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Aircraft/aerospace plastics - the muscular lightweights.

Composites are the key to many exciting existing and future aircraft/aerospace designs. To a large degree, unlocking the door to expanded applications depends on blending the capabilities of the materials with needed advancements in manufacturing cost-effectiveness. WINDOW OF OPPORTUNITY

For the success of the projected supersonic commercial aircraft, Donald Grande, manager, Structures, High Speed Civil Transport (HSCT) Program, Boeing Commercial Airplane Group, says a Mach 2 to Mach 2.5 design provides the best "window of opportunity," in terms of optimum gross weight and economic viability. The materials challenge would be significantly easier between Mach 1.6 and 1.8, he explains, but at those speeds the aircraft would not be as productive and thus would not pay off economically.

Boeing indicates that the airplane must provide the same economic return to the airlines, with about a 10% surcharge to the passenger, compared with subsonic jetliners. In addition, it must be environmentally friendly, with nonpolluting emissions and low noise, or it will not be built. Studies have shown that for the best chance of commercial success, an airplane must have a speed between Mach 2.0 and 2.5, an initial range of 5800 statute miles with a growth version up to 7500 statute miles, and a capacity of more than 250 passengers. Hypersonic speeds (more than Mach 5), requiring exotic structural materials and fuels, would be far too expensive to attempt to achieve.

Lightweight advanced composites are in the forefront of design planning for the first American commercial supersonic transport, which, if built, would be expected to go into service in the first decade of the 21 st century. This may seem far away, but it really is not, considering the many problems that must be solved before such an aircraft can be rolled out for economical service.

Both Boeing and McDonnell Douglas are exploring the viability of such a plane. Grande says that although the differences in temperature levels on the aircraft's exterior surfaces-about 160[deg.]F at Mach 1.6 and up to 315[deg.]F at Mach 2.4-can be substantial, that, in itself, is not daunting. A major challenge is to prove out the composite materials on the basis of the airliner's projected 20 year life, involving about 72,000 hours flying time. Of the total, 60,000 hours would be at supersonic speeds. This raises the problem of viable "real time" testing of aging effects, and how to accelerate those effects so that the materials can be proved out in relatively short development times.

Predicting that an all-composite exterior design will be necessary for the aircraft to "pay off," Grande says the current concentration is on thermosetting materials, although the improved damage tolerance of the thermoplastics and their stability under thermal oxidation is of great interest. Nevertheless, the essential requirement is to build a realistic database, by aging testing, to establish design credibility.

A critical consideration is the cost of the composite structures. "We still do not know how to design and build, even with automation, a low-cost structural composite," Grande maintains, "and this would be essential for success of the supersonic civil transport project." He predicts it will be 1997 to 1999 before many of the critical issues are resolved. THE FORTHCOMING 777

Pointing to real progress in primary composite structures on commercial aircraft, Al Miller, manager, Chemical Technology Unit, Boeing Commercial Airplane Group, cites the use of new toughened epoxy/carbon fiber for the empennage (the horizontal stabilizers and the vertical fin) and the floor beams of the forthcoming 350 to 400 passenger Boeing 777. The composite empennage is basically a laminated structure with stiffeners. In earlier designs with previous generation composite materials, the need for padded structural buildups over the stiffeners precluded use of automatic tape lay-up machines, which optimally require smooth surfaces for proper functioning. But owing to the recent availability of toughened epoxies, composite pads over the stiffeners are not required, thus facilitating the use of structural composite for the empennage application.

Miller says that composites constitute about 10% of the 777's structural weight, representing some 35% of the aircraft's exterior surface area. This compares with 3% by weight of structural composites on the 767. All flight control surfaces, and landing gear doors, non-load-bearing upper and lower fixed panels on the wing, and wing/body fairings will be made of composites.

In potential applications for primary structures, Miller foresees the use of the materials following a pattern of increasing complexity, advancing from the empennage, to the wing, and then to the fuselage. The foremost issue, he stresses, is not technological, but rather the need for overall cost-effectiveness. Typically, a saving of about 30% by weight can be achieved, compared with metal, for a typical composite application.

