Monitoring of fatigue crack propagation by means of distributed fiber optic sensing

Distributed Fiber Optic Sensing (DFOS) is an innovative technique for Structural Health Monitoring (SHM). Taking advantage of the fact that DFOS can conveniently measure mechanical strain continuously along an optical fiber, it is increasingly used in monitoring of concrete bridges and tunnels. However, DFOS still needs research in new application areas, such as monitoring of steel bridges. In the present study, DFOS is used to investigate the potential to monitor fatigue crack initiation and propagation by experiments. In a full-scale test, a steel railway bridge was dynamically excited into resonance, generating fatigue-effective vibration amplitudes. The fiber was glued to the flange of the main beam in several loops to cover a larger area for crack detection. The measured strain signal was compared with results obtained from Finite Element Method (FEM) simulations supported by data acquired from conventional strain gauges and extensometers. The strain measurement with DFOS showed excellent agreement with the simulated strain. In this context, additional information about crack initiation, propagation, opening and length can be obtained indirectly from the DFOS measurement. However, when the crack is crossing the fiber, nonlinear effects come into play. To consider the nonlinear effects, a hysteresis model taking steel-fiber interaction into account was applied. The results of the study are presented and the applicability and potential of DFOS for fatigue crack monitoring in railway bridges is discussed.