[Federal Register Volume 65, Number 114 (Tuesday, June 13, 2000)]
[Notices]
[Pages 37198-37205]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 00-14482]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

[Policy Statement Number ACE-00-23.613-01]


Proposed Issuance of Policy Memorandum, Material Qualification 
and Equivalency for Polymer Matrix Composite Material Systems

AGENCY: Federal Aviation Administration, DOT.

ACTION: Notice of policy statement; request for comments.

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SUMMARY: This document announces an FAA proposed general statement of 
policy applicable to the type certification of normal, utility, 
acrobatic, and commuter category airplanes. This document advises the 
public, in particular manufacturers of normal, utility, acrobatic, and 
commuter category airplanes, of additional information related to 
material qualification and equivalency for polymer matrix composite 
material systems. This notice is necessary to advise the public of FAA 
policy and give all interested persons an opportunity to present their 
views on the policy statement.

DATES: Comments submitted must be received no later than July 13, 2000.

ADDRESSES: Send all comments on this policy statement to the individual 
identified under FOR FURTHER INFORMATION CONTACT at Federal Aviation 
Administration, Small Airplane Directorate, ACE-111, Room 301, 901 
Locust, Kansas City, Missouri 64106.

FOR FURTHER INFORMATION CONTACT: Lester Cheng, Federal Aviation 
Administration, Small Airplane Directorate, ACE-111, Room 301, 901 
Locust, Kansas City, Missouri 64106; telephone (816) 329-4120; fax 816-
329-4090; e-mail: <a href="/cdn-cgi/l/email-protection#2f434a5c5b4a5d014c474a41486f494e4e01484059"><span class="__cf_email__" data-cfemail="aac6cfd9decfd884c9c2cfc4cdeacccbcb84cdc5dc">[email protected]</span></a>.

SUPPLEMENTARY INFORMATION:

Comments Invited

    Interested persons are invited to comment on this proposed policy 
statement, ACE-00-23.613-01, by submitting such written data, views, or 
arguments as they desire. Comment should be marked, ``Comments to 
policy statement ACE-00-23.613-01,'' and be submitted in duplicate to 
the above address. The Manager, Small Airplane Directorate, will 
consider all communications received on or before the closing date for 
comments.

Background

    This notice announces the availability of the following proposed 
policy

[[Page 37199]]

memorandum, ACE-00-23.613-01, for review and comment. The purpose of 
this memorandum is to address certification projects initiated after 
the final date of the memorandum. Certification projects already in 
work do not necessarily need to comply.

Effect of General Statement of Policy

    The FAA is presenting this information as a set of guidelines 
appropriate for use. However, this document is not intended to 
establish a binding norm; it does not constitute a new regulation and 
the FAA would not apply or rely upon it as a regulation. The FAA 
Aircraft Certification Offices (ACO's) that certify normal, utility, 
acrobatic, and commuter category airplanes should generally attempt to 
follow this policy when appropriate. Applicants should expect that the 
certificating officials would consider this information when making 
findings of compliance relevant to new certificate actions.
    Also, as with all advisory material, this statement of policy 
identifies one means, but not the only means, of compliance.
    Because this proposed general statement of policy only announces 
what the FAA seeks to establish as policy, the FAA considers it to be 
an issue for which public comment is appropriate. Therefore, the FAA 
requests comment on the following proposed general statement of policy 
relevant to compliance with 14 CFR part 23, Sec. 23.613, and other 
related regulations.

