modulus and Poisson’s ratio of the polyimide are 1.47 GPa and 0.45, respectively. In the analysis, the glass optical fiber was divided into seven concentric cylinders, and the polyimide coating into two cylinders. Figure 3 shows the longitudinal strain distribution in 0° ply calculated from the positions of transverse cracks in Figure 1(a) obtained from a tensile test. The average tensile stress was 359 MPa. Furthermore, axial strain distributions at the cores of uncoated and polyimide-coated FBG sensors are shown in Figure 3. In the calculation, the strain distribution at the outermost layer was assumed to be the same as that in 0° ply. As shown in Figure 3, although the strain distribution in the uncoated FBG sensor scarcely changed from that in 0° ply, the variation of the strain in the polyimide-coated FBG sensor was attenuated and smoothed because of the soft coating. From the axial strains, the distributions of the grating period and the average refractive index along the FBG sensors were calculated. Then, the reflection spectra were simulated from the distributions. This calculation was conducted using the software ‘IFO_Gratings’ developed by the Optiwave Corporation. This program can calculate the spectrum by solving the couple mode equations using the transfer matrix method. The reflection spectra calculated from the strain distributions in Figure 3 are shown in Figure 4. The deformation of the spectrum of the coated FBG sensor is slightly smaller than that of the uncoated FBG sensor. Peaks and dips of strain distribution in Figure 3 correspond to large wavelength and small wavelength components of the spectrum, respectively. Thus, the relaxation of the non-uniform strain distribution by the polyimide coating affects the intensity of the components away from the center wavelength of the reflection spectrum. However, the two spectra in Figure 4 are almost the same, so that the polyimide-coated FBG sensor can be applied to the detection Figure 3: Longitudinal strain distribution in 0° ply calculated from the positions of transverse cracks in Figure 1(a) and axial strain distributions calculated at the cores of uncoated and polyimide-coated FBG sensors. The average tensile stress was 359 MPa. Figure 4: The reflection spectra of the uncoated and coated FBG sensors calculated from the strain distributions in Figure 3. 0.41 0.43 0.45 0.47 0.49 0 2 4 6 8 10 0ply uncoated coated Strain (%) Position z (mm) z 0° Ply Uncoated Coated 0 0.2 0.4 0.6 0.8 1 1537 1538 1539 uncoated coated Reflectivity Wavelength (nm) Uncoated Coated
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