In-Situ Cure Monitoring of Graphite/Epoxy Composites Using Fiber Optics and Ultrasonics

摘要

Advanced composites can be fabricated by laminating multiple prepreg plies into the desired shape and then cured in an autoclave with simultaneous application of proper heat and pressure. The knowledge of cure process is very important in order to obtain fully cured and high quality composites at reduced production costs [1]. In-situ sensors capable of monitoring the cure process are therefore desirable. The thermal Differential Scanning Calorimetry (DSC) can monitor the exothermic flow of heat of the cure reaction, and a characterization of the state of cure of composites, thus, is possible. For the AS4/3501-6 prepregs, Figure 1 shows the degree of cure (DOC) as a function of cure time with different cure temperatures. As the cure temperature increased, the DOC increased while the time for completion of the cure reaction decreased. At the cure temperature of 176°C, the cure reaction was almost fully complete after 60 minutes, while at 146°C cure, the DOC only reached around 70% after 120 minutes. However, even though the cure reaction was nearly completed, the mechanical properties of the curing composites may not be well developed because the DOC is a process parameter which gives information on the extent of chemical cure reaction while the extent of modulus is a parameter which gives information on the degree of mechanical property development [2]. The critical objective of the process engineer is to know when the material has been "fully processed", and the cure characterization should be reflective of the ultimate material application, thus, the "end-of-cure" should be determined by the completion of the development of not only chemical properties but also mechanical properties of curing composites.