General Statement of Policy

1.1  General

    In the decades since the introduction of advanced composite 
materials for use in aircraft, the material qualification has been a 
costly burden to the airframe manufacturer. For each manufacturer, 
extensive qualification testing has often been performed to develop the 
base material properties and allowables at operating environmental 
conditions, which are used as part of an aircraft's design data, 
regardless of whether this material system had been previously 
certificated by other manufacturers. In addition to the use of such 
data in design, qualification also provides a population basis (e.g., 
in mean and variability statistics) to continuously ensure stable 
material production practices by the material supplier. The practice of 
qualification when performed by each manufacturer for an identical 
material system represents a massive duplication of effort.
    In recent years, NASA, Industry, and the FAA have worked together 
to develop a cost-effective method of qualifying composite material 
systems by the sharing of a central material qualification database. 
This method is built on the existing sections of MIL-HDBK-17-1E, and 
allows credit for FAA witnessed materials testing performed by third 
parties such as material vendors or industry consortia. During the 
development process, the Small Airplane Directorate worked closely with 
members of the NASA Advanced General Aviation Transport Experiment 
(AGATE) research consortium to ensure the acceptability of this method 
of compliance to the applicable airworthiness regulations. Furthermore, 
the FAA and AGATE have maintained a good communication with the 
appropriate MIL-HBDK-17 Working Groups by participating in their 
regular meetings. Valuable thoughts have been shared for the 
development of this method.
    This effort creates a new way of conducting business with airframe 
manufacturers and material suppliers. It enables composite material 
suppliers to work with the FAA to qualify their composite material 
system and receive approval (i.e., material qualification). An airframe 
manufacturer can then select this approved composite material system to 
fabricate aircraft parts and perform a smaller subset of testing to 
substantiate their control of material and fabrication processes 
tailored to a specific application. The terms ``material equivalency'' 
will be used in the current context to describe the sampling process 
for a subset of testing used to confirm equivalent mechanical, physical 
and chemical properties for a particular material or one undergoing 
minor changes. For purposes of example, a minor change would be a new 
material production line, which uses identical raw materials, processes 
and equipment. Another example of a minor change is the substitution of 
a new supplier for the same chemical constituent used to fabricate a 
given fiber or matrix type. A major change would involve more 
significant differences in the fiber type, matrix resin, and pre-
impregnated fabrication process. It is anticipated, significant cost 
saving can be realized for both the industry and the FAA by sharing the 
approved central database and standardizing engineering protocol to 
demonstrate material equivalency.
    As a precursor, efforts to establish protocol for shared material 
databases were documented in a letter, which was disseminated by the 
Small Airplane Directorate to both FAA certification field offices and 
industry in 1998. In that letter, the essential concepts of this method 
have been outlined both in terms of regulatory and technical 
considerations. As a follow-up, the current memorandum is intended to 
serve as a policy and guidance for the implementation of this newly 
developed methodology of qualifying the material systems. It is noted 
that currently this method pertains only to part 23 aircraft.

1.2  Substantiation of Composite Structures

    It has been well recognized that analysis and base material data 
alone is generally not adequate for substantiation of composite 
structural designs. The ``building-block approach'' of testing, in 
concert with analysis, is typically used to fulfill the certification 
requirement. As outlined in Section 2.1 of MIL-HDBK-17-1E for Polymer 
Matrix Composites, the building-block approach consists of several 
levels of activities from both the ``structural complexity'' and ``data 
application'' considerations. The structural complexity is geometry or 
form-based, and may include levels of ``constituent,'' ``lamina,'' 
``laminate,'' ``structural element,'' and ``structural sub-component.'' 
On the other hand, the data application is a specific activity 
performed within the design development and certification process. The 
specific levels of structural complexity required depend on the 
distinct purpose of the data application. For example, structural 
substantiation may use tests and analysis at many different levels of 
structural complexity, whereas material acceptance may only rely on the 
lowest levels (i.e., base material properties).
    The material qualification and equivalency method discussed in this 
memorandum is a data application intended to be at the lower-levels of 
the structural complexity consideration. It includes testing to get 
mechanical and physical properties at the lamina level. Such tests are 
performed using laminates with simple ply stacking sequences to 
characterize the response of the composite material. At this level, the 
key properties represent un-notched and un-damaged base material 
strength allowables for loading in tension, compression, and shear. 
Other important results are the lamina moduli for these load cases. 
This material qualification testing provides 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.

[[Page 37200]]

    For clarification purposes, tests at higher levels (i.e., 
structural laminate, element and sub-component) are typically needed to 
fulfill the remaining parts of the structural substantiation 
requirement. As the design moves closer to application specific, the 
testing program proceeds to a higher level.
    Additional structural laminate specimen and element testing is 
intended to evaluate the ability of the material to tolerate common 
discontinuities. Key properties include open/filled hole tensile/
compression strengths, cutouts, joint bearing and bearing/bypass 
strengths, bonded joint element and attachments strengths, and impact-
damaged element strengths. These strength tests are used to derive the 
design values of the notched, bolted, bonded, and damaged features. 
These design values, in general, would be lower than that of the base 
material strength allowables established via the material qualification 
testing program. However, as the test element size and complexity 
increases, it is more costly to generate variability data. As a result, 
conservative engineering practices are typically applied to utilize 
statistics collected at the lower (specimen) level of tests.
    Furthermore, the structural sub-component (or full scale) testing 
is typically required to confirm load paths (i.e., validate analyses) 
and evaluate the behavior and failure mode of increasingly more complex 
structural assemblies that are considered application specific. At this 
scale, it is unreasonable to think of shared databases due to unique 
features in the design of a given product.

