Aeronautical Technologies for the Twenty-First Century (1992) / Chapter Skim
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9 Materials and Structures
9 Materials and Structures
Pages 187-220
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From page 187... ...
aircraft, now have composite structures ~7
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From page 188... ...
Commercial transports use advanced composites in essential secondary structures such as flaps and control surfaces and in some primary structure such as vertical fins. The advantages of composite materials, as exemplified by their greater strength and stiffness per unit weight, superior fatigue and corrosion resistance for many applications, and potential for lower manufacturing costs through reduced part counts and tooling expenses, make their wide application to U.S.
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From page 189... ...
Similarly, computer-aided design tools make it easier and quicker to consider a much greater variety of alternative structural designs. The use of high-speed, large-memory computers permits, in turn, more detailed internal structural loads analysis for each of the many loading conditions and design alternatives, with fine and analysis determining more precise load paths, stress dis~ibudons, and load deflection charactensUcs for subsequent aeroelashc analysis.
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From page 190... ...
For propulsion systems, higher specific strength and ability to withstand higher temperatures are the principal drivers. These objectives, in turn, require advances in materials, structural design concepts, life prediction methodologies, and fabncabon technologies.
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From page 191... ...
An appropriate program of this kind should be guided by needs that arise in the development of generic aircraft types; it also should, by its results, change the direction of generic aircraft developments. Thus, an appropriate fundamental program of materials and structures research should seek to provide both evolutionary and revolutionary advances in materials and structures, which will be required to sustain a leadership role in both airframe and propulsion technologies.
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From page 192... ...
This class of materials is, in general, very large; it includes polymer matrix, metal matrix, and ceramic matrix composites (CMCs) , as well as continuous and discontinuous fibers.
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From page 193... ...
However, the magnitude of the potential benefits from these matenals for higher-temperature applications, such as uncooled turbine engine components, justifies major research efforts. Both ceramic matrix and ceramic fiber technologies need to be pursued, along with an emphasis on improving fabrication technology.
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From page 194... ...
Improved structural analysis methods capable of exploiting the computational power that win be available In the near future should be a high-priority objective of structural design research. A necessary adjunct of this is development of tools to reduce the cycle time for generating structural analysis models sufficiently that such analyses for both strength and stiffness can accompany the earliest structure design concepts considered by designers.
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From page 195... ...
Applications where weight savings, fatigue life, and corrosion resistance override cost considerations have been limited VTOL and combat aircraft. Essential to the realization of reduced manufacturing costs with composite structures is a reduction in labor costs sufficient to offset higher materials costs.
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From page 196... ...
Significant weight and cost reductions were achieved by using composite sandwich construction in the Airbus A330/A340 rudder. Weight was reduced by 20 percent and cost by 10 percent, compared with the metal design it replaced.
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From page 197... ...
substructure. Experience to date has shown that design and tooling for integrally stiffened son panels should provide for adjustment in the position of the substructure to be attached to skins, to account for tolerances of fit-up between skin panels and felines and stiffeners, for fuselages, and for ribs and spars for wings.
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From page 198... ...
This is also true for bonded joints in metal structures, particularly when the extended useful lives of commercial aircraft are considered. From these considerations, it is apparent that structural design with composites is influenced to a far greater extent by fabrication technologies and materials choices than is the classical design of metal airframe structures.
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From page 199... ...
Although sensitivity and reliability of crack detection need an order-of-magn~tude improvement, both NDE and the damage tolerance of materials and their applications must be advanced before efficient da~nage-tolerant design concepts can become routine for airframes and the critical rotating parts of turbine engines. Integration of NDE into the structural concept/design/fabrication processes and automation of the NDE process also require greater attention.
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From page 200... ...
Programs dealing with aircraft structural integnty, fleet structural management, and aircraft life cycle management and operation are important contributors in this regard, but technology advances are needed in each of these three parts of life management programs. The same basic philosophy in life management programs is common to metallic and composite structures, but the technology advances required are different for structures composed of these two classes of matenals.
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From page 201... ...
Nondestructive inspection techniques for laminated composite structures are not well developed in comparison to Hose for metallic structures. The fact that much of the damage in composite materials occurs below He surface of the structure and can, therefore, not be detected by visual methods hampers nondestructive inspection.
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From page 202... ...
Thus, the financial risks undertaken by private companies when they introduce advanced materials and structures into commercial transport aircraft go beyond liability for passenger safety-as important as those ramifications are and can involve structural maintenance, modification, and repair of fleets worldwide. Airframe Lower structural weight fraction and lower costs are high-payoff aspects of advanced subsonic airframe structures.
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From page 203... ...
Hybnd composite construction does promise the means to do this, with bundles of highstrain-aDowable fibers interspersed at intervals among the high-modulus fibers that provide the bulk of structural properties. In addition, these structural concepts will have to meet damage tolerance and long-life requirements Apical of transport aircraft.
