Subject: Acceptance Guidance on Material Procurement and Process Specifications for Polymer Matrix Composite Systems
Date: 9/19/03 Initiated By: ACE-100
AC No: 23-20 Change:
CONTENTS
P a r a g r a p h P a g e
1. PURPOSE1
2. RELATED REGULATIONS AND GUIDANCE DOCUMENTS1
a. Regulations1
b. Advisory Circulars2
c. Policy Statements3
3. SUPPORTING REFERENCES3
a. DOT/FAA Technical Reports3
b. Industry Standards4
4. BACKGROUND4
a. Scope4
b. Historical Perspectives4
c. Shared Databases5
d. Material Qualification & Equivalency (MQ&E)5
e. Material Control6
f. Importance to Structural Substantiation7
5. MATERIAL SPECIFICATION8
a. Overview8
b. Material Requirements9
c. Material Characterization12
d. Changes to Qualified Materials14
e. Quality Assurance15
f. Material Test Methods17
g. Test Panel Fabrication17
h. Packaging and Shipping Requirements17
i. Use of a Distributor in the Supply Chain18
6. PROCESS SPECIFICATION18
a. Overview18
b. Fabrication Requirements20
c. Quality Assurance22
7. SUMMARY22
8. FUTURE STANDARDIZATION23
ii
1. PURPOSE.
a. This advisory circular (AC) provides information and guidance concerning an acceptable means, but not the only means, of compliance with Title 14 of the Code of Federal Regulations (14 CFR) part 23. It is applicable to the material and process specifications, or other documents, used to ensure sufficient control of composite prepreg materials in normal, utility, acrobatic, and commuter category airplanes. Material in this AC is neither mandatory nor regulatory in nature and does not constitute a regulation.
b. This AC has been developed with the help of industry to ensure adequate composite material control and to promote standardization of material and process specifications. It presents the procedural requirements and technical information from a regulatory perspective. Readers interested in engineering guidelines and recommended criteria should refer to the DOT/FAA Technical Reports provided under the Supporting References in paragraph 3.
c. This AC is issued for guidance purposes and to outline a method of compliance with applicable airworthiness requirements. Because the method of compliance presented in this AC is not mandatory, the terms "should" and "must" used in this AC apply only to an applicant who chooses to follow this particular method.
2. RELATED REGULATIONS AND GUIDANCE DOCUMENTS.
a. Regulations. This AC provides guidance for showing compliance with 14 CFR part 23 requirements relating to material control. The regulations that are directly related to this AC include the following:
• Section 23.601 General
• Section 23.603 Materials and workmanship
• Section 23.605 Fabrication and methods
• Section 23.613 Material strength properties and design values
(1) Section 23.605 requires that "the methods of fabrication used must produce consistently sound structure." Furthermore, "if a fabrication process requires close control to reach this objective, the process must be performed under an approved process specification."
This rule mandates that each new aircraft fabrication method be substantiated by a test program.
(2) Section 23.613 contains specific requirements for material strength properties and design values. The requirements that are particularly relevant to this AC include the following:
• "Material strength properties must be based on enough tests of material meeting specifications to establish design values on a statistical basis." [§ 23.613(a)]
• "Design values must be chosen to minimize the probability of structural failure due to material variability." [§ 23.613(b)] 1
• "The effect of temperature on allowable stresses used for design in an essential component or structure must be considered where thermal effects are significant under normal operating conditions." [§ 23.613(c)] Section 23.603(a)(3) prescribes similar requirements.