2.0  Related Regulatory and Guidance Materials

2.1  Federal Regulations

    This new method for material qualification and equivalency has been 
developed as a means of showing compliance with 14 CFR part 23 
requirements for the field of application defined. The regulations that 
are directly related to this method include:

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

    Section 23.613 contains specific requirements for material strength 
properties and design values. Presented below are the requirements 
that, in particular, are tied to this method:
    <bullet> ``Material strength properties must be based on enough 
tests of material meeting specifications to establish design values on 
a statistical basis.'' [Sec. 23.613(a)]
    <bullet> ``Design values must be chosen to minimize the probability 
of structural failure due to material variability.'' [Sec. 23.613(b)]. 
Section 23.613(b) requires that the design values selected to ensure 
structural integrity need to be characterized by the probability 
depending on the design configurations. That is, A-Basis for single-
load-path design and B-Basis for multiple-load-path.
    <bullet> ``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.'' 
[Sec. 23.613(c)]. Similarly, Sec. 23.603(a)(3) requires ``Take into 
account the effects of environmental conditions such as temperature and 
humidity, expected in service.''
    As discussed in Section 1.2, the database from the qualification 
program includes the base material strength allowables, which represent 
the design basis at the lamina level at appropriate environmental 
conditions. Design values utilized for any specific application still 
need to be established via some combination of additional testing 
programs, rationale engineering assumptions, and validated analyses. 
Nevertheless, the qualification database serves as a foundation upon 
which the material can be controlled and design values for higher-level 
application are derived. For certification purposes, the base material 
allowable is a subset of the aircraft's type design data.

2.2  Advisory Circulars

    The following two FAA advisory circulars (AC's) 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

    AC 20-107A sets forth an acceptable, 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. Guidance information is also presented 
on associated quality control and repair aspects.
    AC 21-26 provides information and guidance pertaining to an 
acceptable, but not the only, means of demonstrating compliance with 
the requirements of 14 CFR part 21 regarding quality control systems 
for the manufacture of composite structures. This AC also provides 
guidance regarding the essential features of quality control systems 
for composites as mentioned in AC 20-107A.

2.3  MIL-HDBK-17

    The MIL-HDBK-17 has been developed and is maintained as a joint 
effort of the Department of Defense (DOD) and the Federal Aviation 
Administration (FAA). This handbook provides guidance in the 
development of base material properties (allowables) and design values 
acceptable to the FAA. This new methodology is derived based on the 
MIL-HDBK-17-1E (Polymer Matrix Composites Volume 1: Guidance). The 
sections that are closely related to this method include:

Section 2.3.2  Material qualification test matrices
Section 2.3.3  Material acceptance test matrices
Section 2.3.4  Alternate material equivalence test matrices
Section 8.4.3  Alternate material statistical procedures

    For the simplicity of this memorandum, the MIL-HDBK-17-1E can also 
serve as a reference for most of the terminology used in this document.
    For standardization purposes, guidance for material database 
presentation, both in terms of format and content, has been well 
outlined in MIL-HDBK-17-2E (Polymer Matrix Composites Volume 2: 
Materials Properties). Presentation of material data per the guidance 
set forth in the MIL-HDBK-17 is highly recommended.

2.4  AGATE Document (DOT/FAA Technical Report)

    The specific methodology outlined in this memorandum has been 
developed through the effort of Work Package 3 (Integrated Design and 
Manufacturing Tasks) of the AGATE program. Technical works have been 
conducted mainly at the National Institute for Aviation Research (NIAR) 
facility affiliated with Wichita State University at Wichita, Kansas. 
Throughout the process, close coordination between the FAA [the Small 
Airplane Directorate, Technical Center and National Resource Specialist 
(NRS)] and the NIAR has been maintained to ensure this method is in 
compliance with the applicable airworthiness regulations.
    Application of this method has been demonstrated for the epoxy-
based pre-impregnated carbon or fiberglass material systems cured at 
250  deg.F with low-pressure curing/processing cycles. This effort has 
resulted in an AGATE technical document entitled ``Material

[[Page 37201]]

Qualification and Equivalency for Polymer Matrix Composite Material 
Systems'' where details of this methodology are presented. To enhance 
the accessibility of this document to the industry in general, an 
effort is underway by the FAA Technical Center to edit and publish it 
as a DOT/FAA Report.

3.0  Material Qualification

3.1  Field of Application

    The developed material qualification methodology is intended, in 
general, for polymer matrix material systems. The purposes of this 
method include:
    <bullet> To solidify and finalize material and process (M&P) 
specifications, including specific acceptance criteria for sampling 
relative to the qualification database
    <bullet> To quantify base material variability
    <bullet> To provide a central database with stabilized material 
processes
    Application of this method has been conducted/demonstrated via the 
effort of the AGATE program. The AGATE program has applied this method 
to material systems that are characterized by the following specifics:
    <bullet> Epoxy-based pre-impregnated carbon or fiberglass
    <bullet> Unidirectional tape or woven fabric
    <bullet> Cure temperature at 240  deg.F or higher
    <bullet> Low-pressure curing/processing cycles (i.e., autoclave and 
vacuum bagging)
    Testing requirements and data reduction procedures needed to 
certify the composite material system for complying with airworthiness 
regulations are presented in the AGATE document. The testing defined in 
the AGATE document represents the minimum requirement. In some cases, 
unique characteristics of a material system or its application may 
require testing beyond that defined by this method (i.e., more rigorous 
procedures and larger qualification databases). In these situations, 
Aircraft Certification Offices (ACO's) may require additional testing 
to demonstrate compliance with the applicable airworthiness 
regulations.

