Signal and mechanical characteristics of FBG sensors embedded into composite structures

Abstract

Fiber Bragg grating (FBG) sensors based on the wavelength division multiplexing technology are ideally suitable for structural health monitoring. In many applications, FBG sensors are widely applied to composite materials through embedding or surface attachments. However, FBG sensors have some problems on signal characteristics, especially peak splits, when they are embedded into composites, and this considerably restricts the application fields and scopes of FBG sensors. For this reason, the signal characteristics of FBG sensors when embedded into composites must be investigated and the related problems also must be solved for more stable and broad applications. In this paper, a new fabrication of FBG sensors with signal stability was developed and the signal characteristics were verified when they were embedded into structures, especially composite structures. For the purpose, the peak signals of FBG sensors were investigated when FBG sensors were influenced by the birefringence or strain gradients. In order to prevent the peak signals of FBG sensors from splitting, the protections of sensor heads such as recoating and protection with a glass capillary tube were introduced and their limitations were investigated. A new fabrication process of FBG sensors was developed using a reflection prism with special coatings on the surface. The signal characteristics of new FBG sensors were investigated and the performance was verified through a comparison with the previous sensors under the birefringence and strain gradients. Furthermore, the correlation between strain gradients and the peak signals of FBG sensors was studied quantitatively. In addition, mechanical strength characteristics of FBG sensors were investigated as the reflectivity and grating lengths changed. The effects of jacket stripping methods on the mechanical strength were also examined. Finally, FBG sensors were embedded into filament wound pressure tanks during the fabrication process and the techniq...