Theoretical and experimental analysis of strain transfer rate in coated fiber Bragg grating strain sensors

Fiber Bragg Grating (FBG) sensors have seen significant development in recent years, especially in the aerospace industry for structural health monitoring due to their versatility and measurement capability. Improvement in sensor's reliability and accuracy, however, continue to be two parameters critical to the eventual implementation of this technology in high value targets and can be enhanced by the effect of both mechanical and optical characteristics of the fiber. This thesis presents an evaluation of the strain transfer capability of different coated FBG strain sensors (i.e. Gold, Polyamide and Acrylic) bonded to metallic host structures. A theoretical relationship for the strain transfer rate from the host structure to the fiber core has been developed. This relationship considers the strain transfer loss through the layers of the system based on their material properties and their geometry. In addition, a simulated analysis using finite element modeling has been developed. Parametric analysis of both the analytical and simulation models revealed the impact of coating material selection, coating thickness selection, and bonding condition on the strain transfer loss. Results illustrate that metallic fiber coatings (i.e. Gold) are more suitable for improved strain transfer than their polymeric counterparts. Additionally a set of experiments were conducted using Acrylic coated FBG sensor bonded to an Aluminium sample to validate the results from theory and the simulation. The strain on the FBG sensor was measured and compared with a calibrated strain gauge mounted on the host structure to characterize the strain transfer loss. The experimental results were compared with the results for the same configuration of the sensor and its host structure, from the theory and simulation and were found to be in good agreement.