Influence of Local Bending of Fresnel-Based Optical Fiber Sensors on Measuring Degree of Cure of FRP280

Abstract

FRP are very light and strong materials and applied to advanced FRP products such as airplanes and automobiles. A recent key issue of manufacturing FRP is cost reduction and in situ monitoring using smart sensing can be used to achieve it. Among such smart sensors, the Fresnel-based optical fiber reflectometer can be used for measuring internal degree-of-cure of thermosetting FRP during cure process and it is known that it has good accuracy and embeddability in FRP. However, the reflection power is affected by the local bending when the sensor is embedded in the complexshaped FRP products and then the quantitative evaluation of effect of the optical bending loss on the measurement accuracy is necessary. In the present paper, the effect of optical loss by local bending on the measurement accuracy of Fresnel-based optical fiber sensors was investigated. Two types of optical fiber sensors, one is a standard 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 the both types of sensors. From this experimental result, it was found that the optical loss was 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 optical fiber sensors with local bending. From this experimental result, the degree-of-cure was hardly affected by the optical bending loss. Next, the degree-of-cure measurements of curved 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 resin impregnation influenced the accuracy but the error is correctable.