(3) This AC focuses on ensuring the control of composite material systems. It closely ties to a policy statement titled “Material Qualification and Equivalency for Polymer Matrix Composite Material Systems” issued by the Small Airplane Directorate.
b. Advisory Circulars. Obtain the ACs listed below from the U.S. Department of Transportation, Subsequent Distribution Office, Ardmore East Business Center, 3341 Q 75th Avenue, Landover, MD 20785. The following two Federal Aviation Administration (FAA) ACs present recommendations for showing compliance with FAA regulations associated with composite materials:
• AC 20-107A Composite Aircraft Structure
• AC 21-26 Quality Control for the Manufacture of Composite Structures
(1) AC 20-107A sets forth an acceptable means, but not the only means, of showing compliance with the provisions of 14 CFR parts 23, 25, 27, and 29 regarding airworthiness type certification requirements for composite aircraft structures. Paragraphs 5 and 6 of AC 20-107A contain valuable guidance relevant to this AC, including the following:
• Material design values, or allowables, which have a high degree of confidence based on experimental evidence, in the appropriate critical environmental exposures expected in service.
• Material and processing variability considerations in deriving proper allowables or design values for use in analysis, and in the analysis of the results of supporting tests.
(2) AC 20-107A also presents guidance on associated quality control and production specifications. It states that specifications covering material, material processing, and fabrication procedures should be developed to ensure a basis for fabricating reproducible and reliable structure.
(3) AC 21-26 provides guidance on 14 CFR part 21 regarding quality control systems for the manufacture of composite structures involving fiber reinforced materials. It also provides guidance on the essential features of quality control systems for composites.
(4) Paragraph 5 of AC 21-26 recognizes that the material properties of a composite structure are manufactured into the structure as part of the fabrication process. Material and process specifications used to produce composite structures must contain sufficient information to ensure that critical parameters in the fabrication process are controlled in production. It also provides information on elements that are to be contained in a typical material and process specification. Furthermore, paragraphs 7 and 8 of AC 21-26 provide an additional description of the requirements relating to both aspects of material and manufacturing controls.
c. Policy Statements. The Small Airplane Directorate issued two policy statements that contain information that ties or closely pertains to the contents of this AC. They are the following:
• “Material Qualification and Equivalency for Polymer Matrix Composite Material Systems" [PS-ACE100-2002-006, September 2003]
• "Static Strength Substantiation of Composite Airplane Structure" [PS-ACE100-2001-006, December 2001]
(1) The first policy statement presents some of the procedures and technical requirements associated with elements of material qualification, equivalency, and continuous quality control. Technical details are referred to in the FAA Technical Reports (DOT/FAA/AR- 00/47 in 2001 and DOT/FAA/AR-03/19 in 2003, see paragraph 3a of this AC).
(2) The policy statement on "Static Strength Substantiation" addresses the composite structural substantiation. It includes higher levels of evaluation to substantiate design-specific detail at structural scales.
(3) The two policy statements listed above discuss the connection/application of the base material database established by the qualification testing for fulfilling the higher-level structural substantiation.
3. SUPPORTING REFERENCES.
a. DOT/FAA Technical Reports. The following technical reports are published by the FAA Technical Center and are available to the U.S. public through the National Technical Information Service (NTIS), Springfield, Virginia 22161. They are also available at the FAA William J. Hughes Technical Center's Full-Text Technical Reports page: actlibrary.tc.faa.gov in Adobe Acrobat portable document format (PDF).
• “Material Qualification and Equivalency for Polymer Matrix Composite Material Systems" [DOT/FAA/AR-00/47, April 2001]
• “Guidelines and Recommended Criteria for the Development of a Material Specification for Carbon Fiber/Epoxy Unidirectional Prepregs" [DOT/FAA/AR- 02/109, March 2003]
• “Guidelines for the Development of Process Specifications, Instructions, and Controls for the Fabrication of Fiber-Reinforced Polymer Composites" [DOT/FAA/AR- 02/110, March 2003]
• “Material Qualification and Equivalency for Polymer Matrix Composite Material Systems: Updated Procedure" [DOT/FAA/AR-03/19, September 2003]
(1) The Technical Report on material qualification and equivalency (MQ&E) presents details developed under the Advanced General Aviation Transport Experiments (AGATE) program. This method enables an airframe manufacturer to qualify composite material systems by sample testing to share a central material qualification database.
