+------+------------------------------------+----------+--------------+
| Subj | **                                 | Date:    | December 21, |
| ect: | [INFORMATION]{.underline}:**Static |          | 2001         |
|      | Strength Substantiation of         |          |              |
|      | Composite Airplane Structure;      |          |              |
|      | Policy Statement                   |          |              |
|      | PS-ACE100-2001-006                 |          |              |
+------+------------------------------------+----------+--------------+
| F    | Manager, Small Airplane            | Reply to | Lester Cheng |
| rom: | Directorate, Aircraft              |          |              |
|      | Certification Service, ACE-100     | Attn.    | 316-946-4111 |
|      |                                    | of:      |              |
+------+------------------------------------+----------+--------------+
| To:  | SEE DISTRIBUTION                   |          |              |
+------+------------------------------------+----------+--------------+

**Purpose of Policy**

> The purpose of this policy, which applies to 14 CFR (Code of Federal
> Regulations) part 23 airplanes, is threefold. First, it provides
> general guidance on large-scale tests and when they are needed to meet
> the certification requirements. Second, it reviews the critical
> factors that affect the static strength of composite airplane
> structures and how they can be addressed. Third, it describes some
> commonly accepted engineering practices used for structural
> substantiation, including the engineering rationale for various means
> of showing compliance. This document also provides further
> clarification of AC 20-107A.
>
> This policy on static strength substantiation for composite structures
> is consistent with small airplane and business jet certification
> programs completed. Note that most of these airplanes have been
> constructed with pre-impregnated, laminated composite material forms.
> As service databases expand and new composite material forms and
> manufacturing processes continue to evolve, future applications may
> need to consider other critical factors important to static strength.
>
> This document describes policy for static strength substantiation of
> primary composite airplane structures that are critical to safety of
> flight. Other structures, whose failure can be shown not to affect
> airplane safety (secondary structures), may not require the same level
> of rigor in engineering analysis and test assessment to ensure
> structural integrity.

**Table of Contents**

**1.0 Introduction**

1.1 General Background

1.2 Factors to Consider

**2.0 Related Regulatory and Guidance Materials**

2.1 Federal Regulations

2.2 Advisory Circulars

**3.0 Building Block Approach**

3.1 Purpose and Application

3.2 Analysis Validation of Load Paths

3.3 Damage, Defects, Repeated Load, and Environmental Effects

3.4 MIL-HDBK-17

**4.0 Environmental Exposure and Repeated Load**

4.1 Long-Term Time Related Degradation

4.2 Solar/Thermal Design Criteria

4.3 Peak Temperature Analysis

4.4 Peak Moisture Content

4.5 Analyses and Tests

4.6 Full Scale Test Considerations

4.7 Other Exposures

**5.0 Static Strength Analysis - "A" and "B" Basis Allowables**

5.1 General

5.2 Policy on Material Qualification and Equivalency

5.3 Detail Design and Notch Sensitivity

**6.0 Static Test Article Fabrication and Assembly**

6.1 General

6.2 Design and Manufacturing Detail Definition

6.3 Conformity Inspections

6.4 Defects and Damage

**7.0 Effects of Impact Damage and Minor Discrete Source Damage**

7.1 Damage at the Threshold of Detectability and Allowed Damages

7.2 Impact Considerations

7.3 Damage Tolerance

**8.0 Material and Process Variability**

8.1 General

8.2 Analytical Substantiation Supported by Tests

8.3 Substantiation Primarily by Tests with Minimal Analyses

8.4 Overload Factors

8.5 Other Considerations

**9.0 Conclusion**

**10.0 References**

**\-- POLICY \--**

**1.0 Introduction**

> Static strength substantiation is an important milestone in the
> certification of composite airplane structure. In the past, different
> approaches have been used to meet the associated Federal Aviation
> Administration airworthiness regulations.

