Abstract

Advanced composites can be fabricated by laminating multiple plies into the desired shape, and then cured in an autoclave with simultaneous application of proper heat and pressure. The knowledge of the cure process is very important in order to obtain fully cured and high-quality composites. In-situ sensors capable of monitoring the change of physical and chemical properties during the cure process are therefore desirable. The major aspects of research in this thesis are as follows: The chemical kinetic and rheological properties of AS4/3501-6 Graphite/Epoxy composites are characterized by using Differential Scanning Calorimetry and Dynamic Mechanical Analyzer. This indicates that at 176C̕, the cure reaction is almost fully completed after 60 minutes, and increasing cure temperature results in a noticeable decrease in loss tan 6 and a profound increase in the Tg. Ultrasonic characterization shows that the moduli of 3501-6 epoxy increase with increasing cure temperature, whereas the moduli of AS4/3501-6 composite reach their maximum around 176C̕. Also, the moduli of the composite increase with increasing cure time, whereas the moduli of the epoxy, after being cured at 176C̕ for 80 minutes, exhibit no change. A state-of-the-art fiber-optic and ultrasonic cure processing monitoring system is developed, which consists of ultrasonic transducers with clad buffer rods, Extrinsic Fabry-Perot Interferometric fiber-optic sensors, and a special autoclave feedthrough. This system is workable in an autoclave at high temperature and gas pressure, has high ultrasonic signal to spurious noise ratio, and is suited for longitudinal and/or shear wave measurement. The development of properties of the curing AS4/3501-6 composite is monitored by using in-situ fiber-optic and/or ultrasonic sensors, indicating that (1) both sensors can sense the gelation period; (2) the ultrasonic sensor is able to determine the end-of-cure, while the fiber-optic sensor is not; (3) during the cool-down period the development of thermal mismatch strains between the mold plates and the composite panel, prior to and after the detachment, can be detected by the fiber-optic sensor.