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Aviation History
1981
1981 - 0150.PDF
140 BOEING 767 PLASTICS Carbon Spoilers inboard and outboard Outboard ailerons Inboard aileron panels Rudder Elevators Weight saved 426kg (9381b) Hybrid Wing fixed TE panels Cowl components Wing-to-body fairing Main landing gear doors Nose landing gear doors Stabilizer fixed TE panels Stabilizer seal plates Lower LE access panels 246kg (5421b) Kevlar Stabilizer tips TE flap linkage fairings Strut fairings ECS ducts Cargo liner Stowage bins Emergency escape system components Lavatories Closets Partitions Inboard flap debris protection 249kg (5481b) Total 921 kg (20281b) ing to Graham Dorey, head of the RAE composite section, the laminates were subjected to 6mm steel-ball or drop-weight impacts of up to 18 joules. The compression strength of Kevlar composites is about one-third that of carbon, perhaps less, but impact- resistance is about three times greater. According to Dorey, onenthird Kevlar can double the composite's impact-strength. Fibres such as carbon, boron and Kevlar, which have a high modulus of elasticity, are useful for maintain ing the shape of control surfaces under aerodynamic load. Glassfibre is useful where more "compliance" is required—in helicopters, for example. Unlike metal, plastics do not absorb energy inelastically by plastic flow, only by fracturing. Nevertheless, the fracture energies of composites com pare favourably with those of metals. Tests show that the highest-modulus fibres such as carbon need relatively little impact energy to fail, whereas fibres such as Kevlar and glass require significantly more. Says Dorey: "It is reasonable to consider combining a high-modulus fibre like carbon, which gives stiffness, with more damage-tolerant fibres such as Kevlar and glass to improve impact- resistance." Kevlar, having a higher elasticity modulus, contributes more than glass in a hybrid laminate to load-carrying capacity; at the same time it has signi ficant energy-absorbing capacity. In a hybrid laminate of Kevlar/ Carbon/Kevlar (KCK), it is found that the Kevlar benefits from the sup port and reinforcement of the carbon, especially in compression, while the carbon benefits from the energy- absorbing and fracture characteristics of the Kevlar. An added bonus is that the material is lighter than carbon so that KCK laminates save about six per cent weight for a given volume. A good compromise is KCK, where the Kevlar outer ply protects the carbon in the sandwich; and one-third K to twonthirds carbon is a good mix, in Dorey's opinion. Du Pont is following up Royal Air craft Establishment work to improve carbonfibre's damage - tolerance and crack-propagation properties by blend ing it with Kevlar in the composite. The key question is whether the trade off in mechanical properties will be acceptable. Du Pont does not claim to have the complete answers yet, despite promising results. Changes to the fibre itself to Brittle carbon composites can be made more battleworthy by laminating with Kevlar. From left to right: carbon, carbonjKevlarjcarbonl Kevlarlcarbon; K/C/K; and K. The top four specimens are seen after impact at 2 Joules, the middle set at 4J, and the bottom specimens at 8J—by a 6mm steel ball in ecch case FLIGHT International, 17 January 1981 improve compression strength is expected to be a difficult problem. Machining Kevlar is a difficult material to cut because of its very toughness. Tools must be sharp and clean, providing an efficient shearing action. Du Pont admits that in the area of machining Kevlar composites, "our goal—to make the material as easy to handle as glassfibre—has not yet been met." A Du Pont book "A Guide to Cutting and Machining Kevlar"^ recom mends tools for sawing, drilling, countersinking, grinding, sanding, chamfering, milling, and turning. The tools include serrated scissors, com mercial scissors, rotary shears, power shears, tungsten carbide razor blades, lasers and water jets. Tools developed for machining Kevlar work well on hybrids of the material, including glassfibre and carbonfibre, but wear-life is adversely averted by the "aggressive" nature of glass or carbon. On the whole, recom mended machining conditions—speeds, feeds, set-up and tooling—are much more critical for Kevlar than for glass or carbon composites. In many ways machining the material is similar to that of wood; to obtain the best results, a shearing action using sharp blades with low heat generation is desirable. Laser and waterjets are already proven methods of cutting Kevlar composites. Conclusion This is how P. L. Wagner of Du Pont sees the future of Nomex and Kevlar: "We see Nomex con tinuing as the choice for honeycomb structures in next-generation aircraft and we see a bright future for it in new applications such as carpets and insulation because of its light weight, flame-resistance and durability. . . . "Because Kevlar is a relatively new material, we believe our first order of business is to provide a solid engineer ing data-base. When complete, this data-base will include static and dynamic properties before and after environmental ageing. "We hope to confirm the significant advantages we see for Kevlar over carbon and glass in damage-tolerance and crack-propagation. We will use these advantages to improve the properties of carbon composites in structural applications and at the same time compensate for the modest compression strength of Kevlar. "We believe our studies on the adhesion of Kevlar to matrix resins will pay off handsomely. Here we are very early on the learning curve versus carbon and glass, and are con vinced that composite properties can be improved substantially by opti mising the resin or the fibre surface. Du Pont has substantial resources committed to making the aramids a success in the aircraft industry, and has high confidence that the R&D effort will pay off."
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