The strategy for the 777's interior is to comply with the U.S. Federal Aviation Administration's FAR 25.853 regulations, regarding the OSU 65/65 specification for heat release, by means of a combination of phenolic/glass fiber and phenolic/carbon fiber for all large surface areas. These applications also use a low-heat-release decorative embossing resin over the substrates. Passenger service units, thermoformed polyetherketoneketone (PEKK) on the 767, are being evaluated for possible injection molding, which may use special polyetherimide grades or polyarylsulfones.

The future of thermoplastic composites is still unclear in the area of subsonic commercial aircraft, since they lack overall cost advantages over thermosets. Thermoplastic composites with continuous carbon fiber-for ribs, spars, and stiffeners-rather than thermosets or metals, have not found application, especially where the impact of manufacturing economics, such as the cost of tooling, becomes significant. Efforts to resolve the overall economic issues continue, notably in tooling costs. Short-fiber-reinforced composites, however, for injection and compression molding, using thermoplastics such as nylon and PEKK, as well as selected compression-moldable epoxy thermosets to replace metal, project a more favorable economic impact. Access doors and small fairings used in numerous areas of the aircraft are examples. Data development is needed for greater penetration into the design community.

Combining material properties with improved design approaches, and simpler manufacturing with greater use of automation are up front in current efforts, Miller says. Incorporation and better understanding of in-process controls in the manufacturing processes is an ongoing goal. "We want to focus," Miller adds, "on what is really important and to standardize many operations to eliminate the need for in-process corrections in manufacturing, such as refinishing and shimming. For example, we are nowhere near the end in developing integrated part making, where a three-dimensional design is manufactured by, say, an automated tow placement technique in which the laying-in of the fiber reinforcements is programmed so that the design itself drives the placement head." THE NEW MD-11

Douglas Aircraft's MD-11, the world's newest and largest wide-cabin trijet, certified in November 1990, contains twenty-one major composite parts, predominantly of epoxy/carbon fiber, for a total weight approaching 10,000 lbs. Composites are used for all the control surfaces, except the rudder. The fuselage, wing, and horizontal and vertical stabilizers are all metal.

Douglas Aircraft's current use of the thermosetting epoxy/carbon systems reflects about 25 years of R&D and application efforts with composites technology, says Moto Ashizawa, group manager, Composites Structural Technology. Referring to a relatively small thermoplastic composite development program now in effect, Ashizawa says that "the materials are still difficult to manufacture economically; raw material costs are comparatively high, and they process at temperatures in the 600[deg.]F to 700[deg.]F range, compared with the 250[deg.]F to 350[deg.]F curing range for thermosets." Replacing the autoclave curing of thermosets, he adds, would require capital intensive thermoplastic forming presses.

Ashizawa says that the design philosophy for the MD-11 composite structures is built around solid-skin construction, emphasizing postbuckled design wherever possible; minimum use of thin-skin honeycomb sandwich construction; limiting strain levels to below the thresholds of damage growth; achieving ultimate load capacity with damage below the threshold of detectability; and avoiding application of metal technology where composites demand their own specific analytical procedures. "Premature failures have occurred in the past because not enough attention was paid to details," he says. "Compared with metals, composites are not as forgiving of design mistakes. Reliability, maintainability, and producibility must always be kept in mind."

For a future MD-12 aircraft, Douglas is projecting use of toughened epoxy/carbon composites. Now being designed with toughened epoxy is a wing box (12 by 8 ft by 15 inches deep) that will use stitched dry preforms and will be processed by resin transfer molding (RTM).

The stitched preform/RTM technology eliminates hand lay-ups, tape-laying machines, or fiber placement, as well as the need for fasteners and autoclave co-curing. The wing box is expected to be tested late in 1993 and, if successful, a 50-ft-long semi-wing structure will be built and tested in 1995. Pending a final decision to proceed, the structure would go into production in 1996 for the Douglas MD-XX twinjet, now in conceptual design.

