Composite constructions are indispensable in current and future society. Fiber Bragg Gratings (FBGs) embedded in composite need to be carefully aligned with the material fibers to reduce inhomogeneous lateral load exerted onto the FBG which occurs due to the inhomogeneous nature of composite materials. Inhomogeneous load causes distortion of the reflection spectrum. We proposed to solve the FBG spectral distortion by incorporating a dedicated design structure inside the optical fiber. This allows the FBG to sense the strain in the axial direction accurately regardless of the optical fiber alignment with respect to the composite matrix. In this paper, the basic design will be discussed and the results of the first prototype of this structured fiber will be presented. Prototype FBGs are embedded in different composite samples of various thicknesses and materials (glass or carbon fiber based). The spectrum before and after curing is measured 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 to the strain applied. During all experiments, the special FBG is found to have a better or comparable spectrum than the standard FBGs. The improvement varies for the different tests. This can be caused by the unknown orientation of the structure inside the fiber. According to the first FEM analysis, this affects the effectiveness depending on the detail design of the structure. Information of the FEM analysis will be used to further optimize the design and for the development of a prototype.
Data from comprehensive thermomechanical tests of shape memory polymers are reported, with specimens tested up to
75% strain and between 30-120°C temperatures. The data is analyzed and key observations are drawn. The stress/strain
behavior during loading at temperatures above glass transition for the Veriflex shape memory polymer tested was
linear and did not show much variation with the actual temperature. When the polymer is cooled with end constraints,
thermally induced tensile stresses developed, but only after the temperature reduced below glass transition and the
material stiffened. When the constraints were then released, 97-98% of the original strain was locked in. Reheating the
shape memory polymer beyond the glass transition temperature resulted in shape recovery (shape memory effect). When
the polymer was reheated while constraining the strain, the full recovery stress developed was about the stress the
polymer was initially loaded to during deformation at high temperature. Examining the Young's modulus at elevated
temperature and low temperature showed that Veriflex softened by around 40-60 times when heated through glass
transition.
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