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Re: bonding aluminum

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  • frederickwilson2000
    VI. Non-Structural Applications on Commercial Aircraft Adhesives are not used just for structural applications on modern aircraft. In fact, the
    Message 1 of 24 , Aug 3, 2004
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      VI. Non-Structural Applications on Commercial Aircraft
      Adhesives are not used just for structural applications on modern
      aircraft. In fact, the number of non-structural applications of
      adhesives vastly outnumbers the structural applications. Adhesives
      are used for everything from assembling lavatory walls to attaching
      the "No Smoking" sign to cabin partitions. Just a sampling of
      adhesive types and applications are discussed below.

      Pressure sensitive adhesives (PSAs) based on acrylic, natural rubber
      and silicone are employed primarily for ease of application. To name
      just a few applications, PSAs bond decals to surfaces, interior
      decorative surfaces to interior panels, interior trim pieces in place
      directly or hook and loop tape for the same purpose, structural shims
      in place during manufacturing and acoustic (sound deadening)
      materials to body skin interior surfaces. Tape products with pressure
      sensitive adhesive on one or both surfaces are used for such
      functions as cargo compartment sealing, as a fluid barrier to prevent
      spills and leaks in the lavatories and galleys from corroding the
      underlying structure and to hold carpets and mats to the passenger
      compartment floor.

      Room temperature curing epoxies are useful for applications that need
      higher strength and durability than adhesives such as PSAs and
      urethanes, but do not require the very high strength of elevated
      temperature curing structural adhesives. Room temperature curing
      epoxies are used for such purposes as bonding rub strips to sliding
      components such as flaps, to apply permanent identification plates to
      major assemblies, and to bond composite sandwich panels together to
      form interior assemblies such as stow bins, lavatories, galleys and
      closets.

      Polysulfides are more commonly used as sealants but are occasionally
      employed as adhesives as well. Polysulfides adhere well to a wide
      range of materials and are useful where some flexibility is needed in
      the joint.

      Contact Adhesives – Natural and synthetic rubber-based contact
      adhesives are used for bonding various interior decorative materials
      such as fabrics and decorative laminates to underlying surfaces.

      Cyanoacrylates – Fast curing cyanoacrylates are used for holding
      shims in place during assembly of components and other applications
      where fast cure is useful.

      Silicones – Silicones are useful where high temperature resistance or
      compatibility with silicone components such as molded seals are
      needed. Silicone firewall insulation materials and silicone gaskets
      and seals are bonded with silicone rubber adhesives.

      VII. Future Technology and Applications on Commercial
      Aircraft
      To some extent, modern applications of adhesive bonding of metal and
      composites exhibit the maturation of adhesive bonding as just one
      component of the aircraft designer's toolbox. Neither technology
      dominates the aircraft; each has its place. Decades of production and
      service experience with adhesive bonding, combined with the
      advancement of other fabrication technologies such as automated
      riveting, high speed machining, welding, super-plastic forming, etc.,
      have given designers a number of options when designing a part. They
      trade the requirements of a specific application against the
      advantages and drawbacks of the available manufacturing options to
      arrive at the lightest, cheapest, most robust design possible.

      Maturation as a technology does not mean that advancement and
      innovation has ceased. Adhesive bonding is so essential to the
      aerospace field that as long as there is a desire to go higher,
      faster and farther more efficiently, there will be an incentive to
      develop new materials and processes for adhesive bonding. Areas of
      particular interest for future applications are high-temperature
      adhesives, fiber-reinforced metal laminates and more efficient bond
      assembly techniques

      There are many applications for which adhesives capable of operating
      at high temperatures for long periods of time are useful, such as on
      aircraft engines. Although the cores of modern turbines operate at
      extremely high temperatures, high bypass ratios mean that a
      significant portion of the engine is at a much lower temperature.

      High-speed aircraft also benefit greatly from high-temperature
      adhesives. Supersonic flight can produce significant frictional
      heating of aerodynamic surfaces. Military craft, with their much
      shorter lives and ability to absorb high fabrication and operational
      costs, have been able to take advantage of metals such as titanium
      and high temperature composites to enable supersonic flight. A cost-
      effective supersonic commercial aircraft, however, is not possible
      without significant advances beyond the current technology of high
      temperature composites and adhesives. The structure of a supersonic
      commercial aircraft would need to withstand tens of thousands of
      hours at temperatures approaching 350F (or higher, depending on
      cruising speed) without significant degradation as well as have
      affordable materials and fabrication costs. There have been advances
      in resin chemistry, particularly with polyimides, that hold out the
      promise of such materials but they are not yet totally proven or
      commercially available at reasonable cost. They also require curing
      at very high temperature (700F and higher) that necessitates the use
      of expensive high temperature tooling and expendable materials and
      increases fabrication costs. Adhesives based on polymers with
      slightly lower temperature capabilities such as bismaleimides have
      been developed and are in use on selected applications.