Ashizawa says that all of the aircraft's exterior-including the wing and all control surfaces-except the fuselage, then would be fabricated of the toughened epoxies. Emphasis would be on the dry stitched preform/RTM processing. "Normally, under the compression loads of RTM, the composite layers would have a tendency to develop cracks. However, the combination of the toughened epoxies and the stitching provides additional resistance to compression and impact. The result is a highly reliable materials and manufacturing system, and one that is not as capital intensive as would be required by a thermoplastic approach."

Phenolic resin is the predominant material in the MD-11 interior. Sandwich panels of phenolic/carbon fiber are used for the floor, all walls, and galleys. Passenger service modules are made of phenolic/glass fiber. Ashizawa explains that the thermoset was selected since it meets the current U.S. Federal Aviation Administration (FAA) requirements and is most cost-effective. PROTECTING THE INTERIORS

Prior to 1988, FAA's flammability regulations regarding all interior materials in commercial aircraft basically involved meeting the vertical burn test requirements of its specification FAP, 25.853, say Karl Liebich, aircraft market manager, and Bruce Torrey, agency programs manager, Structured Products Dept., GE Plastics. The Bunsen burner test requires measurement of the flame's speed of consumption, length of travel along the specimen, and average self-extinguishing time, or start of dripping of the material. The specification applies to everything in the aircraft interior that has a large surface area, such as sidewalls, overhead bins, window reveals, ceilings, and passenger service units, but does not include seats and floor covers.

Gus Sarkos, manager, Fire Safety Branch, FAA Technical Center, says that an FAA regulation specifically for aircraft seat cushions, issued in 1984 and effective since November 1987, is based on a severe two-minute test in which a seat mock-up is exposed to a large burner that simulates the heat output of a jet fuel fire. No more than 10% of the cushions' weight may be consumed, and there must be no flame spread across any surface of the seat. Sarkos says that more than 600,000 seats had to be retrofitted by the regulation's 1987 effective date in order to meet the rigid requirements. Compliance with the regulation was achieved with fire-blocking covers over the polyurethane foam cushions, made basically of PBI felt, PBI/aramid blends, or aramid quilt.

Although similar approaches are still being used, there is a growing trend to specially treated polyurethanes, which can be constructed to meet the test requirements without the need for the weight-adding fire-blocking layers. However, although the special polyurethanes could incur a weight penalty greater than that of the fire blockers, the airlines may be willing to accept it because of the greater simplicity. The FAA is conducting an ongoing program of full-scale tests on completely assembled aircraft seats, thus including other plastics used in the total seat construction.

In 1988, the first of two FAA addenda to the original specification became effective. Built around the Ohio State University (OSU) criteria that measure the heat given off during combustion, it also can be an index of the burning material's flashover potential for rapidly spreading a fire. The regulation applies to aircraft interiors with 20 or more seats. Aircraft with up to 19 seats are still regulated only by the specification's initial vertical burn test.) Two criteria for the heat release test were that the peak heat release over the test's five-minute duration not be greater than 100 kilowatts per square meter, and that the total heat release over the first two minutes be no more than than 100 kilowatts-minute per square meter. A second addendum (OSU 65/65), which became effective in 1990, reduced the OSU test's maximum heat emission in each of the test's conditions from 100 to 65. In addition, the addendum included a smoke-density test requirement, performed in an NBS smoke chamber, in which a value of 200 at four minutes was set as the maximum allowable optical smoke density.

Sarkos emphasizes that while there are currently no specific FAA regulations for emission of toxic gases during an aircraft cabin fire, the full-scale OSU test makes a separate test largely unnecessary. He explains that since the release of toxic gases in a fire is triggered during flashover, delaying the onset of flashover by the OSU heat release standard implicity controls the toxicity. Some aircraft companies have internal standards or guidelines that attempt to track the toxic emissions during plastic combustion of six gases at the four-minute burning point. Some question, however, the applicability of the test methods in realistically gaging toxic gas emissions during a post-crash fire.

The reliance on the OSU test procedures, reflecting a general belief that the critical requirement is to delay the material's flashover point by minimizing the radiant heat emissions, has led to the development and application of more thermally stable materials. Where, before 1988, a range of sheet materials was used for interiors, including flame-retarded polycarbonate, ABS, ABS/PC, and PVC/acrylic blends, today's entries include materials such as modified phenolics, polyetherimide, polyethersulfone, polyetherketoneketone, polyarylsulfone, polyphenylene sulfide, and polyphenylsulfone.

