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The Challenge of large Composite Transport Airplanes

COMPTEST 2011. The Challenge of large Composite Transport Airplanes. Airbus 350 XWB Barrel for Test. Boeing 787 Flight Test. Michel Guillaume RUAG Aviation KOR-TA2011-0064. Scope. Motivation Military Composite Experience Civil Aviation Composite Experience

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The Challenge of large Composite Transport Airplanes

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  1. COMPTEST 2011 The Challenge of large Composite Transport Airplanes Airbus 350 XWB Barrel for Test Boeing 787 Flight Test Michel Guillaume RUAG Aviation KOR-TA2011-0064

  2. Scope • Motivation • Military Composite Experience • Civil Aviation Composite Experience • Fatigue & Damage Tolerance for Composite Structures • Design, Production, and Operation Experiences • Comments & Conclusion Fatigue & Damage Tolerance for Composite Structures RUAG Aviation, Michel Guillaume

  3. Motivation • Military Composite Experience • Civil Aviation Composite Experience • Fatigue & Damage Tolerance for Composite Structures • Design, Production, and Operation Experiences • Comments & Conclusion RUAG Aviation, Michel Guillaume

  4. Evolution of composite for transport planes RUAG Aviation, Michel Guillaume

  5. Evolution of composite for Airbus models RUAG Aviation, Michel Guillaume

  6. Advantage of Compositelighter, cheaper, better Goal for Boeing 787 20% fuel advantage over 767-300: • Structural weight reduction due to composites (-20%)(50% less rivets on the fuselage) • Better wing shape to improve aerodynamics (-3%) • Engine technology contribution -8% and through new electrical systems -3% (no bleed air system) • Cheaper production cost (-20%) • Lower operating costs (-20%) • Fewer scheduled maintenance labour hours (-40%) RUAG Aviation, Michel Guillaume

  7. Motivation • Military Composite Experience • Civil Aviation Composite Experience • Fatigue & Damage Tolerance for Composite Structures • Design, Production, and Operation Experiences • Comments & Conclusion RUAG Aviation, Michel Guillaume

  8. US Navy has long term experience • First applications on F-5, F-14, F-15, F-16 with boron/epoxy monolithic composite honeycomp skins (F-14; 4% of composite of total airframe weight). • F/A-18 A/B/C/D (10% composite)Carbon/epoxy (Hexcel 350l-6) in monolithic and honeycomb structures, wing skins thick monolithic primary structure. • AV-8B VSTOL (24% composite)Four composite types used: graphite/epoxy, graphite/bismaleiemide, glass/epoxy, and glass/bismaleimide • F-35 (40% composite)Wing skins and thick monolithic applications • Experience:Early problems on carriers with repair shop (refrigerator, cure temperature, operational conditions).Composite very durable but in some areas delamination, acoustic fatigue, extensive use of bolted metallic patches on AV-8B VSTOL. RUAG Aviation, Michel Guillaume

  9. Materials used on the F/A-18 C/D vs. E/F F/A-18 C/D USN F/A-18 E/F USN Percent of Structural Weight RUAG Aviation, Michel Guillaume

  10. Lower Lug Hole Surface Adjacent to Lower Lug Landing Gear Well Web Landing Gear Well Web Flange Main Landing Gear Uplock Attach Holes Fuel-Line Hole Hydraulic-Line Hole Control Cable Hole Mold-Line Flange below Lower Lug Drag Longeron Interface Area and Attach Holes Upper Outboard Duct Flange and Lug Back-up Structure Lower Outboard Duct Flange 1 2 3 4 5 15 8 6 1 7 13 8 2 9 12 9 6 10 14 & 11 10 11 & 13 12 & 15 4 7 14 3 5 Titanium wing carry through bulkhead for Swiss F/A-18A lot of very different load paths, complex structure ! US Navy had R&D project for composite wing carry through bulkhead RUAG Aviation, Michel Guillaume