      Fiber-reinforced metal laminates have been developed in recent years
      and are slowly finding their way onto commercial aircraft. Fiber-
      reinforced metal laminates are composite materials comprised of
      layers of fiber-reinforced resins between layers of metal sheet or
      foil. The fiber-resin layers act as an adhesive between the metal
      layers and in some cases the fiber layer matrix is in fact an
      adhesive film. Various fiber layers such as glass, graphite and
      aramid fibers can be combined with metals such as aluminum and
      titanium to tailor the properties of the material to the application.
      Fiber-metal laminates combine the best properties of metallic
      structure such as ease of fabrication, damage visibility and
      lightning strike durability with the fatigue resistance of bonded
      structure and the high specific strength of composites.

      As always, there is constant interest in reducing the fabrication
      costs of bonded structure. On the composites side of the house this
      has resulted in continued research into automated (robotic) means of
      applying composite raw materials to the tool. Although this is most
      often used to lay down composite fabrics and tapes, it is
      occasionally used to apply adhesive film as well. Metallic bonded
      structure has seen the development of concepts such as Grid-Lock ®, a
      combination of the advantages of accurate high-speed machining and
      adhesive bonding. Grid-Lock ® is probably most useful for fabricating
      assemblies such as flight control surfaces. In the Grid-Lock ®
      concept, components of the bonded assembly (skins, ribs, spars, etc.)
      are machined to size with grooves at the bondlines between the
      details. For simple assemblies, two major details can be machined.
      One skin is machined with integral ribs and spars, the other with
      receiving grooves (Figure 43). After surface treatment and priming,
      adhesive in paste form is applied to the grooves, the details
      assembled and the adhesive cured. Because of the self aligning
      grooves and precision machining, Grid-Lock ® assemblies require
      minimal tooling to assemble and little pressure during adhesive cure.


      Commercial transports will undoubtedly follow the lead of their
      smaller business jet brethren and utilize more bonded primary
      structure, probably in the form of advanced composites. As the
      industry gains experience and confidence and as materials and
      manufacturing costs are driven down, the advantages of bonded
      structure become more compelling. Because of continuing advancements
      in other materials and joining technologies we may not see a
      completely bonded airframe, but it seems clear that we will see
      increased use of bonded structure. Both Boeing and Airbus, the
      remaining large commercial transport manufacturers, have plans for
      new aircraft that contain significantly increased amounts of bonded
      primary structure and continue to rely on bonded metal and composite
      secondary structure.

      VIII. Military Applications
      The military has historically led the way in the development and
      application of adhesive bonding on aircraft. This practice continues
      today, primarily with bombers, fighter and attack aircraft where
      weight is a critical consideration, but also with support craft such
      as reconnaissance aircraft and freighters.

      The military embraced adhesive bonding earlier and more extensively
      than the civil aviation industry for two primary reasons, basic
      factors that dominate many of the differences between military and
      civil aviation. Much more so than the commercial world, military
      aviation is concerned with aircraft performance. The advantages that
      adhesive bonding brings, such as weight efficiency and aerodynamic
      smoothness, result in significant airframe performance improvements
      over mechanically fastened structure. Secondly, the military is
      largely insulated from the risk and cost constraints that limit the
      use of new and unfamiliar technology on commercial aircraft. Military
      aircraft designers recognized the potential that adhesive bonding had
      for improving aircraft performance and were willing to spend the
      resources to develop the new technology and continue to do so today.
      Virtually all advancements and innovations in adhesive bonding on
      commercial aircraft were first developed for military craft. In
      addition, the military is less constrained by the inevitable higher
      production and maintenance costs associated with working the bugs out
      of new technologies. As mentioned previously, early adhesive bonded
      structure was notorious for in-service delamination and corrosion
      problems. These problems severely impacted the growth of adhesive
      bonded aluminum structure on commercial aircraft but were much less
      detrimental to the use of bonding on military craft.