Liebich and Torrey of GE say that the newer materials represent dramatic improvements. They indicate, for example, that a pre- 1988 polycarbonate sheet product would have registered 150-plus kilowatts per square meter in the OSU radiant heat test, whereas the company's new grades would measure about 60 kilowatts or under.

Sarkos of the FAA adds that his agency is also exploring the idea of incorporating a water-spray system inside the cabin. Such a system creates some concerns: the extra weight, the storage requirements, the possibility of premature activation, and the potential for corrosive activity after activation in event of a small, or limited, fire. Sarkos says, however, that in the present concept, the system is intended for use only in major crash fires and normally would be disarmed in flight. It would be crew-activated only during takeoff and landings. Full-scale fire tests of the system have been conducted on both wide-body and narrow-body test articles, and except for very severe fires, the FAA results indicate that this type of system provides two to three minutes additional time to escape. The FAA is now addressing practical issues related to installing a system in an aircraft; considering issues of activation and accidental discharge; and developing performance specifications. SLOW GROWTH

"The initial interest in thermoplastic composites has not yet blossomed into major usage," says Charles E. Browning, chief of the Structural Materials Branch, Nonmetallic Materials Division, Materials Directorate, Wright Laboratory, Wright-Patterson Air Force Base (WPAFB). The improved toughness of thermoplastics, and the elimination of the cumbersome autoclave curing that is required for thermosets, are potent inducements. However, a major consideration is the existence of an in-depth manufacturing base for thermoset composites, as against the large-scale capital investments that would be needed for a major shift to thermoplastics.

"As some of the potential applications drew nearer," Browning adds, "perceived and unresolved elements of risk fostered a reluctance to apply the thermoplastics to systems on a broad basis. Instead, applications were limited to specific areas." Nevertheless, a result of the activity with thermoplastic composites was the catalytic effect on the development of a whole new class of toughened epoxies and bismaleimides.

The current emphasis with thermoplastic composites at WPAFB is to develop more cost-effective manufacturing methods for primary fuselage structures, and for secondary structures such as access doors, panels, and leading and trailing edges, which have a higher damage potential in service. The thermoplastics are advantageous here because of their toughness and the inherent opportunities for lower-cost manufacturing of parts requiring relatively complex structures and buildups of the composite layers. For example, ten thermoplastic composite doors are being made to demonstrate low-cost manufacturing of an actual aircraft part. The use of commingled hybrids, involving production of woven fabrics from intertwined strands of resin and reinforcing fibers, or incorporating ground thermoplastic powder over the woven graphite, is another ongoing approach.

A large program coordinated by the Defense Advanced Research Projects Agency (DARPA) is another indication of the continuing search for innovative processes to take better advantage of the potential of thermoplastic composites. For example, work on a hot-head delivery system, basically an extension of automatic tape-laying machines, fosters low-cost goals.

"We are trying to stretch the technology," Browning says. "Under a program for ultra-lightweight materials development, we are pursuing approaches to allow weight reductions by as much as 50%. In 1965, for example, carbon fibers typically had tensile strengths of about 100,000 psi and flexural moduli of 30 million psi, and sold for about 1500/lb. Today, carbon fibers with tensile strengths of 800,000 psi and moduli of 40 million psi and costing less than $ 100/lb are not unusual. Our effort now is to utilize the higher performance carbon fibers in advanced matrices, and to provide much lighter-weight and more cost-efficient aircraft parts. We are also looking to possible use of fibers in the 50 million psi modulus range, while retaining the 800,000 psi tensile strength."

Much still has to be done to gain better understanding of the behavior of the fibers in their matrices, including those under compression, to optimize transfer of the fiber properties into the composite. Among the current efforts is to define a set of target properties, using advanced matrix materials. Included in this group, Browning says, are thermoplastic molecular composites (Dow's polybenzoxazoles, for example), in which high strengths are achieved by inherent orientations in the resin. Molecular composites have the potential of entirely eliminating the need for supplementary reinforcing fibers.

