Fiber optic sensors for flow and health monitoring of resin transfer molded composite structures

This paper proposes the use of fiber optic sensors for flow and health monitoring of resin transfer molded (RTM) composite structures. Both fiber Bragg gratings (FBG) and etched fiber sensors have been embedded in composite panels. To this end, a laboratory scale RTM apparatus was designed and implemented with the capability of visually monitoring the resin filling process as well as embedding fiber optic sensors. Tensile test specimens containing embedded FBG sensors with the capability to measure strain have been produced. These sensors have been characterized and evaluated using strain gage and a tensile testing machine. Furthermore, etched fiber sensors that have the capability of detecting the presence of resin have also been fabricated, embedded and tested in conjunction with the FBG sensors. The combined sensor system has proven to be capable of detecting the presence of resin at various locations as it is injected into the mold thus ensuring the quality of the manufactured component and subsequently monitor the health of the component in service

An experimental study on the effect of length and orientation of embedded FBG sensors on the signal properties under fatigue loading

Fiber Bragg grating (FBG) sensors provide excellent capability for the structural health monitoring (SHM) of load-bearing structures by allowing for local internal strain measurements within structures. However, the integration of these sensors to composite materials is associated with several challenges that have to be addressed to have the correct strain measurement and in turn to perform reliable SHM. One of the most important issues is the presence of uneven strain fields around FBGs, which significantly affect the response of the sensors and hence the reliability of the acquired data. The uniformity of the strain fields around sensors is important for dependable data acquisition; however, to generate such a condition, tow width-to-FBG length relationship, optical fiber configuration with respect to reinforcement fiber orientation, and crack density resulting from fatigue loading are very important factors that have to be considered. In this paper, these issues are addressed by investigating the signal properties of FBG sensors with 1 and 10 mm lengths embedded within the composite specimens during the manufacturing process. After fatigue testing of the specimens, it is shown that 1-mm-long FBGs embedded in-line with adjacent reinforcement fibers with tow widths of ∼2 mm provide much more reliable signals than 10-mm-long FBGs embedded perpendicular to adjacent tows

Prediction of fatigue response of composite structures by monitoring the strain energy release rate with embedded fiber Bragg gratings

Composite materials are becoming increasingly more valuable due to their high specific strength and stiffness. Currently, most components are operated for a number of service cycles and then replaced regardless of their actual condition. Embedded fiber Bragg gratings are under investigation for monitoring these components in real time and estimating their remaining life. This article presents research conducted on a novel technique for prediction of the remaining life of composites under fatigue loading using embedded fiber Bragg grating sensors. A prediction is made of the remaining life at every cycle based on data collected from the sensors and the previous loading history

Fiber Bragg grating and etched optic sensors for flow and cure monitoring of resin transfer molded composite structures

In this study, we present the research conducted on in situ process monitoring (cure and flow) of resin transfer molded glass fiber reinforced polymer composites using fiber Bragg grating (FBG), and etched bare fiber optic sensors. Both FBG and etched fiber sensors are embedded into glass fiber reinforced composites manufactured by the Resin Transfer Molding (RTM) method, and are used to monitor the flow front during the resin injection process and subsequently the cure cycle. The results of this study have shown that both the FBG and etched sensors can be used efficiently for flow and cure monitoring in the RTM process. The experimental results of etched sensors are in accordance with those of FBG sensors for cure monitoring

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

This paper presents research being conducted on the use of a combination of fiber optic sensors for process and health monitoring of resin transfer molded (RTM) composite structures. A laboratory scale RTM apparatus has been designed and built with the capability of visually monitoring the resin filling process and embedding fiber optic sensors into the composite. Fiber Bragg gratings (FBG) and etched fiber sensors (EFS) have been multiplexed and embedded in quasi-2-D panels and 3-D hollow semicircular structures using a novel ingress/egress technique for the purpose of both process and structural health monitoring. The objective of this work is to demonstrate the simultaneous usage of FBGs and etched fiber sensors on a single optical fiber for resin flow monitoring and strain monitoring throughout the life of the composite. Three specimens are presented: one quasi-2D panel with two FBGs and three etched fiber sensors, one semicircular tube with two etched fiber sensors and one semicircular tube with two FBGs. Etched fiber sensors have been correlated with visual inspection to detect the presence of resin. Specimens with embedded FBG sensors have been tested in a tensile test machine to characterize the FBGs for strain monitoring

An experimental study on the process monitoring of resin transfer molded composite structures using fiber optic sensors

Research conducted on in situ process monitoring of resin transfer molded composites with fiber optic sensors is presented. A laboratory scale resin transfer molding (RTM) apparatus was designed and built with the capability of embedding fiber optic sensors and visually observing the resin filling process. Both fiber Bragg grating (FBG) and etched fiber (EF) sensors are embedded into glass fiber reinforcements in the RTM mold and used to monitor the resin flow front and cure. The cure cycle of the resin system utilized in this work is also studied using a Fresnel reflection refractometer (FRR) sensor. The results of this study show that both the FBG and EF sensors can be used efficiently for flow and cure monitoring of the RTM process. The experimental results of the EF and FRR sensors agree with those of the FBG sensors for cure monitoring