      Because of this continued emphasis on adhesive bonding technology
      development over the years, the airframes of modern front-line
      aircraft such as the B-2 bomber and the F-117 and F-22 fighters are
      largely structurally bonded advanced composites. They tend to be
      comprised of materials that are more advanced (expensive) than
      commercial aircraft such as carbon and boron fiber reinforcements
      with cyanate esters, bismaleimides, polyimides or other high-
      temperature resin matrices and adhesives.

      IX. Space Applications
      Space applications have traditionally pushed the envelope of adhesive
      bonding technology in selected areas, and continue to do so. The
      extreme weight sensitivity of spacecraft is the primary driving force
      behind this interest in bonding. The very high cost of boosting
      spacecraft structure into orbit makes it cost effective to spend
      significant resources to save weight. Exotic materials and processes
      that are too expensive for use on commercial aircraft are commonplace
      in the space vehicle industry.

      Typical requirements for adhesively bonded structure for space
      applications vary widely and differ substantially from those for
      atmospheric vehicles. Because of widespread use of cryogenic rocket
      fuels, adhesives near tank structure must maintain adequate
      properties at very low temperatures. At the other extreme, adhesives
      have been used to bond ablative or insulative heat shields to the
      bottom of re-entry vehicles since the advent of manned space flight
      [ix]. The expected service life of bonded structure on spacecraft
      varies tremendously as well. Adhesives on expendable booster
      structure may have a life measured in minutes, while reusable
      structure such as the shuttle has a much longer life. Adhesives that
      are used on structure that reach and remain in orbit for some time,
      such as satellites and space stations, can be subject to large swings
      in temperature from varying sun load and must be protected from
      attack by atomic oxygen. The widely varying requirements are
      mentioned merely to illustrate the futility of making broad, all-
      encompassing summaries of adhesives on space vehicles.

      Virtually all of the early manned space flight programs (Mariner,
      Mercury, Apollo) used structural adhesive bonding of large structural
      assemblies in both booster and payload modules[x]. Urethane adhesive
      was used on some early rocket structure because of low modulus at
      cryogenic operating temperature[xi]. Elevated temperature cure epoxy
      film and paste adhesives supplanted the urethanes and are standard
      today in areas of low to moderately high temperature exposure because
      of their ease of fabrication and high strength. Other adhesives such
      as silicone elastomers, cyanoacrylates and room-temperature epoxies
      are used to bond many non-structural joints.

      As with commercial aircraft, modern spacecraft take advantage of the
      extreme weight savings potential of bonded composite structure. Solid
      and liquid tank structure are often filament-wound or fiber placed
      carbon fiber epoxy composite construction, which lends itself well to
      secondary bonding of support structure with epoxy adhesives. Where
      service temperatures allow, other graphite epoxy composites
      components such as satellite structural members are bonded with epoxy
      film and paste adhesives to metallic (typically titanium) and
      composite components. In bonded structure with higher temperature
      requirements such as payload fairings (aerodynamic heating) or
      satellite structural radiators (waste heat from satellite
      electronics), more heat-resistant cyanate esters and bismaleimides
      are used.

      There has been concerted effort in recent years to develop next
      generation launch vehicles that are significantly less expensive to
      operate than traditional expendable boosters. Many of these efforts
      center on reusable launch vehicles with very low structural mass.
      Although there have been setbacks such as the failure of the X-33
      composite fuel tank during development testing and subsequent program
      cancellation, it is quite likely that any successful vehicle will
      contain significant bonded composite structure. The weight savings
      potential is too great for spacecraft designers to ignore. However,
      as the X-33 program also illustrated there is significant development
      work remaining before we are able to take advantage of adhesive
      bonding for such large, complex structure.



      Bibliography:

      Adhesive Bonding Alcoa Aluminum, Aluminum Company of America,
      Pittsburgh, PA, 1967

      AGARD Lecture Series No. 12, Bonded Joints and Preparation for
      Bonding, North Atlantic Treaty Organization Advisory Group for
      Aerospace Research and Development, London, 1979

      Alldredge, J. D. and Holmquist, H. W., Adhesive Bonding, Boeing
      Airliner, April-June 1985, pp. 6-8

      Bishop, John A., The History of ReduxÒ and the Redux Bonding Process,
      Int. J. Adhesion and Adhesives, V17, No. 4, 1997, pp. 287-301

      Bodnar, Michael J., Structural Adhesives Bonding, Interscience
      Publishers, New York, 1966

      Bulletin ANC-19, Wood Aircraft Inspection and Fabrication, Munitions
      Board Aircraft Committee, Washington D. C., 1951

