Resin flow monitoring in vacuum-assisted resin transfer molding using optical fiber distributed sensor

Abstract

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.

Multiplexed FBG and etched fiber sensors for process and health monitoring of 2-&3-D RTM components

“…When the light reaches the section of fiber where the cladding is removed a small portion of it escapes while most is transmitted. 24 When resin contacts the core its refractive index being higher than that of the glass causes more light to escape. Therefore, the amount of light transmitted through the fiber is less.…”

“…Due to the arrangement of the sensors on the fiber (FBG before EFS) no change in power was noticed when resin contacted the EFS sensors, therefore, demonstrating that the EFS do not effect the FBG sensors as long as they are situated after the FBGs on the fiber. The FBG output is not included here, however this technique has been thoroughly researched by others such as Novo et al 23 and Eum et al 24,25 Flow monitoring of 3-D semicircle specimen To demonstrate the versatility and applicability of these sensors to more realistic structures, an optical fiber containing two EFS sensors was embedded in a hollow tube with a 3-D semicircle cross-section using the aforementioned RTM apparatus. The sensors were embedded between the fourth and fifth layers on the curved surface of the semicircle.…”