  11. Motivation • Military Composite Experience • Civil Aviation Composite Experience • Fatigue & Damage Tolerance for Composite Structures • Design, Production, and Operation Experiences • Comments & Conclusion RUAG Aviation, Michel Guillaume

  12. Since 1985 increased usage of composite • First serial production of vertical tail, and spoilers for Airbus A310/300. • Increased use of composite on Airbus 320 for vertical tail, horizontal tailplane, all moveables, belly section, landing gear doors, and engine nacelles; different types used CFRP, AFRP, GFRP, QFRP. • Composite for Airbus and Boeing commercial planes used for similar parts as for the Airbus 320; approximately 10 to 18% of airframe weight. • Increased use of composite for Airbus 380 for pressure bulkhead, aft section; and for the first time GLARE material was used on upper fuselage shells. • New area for composite starts with the development of the Boeing 787 with fuselage barrel sections made fully out of carbon laminate produced in big autoclaves. • Airbus followed the same trend for the A350 XWB but with 4 shells made of carbon laminates for a barrel section. The cockpit nose section is still made of aluminium alloy. RUAG Aviation, Michel Guillaume

  13. R&D projects • USA ACEE project (1971):Improvement of the aerodynamics, engine efficiency and structure weigh saving (composite for 20% weight reduction) & certification issues.Technology demonstration for Boeing 737 horizontal tail, DC-10 fin (Mc Donnell Douglas) and L-1011 fin (Lockheed). • Europe TANGO project (1990):One big task composite fuselage with goal of 20% weight & cost saving.Technologies: AFP, RFI, LRI, RTM, and thermoplastic applications. TANGO Consortium RUAG Aviation, Michel Guillaume

  14. Composites for primary structures A320-200 B-787 RUAG Aviation, Michel Guillaume

  15. Motivation • Military Composite Experience • Civil Aviation Composite Experience • Fatigue & Damage Tolerance for Composite Structures • Design, Production, and Operation Experiences • Comments & Conclusion RUAG Aviation, Michel Guillaume

  16. Lessons learned in metals from accidents • 2 crashes of De Havilland Comet in 1954, fuselage fully desintegrated • Loss of upper shell section in 1988 in Aloha Boeing 737 Concern of metal fatigue: design principles, testing Wide spread fatigue damage: Demonstration with sufficientfull scale testing at least 2design life times (FAR 25-571 amendment) RUAG Aviation, Michel Guillaume

  17. Damage tolerance philosophy for metals • US Air Force bomber F-111 crashed in 1970 after 107 FH with cracked pivot in wing structure due to initial defect during production. • US Air Force introduced damage tolerant design philosophy, initial flaw concept with slow crack growth to ensure structural integrity. (MIL STD 1530) critical crack size H a0 H/2 a0 = initial flaw size = 0.05 inch = 1.27 mm First inspection at H/2 RUAG Aviation, Michel Guillaume

  18. Failures in composite structures • Fatigue failure by disbonding in the test of the A320 vertical tail fin. • Airbus A300-600 AA flight 587 crashed after separation of vertical tail fin. No real composite failure with loss of airplane reported. A300-600 crash due to over-stressing of the rudder beyond material limits.(wake behind B-747 with turbulent air and aggressive rudder inputs) RUAG Aviation, Michel Guillaume

  19. What fatigue failure means with composites • Fatigue in metals well understood more than 554 references listed in Walter Schütz paper entitled ¨A History of Fatigue¨ published in 1996. • Crack nucleation in metals to what does it correspond for composites at the same micro scale of either the matrix itself or the fiber-matrix interface? • There is a very complex phenomenon involving numerous translaminar and interlaminar cracks plus likely interface cracks and fiber failures at a lower scale level.At the end, fatigue damages with composites are mainly delaminations which are by nature oriented parallel to the axial forces. • The failure mode for a composite will be an unacceptable the loss of stiffness. For an element loaded in compression this may lead to a static failure by buckeling. RUAG Aviation, Michel Guillaume

