Fiber-optic and ultrasonic measurements for in-situ cure monitoring of graphite/epoxy composites

To produce high-quality composites with high density and low void content, the knowledge of cure process is very important, and the sensors capable of monitoring the cure process are therefore desirable. Since the term of "fully processed" should be a reflection of the ultimate material application, the objectives of the present work are to monitor how material properties have been developed during the cure and justify when " end-of-cure" has been achieved by measuring chemical and mechanical propert ies of the curing composites. As a reference, differential scanning calorim etry is used to evaluate the degree of cure while fiber-optic sensors are u sed to measure the evolution of process-induced strains, and ultrasonic sen sors are used to monitor the development of viscoelastic properties of the curing composites. Moreover, an ultrasonic cure monitoring system has been developed, by using conventional broadband ultrasonic sensors together with clad buffer rods. The major merits of this system can be summarized as (i) workable inside an autoclave at high temperature (for example, up to 250 d egrees C if by water-cooling) and internal pressure (for example, 100 psi): (ii) high signal to spurious ultrasonic noise ratio; and high signal stren gth; (iii) suitable for longitudinal or shear wave measurement in reflectio n/transmission mode.