(2) The information contained in the other two technical reports provides guidelines for material and process control, which is a logical advancement of the AGATE method. The FAA and National Aeronautics and Space Administration (NASA) worked jointly with industry experts to collect inputs on key properties that need to be controlled to ensure stable composite materials. The criteria/guidance presented in these two reports, which are applicable to part 23 products, form the core contents of this AC.
b. Industry Standards. MIL-HDBK-17 (current version) has been developed with the help of industry and is maintained as a joint effort of the Department of Defense (DOD) and the FAA.
(At the time that this AC was first released, Version F of MIL-HDBK-17 was current.) The handbook provides guidance for developing base material properties (allowables) and the design values acceptable to the FAA. Valuable information closely related to this AC is contained under Chapter 2 in Volume 1 of this handbook (i.e., Guidelines for Property Testing of Composites). MIL-HDBK-17 can also serve as a reference for most of the terminology used in this document. MIL-HDBK-17 can be obtained from the Department of Defense Single Stock Point (DODSSP), 700 Robbins Avenue, Building 4D, Philadelphia, PA 19111. This handbook can also be purchased through ASTM International's website at "www.astm.org" [note: search using keyword "MIL-17"].
(1) The material and process specifications normally refer to industry test methods or standards such as American Society for Testing and Materials (ASTM International). These standards are available through ASTM at either their Internet site address, which is "www.astm.org", or their mailing address: ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, Pennsylvania 19428-2959.
(2) Consistent and repeatable industry engineering practices would help assure compliance with the requirements for material control. The purpose of building on the current FAA/NASA effort is to standardize these specifications with MIL-HDBK-17, ASTM International, and Society of Automotive Engineering (SAE International) Aerospace Materials Specifications (AMS) Committee P-17.
4. BACKGROUND.
a. Scope. The guidance in this document pertains to prepreg materials, but many of the basic concepts are generic for composite material control. It was derived from certification experiences with the help of industry groups, which represent composite material suppliers and manufacturers for small airplanes (part 23), transport airplanes (part 25), and rotorcraft (parts 27 and 29) product categories. Future efforts to continue development of reliable industry standards for the control and testing of composite materials are needed. The FAA supports international initiatives in this area.
b. Historical Perspectives. Composite materials have not achieved the same level of standardization as metallic materials. This includes a general lack of industry material and process specifications and shared databases for basic properties, as well as test standards that continue to evolve. Composite manufacturers must face these issues as they establish the necessary base material control to ensure repeatable production of their aircraft product. Such 4
control must be achieved in the quality assurance procedures developed by each user and their selected material suppliers.
(1) In the decades since the introduction of advanced composite materials for use in aircraft, the cost associated with material control has been a significant burden to the industry.
The cost burden begins with the need to conduct qualification testing for each material system to be used by an airframe manufacturer. The cost may be further escalated by instabilities originating from the sources associated with material supply. Examples of instabilities may include changes in material and/or process from the supplier end and, on a larger scale, the reshaping of the supplier industry as new users and markets evolve.
(2) Traditionally, each manufacturer conducted extensive qualification testing to develop the base material properties and allowable strengths, which are part of their aircraft design data.
The practice of qualification, when performed by each manufacturer for an identical material system, represents a duplication of effort. The same can be stated for development of those portions of the composite material and process specifications used to control the source of raw materials.
c. Shared Databases. In recent years, NASA, the FAA, and industry worked together through the AGATE program to develop a cost-effective method of qualifying composite material systems and sharing a central material qualification database. This method is built on existing sections of MIL-HDBK-17, and it allows credit for FAA approved materials testing performed by third parties such as material suppliers.
(1) To use an approved composite material system, airframe manufacturers need to perform a smaller subset of testing (i.e., equivalency) to substantiate their control of material and fabrication processes tailored to a specific application. Throughout its development, the FAA closely participated in the AGATE program to ensure that this method is in compliance with the applicable airworthiness regulations. Use of the shared database presents not only a significant cost saving opportunity for airframe manufacturers, it also provides the stability needed in the material suppliers environment.
