CompositesWorld
Issue link: https://cw.epubxp.com/i/630867
FEBRUARY 2016 16 CompositesWorld OUT OF AUTOCLAVE SUPPLEMENT Read this article online | short.compositesworld.com/OOA-RandD comparable bulk, wrinkling issues and how to translate USC's material concept to unidirectional prepreg plies are still challenges: Work is continuing on visualization studies that track air removal in prepregs, co-cure of OOA sandwich structure facesheets and adhesives, and VBO processing of complex parts with corner angles. Mikhail Grigoriev, a senior process engineer at Aero- space Materials Processing (Redondo Beach, CA, US), has investigated ways to reduce surface porosity and pitting on the tool side of OOA parts, with funding from the Air Force Research Laboratory (AFRL, Dayton, Ohio). "Our motivations were to achieve equal properties with auto- claved composites, at reduced part cost, and with no limit in part size," says Grigoriev. To avoid having to use unwieldy release flms that are tough to apply on complex shapes, Grigoriev's team has investigated etching micro-structured patterns onto tools. Te research has shown that the height of the etched features was the key element. Etched patterns between 12 and 14 microns high reportedly provide adequate pathways for gas escape, reducing surface pitting and overall porosity. Work is continuing on larger tools and proprietary etchings, he says. Although autoclave-cured prepregs are well- modeled and characterized, OOA prepregs can have diferent tool/part interaction and thermal profles. AFRL's Gregory Ehlert has presented information on the use of gold nanorods, 40 to 60 nm in length, mixed with epoxy prepreg resin, that change shape gradually, and irreversibly, with heat. Using Raven simulation software from Convergent Manufacturing Technologies Inc. (Vancouver, BC, Canada), Ehlert's group was able to model degree of cure out of the autoclave as a function of temperature and time, confrmed as nanorods change from rods to small spheres: "It's a passive technique that has utility for monitoring cure for OOA processing," says Ehlert. Te University of Delaware's Center for Composite Materials' (CCM, Newark, DE, US) post-doctoral researcher Tomas Cender has studied the physics of air bubble formation and migration during vacuum application, and the conditions under which the air bubbles within the resin can fow faster than the average resin velocity, and be evacuated with the applied vacuum. Te CCM group is developing models to characterize the way that bubbles (which form voids in the cured part) can be eliminated. "We want to establish how the pressure gradient, fabric permeability and fuid properties can be optimized to increase bubble mobility so that they can be removed during the process," says Cender. NASA has investigated the issue of out-time of OOA prepregs, compared to autoclave-curable prepregs, from the standpoint of their impact on the ability to fabricate very large structures within the time window aforded by the prepreg itself. Past research has shown that while OOA composite test parts have comparable mechanical and thermal performance to autoclaved parts, when both are made with "fresh" prepreg, OOA prepregs' thermal and mechanical performance and tack was signif- cantly reduced when out-life exceeded 21 days. A current NASA project, the Long Out-time, Out-of- Autoclave Cure Composites Project, is committed to developing a prepreg matrix system for OOA parts with an out-time of one year, during which produced parts uniformly exhibit good part proper- ties and damage tolerance. A N N I V E R S A R Y 0 2 HPC Charter Advertiser