DEVELOPMENT OF DEGREE-OF-CURE MEASUREMENT METHOD BY THIN-DIAMETER FRESNEL’S REFLECTION OPTICAL FIBER SENSOR

Fiber-reinforced polymer (FRP) has superior mechanical properties such as lightweight, high specific strength, and high specific rigidity. Recent important issues of manufacturing FRP are cost reduction and high cycle manufacturing of high-quality products. It is expected that in-situ process monitoring using a smart sensor can be used to solve the issues. Therefore, we paid attention to the monitoring method using an optical fiber sensor because it has good accuracy and embeddability. Up to the present, we have been developing a degree-of-cure (DOC) monitoring method for large and complex-shaped FRP products by Fresnel’s reflection optical fiber sensor. This sensor was based on Fresnel’s reflection due to the mismatch of refractive-index between glass and resin. In the previous study, the effect was investigated that the optical bending loss on the DOC measurement using a 𝜑125 μm optical fiber sensor. It was confirmed that a 𝜑125 μm optical fiber was sometimes broken at less than a 2 mm bending radius as one of the results of previous study. However, it is needed that stable measurement of DOC at very severe embedding condition if we want more expansion of the application range of FRP in the future. Thus, we aim to develop a DOC measurement system that can be measured by severe bending conditions using 𝜑80 μm optical fiber in this study. The optical loss property was measured by winding the fiber around the jig whose radiuses were 1, 1.5, 2, 3, 4, and 5 mm. From the result, the 𝜑80 μm optical fiber sensor didn’t break if it was bent less than 2 mm bending radiuses. Besides, it was found that the optical loss rate 𝑑𝐿/𝑑𝑥 was increased with the decrement of the bending radius 𝑅, and 𝐿𝑛 𝑅 and 𝐿𝑛 𝑑𝐿/𝑑𝑥 have a linear relationship. The DOC of epoxy resin was measured by using the 𝜑80 μm optical fiber sensor. The DOC curves were calculated from the measured refractive-index curves. From the result, it was confirmed that the DOC curve of the 𝜑80 μm optical fiber sensor agreed very well with the simulation curve by the Kamal model. Therefore, it can be estimated that appropriate the measurement result of DOC by the 𝜑80 𝜇𝑚 optical fiber sensor.