  20. Certification of composite materials • Certification for metals were established in the last 70 years over fail-safe, damage tolerance with a lot of amendments which cover wide spread fatigue and aging airplanes (limit of validity). • Composite are different materials, high strength, noteworthy with no fatigue or corrosion found, but just accidental impacts, and some ligthning strike damages. • For transport airplanes paragraph 25-571 (Aviation Rulemaking advisory Committee) states: (a) General. An evaluation of strength, detail design, and fabrication must show that catastrophic failure due to fatigue, corrosion, manufacturing defects, or accidental damage, will be avoided throughout the operational life of the airplane. RUAG Aviation, Michel Guillaume

  21. Certification of composite materials (b) Damage Tolerance (fail-safe) evaluation. The evaluation must include a determination of probable locations and failure modes due to fatigue, corrosion or accidental damage […] The residual strength evaluation must show that the remaining is able to withstand loads (considered as static ultimate loads) […] (c) Fatigue (safe-life) evaluation. … This structure must be shown by analysis, supported by test evidence, to be able to withstand, the repeated loads of variable magnitude expected during its service life without detectable cracks. […] RUAG Aviation, Michel Guillaume

  22. AC 20-107 for composite structures • In July 1978 first Advisory Circular covering composite structures was published in 1978 by FAA under AC 20-107.Damage tolerance fail-safe evaluation:… The tests should demonstrate that residual strength of the structure can withstand the limit loads (considered as ultimate) with a damage extend consistend with initial detectability and subsequent growth under repeated loads including the effect of temperature and humidity. Growth rate data should be used in establishing a recommended inspection program. Copied pasted from the methods under development with metals, the fact is that slow growth principle would not be practicable with composites. RUAG Aviation, Michel Guillaume

  23. Revision A of AC 20-107 (1984) In a revision A in 1984 the text became: Structural details, elements, and subcomponents of critical areas should be tested under repeated loads to define the sensitivity of the structure to damage growth. This testing can form the basis for validating a no-growth approach to the damage tolerance requirements. Of equal importance, the revision introduced the specific concern of low velocity accidental impact damage with composites. • This Advisory Circular AC 20-107A really needed to be updated to be more reflective of lessons learned and today’s practices, work is in public review. • Direction for the proof of structure fatigue/damage tolerance:For the case of the no growth design concept, inspection intervals should be established as part of the maintenance program. In selecting such intervals the residual strength level associated with the assumed damages should be considered. RUAG Aviation, Michel Guillaume

  24. Structure fatigue/damage tolerance • No-growth approach for composite:New development of probalistic approaches requiring an assessment of impact damage risks in the form of damage severity of occurrence. Possible long duration below UL RUAG Aviation, Michel Guillaume

  25. The scatter factor issue • In paragraph § 7 a the AC 20.107 states:The number of cycles applied to validate a no-growth concept shoud be statistically significant, and may be determined by loads and/or life considerations. The no-growth evaluation should be performed by analysis supported by test evidence or by tests at the coupon, element, or subcomponent level. S/N curve at 50% probability failure factor on loads factor on life conservative S/N curve at (1-p) probability failure For variability of composite Whitehead method used, the test factor should cover typical gap between S/N curve (50%) and B-basis (90% curve). RUAG Aviation, Michel Guillaume

  26. Factor on life and load enhancement αL = scatter factor of composite properties on life αR = scatter factor of static strenght properties N = number of test articles N = coefficinet applied on life p = survival probalility γ = is confidence level of 0.95 • Load enhancement factor (LEF) based on two parameter Weibull distribution αL = 1.25 for Fatigue αR = 20 for static Test life will be in the range of 13 – 14 for a LEF of 1 ! RUAG Aviation, Michel Guillaume

  27. Test duration & challenge for hybrid structures • Test duration: for LEF = 1 test duration of 13.3 times the service life is impractical! • The combination of a 1.5 factor on life with a 1.15 LEF is widely used in European programmes. • Specific issue for hybrid structures of composite & metals: Fatigue Test spectrum very different requirements on loads ? Less refined spectrumstepping for composites Higher omission level for composites Clipping not allowedfor composites More severe factors to cover scatter for composites Emphasis on compressionloads for composites andtension loads for metallics RUAG Aviation, Michel Guillaume

