Strain monitoring in thermoplastic composites with optical fiber sensors: embedding process, visualization with micro-tomography, and fatigue results

This study investigates the possibility of using optical fibers with Bragg gratings for measurements under fatigue loading conditions. Detailed information is given on the principle of optical fiber measurements, the embedding process, and the fatigue tests. To verify the strain derived from the optical fiber, the strain is compared with extensometer measurements. A special design of the blades of the extensometer is presented, since the standard blades suffer from a loss of grip on the surface of the specimen. Furthermore, X-ray micro-tomography is discussed and used for the visualization of the optical fibers and damage in the composite material. The material used for this study is a carbon fiber- reinforced polyphenylene sulfide. It can be concluded that the optical fiber survives over half a million loading cycles, without de-bonding of the fiber. Furthermore, the resolution of the micro-tomography is high enough to visualize not only the optical fiber, but also damage in the material.