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.
In this study, we implemented resin flow monitoring by using an optical fiber sensor during vacuum assisted resin transfer molding (VaRTM).We employed optical frequency domain reflectometry (OFDR) and fiber Bragg grating (FBG) sensor for distributed sensing. Especially, long gauge FBGs (about 100mm) which are 10 times longer than an ordinary FBG were employed for more effective distributed sensing. A long gauge FBG was embedded in GFRP laminates, and other two ones were located out of laminate for wavelength reference and temperature compensation, respectively. During VaRTM, the embedded FBG could measure how the preform affected the sensor with vacuum pressure and resin was flowed into the preform. In this study, we intended to detect the gradient of compressive strain between impregnated part and umimpregnated one within long gauge FBG. If resin is infused to preform, compressive strain which is generated on FBG is released by volume of resin. We could get the wavelength shift due to the change of compressive strain along gauge length of FBG by using short-time Fourier transformation for signal acquired from FBG. Therefore, we could know the resin flow front with the gradient of compressive strain of FBG. In this study, we used silicon oil which has same viscosity with resin substitute for resin in order to reuse FBG. In order to monitor resin flow, the silicon oil was infused from one edge of preform, the silicon oil was flowed from right to left. Then, we made dry spot within gauge length by infusing silicon oil to both sides of preform to prove the ability of dry spot monitoring with FBG. We could monitor resin flow condition and dry spot formation successfully using by FBG based on OFDR.
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