(2) Two FAA Technical Reports [DOT/FAA/AR-00/47 and DOT/FAA/AR-03/19] have been published to present the technical details of the AGATE method. Based on this AGATE method, the Small Airplane Directorate issued a policy statement titled "Material Qualification and Equivalency for Polymer Matrix Composite Material Systems." This policy statement presents both the procedure and the technical requirements associated with this method. Since the proposed version was issued, in May 2000, several workshops and presentations have been conducted for audiences representing the authorities and the industries, both domestic and foreign. The AGATE method made a big step in achieving the objective of stabilizing the composite materials, while enhancing efficacy both in terms of cost and cycle time.
d. Material Qualification & Equivalency (MQ&E). A review of the AGATE method provides a clear purpose for material qualification and equivalency. Material qualification is a process of establishing the database to control an "original" material system. The qualification 5
method documented by AGATE presents guidelines for base material properties with several purposes:
• To solidify and finalize material and process specifications, including specific acceptance criteria for sampling relative to the qualification database;
• To quantify base material variability; and
• To provide a central database of material properties for stable processes.
(1) Material equivalency is a process to substantiate material properties. (See paragraph 5d of this AC for expanded discussion.) This is achieved by test sampling and passing the acceptance criteria, which were derived from a larger population of material data. The equivalency method documented by AGATE presents guidelines for engineering practices with several purposes:
• To share and make use of the central database by a new user (i.e., original material qualification);
• To continue surveillance of material and process;
• To show that minor changes to material and processes do not affect base material properties; and
• To make final adjustment on material and process specifications for specific application, and demonstrate that it has minor effect on base material properties.
(2) The AGATE MQ&E protocol for shared databases provides a basis in consistent engineering practices for composite material control. Following this basis, fundamental requirements for material control should include the following:
• Qualification database should be the statistical basis for equivalency (for new users and changes) and quality control acceptance requirements;
• Documentation and databases should exist for each unique material; and
• Property drift should be minimized.
e. Material Control. The FAA and NASA performed efforts, which go beyond AGATE, in composite material control. The primary purpose of these expanded efforts was to establish the guidance for developing composite material and process specifications. Shared composite databases and specifications throughout industry would improve the efficiency of material suppliers, end-users, and regulators.
(1) The FAA/NASA work was administered through the FAA Technical Center and Airworthiness Assurance Center of Excellence (AACE) at Wichita State University (WSU).
Industry experts that have extensive experience with material specifications, part processing, qualification programs, and design allowables were invited to participate. They worked together in drafting detailed guidelines for material control. This task covers product categories of small airplanes (part 23), transport airplanes (part 25), and rotorcraft (parts 27 and 29). In August 2002, a workshop was conducted in Chicago that provided a forum for experts, both from government and industry, to discuss the most efficient ways to achieve composite material control.
(2) The inputs generated through the FAA/NASA/industry joint effort have been compiled and published in two DOT/FAA Technical Reports. This effort was driven by the following goals:
• Greatly reduce the number of material and process specifications for identical composite material systems;
• Develop property databases that uniquely define a given material;
• Establish material batch testing and process monitoring sufficient to minimize variability and preclude property changes over time; and
• Reduce costs through common documentation and shared databases of basic material properties.
f. Importance to Structural Substantiation. Analysis and base material data alone is generally not adequate for substantiation of composite structural designs. The "building-block approach" is typically used to fulfill the certification requirement. While details of the "buildingblock approach" appear in MIL-HDBK-17, the portion that pertains to part 23 certification has been discussed in two policy statements issued by the Small Airplane Directorate.
(1) The AGATE method is typically used to develop the design database and meet certification requirements at the lower level of structural complexity (i.e., coupon tests). The material qualification testing provides a means for quantitative assessment of the variability of key base material properties, leading to various statistics that are used to establish material acceptance, equivalence, quality control, and design basis.