      Grimes, David L., Application of Structural Adhesives in Air
      Vehicles, Advisory group for Aeronautical Research and Development,
      North Atlantic Treaty Organization, Paris, 1958

      Higgins, A., Adhesive Bonding of Aircraft Structures, Int. J.
      Adhesion and Adhesives, V20, 2000, pp. 367-376

      Kuperman, Murray, and Tony Seidl, Improved Structural Adhesive
      Bonding at United Airlines, Boeing Airliner, April-June 1985, p. 9

      Lockshaw, James J., et al., United States Patent 5,273,806,
      Structural Element with Interlocking Ribbing, United States Patent
      Office, 1993

      Minford, J. Dean, Handbook of Aluminum Bonding Technology and Data,
      Marcel Dekker, Inc., New York, 1993.

      National Materials Advisory Board, National Research Council,
      Structural Adhesives with Emphasis on Aerospace Applications, Marcel
      Dekker,Inc. New York, 1976

      Papers from the Structural Adhesive Bonding Conference Presented
      March 15-16, 1966, NASA Marshall Space Flight Center, Clearinghouse
      for Federal Scientific and Technical Information, 1966

      Potter, D. L., et al., Primary Adhesive Bonded Structure Technology
      (PABST) Design Handbook for Adhesive Bonding, Report AFFDL-TR-79-
      3129, Douglas Aircraft Co., Air Force Flight Development Laboratory
      (FBA) Air Force Systems Command, WPAFB, (November, 1979)

      Schliekelmann, Rob J., Adhesive Bonding in the Fokker-VFW F-28
      Fellowship, National Technical Information Service, Springfield,
      Virginia, 1973

      Structural Adhesive Bonding, Boeing Airliner, February 1959, pp. 3-8

      Symposium on Structural Adhesives and Sandwich Construction, Wright
      Air Development Center, Wright Patterson Air Force Base, Ohio, 1957

      Thrall, Edward W., and Raymond W. Shannon, Adhesive Bonding of
      Aluminum Alloys, Marcel Dekker, Inc., New York, 1985

      Truax, T. R., Technical Bulletin No. 205, Gluing Wood in Aircraft
      Manufacture, United States Department of Agriculture, Washington
      D.C., 1930

      Truax, T. R., Technical Note No. 291, Gluing Practice at Aircraft
      Manufacturing Plants and Repair Stations, National Advisory Committee
      for Aeronautics, Washington D.C., 1928


      Footnotes:
      ----------------------------------------------------------------------
      ----------

      [i] T. R. Truax, Technical Note No. 291, Gluing Practice at Aircraft
      Manufacturing Plants and Repair Stations, National Advisory Committee
      for Aeronautics, Washington D.C., 1928

      [ii] T. R. Truax, Technical Bulletin No. 205, Gluing Wood in Aircraft
      Manufacture, United States Department of Agriculture, Washington
      D.C., 1930

      [iii] John A. Bishop, The History of ReduxÒ and the Redux Bonding
      Process, Int. J. Adhesion and Adhesives, V17, No. 4, 1997, pp. 287-
      301

      [iv] Ibid.

      [v] A. Higgins, Adhesive Bonding of Aircraft Structures, Int. J.
      Adhesion and Adhesives, V20, 2000, pp. 367-376

      [vi] A. Sorenson, Sonic Fatigue Testing and Development of Aircraft
      Panels, Symposium on Structural Adhesives and Sandwich Construction,
      Wright Air Development Center, Wright Patterson Air Force Base, Ohio,
      1957

      [vii] Edward W. Thrall, Jr., Failures in Adhesively Bonded
      Structures, AGARD Lecture Series No. 12, Bonded Joints and
      Preparation for Bonding, North Atlantic Treaty Organization Advisory
      Group for Aerospace Research and Development, London, 1979

      [viii] Murray Kuperman, Tony Seidl, Improved Structural Adhesive
      Bonding at United Airlines, Boeing Airliner, April-June 1985, p. 9

      [ix] National Materials Advisory Board, National Research Council,
      Structural Adhesives with Emphasis on Aerospace Applications, Marcel
      Dekker,Inc. New York, 1976, pp. 33-34

      [x] J. Dean Minford, Handbook of Aluminum Bonding Technology and
      Data, Marcel Dekker, Inc., New York, 1993, p. 546

      [xi] Richard L. Long, Cryogenic Adhesive Application, Papers from the
      Structural Adhesive Bonding Conference Presented March 15-16, 1966,
      NASA Marshall Space Flight Center, Clearinghouse for Federal
      Scientific and Technical Information, 1966
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