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 properties of the curing composites. As a reference, differential scanning calorimetry is used to evaluate the degree of cure while fiber-optic sensors are used to measure the evolution of process-induced strains, and ultrasonic sensors 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 degrees C if by water-cooling) and internal pressure (for example, 100 psi): (ii) high signal to spurious ultrasonic noise ratio; and high signal strength; (iii) suitable for longitudinal or shear wave measurement in reflection/transmission mode.