3.2  Qualification Approval Procedures

    Material qualification bears the objective of establishing the FAA 
approved base material properties of an ``original'' material system. 
Test materials are fabricated using ``original'' process 
specifications. This effort may be part of ongoing certification 
programs and can be managed by the appropriate project ACO. In some 
cases, such as a consortium crossing geographic boundaries, the Small 
Airplane Directorate may manage this effort.
    All specimen shall be fabricated according to the appropriate 
process specification to the geometry described in the AGATE document. 
Prior to testing, conformity of the test specimen must be performed by 
Manufacturing District Inspection Office (MIDO) inspectors at the 
request of ACO engineers. The MIDO inspector may elect to delegate this 
responsibility to a Designated Manufacturing Inspection Representative 
(DMIR) or Designated Airworthiness Representative (DAR).
    Testing must be witnessed by the FAA. Witnessing can be performed 
by ACO engineers, or they may delegate this responsibility to a 
Designated Engineering Representative (DER) or MIDO inspector.

3.3  Environmental Conditions

    In order to substantiate the environmental effects with respect to 
the material properties, several environmental conditions are defined 
to represent extreme cases of exposure. The selection of these 
conditions shall be based on the nature of the material system and its 
intended application as well.
    To illustrate, the conditions defined as extreme cases for the 
AGATE program are as follows:

<bullet> Cold Temperature Dry (CTD)....  -65 deg. F (<plus-minus>5
                                          deg.F) with an ``as
                                          fabricated'' moisture content.
<bullet> Room Temperature Dry (RTD)....  ambient laboratory conditions
                                          with an ``as fabricated''
                                          moisture content.
<bullet> Elevated Temperature Dry (ETD)  180 deg. F (<plus-minus>5
                                          deg.F) with an ``as
                                          fabricated'' moisture content.
<bullet> Elevated Temperature Wet (ETW)  180 deg. F (<plus-minus>5 deg.
                                          F) with an equilibrium
                                          moisture weight gain in a 85%
                                          relative humidity (<plus-
                                          minus>5% R.H.) environment.
 

    Properties for less extreme temperature conditions are determined 
through documented interpolation procedures.

3.4  Material Quality Control

    As part of material qualification, physical and chemical property 
tests are recommended for each batch of material received from the 
material vendor. These tests should be traceable to each referenced 
test. Prior to a significant investment in material qualification 
testing, the quality control procedures of the material vendor should 
be reviewed to ensure that quality control programs are in place for 
the fiber and neat resin, as well as pre-impregnation of the material 
form (e.g., tape or fabric). The recommended testing items (e.g., resin 
content, fiber areal weight, and gel time), along with the test 
methods, are presented in the AGATE document.
    In order to support the maximum operational temperature (MOT) limit 
of the material system and the specific data to be used in the 
statistical design allowable generation, cured lamina physical property 
tests (e.g., glass transition temperature, fiber/resin volume, and void 
content) are also required. These tests, along with the test methods, 
are defined in the AGATE document.

3.5  Batch-to-Batch Variability

    For a composite material system base properties (allowables), 
several batches of material must be characterized to establish the 
statistically-based material property for each of the material systems. 
For this qualification method, a minimum of three (3) batches of 
material are required to establish a B-basis design allowable. For an 
A-basis design allowable, three (3) batches may also be used, but five 
(5) batches of material are highly recommended to establish more 
statistically stable properties. It is noted that the minimum number of 
batches used in AGATE methodology is less than that recommended in MIL-
HDBK-17-1E.
    In order to account for processing and panel-to-panel variability, 
the material system being qualified must also be representative of 
multiple processing cycles. For this qualification method, each batch 
of material must be represented by a minimum of two independent 
processing/curing cycles (e.g., low-pressure autoclave and vacuum 
bagging). One engineering observation, which led to this AGATE 
methodology, was that the variation from composite panel processing can 
be as important as batch-to-batch material variability.

