Title: Ultrasonic Cure Monitoring for Autoclave Cured Composites

Abstract:
The objective of this work was to demonstrate the use of ultrasonics to determine the end-of-cure for autoclave cured, graphite/epoxy composite laminates. The fundamental enefit of this work will be understanding when to complete he temperature hold and cool down of the autoclave and, therefore, consistently produce composite laminates with the desired material properties. An additional benefit is the bility to follow the changing viscosity of the resin during the inital part of the cure.

The general approach to this program involved using pulse-echo ultrasonics to measure the transit time for longitudina1 ultrasonic waves to pass through a raphite/epoxy composite laminate during cure. Sixteen, 32 nd 64 ply [0/90]s graphite/Fiberite 934 epoxy panels were fabricated and cured to various end-of-cure conditions. Panels were cured to the manufacturer's recommended cure cycle (2 hour hold at 350F) and to the over- and under-cured states. Additonally, variations on onetime were performed. Ultrasonic measurements were made in real-time on all the anels using an ultrasonic system developed by United technologies Research Center under a program with the National Center for Manufacturing Sciences. Sound speed was calculated using the panel thickness (number of plies multiplied by the manufacturer specified thickness of each ply) divided by the measured transit time.

Mechanical (flexural strength and modulus) and pass transition temperature (Tg) tests have been performed on all the fabricated panels. Determining the relationships etween the velocity profiles, mechanical properties, and Tg late is in progress. Ultimately, the goal is to understand the relationship between the ultrasonic data and state-of-cure, Ed to incorporate the ultrasonic sensors into an operational expert system for sensor feedback control of the autoclave Process.

Source: Proceedings of the 'NDE applied to Process Control of Composite Fabrication' - Conference, 1-2 Oct 1996 St. Louis, Missouri. Publisher and Organizer: Nondestructive Testing Information Analysis Center (NTIAC) Texas Research Institute Austin, Inc. Austin, Texas [http://www.ntiac.com] [Buying the Proceedings]