In-situ resin flow monitoring in VaRTM process by using optical frequency domain reflectometry and long-gauge FBG sensors

The vacuum-assisted resin transfer molding (VaRTM) process has attracted increased attention because of its ability to provide fast production times for large structures while maintaining a high quality. However, the occurrence of process-induced defects during the VaRTM process significantly affects the mechanical properties of the composites, resulting in a decrease in the reliability of the process. Therefore, flow monitoring should be performed during the manufacturing process to eliminate process-induced defects, such as dry spots and unimpregnated areas in the preform. Herein, we report a facile method for monitoring resin flow using a distributed strain sensing method during the VaRTM process. For the distributed strain sensing method, optical frequency domain reflectometry (OFDR) and a long-gauge fiber Bragg grating (FBG) sensor were used simultaneously. To analyze the change in the sensor signal according to the resin impregnation state, the strain along the in-plane direction of the preform during the VaRTM process was investigated through a numerical simulation and compared with the strain change measured by the OFDR system. The strain rate from the FBG sensor showed a positive value in the impregnated region but a negative value in the unimpregnated region, which means that the point at which the reversal of the strain rate occurs is the same as the position of the flow front. Thus, not only the flow front over the entire gauge length but also the approximate position and size of the unimpregnated area were accurately monitored.