3.6  Property Testing Requirement

    The required material property tests are specified in the AGATE 
document, along with the recommended test method and the required 
number of batches/replicates per environmental condition (i.e., CTD, 
RTD, ETW and ETD). In the AGATE document, a format

[[Page 37202]]

has been defined to represent the required number of batches and 
replicates per batch. The format reads: # x #, where the first # 
represents the required number of batches and the second # represents 
the required number of replicates per batch. For example, ``3 x 6'' 
refers to three batches of material and six specimen per batch for a 
total requirement of 18 specimen.
    To illustrate, the tests required by the AGATE document for 
qualification at the environmental condition of ``Room Temperature Dry 
(RTD)'' are listed as follows:

------------------------------------------------------------------------
        No.                      Test                  Specimen (RTD)
------------------------------------------------------------------------
 1.................  0 deg. (warp) Tensile         3 x 4
                      Strength.
 2.................  0 deg. (warp) Tensile         3 x 2
                      Modulus, Strength and
                      Poisson's Ratio.
 3.................  90 deg. (fill) Tensile        3 x 4
                      Strength.
 4.................  90 deg. (fill) Tensile        3 x 2
                      Modulus and Strength.
 5.................  0 deg. (warp) Compressive     3 x 6
                      Strength.
 6.................  0 deg. (warp) Compressive     3 x 2
                      Modulus.
 7.................  90 deg. (fill) Compressive    3 x 6
                      Strength.
 8.................  90 deg. (fill) Compressive    3 x 2
                      Modulus.
 9.................  In-Plane Shear Strength.....  3 x 4
10.................  In-Plane Shear Modulus and    3 x 2
                      Strength.
11.................  Short Beam Shear............  3 x 6
------------------------------------------------------------------------

3.7  Base Material Allowable Generation

    Upon completion of the property testing, the statistical base 
material allowable can be generated for each mechanical strength 
property per the data reduction procedure described in the AGATE 
document. Software for the data reduction procedure has been made 
available in the form of a disk-file as an attachment to the AGATE 
document. Raw test values are normalized to a specified fiber volume as 
the fibers are the primary load-carrying component of the composite 
material. This provides a consistent basis for property comparisons and 
generally reduces variability in fiber-dominated properties. The 
procedure used for this is consistent with that recommended by MIL-
HDBK-17-1E.
    Proper consideration of the inherent material property variability 
in composite materials needs to be addressed in assigning design basis 
value to each mechanical property. Although the statistical procedures 
presented in the AGATE document may account for most common types of 
variability, these procedures may not account for all sources of 
variability.
    B-basis and A-basis material allowables are determined for each 
strength property using the statistical procedures outlined in the 
AGATE document. The specific procedures used assume a normal 
distribution for the population and take advantage of pooling of data 
between environments in calculating statistical variations. The latter 
is dependent on the assumptions that the failure mode for a given type 
of test does not vary significantly between environments and that the 
material variability across environments is comparable. The AGATE 
document describes the additional statistical tests and engineering 
data analysis needed to ensure all assumptions are not violated for a 
given material system. If evidence of deviations from the assumptions 
exists, more general procedures in MIL-HDBK-17-1E should be followed. 
For the moduli and Poisson's ratio, the average value of all 
corresponding tests for each environmental condition is used.
    If maximum strain material allowables are required, simple one-
dimensional linear stress-strain relationships may be employed. The 
linear assumption works well for tensile and compressive strain 
behavior but may produce rather conservative strain values in shear due 
to nonlinear behavior. More realistic engineering guidelines to derive 
shear strain allowables are given in MIL-HDBK-17-1E (Section 5.7.6).

3.8  Material Performance Envelope

    Referring back to the discussions in Sections 1.2, 2.1, and 3.1, 
base material strength allowables and elastic moduli generated by the 
procedures given in the AGATE document serve a purpose in stable 
composite material control within the industry and certification of 
specific aircraft products. Standard test methods and accepted 
statistical data treatment facilitate their use for the former, where a 
wide segment of the material supplier and aircraft manufacturing 
industry can share in the cost of generating the database. When it 
comes to the use of this data for the development and certification of 
structure for a specific aircraft, complementary test data and analysis 
is needed to account for the effects of design detail, structural 
scale, and damage.
    Using the statistical allowables, a base material performance 
envelope can be generated for a material system by plotting these 
values as a function of temperature. Each specific aircraft application 
of the qualified material may have a different maximum operational 
temperature (MOT) limit than those tested for the material 
qualification. Some applications may require a reduced MOT. For these 
cases, interpolation may be used to obtain the corresponding basis 
values at the new application MOT.
    Interpolation schemes and examples are presented in the AGATE 
document. The schemes provided in the document are practical for 
materials obeying typical mechanical behavior. In most cases, some 
minimal amount of testing may also be required to verify the 
interpolated values.
    Since unforeseen material property drop-offs with respect to 
temperature and environment can occur, extrapolation to a higher MOT 
should not be attempted without additional testing and verification.

4.0  Material Equivalency

    For clarification purposes, the terms ``material equivalency'' used 
in the current memorandum refer to the process of substantiating 
material properties for purposes of sharing a composite material 
qualification database and/or demonstrating that minor changes in 
material production processes have a negligible effect. This is 
achieved by test sampling and passing the acceptance criteria, which 
were derived from a larger population of material data.