  28. Proposal of test procedure for hybride structures • Combination of static strength and fatigue tolerance substantiation to meet hybrid structures structural integrity. Limit loads test(not regulatory) Residual loads testwith a factor of 1.2 Ultimate Loads Testwith a factor of 1.5after damage repair one service life time with scatter factor of 2and LEF of 1.13 one service life time with scatter factor of 2 Durability life demo to showcrack nucleation for metals Damage tolerance demo, for intial flaws in metal & service composite damage (no-growth) Introduce intial flaws in metal parts (crack growth) & detectable accidental damage for composite parts Start with a strucrure represantive of the minium quality allowed Goal one test article for fatigue life substatiation to meet fail-safe damage tolerance of metals & composite parts. RUAG Aviation, Michel Guillaume

  29. Motivation • Military Composite Experience • Civil Aviation Composite Experience • Fatigue & Damage Tolerance for Composite Structures • Design, Production, and Operation Experiences • Comments & Conclusion RUAG Aviation, Michel Guillaume

  30. Successful operation of composite structures • Design of composite structures requires a lot of knowledge much more than metals (processes, material data base, quality issues, tolerances). • Airbus & Boeing fleet with 10 to 18% weight ratio of composite succesful in operation. • Wings and fuselage of composite structure will lead to new experience (impact damage on ground, lightning strikes, debris from run ways).RUAG maintenance experiences much more damages on the F/A-18 composite skins compared with F-5 and Mirage III fighters. • Concepts for inspection for barely visible impact damage to ensure structural integrity. • For 50% of aircraft structure made of composite, most metal applications are concentrated to ¨structural knots¨, heavily loaded and of such complexity in their design that so anisotropic composites will never be able to accommodate. • Repairs for large composite structures will require new solutions which have to be tested and qualified, and crashworthiness is also an issue. • High production rate of airplanes with new technology is a real challenge. RUAG Aviation, Michel Guillaume

  31. Damages and repairs on Swiss F/A-18 Hornet RUAG Aviation, Michel Guillaume

  32. Environmental project ECO Design prepreg Technology Area: Hybrid-Prozess and Out-of-Autoclave • Combination of RTM and OoA prepreg • Low temperature curing out of autoclave • New possibilities for integral construction • Less subsequent work in combination with RTM Technology Area: Cold bonding • Long open times • New curing techniques  time and energy savings • Replacement of film adhesives • Shim application with high pressure stability for the reduction of need shims Technology Area: Use of recycled Fibers • For aircraft non structural parts Swiss cluster managed by RUAG with RUAG Aviation, Michel Guillaume

  33. Motivation • Military Composite Experience • Civil Aviation Composite Experience • Fatigue & Damage Tolerance for Composite Structures • Design, Production, and Operation Experiences • Comments & Conclusion RUAG Aviation, Michel Guillaume

  34. Testing of composite structures is very important, but which test procedure for hybrid structures (full scale test procedures)? • Certification is key step for the use composite structures, more experience and requirements are needed to ensure structural integrity through fail-safe damage tolarance. • More R&D for better modeling of 3D stresses in the laminates, a better prediction of delamination onset allowing for inherent material variability, and a better calculation of residual stresses in the presence of damages. • Nano technology will further boost the composite technology in aerospace applications. • The service experience of the Boeing 787 and Airbus 350 XWB will demonstrate the reliability and will show the impact on future programmes. • Composite will have a big future it is just a question of time. RUAG Aviation, Michel Guillaume

  35. Tests can show what happens, analysis can show why. To make a good decision you need both. (Steven G. Rensinger, Experimental Techniques, 1988) Acknowledgement Jean Rouchon, retired CEAT, Toulouse Hans-Jürgen Schmitt, retired EADS Airbus, Hamburg Bob Griffiths, K. Das, Boeing, Seattle Robert Eastin, FAA, Los Angelos RUAG Aviation, Michel Guillaume

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