Structural health monitoring using fiber optic distributed sensors for vacuum-assisted resin transfer molding

In this study we implemented manufacturing process and strain monitoring of a composite structure by optical fiber sensors for vacuum-assisted resin transfer molding (VaRTM). Optical fibers with fiber Bragg gratings were embedded into a glass fiber reinforced plastic specimen made by VaRTM and the applicability of structural health monitoring with fiber Bragg grating (FBG) sensors based on optical frequency domain reflectometry (OFDR) was investigated. In this study, long-gage FBGs which are 10 times longer than ordinary FBGs (which are about 10 mm long) were employed for distributed sensing. We can easily map the strain or temperature profile along gratings by OFDR and the spatial resolution of this sensing technique is about 1 mm. The resin flow process in VaRTM could be monitored by measuring the difference in temperature between the resin and preform. Then, the shrinkage of resin could be also monitored during the curing process. The specimen was then subjected to a bending load in a three-point bending test and the strain distributions along the FBGs were measured. From these results we could show the applicability of distributed sensors to quality assurance of a composite structure made by VaRTM and assessment of the structural integrity of in-service composite structures.