4.1  Field of Application

    Composite material equivalence testing, which constitutes reduced 
data sampling (e.g., a single batch), may be performed by a 
manufacturer to establish a link with the original qualification 
database and associated specifications. Depending on the manufacturer's 
use of the qualification database, specifications for processing a 
particular product and the associated design data may even change 
significantly after establishing the link. For example, if the only 
intent of a link with the qualification database is to establish a 
population from which acceptance criteria are derived for standard 
tests performed in base material control, then significant changes in 
processing for a particular product may be allowed. On the other hand, 
if the base material qualification database has greater use in design 
(e.g., applied in deriving design values), then additional testing may 
be needed to show equivalency with the process variations. In short, 
the role of material equivalency testing in certification will depend 
on details of the particular project.
    For example, consider the use of a given material in sandwich 
construction, which may have process variations (e.g., lower autoclave

[[Page 37203]]

pressures) and changes in laminate characteristics resulting from the 
sandwich panel design configuration (e.g., dimpling of the face-sheets 
on honeycomb cells). In such a case, standard tests for base material 
properties in the AGATE approach use flat laminates, which may yield 
different properties than occur in sandwich panels. If the 
manufacturer's intended use of the qualification database is limited to 
control of the base material as purchased, the manufacturer may elect 
to demonstrate equivalency using original specifications. On the other 
hand, if the qualification database will have greater use in design, 
then equivalency testing should expand to consider the effects of 
product process and design variations on the base material properties. 
Alternatively, subsequent tests within the building block approach used 
for certification may also be defined to account for such differences. 
Again, the role of material equivalency testing in certification will 
depend on details of the particular project.
    The material equivalence testing may also be used to assess the 
effects of minor changes in constituent(s), the constituent 
manufacturing process, and/or the resin pre-impregnation process, for 
the purpose of utilizing the existing material qualification database. 
This testing evaluates the key properties for test populations large 
enough to provide a definitive conclusion but small enough to provide 
significant cost savings as compared to establishing a new database.
    Note that MIL-HDBK-17-1E goes beyond the discussions in this 
memorandum to describe methods for demonstrating alternate material 
acceptance. The discussion can be found in Section 2.3.4. Although the 
term equivalence is used in this section of MIL-HDBK-17-1E, the test 
matrices presented are much more extensive, highlighting additional 
issues for the problems being addressed (i.e., changes in fiber type, 
fiber tow size, resin, and pre-impregnated manufacturer). Table 
2.3.4.1.3 of this volume covers a wide variety of changes to a material 
system and highlights the fact that the performance of a material 
system is determined by both the materials and processes used in its 
manufacture.
    The AGATE methodology of demonstrating material equivalency is 
derived from MIL-HDBK-17-1E. This methodology only applies to 
situations with minor changes to the ``original'' material system in 
terms of material constituents and/or manufacturing processes. These 
situations may include:
    <bullet> Identical materials, processed by same manufacturer using 
identical fabrication process at different locations;
    <bullet> Identical materials, processed by different manufacturer 
using a ``follow-on'' process that is equivalent to the ``original'' 
fabrication process;
    <bullet> Identical materials, processed by different manufacturer 
using a ``follow-on'' process that is slightly different to the 
``original'' fabrication process;
    <bullet> Minor changes in constituent(s) and/or constituent 
manufacturing process, processed by same/different manufacturer using a 
``follow-on'' process that is slightly different to the ``original'' 
fabrication process;
    <bullet> Combinations of the above.
    In summary, the purposes of this equivalency method include:
    <bullet> To share and make use of the central database by a new 
user (i.e., original material qualification);
    <bullet> To continue surveillance of material and process (e.g., 
Section 5.0 as applied in material quality control);
    <bullet> To show that minor changes to material and processes do 
not affect base material properties;
    <bullet> To make final adjustment on material and process 
specifications for specific application and demonstrate that it has 
little affect on base material properties.

4.2  Equivalency Approval Procedures

    For the ``follow-on'' applicants to use the database, they need to 
develop their own material and process specifications based on the 
``original'' material and process specifications. The applicants submit 
these specifications along with the necessary test plans to their 
geographically responsible ACO for review. In all cases of material 
equivalency, an ``original'' should exist that contains base material 
mechanical properties and strength allowables, as well as the chemical 
and physical properties, for the initially qualified material system.
    As is the procedure on any certification program, the ACO reviews 
the test plans and the updated material/process specifications prior to 
the initiation of testing. The review of the applicants' specifications 
should determine if they meet the application limitations outlined in 
Section 4.1, and are, therefore, candidates for material equivalency 
testing. Since the basis properties of a composite material system are 
sensitive to both its material constituents and manufacturing process, 
vigilant engineering judgement must be exercised during the evaluation 
process. The fabrication methods of the applicants' structure must meet 
the applicable airworthiness regulations including, but not limited to, 
Secs. 23.603 and 23.605.
    Testing is required to qualify the ``follow-on'' material system by 
demonstrating material equivalency to the ``original'' material system. 
Testing must be witnessed by the FAA. Testing requirements, data 
reduction procedures, and material equivalency criteria/guidance are 
presented in the AGATE document.
    In addition to the base material level coupon testing, 
certification programs may require some element or sub-component 
testing in demonstrating equivalency for minor changes in the material 
production processes over time, which are suspected to have some effect 
on part manufacturing processes. These requirements will depend on the 
degree of change as well as on the application (e.g., complexity of the 
components or parts to be manufactured).