1.1 General Background

> Many factors contribute to the strength exhibited by built-up
> composite structure, which utilizes bonding and other manufacturing
> technologies to minimize parts count and achieve performance goals.
> Traditionally, applicants used large-scale structural tests at the
> component level to evaluate the complex load paths and failure
> mechanisms of such structure. As in the case of new metal structural
> designs, such tests are often used to ensure sufficient ultimate
> strength. However, it becomes unwieldy to address all issues affecting
> composite static strength in large-scale tests.
>
> The combination of lower-scale tests and analyses has proven useful to
> quantify many static strength issues, minimize uncertainties, and
> mitigate risks before large-scale tests. These tests may also prove
> essential in addressing common manufacturing and service issues that
> should be expected following type design certification.

1.2 Factors to Consider

> When substantiating the static strength of a composite design,
> applicants should consider the following:
>
> \- Critical load cases and associated failure modes;
>
> \- Effects of environment, repeated loading, manufacturing tolerance,
> and material and process variability;
>
> \- Manufacturing defects and service damage that are not detectable by
> the anticipated inspection methods (as well as those defects or damage
> that are permitted by the quality control or maintenance documents of
> the product); and
>
> \- Desired repair scenarios.
>
> The static strength demonstration should include an ultimate load test
> for each major structural component, unless the adequacy of the
> analysis is substantiated by sub-component tests or component tests to
> appropriate lower load levels. These tests should include, but are not
> limited to, configured wing, empennage and fuselage structure. The
> necessary experience to validate an analysis should include previous
> component tests with similar designs, material systems, and load
> cases.
>
> Although not specifically discussed in this memo, factors that affect
> static strength may also impact the vibration characteristics
> (stiffness, damping, and mass) of the composite structure. For this
> reason, applicants need to be aware of these factors when they verify
> the integrity of the composite structure against flutter and other
> aeroelastic mechanisms.

**2.0 Related Regulatory and Guidance Materials**

2.1 Federal Regulations

> The regulations that are directly related to this policy include the
> following:

**14 CFR Part 23 Subpart C -- Structure**

  ------------------------------- ---------------------------------------
  Section 23.305                  Strength and deformation

                                  

  Section 23.307                  Proof of structure

                                  

  Section 23.573(a)(1)            Damage tolerance and fatigue evaluation
                                  of structure
  ------------------------------- ---------------------------------------

**14 CFR Part 23 Subpart D - Design and Construction**

  ---------------------------------- ------------------------------------
  Section 23.601                     General

                                     

  Section 23.603                     Materials and workmanship

                                     

  Section 23.605                     Fabrication methods

                                     

  Section 23.609                     Protection of Structure

                                     

  Section 23.613                     Material strength properties and
                                     design values

                                     

  Section 23.619                     Special factors
  ---------------------------------- ------------------------------------

> Section 23.573(a) specifically sets forth the requirements for
> substantiating the primary composite airframe structures, including
> considerations for damage tolerance, fatigue, and bonded joints.
> Paragraph (a)(1), which prescribes \"that the structure is cable of
> carrying ultimate load with damage up to the threshold of
> detectability considering the inspection procedure employed,\" is
> particularly pertinent.
>
> Following § 23.573(a)(1) ensures sufficient ultimate load capability
> with undetected manufacturing defects, as well as impact damage, that
> can be realistically expected from production and service. Do not
> confuse it with damage tolerance requirements that ensure sufficient
> residual strength at limit loads.
>
> Section 23.613 contains specific requirements for material strength
> properties and design values. An earlier policy document issued by the
> Small Airplane Directorate contains detailed discussion of these
> requirements (see reference (1) under 10.0).
>
> Sections 23.603(a)(3) and 23.613(c) address the requirements relating
> to the effects of environmental conditions, which includes both the
> temperature and humidity expected in service. These requirements are
> of particular significance to composite aircraft structures.

