FRP are very light and strong materials and applied to advanced FRP productssuch as airplanes and automobiles. A recent key issue of manufacturing FRP is costreduction and in situ monitoring using smart sensing can be used to achieve it. Amongsuch smart sensors, the Fresnel-based optical fiber reflectometer can be used formeasuring internal degree-of-cure of thermosetting FRP during cure process and it isknown that it has good accuracy and embeddability in FRP. However, the reflectionpower is affected by the local bending when the sensor is embedded in the complexshapedFRP products and then the quantitative evaluation of effect of the opticalbending loss on the measurement accuracy is necessary. In the present paper, theeffect of optical loss by local bending on the measurement accuracy of Fresnel-basedoptical fiber sensors was investigated. Two types of optical fiber sensors, one is astandard type and another is a highly-flexible type, were used for the measurements.First, optical loss by bending with radius of 2,3,4,5 and 6mm was measured for theboth types of sensors. From this experimental result, it was found that the optical losswas a linear function of bending length and influenced strongly by the bending radius.In addition, it appeared that the optical loss by the embedment could be estimated.Second, degree-of-cure measurements of epoxy resin were conducted by the opticalfiber sensors with local bending. From this experimental result, the degree-of-cure washardly affected by the optical bending loss. Next, the degree-of-cure measurements ofcurved FRP plates molded by VaRTM were conducted. From the experimental results,it was found that the local bending hardly affected the measurement accuracy.Additionally, it appeared that the movement of the optical fiber due to resinimpregnation influenced the accuracy but the error is correctable.
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