4.3  Equivalency Testing Requirement

    As described in Section 4.1, the AGATE material equivalency 
methodology is derived based on the most compatible situations 
existing, as discussed in MIL-HDBK-17-1E (i.e., an identical material 
is used or changes in the material are minor). Based upon the batch-to-
batch variability established in the original qualification database, 
material equivalency testing should be conducted to investigate the 
processing or panel-to-panel variability inherent in the follow-on 
manufacturer or location. As a minimum requirement to initiate such an 
exercise, the material and process controls used to generate the 
initial database must be known (i.e., the ``original'' material and 
process specifications or ``pedigree'' must be known). This issue has 
come up relative to some of the data that has been published in MIL-
HDBK-17-2E, and a plan has been initiated to ensure such information is 
available for data utilization.
    The equivalency tests required are presented in the AGATE document 
along with the recommended test methods and the required number of 
batches/replicates per environmental condition (i.e., RTD and ETW). One 
(1) batch of material is the minimum required for this testing program. 
As with material qualification, two separately processed panels are 
used in obtaining specimen for strength tests.
    To illustrate, the tests required by the AGATE document to 
demonstrate equivalency under the environmental condition of ``Room 
Temperature Dry (RTD)'' are listed as follows:

[[Page 37204]]



------------------------------------------------------------------------
                                                                Specimen
           No.                             Test                   (RTD)
------------------------------------------------------------------------
 1.......................  0 deg. (warp) Tensile Strength.....         8
 2.......................  0 deg. (warp) Tensile Modulus and           4
                            Poisson's Ratio.
 3.......................  90 deg. (fill) Tensile Strength....         8
 4.......................  90 deg. (fill) Tensile Modulus.....         4
 5.......................  0 deg. (warp) Compressive Strength.         8
 6.......................  0 deg. (warp) Compressive Modulus..         4
 7.......................  90 deg. (fill) Compressive Strength         8
 8.......................  90 deg. (fill) Compressive Modulus.         4
 9.......................  In-Plane Shear Strength............         8
10.......................  In-Plane Shear Modulus.............         4
11.......................  Short Beam Shear...................         8
------------------------------------------------------------------------

