Long period gratings as flow sensors for liquid composite molding

One of the most important issues in liquid composite molding (LCM) is the complete saturation of the preform by the resin to eliminate voids or dry spots in the structure which could later adversely affect the structural integrity of the part. While there have been efforts in developing reliable mold filling simulations for LCM, very few successful flow sensing systems exist for detecting actual resin arrival during mold filling. In this study, the feasibility of using optical fibers with long period gratings (LPG) as sensors for monitoring flow in the LCM process was investigated. An advantage of using LPGs is that they are more robust and less susceptible to background noise than simple bare fibers. Furthermore, the location of resin arrival can be easily identified as the signals from each LPG uniquely correspond to predetermined wavelengths along the source spectrum. The LPGs are sensitive to changes in the refractive index and register a strong signal change when covered with resin. In this study, the LPG sensors were placed in the middle of a preform stack inside a mold and the sensor response after the mold was properly closed, and when the resin covered a particular LPG was monitored. An assortment of preforms, which included random mats and unidirectional fabrics, with a series of fiber volume fractions were used to determine their effects on the sensor response.