interpretation of strain transfer mechanisms, as well as quantification of structural cracks require information about the mechanical characteristics of 2 the optical fibers. Practical implementation of distributed sensor systems in SHM requires monitoring strain distributions, detection and growth of damage, changes in structural stiffness, and others. Hence, there is a need for distributed optical fiber sensors to extend their capabilities beyond strain measurements, especially in terms of the quantification of cracks. Hence, a method for conversion of the distributed strain measurements to crack opening displacements in structural elements has been investigating in this research. A hybrid approach involving theoretical and experimental strain distribution at the crack location was employed to formulate the conversion equation. The viability of the method was accomplished by experiments involving a 15-meter steel beam with prefabricated flaws. FBG-based displacement sensors were also used for direct measurement of crack opening displacements at the locations of the flaws and comparison with the theoretical calculations. Finally, field tests were carried to test the feasiability of the distributed monitoring of operating structures. A bridge monitoring approach was investigated for distributed detection and quantification of crack opening displacements along the lengths of large operating bridges. FBG-crack sensors were also employed for verification of the COD measurements based on direct acquisition of distributed strains. At the same time, a method for the out-put only modal analysis of structures based on distributed strains was proposed. The BOTDA modal shapes were first computed through laboratory experiments on a three-span beam and compared with canonic accelerometers results. The procedure was then applied on an opterating five-span bridge.