2.2 Advisory Circulars

> The following two FAA advisory circulars (AC\'s) present
> recommendations for showing compliance with FAA regulations associated
> with composite aircraft structure:

  ------------------------------- ---------------------------------------
  AC 20-107A                      Composite Aircraft Structure

                                  

  AC 21-26                        Quality Control for the Manufacture of
                                  Composite Structures
  ------------------------------- ---------------------------------------

> AC 20-107A sets forth an acceptable means, but not the only means, of
> showing compliance with the provisions of 14 CFR parts 23, 25, 27, and
> 29 regarding airworthiness type certification requirements for
> composite aircraft structures. Section 6 (Proof of Structure --
> Static) is particularly relevant to this policy where the general
> guidance for conducting the component ultimate load tests is
> presented. Section 6 also describes the factors important to composite
> structure, which include:
>
> \- Material and process variability;
>
> \- Strength degradation due to repeated loading and environmental
> exposure; and
>
> \- Impact damage expected from manufacturing and service.
>
> AC 21-26 provides information and guidance pertaining to an acceptable
> means, 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.

**3.0 Building Block Approach**

3.1 Purpose and Application

> Within the composite engineering community, the structural
> substantiation process, which uses testing and analysis at
> increasingly complex levels, has become known as the "building block
> approach." Traditionally, such an approach has been used to address
> durability and damage tolerance as well as static strength for both
> metal and composite aircraft structure.
>
> Figure A provides a conceptual schematic of tests included in a
> building block approach for wing structure. As suggested by the
> figure, lower levels of testing are more generic and likely to apply
> to other airplane parts and products. Generally, more lower-level test
> repetitions provide a statistical basis for material performance.

![](media/image1.png){width="4.885416666666667in" height="4.3125in"}

Figure A. Schematic Diagram of Building Block Tests.

> Since some lower levels of building block tests can be considered
> generic, the concept of databases shared between programs is
> reasonable. Engineering protocol for base material qualification and
> the equivalency testing to use shared composite databases has been
> published previously (see references (1) and (2) under 10.0). As
> discussed in these references, each certification project will have
> its own certification plan and methods approved by the local Aircraft
> Certification Office.
>
> The integration of the design and manufacturing process becomes
> evident in building block levels, which are above coupons and
> elements. The larger scales of testing are needed to address the
> effects of more complex loads and geometry.
>
> As implied by Figure A, fewer tests are performed at larger scales.
> These tests are relevant because they address specific structural
> details.
>
> Applicants should place design attention on avoiding out-of-plane
> loads and related failure modes, which may occur with significant loss
> of local stability. Small out-of-plane load conditions, which have
> little impact on bolted structures, can be significant for bonded
> structure. These conditions are often not easily analyzed.

3.2 Analysis Validation of Load Paths

> Analysis validation is an important part of the building block process
> because it provides a basis to expand beyond the specific tests
> performed in development and certification. This validation starts
> with prediction of the structural stiffness, internal load paths, and
> stability.
>
> In order to perform these analyses, applicants need to establish the
> material stress-strain curve to failure (or a strain cutoff in the
> test methods) for each composite material used in the design. Analysis
> has proven reliable to minimize the number of tests needed to define
> this characteristic for laminated composite material forms.
>
> Verifying internal load paths may require additional building block
> tests, which are designed to evaluate load share between bonded and
> mechanically attached elements of a design. As failure is approached,
> some nonlinear behavior can be expected. Combined load effects can
> further complicate the problem of analytical predictions.

3.3 Damage, Defects, Repeated Load, and Environmental Effects

> Predicting the effect of multiple influences (environment, repeated
> loads, damage, and manufacturing defects) on the failure modes that
> affect structural strength traditionally relies on the building block
> tests. Often, semi-empirical analyses have been adopted for composite
> strength. Special consideration is given to structural discontinuity
> (for example, joints, cutouts or other stress risers) and other design
> or process-specific details.
>
> One of the most important parts of the building block analysis and
> test development comes in providing engineering databases to deal with
> the manufacturing defects, field damages, and repairs likely to occur
> in production and service. Traditionally, not enough attention was
> given to these issues during composite product development and
> certification. This has caused significant work slowdowns and
> increased costs for subsequent product manufacturing and maintenance.
>
> Without sufficient analysis and test database to cover commonly
> allowed manufacturing defects, damages, and repairs, engineers will be
> forced to either adopt conservative assumptions (part rejections or
> expensive repairs) or generate the data as it is needed (leading to
> down time and associated cost or lost revenue).
>
> It is difficult to plan for unanticipated defects and damage.
> Production and service experiences with new technologies such as
> composite materials are often needed to completely define the
> problems. Nevertheless, an awareness of the likely production and
> service issues will help define practical levels of building block
> tests and analyses to be performed as part of structural
> substantiation.

