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Aviation History
1981
1981 - 0148.PDF
lis mm To wards the plastic fUCHT .17 IN Geneva Du Font invited Kevlar and other plastics users to share their experience with the airframe and missile makers. J. M. RAMSDEN was there to report the papers and dis cussions*. KEVLAR is a very light material, having a density of about 1-44 compared with 1-74 for carbon (graphite in US parlance), 2-54 for glassfibre, 2-7 for aluminium and 7-8 for stainless steel (gm/cm3). It has high impact-resistance and toughness, which make it a better crack-stopper and vibration-damper than glassfibre or carbonfibre. The material has the highest specific tensile strength of all commercial fibres, though it has a rela tively limited compression strength. Compared with glassfibre, for ex ample, Kevlar is stronger, has twice the elasticity modulus, and is more than 40 per cent lighter. Kevlar 49, as produced by Du Pont for the aero space industry, has a tensile strength of more than 500,0001b/sq in. Like other synthetic materials it is highly corrosion-resistant. Though it has an inherent moisture "regain", or pickup, the material is claimed to be no more of a problem in this respect than are other composites. Depending on the epoxy resin, tests by Nasa on TriStar components indi cate a moisture pickup slightly higher than that of glassfibre and carbon- fibre. But, after three years' testing in tropical conditions, including ultra violet exposure, mechanical properties are claimed to have been substantially retained. Kevlar, glassfibre and carbon- fibre in two epoxy resins have been examined over 21 days at 82°C and 95 per cent humidity. Initial results indicate responses similar to those of glassfibre and carbonfibre. Kevlar and Nomex, both Du Pont inventions, are closely related chemi cally, being aromatic polyamides or "aramids". Both are inherently flaime- * The papers are available as a book from Garry Lambert, Du Pont de Nemours International. 16 Rue Alexandre-Gavard, CH-1211, Geneva 24, Switzerland; Telex: 22512 Geneva; Telephone: (022) 278111. "Towards the plastic Aeroplane—1," based on Ciba-Geigy experience, was published in "Plight" for July 19, 1980. Kevlar is a Du Pont tradename. resistant, requiring more than 27 per cent oxygen to sustain combustion. They are self-extinguishing in a nor mal atmosphere when the flame source is removed. Smoke-generation is quite low compared with most organic poly mers, but "the toxicity of any com posite material is still a very open question", to quote a Lockheed expert. Another kindred non - flammable material is Du Pont's Kapton, a poly- mide film used for wire and cable insulation, and for encasing flammable upholstery foam or insulation blankets. Kapton does not burn, melt, smoke or propagate flame, and is said to remain serviceable up to 400°C. Fabrics are the most common appli cation of Kevlar 49, usually in "pre- preg" (resin pre-impregnated) form. Like glassfibre it is laid up in sheets which can be converted into solid laminates or bonded to a honeycomb core. Honeycomb made of the kindred material Nomex paper has been used as a sandwich core on commercial aircraft for many years, faced with glassfibre or carbonfibre. Peel- strength problems have been dealt with by adding more resin to the aramid fabric or redesigning the weave. Adding more resin before laminating to the honeycomb costs weight, but a Kevlar sandwich is still considerably lighter than one of glass. Nomex-fibre carpets for airliner cabins are as fire-resistant as are wool/nylon carpets, and are lighter and easier to clean. The lightness is claimed to compensate for the higher price. Flame-resistance is at least equivalent to that of wool/nylon, and there is less smoke-generation. Nomex and Kevlar felts are being used also for acoustic insulation, air-condition ing ducts and carpet underlay. Kevlar TriStar On the TriStar, which in 1972 was the first major airframe application of Kevlar to enter passen ger service, Lockheed says that 2,5001b of the material has saved 8001b. Kevlar is used for TriStar cabin interiors, control surfaces, fixed wing leading and trailing edges, elevator and rudder trailing edges, and other secondary structural components in cluding wing-to-body and aft-body fairings. All have performed satis factorily for more than 20,0O0hr. Eighteen fairings are being flight- tested by three airlines: Eastern, TWA and Air Canada. From early 1973 to the end of the 1980 summer, reports Lockheed, three shipsets had flown a total of 52,000hr with no significant defects or major damage incidents; minor impact damage has been repaired or filled with resin. The only adverse condition not noted on glassfibre parts is some fraying of fastener holes, reflecting 1972 machin ing state-of-the-art when the parts were made. Lockheed finds the material compat ible with standard glassfibre resins and—except for machining—similar in its forming characteristics. Com pression strength is lower than that of glassfibre, but impact strength is better. Summarising seven years of service with Kevlar components, Lock heed says that "the significant result is the absence of any major defect." The test programme continues until 1983, to complete ten years' experi ence with Kevlar components in regu lar TriStar service. On the Lockheed Trident missile, the material is said to save 8001b by replacing glassfibre for the motor cases of all three stages. The new MX missile will have a similar structure. Boeing 757/767 Similar Kevlar ap plications are being made by Boeing to tne 757 and 767. Boeing intends to
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