Design of Flat Optical Fiber Sensor for Triaxial Strain Monitoring in Composite Laminates

A fiber Bragg gratings sensor based on a novel flat optical fiber is designed for through-thickness, in-plane and tensile strains monitoring within unidirectional carbon fiber reinforced polymer laminates. An opto-mechanical investigation based on finite element method is carried out. The behavior of the embedded optical fiber and the composite material is simulated through a multiphysical model. The electromagnetic investigation is focused on the effect of the birefringence induced by the axial stresses on the propagation modes. The Bragg wavelength shift and the triaxial strain sensitivities are derived by considering transfer matrix method based on the coupled mode theory. A refined design of the flat fiber with a thinned asymmetric cladding is performed to obtain two different strain transfers between the sensing regions, improving the discrimination of triaxial strains. Moreover, the advantages of the designed optical fiber sensor with respect to standard optical fibers are discussed, paving the way for future developments.