3.4 MIL-HDBK-17

> MIL-HDBK-17 (Composite Material Handbooks) is available at
> http://www.mil17.org/. This handbook provides detailed background on
> the engineering practices that have been successfully applied with
> composite materials used in airplane structures. Chapters on the
> building block approach for substantiation of composite structures are
> most useful for the current discussion. Chapter 2.1 from Volume 1 of
> the Polymer Matrix Composite (PMC) provides some introduction to this
> subject, including a synopsis of test levels and data uses.
>
> There is more information on the building block approach that can be
> found in Chapter 4, Volume 3 of PMC for the most recent revision of
> MIL-HDBK-17 (Revision F). This chapter outlines rationale for the
> traditional multi-level testing and analysis development approach used
> for metallic and composite structures, particularly in the aerospace
> industry. It also contains guidance and example test programs for
> various applications.
>
> Many of the engineering practices outlined in MIL-HDBK-17 were derived
> from composite applications to military and commercial transport
> structures. The composite material types, structural design details,
> and associated manufacturing processes selected for such applications
> may have significant differences from those used for small airplanes.

**4.0 Environmental Exposure and Repeated Load**

4.1 Long-Term Time Related Degradation

> The effects of environmental exposure and repeated loading, which may
> result in static strength degradation, should be addressed. Reduction
> in composite static strength as a function of environmental exposure
> and repeated loads can occur over long periods of time. For purposes
> of this policy, these considerations will be limited to reductions in
> static strength, which come without detectable damage. As a result,
> damage tolerance evaluation methods and maintenance practices cannot
> cover such degradation. Instead, sufficient static strength must be
> shown following exposures to environmental effects and repeated loads.
>
> The effect of repeated loads on base composite material properties is
> typically not as severe as metal fatigue (relatively flat curves for
> critical fatigue stress versus number of cycles). Degradation
> mechanisms for a composite subjected to repeated loads are typically
> not the same as self-similar crack growth behavior in metals. Instead,
> the composite damage associated with repeated loads can be more
> dispersed with minimal drop in local stiffness and residual strength
> until immediately prior to failure. As a result, composite structures
> are usually designed to working stress levels (highest loads in the
> spectrum) below that where significant damage accumulation occurs. In
> adopting such design practice, the degradation in residual strength
> with repeated loads should be minimal.
>
> Fatigue tests with open hole and impact damaged specimens may be used
> to determine damage accumulation stress levels for laminated composite
> materials. Some static strength testing following repeated loads can
> also demonstrate long-term resistance for specific materials and
> design details.
>
> Typical environmental exposures, which reduce static strength, include
> high temperatures and long-term moisture conditioning (often referred
> to as a hot/wet condition). Composite airplane structures absorb
> moisture in the service environment and tend to reach an equilibrium
> condition after some period of time, which depends on design details
> (for example, part thickness) and exposure.
>
> Composite structural properties greatly affected by hot/wet
> conditioning include those that decrease with matrix stiffness (for
> example, compression strength). Other composite properties may be more
> strongly affected by cold/dry conditions. Coupon data generated for
> material allowables are generally a good basis for understanding the
> critical environments for different load types.