In the area of ultrahigh-temperature resins, Wright-Patterson AFB started a program a few years ago to foster development of a polymeric material for composites with continuous-use temperature in air to 700[deg.]F, and which would also be cure-processable in 200 psi autoclaves. The goal was to advance beyond the state of the art, to the 700[deg.]F level, from the existing PMR-15 polyimide, with use temperatures of 550[deg.]F to 600[deg.]F.

A progeny of the ultrahigh-temperature program, fluorine-based AFR700B, developed by TRW under contract to the U.S. Air Force, retains 50% or more of its room-temperature properties at 700[deg.]E Various companies are developing and testing parts.

The higher threshold opens possibilities for replacement of titanium for weight reduction in static structural areas of the aircraft engines as well as specific areas of the aircraft structure. In addition, replacing die metal with composites, where it was not possible before because of temperature limitations, would reduce radar observability of the aircraft-in situations where this is necessary.

Browning adds that during the next few years cost reduction will be at the forefront of technology and application development. One area of emphasis will be to make the autoclave curing cycle for thermoset materials shorter and more efficient. Introduction of more sophisticated sensor-based processing and computer controls in the cycle is expected to result in enhanced part consistency and reliability and greater production throughout. OUT OF THE LABORATORY

"One of our missions is to effect a transition of thermoplastic composite technology out of the laboratory and into industry, by demonstrating the feasibility of economically producing full-size structural primary and secondary aircraft components," says Diana Carlin, program manager, Manufacturing Technology Directorate, Wright Laboratories, Wright-Patterson AFB. "In the last few years we have validated numerous structural thermoplastic parts based on improved manufacturing approaches that have emphasized higher productivity and lower costs."

Carlin says that Wright Laboratories has sponsored several programs in manufacturing technology aimed at design, manufacturing, and confirmation of composite costs. The programs are built around analysis, first, of small-sized parts, and then, of full-scale parts, in limited production runs. Included are investigations for adapting autoclaving, thermoforming, superplastic forming, and tape-placement techniques to the particular higher temperature requirements of the thermoplastic materials.

"We have learned that there are really no generic answers for achievement of a low cost structural aircraft part," Carlin says. "To optimize cost, each component or subassembly must be specifically assessed, based on complexity and functional requirements, for proper selection of preform material and manufacturing method."

To facilitate the selection of materials and manufacturing processes, Carlin says, the Manufacturing Technology Directorate has recently awarded a contract to the Northrop Corp. for developing and validating a computerized "Integrated Product Manufacturing System" for faster and lower-cost production of spare thermoplastic aircraft parts. The program is intended to allow retrofitting from an original metal to a thermoplastic noncritical, secondary structural part, such as a door, control surface, or fairing, without previous existence of the plastic design. Although some preliminary design will be necessary for the new thermooplastic spare part, Carlin explains, the expert system's database then will be able to aid in selecting the material, additional design parameters, and the manufacturing process, and preparing a cost analysis.

The computerized system will centralize access to multiple existing databases and consolidate diem to optimize their functionality and the system's capabilities for making design and production suggestions. Full-size structural components developed with the system will be built and evaluated from both functional and cost standpoints. Carlin targets availability of a validated, computerized product manufacturing system for 1996. THE AERO-SPACE PLANE

If development proceeds on its present course, advanced composites also will have a significant role in selected areas of the U.S. National Aero-Space Plane (NASP), a joint effort of the Department of Defense (DOD) and National Aeronautics and Space Administration (NASA), and supported by the aerospace industry. The program's goal is a two pilot, liquid-hydrogen-powered, single-stage-to-orbit vehicle (designated the X-30) capable of taking off and landing horizontally, flying directly into orbit like a spacecraft, and long range cruising at hypersonic speeds within the atmosphere. Technological advances supporting the goal of achieving an aero-space plane before the year 2000 include development of high strength, lightweight, high temperature materials; increased understanding of supersonic combustion phenomena; and the availability of supercomputers for engine/airframe design integration.

