Appendix 3 to AMC 20-29 – Change of Composite Material and/or Process

ED Decision 2010/003/R

1.         It is necessary to re-certify composite structures, which during production, incorporate substitutions of, or changes to, the materials and/or processes from those originally substantiated at the time of initial certification. For example, the original material supplier may either change its product, or cease production. Manufacturers may also find it necessary to modify their production processes to improve efficiency or correct product deficiencies. In either case, care must be taken to ensure that modifications and/or changes are adequately investigated to ensure the continued adequacy of already certificated composite structure. This appendix covers such material and/or process changes, but does not address other changes to design (e.g., geometry, loading). The definition of the materials and processes used is required in the specifications by Part 21.A.31. Changes to the material and process specifications are often major changes in type design and must be addressed as such under Part-21, subpart D or E as applicable.

2.         The qualification and structural substantiation of new or modified materials and/or processes used to produce parts of a previously certified aircraft product requires:

a.         The identification of the key material and/or process parameters governing performances;

b.         The definition of the appropriate tests able to measure these parameters; and

c.         The definition of pass/fail criteria for these tests.

3.         ‘Qualification’ procedures developed by every manufacturer include specifications covering: 

a.         Physical and chemical properties,

b.         Mechanical properties (coupon level), and

c.         Reproducibility (by testing several batches).

4.         Specifications and manufacturing quality procedures are designed to control specific materials and processes to achieve stable and repeatable structure for that combination of materials and processes. However, the interchangeability of alternate materials and processes for a structural application cannot be assumed if one only considers the properties outlined in those specifications (as it could be for materials that are much less process dependent, e.g., some metallic material forms). A structure fabricated using new or modified materials and/or processes, which meet the ‘qualification’ tests required for the original material and process specifications, does not necessarily produce components that meet all the original engineering requirements for the previously certified structure.

5.         Until improvements in identifying the complex relations between key material parameters that govern composite processing occurs, there will be a need for extensive and diverse testing that directly interrogates material performance using a range of representative specimens of increasing complexity in building block tests. Furthermore, failure modes may vary from one material and/or process to another, and analytical models are sometimes insufficiently precise to reliably predict failure without sufficient empirical data. Therefore, a step-by-step test verification with more complex specimens may be required.

6.         Classification of Material or Process Change

Material and/or process changes require appropriate classification in order to aid the determination of the extent of investigation necessary. Some minor changes may only require material equivalency sampling tests to be completed at the base of the test pyramid, whilst more significant changes will require more extensive investigations, including possibly a new structural substantiation.

a.         Any of the following situations requires further investigation of possible changes to a given composite structure:

(1)       Case A: A change in one or both of the basic constituents, resin, or fibre (including sizing or surface treatment alone) would yield an alternate material. Other changes that result in an alternate material include changes in fabric weave style, fibre aerial weight and resin content.

(2)       Case B: Same basic constituents, but any change of the resin impregnation method. Such changes include: (i) prepregging process (e.g., solvent bath to hot melt coating), (ii) tow size (3k, 6k, 12k) for tape material forms with the same fibre areal weight, (iii) prepregging machine at the same suppliers, (iv) supplier change for a same material (licensed supplier).

(3)       Case C: Same material, but modification of the processing route (if the modification to the processing route governs eventual composite mechanical properties). Example process changes of significance include: (i) curing cycle, (ii) bond surface preparation, (iii) changes in the resin transfer moulding process used in fabricating parts from dry fibre forms, (iv) tooling, (v) lay-up method, (vi) environmental parameters of the material lay-up room, and (vii) major assembly procedures.

b.         For each of the above cases, a distinction should be made between those changes intended to be a replica of the former material/process combination (Case B and some of Case C) and those which are “truly new material” (Case A and some of Case C). So, two classes are proposed:

(1)       “Identical materials/processes” in cases intended to create a replica structure.

