Indicating cure and stress in composite containers using optical fibers

Long optical fibers embedded inside a composite material enable on-line monitoring of the manufacturing process and can also be used to assess damage by monitoring strain and cracks in the structure.
We report on measurements of the microbending loss in long single-mode and multimode telecommunication fibers that were successfully installed inside the walls of composite containers during the normal manufacturing process.
The work was largely empirical, and theoretical aspects were not investigated thoroughly. The measurements were carried out using the optical through power technique and a commercial optical time-domain reflectometer that gives not only the attenuation but also spatial information.
We found that the coating material and its thickness have an effect on loading sensitivity and on the applicability of the method for cure monitoring.
Although we discovered that repeatability issues are a problem, the measurement data indicate that the end-of-curing process of the composite can be determined by optical sensing fibers and damaging external loading of the structure can be detected.