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.