PSI - Issue 28

J. Weiland et al. / Procedia Structural Integrity 28 (2020) 1249–1257 Weiland et al. / Structural Integrity Procedia 00 (2019) 000–000

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2. Structural Health Monitoring with Polymer Optical Fiber In contrast to glass fibers or metallic-electronic sensors, the mechanical properties and especially the Young’s modulus of the POF can be adjusted to the mechanics of the adhesive layer by selecting a suitable POF material. Typically, Young’s modulus of any POF is much lower than of silica glass fibers. This results in a simultaneous deformation of the fiber cross-sectional shape induced by the deformation of the adhesive layer. Typically, the deformation of the POF manifests itself in an ovalization of the fiber cross-sectional shape, see figure 1. This results in a change in light propagation in the POF. Because of its large diameter in the range of one millimeter, many individual modes of the light can propagate inside of the fiber core. In a ray-optics model of such multimode fibers, each mode is represented by two angles in space of a light ray in respect to the longitudinal fiber axis. Due to the changed fiber cross-sectional shape, some light rays violate the total internal reflection conditions and light is emitted from the cladding material. In addition, a deformation of the cross-section will change the modal distribution, i.e. the distribution of the total optical power on individual modes. Another aspect is the change in optical material properties, such as the refractive index. The transmitting deformation causes stresses in the POF, mainly in the core, which can influence light propagation by the photo-elastic effect. Optoelectronic measuring devices at the fiber output can easily detect these changes in light propagation. [21]

Fig. 1: Principle of SHM with POF – Schematic view of the mechanical behavior of the fiber-adhesive-composite

3. Materials, mechanical properties and manufacturing 3.1. Polymer Optical Fibers

Polymer Optical Fibers (POF) have a core material, which is coated with a thin cladding of about 10 µm, see figure 2. Based on Ziemann et al. [23] standard materials for commercially available POF are currently polymethylmethacrylate (PMMA), polycarbonate, silicones, cross-linked acrylic resins and perfluorinated butene vinyl ethers. Due to the different refractive indices of the core n co and the cladding n cl , POF can guide light in the core by total reflection, if the condition n co > n cl is satisfied. A ray of light in air and entering the fiber core at an angle of Θ in respect to the fiber axis is refracted at the air-core interface as described be the law of Snellius. If the refracted light inside of the core satisfies the condition for total internal reflection (TIR) at the core-cladding interface, the light is guided inside the core by repeated TIR. Therefore, a maximum angle of the rays in respect to the axis of the fiber must not be exceeded, and thus, only light entering the fiber from air within the acceptance angle Θ ��� is guided. Usually, the so-called numerical aperture of the fiber, defined according Ziemann et al. [23], expresses this