(2) Structural certification is used to ensure acceptable materials and repeatable manufacturing processes are used for all parts that are integrated into the production. The material and process controls are essential prerequisites for a repeatable and safe product.
Material and process specifications are interwoven throughout the certification process. Material specifications define the material’s attributes and define the qualification tests. Subsequently, materials used for production are purchased under the material specification.
(3) Aircraft manufacturers use the material specification to ensure that the delivered aircraft consists of materials that are of the same quality and performance standards achieved in the certification process. Process specifications define and control the processes used to convert materials into structural components. The base material properties of composite laminates are directly determined by the specific processes used for their fabrication. Quality control procedures, which often make use of the lower levels of building block tests, ensure the material used to fabricate production aircraft parts remains invariant.
(4) Building on the base material database, tests at higher levels of the scale (i.e., structural laminate, element and sub-component) are typically needed to fulfill the remaining parts of the structural substantiation requirement. Structural laminate data may be shared between applications and can become part of a central database. As the program moves closer to application-specific design detail, data sharing typically becomes more difficult.
5. MATERIAL SPECIFICATION.
a. Overview.
• The qualification of databases and specifications are recognized as the prerequisites to ensure a stable raw material source for aircraft products. Detailed background and policy for material qualification, shared databases, and equivalency testing of polymer matrix composites have been published.
• A recent joint FAA/NASA effort defined criteria and guidelines for the related specifications for effective material control. These criteria and guidelines have been published in two FAA Technical Reports. One report focuses on material specification [i.e., DOT/FAA/AR-02/109], and the other one focuses on related process instructions and controls [i.e., DOT/FAA/AR-02/110]. This paragraph gives regulatory guidance on the former.
• Composite material and process specifications are closely interrelated. A material specification typically defines the composite material and its qualification tests.
Many of the tests require specimens fabricated using the process specification. Once qualification is completed, requirements in the material specification are finalized.
The material specification is used to procure the production material and maintain levels of quality attained at certification.
( 1) Standard Specification Format.
(a) The FAA Technical Report [DOT/FAA/AR-02/109] contains the essential criteria and guidelines to develop a material specification. This guidance is applicable to polymer matrix material systems. It also contains some technical details that only apply to a particular material form (unidirectional prepregs).
(b) A standard format for the outline of a material specification can be found in the FAA Technical Report [DOT/FAA/AR-02/109]. This format was derived from SAE International recommendations. As a result, format information contained in the FAA report may be superseded by future changes in the SAE International recommended format.
(c) Initial sections of a material specification typically provide general information, which is essential to users of the specification. The remaining sections contain the engineering criteria and guidelines. The particular order and names for sections in a given material specification may differ from industry standards; however, similar content should be included in material or other specifications and related documents used to control materials.
(2) General Information. The general information essential for a material specification includes the following:
(a) Describe all materials controlled by a specification in the front of the document.
This should include details that define the materials (e.g., form, resin content, cured ply thickness, fiber, areal weight, and fiber type) and cure conditions needed for processing, as well as a general description of applications and environmental use limits.
(b) List (usually near the front) all references applicable to the specification. These may include drawings, specifications, standards, company-specific test procedures and methods used in following the specification. Standard methods and procedures available to the public and developed or approved by industry organizations are desirable, but they are not required.
(c) Provide definitions for terms or abbreviations that need to be understood to follow the specification. Any terms essential to raw material control (e.g., prepreg allowed defect levels) and processing (e.g., storage life and out life) need to be clearly defined with no room for interpretation. Where possible, use definitions from industry standards.
b. Material Requirements. The requirements used to control materials need to be well documented. Some requirements will only appear in documents that are proprietary to suppliers while other requirements are also documented in the material specification.
(1) Process Control Document.
