<title>End-of-cure sensing using ultrasonics for autoclave fabrication of composites</title>

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 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 ofthe 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-ofcure 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. * formerly of the carderock Division Naval Surface Warfare Center, Annapolis, MD 21402 72 / SPIE Vol. 2948 O-8194-2352-1/96/$6.OO Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/21/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspxMechanical (maximum fiber stress and maximum strain) and glass transition temperature (Tg) tests are being performed on all the fabricated panels. Determining the relationships between the velocity profiles, mechanical properties, and Tg data is in progress. Ultimately, the goal is to understand the relationship between the ultrasonic data and state-of-cure, and to incorporate the ultrasonic sensor into an operational expert system for sensor feedback control of the autoclave process.BACKGROUND

Ultrasonic measurements of polyimide materials during heating

“…Fabrication processes account for ~80% of the finished cost, a clear target for cost reduction [1]. Monitor and control of process/material parameters (temperature, pressure, viscosity, degree of cure, resin flow) have been successfully applied to single-stage epoxy composites [2][3][4] in reducing process development time and improving product quality. Many of these sensor techniques are being applied to HTPMC processing with success except for detection of resin boiling during the initial solvent removal stage.…”