[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: [email protected].
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:
``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)]
``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.
``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:
To solidify and finalize material and process (M&P)
specifications, including specific acceptance criteria for sampling
relative to the qualification database
To quantify base material variability
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:
Epoxy-based pre-impregnated carbon or fiberglass
Unidirectional tape or woven fabric
Cure temperature at 240 deg.F or higher
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:
Cold Temperature Dry (CTD).... -65 deg. F (5
deg.F) with an ``as
fabricated'' moisture content.
Room Temperature Dry (RTD).... ambient laboratory conditions
with an ``as fabricated''
moisture content.
Elevated Temperature Dry (ETD) 180 deg. F (5
deg.F) with an ``as
fabricated'' moisture content.
Elevated Temperature Wet (ETW) 180 deg. F (5 deg.
F) with an equilibrium
moisture weight gain in a 85%
relative humidity (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:
Identical materials, processed by same manufacturer using
identical fabrication process at different locations;
Identical materials, processed by different manufacturer
using a ``follow-on'' process that is equivalent to the ``original''
fabrication process;
Identical materials, processed by different manufacturer
using a ``follow-on'' process that is slightly different to the
``original'' fabrication process;
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;
Combinations of the above.
In summary, the purposes of this equivalency method include:
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 (e.g.,
Section 5.0 as applied in material quality control);
To show that minor changes to material and processes do
not affect base material properties;
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: ``'' (probability of rejecting a good material) and ``n''
(number of specimen to be tested for the property of interest).
A selection of = 0.01, for example, represents 1% of the
chance of wrongly rejecting a good material. A higher ``''
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 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 ``'' 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
``'' 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 ( = 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 ``'' 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
``'' 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 ``''.
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