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From page 204... ...
The families of materials to be considered for engine applications, in the general order of increasing temperature environment, are PMCs, aluminum MMCs, advanced titanium alloys, titanium MMCs, superalloys, titanium and nickel aluminides, intermetallic matrix composites (IMCs)
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From page 205... ...
MMC disks should provide the improved temperature capability desired. Managing He cost to manufacture these disks win be crucial; ensuring long-term reliability will be essential (through damage tolerance and adequate creep resistances; and providing efficient joining techniques, which allow the rotor to be built up from many individual pieces, will be necessary.
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From page 206... ...
. As in compressor applications, additional turbine structural challenges include developing new design concepts that capitalize on the unique properties of composite materials.
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From page 207... ...
Figure 9-2 shows the expectations for increased temperature capability of bearing Target Matenal bulk temperature capability at stress 1950 1960 1970 1980 Service Entry (year) 2000 2010 FIGURE 9-2 Expected temperature capability of turbine engine bearing systems as a function of service entry year.
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From page 208... ...
They clearly must be made of composites if advanced engine weight goals are to be achieved. Their temperature requirements are modest, so that polymer matrix composites can be applied.
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From page 209... ...
Thus, the materials technology program required to meet HSCT requirements should focus on PMC, advanced titanium alloys, and the development of cost-eff~cient design concepts for titanium and hybrid laminates. Not only are operating economics directly affected, but current Currently, there are no proven PMC materials or aluminum alloys capable of 60,000 hours of service in an airframe structural environment at temperatures in the 225-375 °F range.
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From page 210... ...
Polymer Matrix Composites Technology PMC technology development should include high-temperature thermosetting and thermoplastic matrix resins. Significant improvements in both processibility and high-temperature stability are required for the HSCT mission.
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From page 211... ...
The present NASA program embodies many charactenshcs needed to achieve these goals, but the major emphasis to date has been on subsonic aircraft requirements. Much of technology development involving new structural concepts is applicable to both subsonic and supersonic designs, but the research program should be balanced to ensure that materials and ma~nufactunng process development will include those compatible with the more extreme requirements of the HSCT.
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From page 212... ...
In addition to materials with higher-temperature capability, structural concepts must be developed that avoid high thermally induced strains at points of attachment. Advanced aerothermal and structural computational codes are likely to be needed to achieve optimized designs, and appropriate testing is required to prove the feasibility of advanced combustor configurations.
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From page 213... ...
All others are of foreign design and manufacture. Low-weight composite and/or superplastically formed metallic airframe structures, with costs substantially below those of aluminum structures, could provide a competitive edge, helping U.S.
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From page 214... ...
In addition, active rotor controls can reduce vibrations generated by He rotor of tiltrotor aircraft in cruise flight, which are caused by rotor operation in the wing's nonuniform flow field. Suppression of Tiltrotor Aircraft Wh~r' Fluder The highly coupled behavior of the ~ciltrotor a~rcrah's rotor and the flexible wing on which it is mounted calls for active control applications to suppress whir]
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From page 215... ...
Airworthiness requirements fatigue resistance, fail-safe characteristics, damage tolerance, inspectability, repairability, and resistance to environment effects require major attention in the nation's matenals and structures technology programs. This should include consideration of how compliance with a~nvor~iness regulations can be demon straw on a practical basis during aircraft certification programs.
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From page 216... ...
Furthermore, the time between conception and application of new structural materials is very long, largely because ultraconservatism must be exercised by responsible structural designers. There is also a need for extensive data bases adequate to ensure substantiation.
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From page 217... ...
Engine Applications New, high-temperature-capable materials needed for advanced engine developments are often cited as including metal matrix composites, ceramics, ceramic matrix composites, and ntermetallics. All of these advanced materials hold considerable pro~se, despite their high costs, of jet engine nozzle applications-particularly in HSCT aircraft.
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From page 218... ...
Reduction in size, weight' and cost of the components constituting these systems, through fiber optics, microprocessors, and smart material sensors and actuators, will allow the redundancies necessary for operations in keeping with commercial transport safety standards. It is noted, however, that before any diagnostic means for increasing structural integrity can be useful, the damage tolerance of composite materials needs to be increased substantially.
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From page 219... ...
NASA's program of materials and structures research for the HSCT should give high priority to developing basic composite and metallic materials and design concepts for 225-375°F operations that save significant weight relative to current metal structures, can be produced at costs acceptable for airframe applications, and have durability arid toughness that resist degradation for 20 years of operation. This program is seen as involving large-scale test components and, preferably, full-scale service testing.
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From page 220... ...
These may encompass, for example, stabilizing aeroelastic phenomena, internal noise suppression, and rotorcraft vibration reduction. Eng~ne-Specif~c Research NASA should assign highest priority, in its long-range engine materials research program, to the development of ceramic matrix composites, including fabrication technology.
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