(2)       “Alternative materials/processes” in cases intended to create truly new structure.

c.         Within the “identical materials/processes” class, a sub-classification can be made between a change of the prepregging machine alone at the supplier and licensed production elsewhere. For the time being, a change to a new fibre produced under a licensed process and reputed to be a replica of the former one, will be dealt with as an “alternative material/process”.

d.         Some minor changes within the class representing identical materials/processes may not interact with structural performances (e.g., prepreg release papers, some bagging materials, etc.) and should not be submitted to the Agency as part of the change. However, the manufacturers (or the supplier) should develop a proper system for screening those changes, with adequate proficiency at all relevant decision levels. Other minor material changes that fall under Case B may warrant sampling tests to show equivalency only at lower levels of building block substantiation.

e.         Case C changes that may yield major changes in material and structural performance need to be evaluated at all appropriate levels of the building block tests to determine whether the manufacturing process change yields identical or alternate materials. Engineering judgment will be needed in determining the extent of testing based on the proposed manufacturing change.

f.          Case A (alternative material) should always be considered as an important change, which requires structural substantiation. It is not recommended to try a sub-classification according to the basic constituents being changed, as material behaviour (e.g., sensitivity to stress concentrations) may be governed by interfacial properties, which may be affected by either a fibre or a resin change.

7.         Substantiation Method. Only the technical aspects of substantiation are addressed below.

a.         Compliance Philosophy. Substantiation should be based on a comparability study between the structural performances of the material accepted for type certification, and the second material. Whatever the modification proposed for a certificated item, the revised margins of safety should remain adequate. Any reduction in the previously demonstrated margin should be investigated in detail.

(1)       Alternative Material/Process: New design values for all relevant properties should be determined for any alternate material/process combination. Analytical models initially used to certify structure, including failure prediction models, should be reviewed and, if necessary, substantiated by tests. The procurement specification should be modified (or a new specification suited to the selected material should be defined) to ensure key quality variations are adequately controlled and new acceptance criteria defined. For example, changing from first to second generation of carbon fibres may improve tensile strength properties by more than 20% and a new acceptability threshold will be needed in the specification of the alternate material to ensure the detection of quality variations.

(2)       Identical Material: Data should be provided that demonstrates that the original design values (whatever the level of investigation, material or design) remain valid. Statistical methods need to be employed for data to ensure that key design properties come from the same populations as the original material/process combination. Calculation models including failure prediction should remain the same. The technical content of the procurement specification (Case B) should not need to be changed to properly control quality.

b.         Testing.

(1)       The extent of testing needed to substantiate a material change should address the inherent structural behaviour of the composite and will be a function of the airworthiness significance of the part and the material change definition. For example, the investigation level might be restricted to the generic specimens at the test pyramid base (refer to figures in paragraph 7) for an identical material, but non-generic test articles from higher up the pyramid should be included for an alternative material. Care needs to be taken to ensure that the test methods used yield data compatible with data used to determine properties of the original structure.

(2)       The testing that may be required for a range of possible material and/or process changes should consider all levels of structural substantiation that may be affected. In some instances (e.g., a minor cure cycle change), possible consequences can be assessed by tests on generic specimens only. For other changes, like those involving tooling (e.g., from a full bag process to thermo-expansive cores), the assessment should include an evaluation of the component itself (sometimes called the “tool proof test”). In this case, an expanded NDI procedure should be required for the first items to be produced. This should be supplemented – if deemed necessary – by “cut up” specimens from a representative component, for physical or mechanical investigations.

c.         Number of Batches.

(1)       The purpose for testing a number of batches is the demonstration of an acceptable reproducibility of material characteristics. The number of batches required should take into account: material classification (identical or alternative), the investigation level (non-generic or generic specimen) the source of supply, and the property under investigation. Care should be taken to investigate the variation of both basic material and the manufacturing process.

(2)       Existing references (e.g., The Composite Materials Handbook (CMH-17) Volumes 1 and 3, FAA Technical Report DOT/ FAA/AR-03/19), addressing composite qualification and equivalence and the building block approach, provide more detailed guidance regarding batch and test numbers and the appropriate statistical analysis up to laminate level. Changes at higher pyramid levels, or those associated with other material forms, e.g., braided VARTM (Vacuum-Assisted Resin Transfer Moulding) structure, may require use of other statistical procedures or engineering methods.

d.         Pass/Fail Criteria. Target pass/fail criteria should be established as part of the test programme. For strength considerations for instance, a statistical analysis of test data should demonstrate that new design values derived for the second material provide an adequate margin of safety. Therefore, provision should be made for a sufficient number of test specimens to allow for such analysis. At the non-generic level, when only one test article is used to assess a structural feature, the pass criteria should be a result acceptable with respect to design ultimate loads. In the cases where test results show lower margins of safety, certification documentation will need to be revised.

e.         Other Considerations. For characteristics other than static strength (all those listed in AMC 20-29, paragraphs 8, 9, 10 and 11), the substantiation should also ensure an equivalent level of safety.

[Amdt 20/6]