Test results of lateral load insensitive FBGs embedded in composites to　suppress spectral distortion

摘要

Composite constructions are indispensable in current and future society. Fiber Bragg Gratings (FBGs) embedded incomposite need to be carefully aligned with the material fibers to reduce inhomogeneous lateral load exerted onto theFBG which occurs due to the inhomogeneous nature of composite materials. Inhomogeneous load causes distortion ofthe reflection spectrum. We proposed to solve the FBG spectral distortion by incorporating a dedicated design structureinside the optical fiber. This allows the FBG to sense the strain in the axial direction accurately regardless of the opticalfiber alignment with respect to the composite matrix. In this paper, the basic design will be discussed and the results ofthe first prototype of this structured fiber will be presented. Prototype FBGs are embedded in different compositesamples of various thicknesses and materials (glass or carbon fiber based). The spectrum before and after curing ismeasured and direct comparisons are performed with embedded standard commercial FBG to verify the improvement.Effects of depth of the embedding and FBG direction with respect to the composite material fiber are investigated.Bending and tension tests are performed to ensure the special FBG in the structured fiber has the directional sensitivity tothe strain applied. During all experiments, the special FBG is found to have a better or comparable spectrum than thestandard FBGs. The improvement varies for the different tests. This can be caused by the unknown orientation of thestructure inside the fiber. According to the first FEM analysis, this affects the effectiveness depending on the detaildesign of the structure. Information of the FEM analysis will be used to further optimize the design and for thedevelopment of a prototype.