Durability of Functionalized Carbon Structures with Optical Fiber Sensors in a Highly Alkaline Concrete Environment

The paper presents an investigation into the durability of functionalized carbon structures (FCS) in a highly alkaline concrete environment. First, the suitability of optical fibers with different coatings—i.e., acrylate, polyimide, or carbon—for the FCS was investigated by subjecting fibers with different coatings to micro/macro bending and a 5% sodium hydroxide (NaOH) (pH 14) solution. Then, the complete FCS was also subjected to a 5% NaOH solution. Finally, the effects of spatial variation of the fiber embedded in the FCS and the bonding strength between the fiber and FCS was evaluated using different configurations —i.e., fiber integrated into FCS in a straight line and/or with offsets. All three coatings passed the micro/macro bending tests and show degradation after alkaline exposure, with the carbon coating showing least degradation. The FCS showed relative stability after exposure to 5% NaOH. The optimum bonding length between the optical fiber and the carbon filament was found to be ≥150 mm for adequate sensitivity.

The Tensile Behavior of High-Strength Carbon Fibers

Carbon fibers exhibit exceptional properties such as high stiffness and specific strength, making them excellent reinforcements for composite materials. However, it is difficult to directly measure their tensile properties and estimates are often obtained by tensioning fiber bundles or composites. While these macro scale tests are informative for composite design, their results differ from that of direct testing of individual fibers. Furthermore, carbon filament strength also depends on other variables, including the test length, actual fiber diameter, and material flaw distribution. Single fiber tensile testing was performed on high-strength carbon fibers to determine the load and strain at failure. Scanning electron microscopy was also conducted to evaluate the fiber surface morphology and precisely measure each fiber’s diameter. Fiber strength was found to depend on the test gage length and in an effort to better understand the overall expected performance of these fibers at various lengths, statistical weak link scaling was performed. In addition, the true Young’s modulus was also determined by taking the system compliance into account. It was found that all properties (tensile strength, strain to failure, and Young’s modulus) matched very well with the manufacturers’ reported values at 20 mm gage lengths, but deviated significantly at other lengths.

Integrative Manufacturing of Textile-Based Sensors for Spatially Resolved Structural Health Monitoring Tasks of Large-Scaled Composite Components

For the continuous and non-destructive structural health monitoring (SHM) of fiber reinforced plastics (FRP), a one-step integration of one-or two-dimensional strain sensors based on piezo-resistive carbon filament yarns (CFY) into textile reinforced structures of subsequent FRP components has been realized during textile-technological manufacturing processes. The two-dimensional alignment of the sensor layouts is realized by a special process-integrated warp yarn path manipulation (WPM). With suchlike manufactured semi-finished reinforcement structures, a functional model of a small wind turbine blade in glass-fiber thermoset composite design has been build up. Using the CFYs’ piezo-resistive effect, mechanical strains can be measured and visualized due to a correlative change of the carbon filaments resistance. Performing quasi-static load tests on the blade and additional test specimens, comprehensible results of the electro-mechanical behavior and spatially resolving capacity of different sensor integration lengths have been achieved. The performed tests demonstrate, that global and even local mechanical stresses within complex FRP components can be measured spatially resolved using the approach of textile technologically integrated textile sensors.