(a) The material supplier establishes a process control document (PCD) or equivalent, which contains a description of all the critical details that are used to ensure repeatable raw material fabrication. Usually, a PCD is proprietary to the material supplier, who is responsible for its maintenance. The PCD is referenced by the material specification. The PCD contains more details on raw material processes than potential users need to know;
however, customers usually review the information to assess material quality assurance.
(b) The PCD lists all critical raw material ingredients and the associated suppliers.
All resin components and their manufacturers are listed, including multiple sources. The resin component specifications, quality sampling plan, and test methods for properties are referenced in the PCD. The fiber specification, quality sampling plan, and test methods for fiber properties are also referenced in the PCD. Backing paper and film is controlled like any other critical raw material.
(c) The PCD defines key process parameters and establishes statistical process control (SPC) procedures. The PCD will contain the requirements for ingredients (fiber, resin, and material form), uncured prepreg, and cured prepreg. The process limits, requirements, and continuous inspection procedures are given in the PCD and/or specification. Internal documents such as company-specific test procedures that are used for material control are referenced in the specification and included in the PCD.
(2) Prepreg Ingredient and Process Requirements.
(a) The material specification includes requirements that define specific raw constituents (i.e., resin and fiber). There is only one resin formulation and a single fiber class
(i.e., specific fiber type, manufacturer, and facility) associated with the unique set of requirements. A designation that represents one combination of ingredients and a single mixing process is given to the resin. Multiple sources of the same resin ingredient must be validated through chemical and physical evaluations. Fiber used in the prepreg is purchased to a separate 9
fiber material specification. If multiple fiber sources are included in the specification, then the specific prepreg materials that use each fiber type should have a unique specification, prepreg designation (i.e., class), and associated prepreg requirements. The fiber sizing, process to add sizing, and sizing content are considered to be an integral part of the fiber.
(b) Resin composition and all steps in the mixing process need to be defined before qualification. This includes tolerance limits that are validated by chemical and physical testing.
Resin ingredients can be blended as long as individual storage and handling requirements are met. Chemical and physical testing is used to validate any blending of mixed resin batches. The goal is to ensure that all variations in the resin composition and mixing process steps, which are within specified limits, yield repeatable resin to produce a consistent prepreg.
(c) Details for resin mixing steps are defined in the PCD with specific requirements that control these processes. Key attributes of the final mix or any pre-mixes are documented as requirements. These include gel time, viscosity, and a chemical analytical signature. The resin cure kinetics and rheology are also well characterized for limits allowed within the mixing processes. A summary of the resin property data, which can be used as requirements to control the resin processes, is given in DOT/FAA/AR-02/109. This table also gives some cured resin properties (moisture diffusion and resistance to thermal micro cracking), which may be of interest to users.
(d) Fiber requirements are documented. The material specification includes requirements that define the specific mechanical and physical properties of the fiber, including tensile strength, tensile modulus, and density. The specification also identifies the fiber form and tow count. Key fiber properties are listed in DOT/FAA/AR-02/109.
(e) Fiber storage and handling instructions normally appear in the PCD. A shelf life requirement for the fiber sizing is recognized in the processes if the sizing ages during storage.
Fiber batch blending is allowed in production, providing each fiber batch can be traced.
(f) The material specification includes requirements that define the processes for producing the prepreg. Details of the resin filming and prepreg processes are documented in the PCD. Process conditions have maximum and minimum limits that are monitored with the SPC procedures. Resin filming process limits maintain the chemical signature while ensuring film quality (e.g., film thickness and visual characteristics). The prepreg process achieves a consistent thickness while controlling resin advancement, degree of impregnation, handling characteristics, defects, and fiber/resin distribution.
(g) Proof is needed that prepreg manufactured at the minimum and maximum limits of allowed thermal history could still be processed within limits of the recommended baseline cure cycle. This effort is needed to ensure an acceptable baseline cure cycle for cured prepreg requirements (see paragraph 5b(4) of this AC). Cured prepreg mechanical property tests in a hot/wet condition and an assessment of the degree of cure provide the necessary validation.