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FAA Report on Proposed Industry Prepreg Specifications

FAA Report on Proposed Industry Prepreg Specifications. Ric Abbott Abbott Aerospace Composites Wichita, Kansas. THE “INDUSTRY COMMITTEE”. The report suggests that the committee will be approving certification data The ACO should be the first point of contact

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FAA Report on Proposed Industry Prepreg Specifications

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  1. FAA Report on Proposed Industry Prepreg Specifications Ric Abbott Abbott Aerospace Composites Wichita, Kansas

  2. THE “INDUSTRY COMMITTEE” • The report suggests that the committee will be approving certification data • The ACO should be the first point of contact • Assistance for the ACO may come from the NRS office, FAA Tech Center, NASA, academia….

  3. SINGLE RESIN/FIBER ea SPEC • The report methodology is founded on a single resin/fiber combination cover by each specification • This is not very practical for large scale production • It is usual to allow the prepreg supplier some flexibility for fiber supply • Also, the end user usually needs flexibility in procurement to a given class of performance, such as 250° F/autoclave cure

  4. EQUIVALENCE • End user required to show equivalence to previous data base? • Should say data base generated other than under end user’s qualification plan • Otherwise: B-767 data need to be re-approved for B-777? • The regulations require statistical material properties “sufficient” to show positive margins with the applied internal loads (including factor of safety)

  5. RECALCULATION AFTER “N BATCHES” • The recalculation of allowables after n batches is essentially requiring n batches for qualification • So, three batch (AGATE) method is only a provisional approval? • Test article parts, including flight hardware, were already fabricated after 10 batches on the Premier program • Acceptance testing, today, does not usually include environmental effects

  6. STATISTICAL ANALYSIS • The AGATE statistical analysis is very desirable to produce reasonable allowables when reduced qualification testing of three batches is employed • Whereas, the report recommends Mil Hnbk-17 analysis

  7. FORM/TYPE/GRADE • End user specs usually include all forms of the prepreg in one spec, eg., tape, tow, PW fabric, 5HS fabric, etc. • Tow count is also part of the basic designator • Resin content is usually defined in the body of the spec as this may be varied based on fabrication needs

  8. OUTER LIMITS OF PROCESS WINDOW • Laminar properties should not involve deliberate process variations within the spec window • Conformity inspection is based on panels built to the test process requirements • Manufacturing process is the subject of the next level of the building block, element testing • Element testing is the level at which critical lay-ups, allowable process variations, and QA acceptance standards are investigated

  9. LAMINA and LAMINATE TESTS • Tables 5a and 5b confuse lamina and laminate tests • Bearing and CAI (as part of material qualification) are typically conducted on pseudo-isotropic laminates fabricated from each material form under consideration • Bearing test should not specify countersunk fasteners; this is part of detail joint design • Element test level should deal with detail design: eg., local joint laminates maybe built up with combinations of tape, fabric, adhesive plies, etc.

  10. DURABILITY and SERVICE LIFE TESTS, TABLE 7 • Fracture toughness (on all-zero direction plies) can be useful to compare materials, but has little practical value in structural design and certification • Fatigue testing on arbitary lay-ups without damage is not useful in compliance with damage tolerance regulations; flaw growth testing is needed and is best done on critical laminates and joints specific to the particular project (R = -1 is more severe than most civil aircraft load cycles) • Similarly, impact damage is required for certification, but to detectable damage or applicable threat levels

  11. SUMMARY COMMENTS • The report seems to propose a simplified composite structural certification approach for emerging airframe fabricators • In place of the building block approach the supplier conducts much more testing than usual for lamina properties • The end user may go straight to full scale validation using load enhancement factors without internal load analysis • Therefore much greater assurance is needed of the exact material used

  12. SUMMARY COMMENTS…contd • The report seems to propose a simplified composite structural certification approach for emerging airframe fabricators • Therefore the report should be an AC which presents acceptable means of compliance with each applicable regulation in CFR 14 part 23, 27….

  13. SUMMARY COMMENTS…concluded • The report, as it stands, will probably be applied in future as “FAA Policy” and enforced as the only acceptable means of compliance with CFR 14, part 25.613 • Therefore the recommendations should be within an Advisory Circular (or revisions to existing AC’s) with much wider and more formally documented industry and public input

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