4.4  Success Criteria for Equivalency

    Results derived from the equivalency testing are compared with the 
original qualification database. The statistical procedures and the 
success criteria for equivalency are presented in the AGATE document. 
As with qualification, the acceptance criteria adopted by AGATE to 
demonstrate equivalency assumes a normal distribution. If a normal 
distribution was not confirmed by checks performed as part of the 
``original'' material qualification, the acceptance criteria will need 
to change to reflect the statistical distribution that was adopted for 
the population. In such a case, the more general procedures in MIL-
HDBK-17-1E should be followed.
    First, the qualification database shall present the property of 
interest in terms of ``mean'' and ``standard deviation.'' For base 
material strength properties, the qualification database also provides 
B-basis and/or A-basis values, which can be used for purposes of 
comparison in establishing specific acceptance criteria. In addition, 
two statistical parameters for sampling need to be defined, and they 
are: ``<greek-a>'' (probability of rejecting a good material) and ``n'' 
(number of specimen to be tested for the property of interest).
    A selection of <greek-a> = 0.01, for example, represents 1% of the 
chance of wrongly rejecting a good material. A higher ``<greek-a>'' 
value represents a more conservative criteria, yet at the expense of a 
higher chance of rejecting a good material. Also, as the number of 
specimen increases, the chance for the mean of the specimen (tests 
sample) to appear different from the original qualification data 
decreases. Statistically, the two parameters reflect the Type I errors 
in test on either means or minimum individual values. The Type I error 
refers to the situation of rejecting the null hypothesis when it is 
true. The B-basis and A-basis values, which were derived in population 
testing, have limited statistical meaning when assessing the 
equivalency from a small sample size. However, they may have some 
engineering value in setting the <greek-a> for a particular 
application.
    For strength properties, material equivalency is established by 
using both the means and the minimum individual values as the 
acceptance criteria. The material equivalence is not acceptable when 
either one of the two comparisons fails. The ``<greek-a>'' represents 
the probability of failing either one of the two, or both, comparisons.
    Based on a limited ``round robin'' testing program, the AGATE 
method currently recommends an ``n'' value of ``8'', and an 
``<greek-a>'' value of ``0.05'' for material equivalency tests to link 
with the complete material qualification database. As the exposure and 
experience increase through time, the values for these two parameters 
may be revised from lessons learned. Also, considering the intrinsic 
difference both in terms of the nature of material system and the 
specific of application, the certification offices (ACO's) may adjust 
this set of values reflecting their unique circumstances.
    Although specific criteria are not given, strength properties from 
equivalency testing should also not be excessively higher than those 
obtained for the original qualification database. Engineering judgement 
should be used to detect such increases in base strength, which may 
affect structural failure modes or reductions in untested strength 
properties. For example, un-notched (or small notch) tensile strength 
properties have been found to be inversely related to the tensile 
residual strength of composite structure with larger flaws.
    For modulus, a simple comparison of means is used. The criterion is 
not satisfied when either the test sample mean is too high or too low 
in reference to the original maximum/minimum mean of the qualification 
database.
    There are also statistical tests that interrogate the new samples 
as to their equivalency to the baseline sample qualification database. 
These can be used as an alternative to the test on means and minimum 
individual values described above. MIL-HDBK-17-1E recommends the k-
sample Anderson-Darling (A-D) statistical test (Section 8.3.2.2) or the 
ANOVA (analysis of variance) method described in Section 8.4.3.1. The 
k-sample A-D test can be used for unequal sample sizes that will be 
encountered when comparing the baseline data to the new data. 
Discussion on the use of a significance level of (<greek-a> = 0.05 is 
given in MIL-HDBK-17. The value chosen should be agreed upon by the 
particular application and should be the same if the ANOVA method is 
used.
    Other alternate tests (if normal distribution is assumed) are to 
use the F-test to show equivalency of the means (Section 8.3.5.2.2) and 
Levene's test to show equivalency of the variances (Section 8.3.5.2.1). 
An ``<greek-a>'' value for these tests must also be selected.
    Successful completion of the equivalency testing allows the 
applicant to use the properties contained in the original qualification 
database. In the case when the testing of the first batch fails, a 
second opportunity using a different batch of material can be allowed 
for this equivalency testing. In order to limit the undesirable, 
statistically termed as the Type II error, only permission of retest to 
the 2nd batch is recommended. The Type II error refers to the situation 
of accepting the null hypothesis when it is false.
    Should the applicant fail criteria for equivalency testing of the 
second batch, the original base material allowable database can no 
longer be used, and a new base material allowable database needs to be 
established per material qualification procedures. Such a scenario 
requires engineering to identify material and/or processing 
differences, which led to changes in the base material properties, and 
the associated update to specifications (i.e., a new material 
qualification). In addition, careful planning of material procurement, 
panel fabrication and testing may be considered at the start of a 
material equivalency exercise to ensure that equivalency testing of a 
first and second batch can be expanded to be part of a new 
qualification if required. For example, the material order and panel 
sizes fabricated for a particular batch of material may be sufficiently 
large enough to yield additional specimens, as needed for the larger 
test matrix in a qualification effort.

5.0  Continuous Quality Control

    Material supplier and purchaser tests performed as part of a 
continuous quality control process may be considered a special case of 
material equivalency testing. In this case, the sample size is 
typically smaller than recommended for the material equivalence 
exercise described in Section 4.0. Nevertheless, the tests are 
typically performed on a per batch basis and a link with the 
qualification database can be developed using the same statistical 
methods (Section 4.4).
    For purposes of continuous quality control, a recommended 
``<greek-a>'' value of

[[Page 37205]]

0.01 (i.e., 1% probability of rejecting ``good'' material) and an ``n'' 
value of 3 to 5 are appropriate. Note the less stringent requirement 
here than for obtaining access to the ``original'' qualification 
database discussed in Section 4.4. In the latter case, all future 
batches of material are being admitted while in the former case only 
one batch is under scrutiny. As the exposure and experience along this 
line increase through time, a new set of values for these two 
parameters may be provided. Also, considering the intrinsic difference 
both in terms of the nature of the material system and the specifics of 
application, the certification offices (ACO's) may adjust this set of 
values reflecting their unique circumstances.
    If quality control testing fails, engineering evaluation can be 
performed to justify a retest of the same batch of material. As part of 
this effort, engineers should search for other reasons to believe the 
material is ``bad'' or identify a problem in specimen fabrication and/
or testing. The number of ``retests'' should be limited to one which, 
from a purely statistical perspective, yields a probability of 
rejecting good material in two sets of receiving inspection tests for 
the same batch is only 0.01% for the recommended ``<greek-a>''.

    Issued in Kansas City, Missouri, on May 30, 2000.
Marvin Nuss,
Acting Manager, Small Airplane Directorate, Aircraft Certification 
Service.
[FR Doc. 00-14482 Filed 6-12-00; 8:45 am]
BILLING CODE 4910-13-U


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