Following evaluation of preliminary designs, a team was chosen to continue the work and to build and test major engine and airframe components for the research vehicle. The group, now working as a cooperative consortium, consists of three airframe contractors (General Dynamics Corp., the North American Aviation Operations Division of Rockwell International, and McDonnell Douglas Corp.) and two engine contractors (Rockwell's Rocketdyne Division, and Pratt and Whitney, a Division of United Technologies). Present designs project a plane the size of the shuttle orbiter or a 727 airliner with propulsion modules that integrate ramjet/scramjet engines along with small rocket motors. A decision, expected in 1993, to proceed with the next phase of the program, construction of the X-30, would initiate a schedule leading to a two-year flight test program beginning in 1997.

Terry Ronald, head of Materials Technology, NASP Joint Program Office, Wright-Patterson AFB, says that the aero-space plane has two major material requirements. First, at hypersonic speeds, the exterior materials must withstand very high temperatures -the vehicle's leading edges could "see" temperatures in the range of 4000[deg.]F to 5000[deg.]F, and other sections of the airframe, anywhere from 1000[deg.]F to 3000[deg.]F. Second, light weight is critical, in order to maximize the vehicle's fuel capacity and efficiency.

The X-30's fuel tank, made of thin-wall epoxy/graphite composite, will occupy almost the complete interior of the vehicle. Ronald says it is essential that there be no diffusion through the tank wall, and it appears the composite will meet the requirement. Polyimide, aluminum alloys, and designs involving multilayer thermal insulation also were considered before the epoxy/graphite composite was selected.

In a basic heat-balance approach, the flow of the cryogenic liquid hydrogen, starting at -423[deg.]F, is routed so that it cools areas of the airframe structure and most of the engine structure. Ronald says that some of the support structure for the exterior skin could be provided by polyimides, and other areas would be protected with thin carbon/carbon composite panels. Because of their excellent conductivity, copper/graphite fiber composites, using pitch-based fibers with moduli of 100 million psi or more, are under consideration for use near the vehicle's leading edges to provide quick heat transfer. In production, the copper-coated graphite fibers are positioned in a mold and hot-pressed to form a fully dense material.

Ronald says the aero-space plane's developing technological base will serve both military and civilian needs in the future. The copper-based composites, for example, could be of interest in the electronics industry, where faster heat transfer in circuit boards is needed. NEW BACKBONES

The development of engineering thermoplastic materials with improved toughness, or damage tolerance, accelerated programs to provide toughened thermoset resins that would match or exceed the challengers' performance. The focus was on new resin backbone structures and formulation adjustments, often with thermoplastic modifiers, to yield toughened materials, often with higher temperature resistance, achieved either by increasing the Tg of the resin or by reducing its tendency to absorb moisture.

Chuck Swartz, market manager, Aerospace, Dow Chemical Co., says that the new epoxy, polycyanate, and bismaleimide systems are a direct result of the effort to counter the threat from the thermoplastics to the traditional thermoset composite markets. In addition, Swartz points out, prepreg formulators began to incorporate new thermoplastic or second-phase rubber modifiers (10 to 30 wt%) that significantly improved the performance of thermoset resins. The development effort also produced hybrid systems with handling and storage characteristics of the traditional thermoset resins and with the toughness of the thermoplastics. BROAD DEVELOPMENT PROGRAM

Many believe that greater fulfillment of the potential of composites is critical for the retention and enhancement of U.S. leadership in aircraft design and manufacturing, The creation in 1990 of the NASA Advanced Composites Technology program reflects this awareness. Integrating industry, government, and academia to foster the development of large composite primary airframe structures and cost-effective fabrication, the program includes airframe builders, universities, and material suppliers, with the aim of increasing capabilities and reducing the cost of composite structures through materials and process development. The program is divided into three technical work elements: advanced materials and processes, advanced structural mechanics, and advanced structural concepts development.

Composites technology is advancing on many fronts and making it possible to cross new design thresholds. It is a technology that is simultaneously here and rapidly expanding beyond its current boundaries.
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Author:Wigotsky, Victor
Publication:Plastics Engineering
Date:Dec 1, 1991
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