CompositesWorld

JUL 2018

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JULY 2018 6 CompositesWorld COMPOSITES: PAST, PRESENT & FUTURE » A significant milestone occurred in thermoplastic compos- ites recently, and hardly anybody noticed. Gulfstream Aerospace (Savannah, GA, US) delivered its 300 th Gulfstream 650 aircraft. is twin-engine business jet, which began production in 2012, is the first commercial airplane to use critical control surfaces made from thermoplastic composites. Airbus has successfully employed thermoplastic composites on the leading edges of its A300-series aircraft for decades, but these are not critical control surfaces. If a leading edge falls off the plane, then the plane still lands without a problem and every- body stays safe. If a critical control surface fails, then probability of a catastrophic landing increases substantially. ermoplastic compos- ites were not considered for critical or major structural compo- nents in aircraft for many years. is was true for several reasons. First, thermosets are in the comfort zone for many — they're structural and stable and have 40+ years of flight-allowable data- bases behind them. e application of continuous-fiber compos- ites is almost completely structured around thermoset resins. Major composites manufacturers use autoclaves (and now OOA ovens), and other thermoset-driven capital equipment. Along with the thermoset-focused database and capital equipment, most composites engineers have lived in the thermoset comfort zone for their entire careers. ey've designed or tailored a process around a handful of off-the shelf, flight-certified prepregs. Shop technicians are experts in vacuum bagging, bonding or other processes based on thermoset use. e customers only wanted to use thermosets, because they knew nothing about those "exotic" materials called thermoplastics. is comfort zone in a necessarily conservative community is a major reason for the aerospace industry's slow progress in exploiting the advantages of thermoplastics. Even when a ther- moplastic prepreg starts at less than 0.5% porosity (some of them do), and the AFP part made from the prepreg is at a similar porosity, some still want to put the final part into an autoclave to ensure consolidation. Heck, even some well-versed thermoplastic composites engineers like the security associated with ensuring consolidation via autoclave. If you find a thermoplastic composite in a database, it's likely a PEEK that is autoclave-consolidated. When you do that, you lose the price advantage of thermoplastics. Back to the G650. Its elevator and vertical tail rudder are made with carbon fiber/PPS composite and then assembled using induction welding via an FAA-certified process. at one sentence describes three milestones associated with the parts. First the elevator and tail rudder are critical for maintaining control of the aircraft, and the FAA would not certify them without substan- tial proof of performance. Second, the use of PPS — not a poly- ketone — on a critical part, was, when these structures were designed, almost inconceivable. Sure, PPS had been used on leading edges, but the resin only has a glass transition tempera- ture (T g ) of 90°C. On a hot summer day in the Mojave desert, at a location on the plane near the engine exhaust, one can be sure the material surface temperature will come dangerously close to 90°C. Wouldn't design of a critical control surface with such a low-T g material create unnecessary risk? Fortunately, PPS (and polyketones) are semi-crystalline polymers. e chain structure within the polymer enables them to retain a significant portion of their strength and stiffness above their T g . In contrast, when a thermoset, such as epoxy, is exposed to temperatures above its T g , it decomposes. PPS, in fact, has been used in underhood automotive applications at temperatures of more than 140°C for many years. An older composites engineer (like myself ) would have had a hard time selecting a matrix material that could operate above its T g . But some young, upstart engineer that "didn't know any better" got it to work, and that was a major milestone. Now for the third milestone. A major advantage of thermoplas- tics is that they can be welded, thereby eliminating the need for bonding and riveting and the cost and weight issues associated with each of these. For a welded, critical thermoplastic composite to be FAA-certified, it would have to be proven to meet spec every time. KVE Composites Group (e Hague, e Netherlands) devel- oped the welding process for part manufacturer Fokker Tech- nologies (e Hague, e Netherlands) using TenCate Advanced Composites (Nijverdal, e Netherlands) CETEX laminate prepreg. (Guess where? Yes, those A300-series leading edges.) And it was good enough to become FAA-certified. (As a side note, every thermoplastic composite engineer should thank God for the Dutch, but that's a topic for another day.) So, despite the major technical milestone at Gulfstream that started production more than five years ago, why is the aerospace composites industry still operating in the thermoset comfort zone? One reason is an education gap: I sat on a SAMPE panel a couple of years ago with a professor from a major US university that has a heavy composites curriculum. One of his slides claimed there were The real impediment to use of thermoplastic composites in critical control surfaces is an education gap. Thermoplastic composites in aerospace — the future looks bright

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