End-of-cure sensing using ultrasonics for autoclave fabrication of composites

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 benefit of this work will be understanding when to complete the temperature hold and cool down the autoclave and, therefore, consistently produce composite laminates with the desired material properties. An additional benefit is the ability to follow the changing viscosity of the resin during the initial part of the cure. The general approach to this program involved using pulse-echo ultrasonics to measure the transit time for longitudinal ultrasonic waves to pass through a graphite/epoxy composite laminate during cure. Sixteen, 32 and 64 ply [0/90]s graphite/Fiberite 934 epoxy panels were fabricated and cured to various end-of-cure conditions. Additionally, panels with various starting conditions were run. 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.