Fibre Bragg gratings: monitoring of infusion process in liquid composite molding manufacturing

For over three decades, fibre Bragg gratings (FBGs) have been incorporated into buildings and various structures including those made from fibre reinforced polymer (FRP) composites as sensors to monitor the structural health. Furthermore, as FRP composite technology has matured the maximum size of these structures has rapidly increased and thus the manufacturing process itself has become a critical factor in the overall success of these projects both financially from a quality control prespective. An example is ensuring that the infusion of resin into the glass reinforcement mats within the mould has completely wet all the fibre surfaces without leaving dry patches. Checking this issue is problematic as checks can only take place after infusion has completed. We report the use of FBGs to monitor and locate the flow front of a resinlike liquid through glass reinforcement mats within the mould during the infusion process of Resin Transfer Moulding (RTM). During the infusion process, the liquid flow front velocity generates a viscous force that causes the FBGs to produce both a blue and red wavelength shift. We found the wavelength shifts are small but still significant and reasonably repeatable for the same conditions, the same depth, location of sensor within the mould and the same velocity of flow front yielding an error of ±1.5pm.The blue wavelength shifts ranged from 8pm to 17pm depending on the flow front velocity for the "resin-like"liquid. Evidence is presented showing that the main factor of this FBG spectral behaviour is the viscous force.