CompositesWorld
Issue link: https://cw.epubxp.com/i/517026
JUNE 2015 30 CompositesWorld WORK IN PROGRESS » Te autoclave is, perhaps, the most beloved and most maligned piece of equipment employed in the fabrication of composite aerostructures. It is beloved because, when used properly, it brings to bear on a carbon fber laminate the brute force required to compress to a very small and harmless size the entrapped air and volatile gases that form voids in cured resin that otherwise would be problematic in a fnished composite part. It is maligned, however, because the application of that brute force costs dearly — in capital, energy and time. Compos- ites fabricators in aerospace and other large industries, such as automotive and wind energy, therefore, are in search of out-of- autoclave (OOA) manufacturing processes that can cross the mandated 1% void content threshold with less expensive, more efcient equipment yet achieve autoclave-quality composite parts for critical structural applications. When it comes to the aerospace sector, however, OOA usually means vacuum-bag consolidation of carbon fber prepregs, followed by oven cure. Te term OOA implies that the process is simply a non-auto- clave version of the autoclave-based original. Indeed, OOA processing is attractive because it ofers a more sustainable manufacturing pathway, based on the cost savings that accrue when foregoing the use of the autoclave, while producing auto- clave-quality composites. But OOA processing of aerostructures forces the fabricator to cope with variables that were, for the most part, obviated by the use of the autoclave. And variable numbers one and two are entrapped air and air bubbles, the mismanage- ment of which can quickly turn a carbon fber laminate into an expensive, void-flled, out-of-compliance paperweight. Managing air and bubbles, however, requires some under- standing of air and bubble behavior, and such understanding is in limited supply in the composites industry. It is, however, the focus of in-depth experimental work by the University of Delaware (UD, Newark, DE, US), where researchers are trying to shed some light on how air, resin and fbers interact during vacuum-based manu- facturing — and the conditions under which air and air bubbles are most likely to escape before resin cure is complete. Te work, Research sheds light on air and air-bubble behavior in OOA aerospace prepregs. Out-of-autoclave processing: <1% void content? By Jef Sloan / Editor-in-Chief it is hoped, will help OOA processors more easily achieve <1% void content. Te research team at UD is led by Dr. Suresh Advani, George W. Laird professor and department chair of mechanical engineering and associate director of the univer- sity's Center for Composites Materials. He was assisted by Ph.D graduate candidate Tom Cender, Dr. John Ganglof (then a Ph.D. candidate, now a Science and Technology Policy Fellow at the U.S. Department of Energy – Fuel Cell Technologies Ofce) and Dr. Pavel Simacek, a research scientist at the Center. Assistance regarding bubble behavior was provided by Dr. Volkan Eskizeybek, assistant professor at the Department of Materials Science and Engineering, Çanakkale Onsekiz Mart University (Çanakkale, Turkey). Cender frst reported on the group's work in a paper presented at SAMPE 2014 (Seattle, WA, US) titled, "Void reduction during out-of-autoclave thermoset prepreg composite processing." Full prepreg vs. semipreg OOA manufacturing using a vacuum-based process is typically done with a fully impregnated fber reinforcement. However, without the heft of an autoclave, special care must be taken to manage air and void evacuation, and this is typically done with a semi-permeable layer of some sort. Alternatively, a fabricator can choose to use a fber reinforcement that is partially impregnated (a semipreg) with a resin in flm form. Te partial impregnation allows air and voids to escape the fabric as vacuum is drawn, before full consolidation is complete. It's on semipreg technology that the UD team has focused, eval- uating the use of resin flm with, initially, 2x2 twill fabric. Specif- cally, the team is evaluating the relationship between various levels of impregnation of the resin flm in OOA semipregs and the utility of intra- and inter-fber pathways for evacuation of gasses that are trapped in the laminate before consolidation and cure (see Fig. 1). FIG. 1 University of Delaware research has assessed air and air-bubble evacuation in out-of-autoclave (OOA) manufacturing at various levels of impregnation, using flm-type resin and a vacuum- based process. A camera under a clear mold (table) enabled documentation of air and bubble behavior. F I G . 1 U n i v e r s i t y o f D e r e s e a r c h h a s a s s a n d a i r - b u b b l e Clear Tables Applied Pressure & Heat Entrapped Bubbles Resin Film Bagging Film