4.2 Solar/Thermal Design Criteria

> In 1990, the Small Airplane Directorate issued an FAA internal policy
> letter presenting the solar/thermal design criteria to be used for
> composite aircraft certification. This guidance supports the
> determination of the peak temperature for a particular composite
> airplane structure (Section 4.2 of this document) [^1].
>
> The \"Guidance for Composite Aircraft Solar and Thermal Design
> Certification Criteria\" is presented below:
>
> Applicants may base the thermal environmental analysis on a parametric
> study of the following data to identify the highest structural
> temperature:

  -------------- ----------------------------- --------------------------
  **Hour**       **Ambient Temperature**       **Solar Radiation**

  1100           111°F                         330 Btu/ft^2^/hr

  1200           114                           355

  1300           119                           355

  1400           122                           330

  1500           123                           291

  1600           124                           231

  1700           123                           160
  -------------- ----------------------------- --------------------------

> The above temperature values would not be exceeded 99.9 percent of the
> time, as derived from MIL-STD-210C statistical data. For the above
> data, the wind speed was 14 feet per second (for consideration of heat
> dissipation by convection). The relative humidity was 3 percent.
>
> The effect of cooling airflow may be considered. The FAA recommends
> the following:
>
> After heat soak at the critical condition, the airplane taxis, takes
> off, and climbs to 1,000 feet above sea level. The airplane then
> accelerates in level flight to:

\(1\) The lesser of the design maneuvering speed (V ~A~ ), or the
aircraft operating speed limit (in § 91.117(b)) if maneuver loads are
critical;

or

\(2\) The lesser of the design cruise speed (V ~C~), or the aircraft
operating speed limit (in § 91.117(b)) if gust loads are critical.

> In the case of a commuter category airplane, the design speed for
> maximum gust intensity, (V ~B~), applies instead of the design cruise
> speed (V ~C~).
>
> Per § 91.117(b), the aircraft operating speed limit is 200 knots or
> the applicable maximum airspeed of the aircraft, whichever is lower.
> This applies to major structure and may not be applicable to certain
> structures such as flaps and landing gear doors, which would be
> subject to limit loads at an earlier time in the flight profile. For a
> small airplane, a maximum taxi speed of 10 miles per hour is
> recommended. A four-minute taxi time would be reasonable.

4.3 Peak Temperature Analysis

> Applicants may use analyses, tests, published data or some combination
> of the three to obtain the peak temperature for a particular composite
> airplane structure. Thermal analysis should use the environmental
> conditions outlined previously in Section 4.2. Tests for conditions
> other than those described in this section may also be used to
> validate the analysis, which would then need to be applied for the
> critical conditions.
>
> Structural temperatures typically rise well above the ambient
> conditions under high solar radiation. In performing thermal analysis
> for a structure, consider allowed paint colors that provide the
> highest temperature. Account for the combined effect of structural
> orientation and the reflection from adjacent structures and the
> ground. For example, adjacent structure painted with a light color may
> reflect radiation and increase the temperature of adjacent structure
> with dark colored paint.
>
> For most paint colors, a default critical structural temperature of
> 180 o F can be assumed without supporting tests or analyses. Dark
> colors or black, which may yield higher structural temperatures, are
> an exception.
>
> In addition to the solar/thermal consideration outlined above,
> consider other thermal sources when assessing the temperature effect
> to the composite airplane structure. Depending on the design
> configuration and installation arrangement, these thermal sources may
> require attention. For example:
>
> \(1\) Business jets may have air cycle machines for air conditioning,
> and there may be high temperature exhaust from the heat exchangers.
>
> \(2\) Engine bleed air is used for anti-ice. Consider system
> insulation and failure conditions. Operational experience indicates
> that bleed air leaks can cause severe local damage to composite
> structure.
>
> \(3\) Engine nacelles and cowlings may have higher operating
> temperatures due to the heat from the engine.

\(4\) Other structure in the proximity of engine exhaust.

4.4 Peak Moisture Content

> Applicants may use analysis, tests, published data or some combination
> of the three to determine peak moisture content for a particular
> composite airplane structure. Moisture diffusion analysis and test
> conditioning should assume a relative humidity on the order of 85
> percent as characteristic of past studies from long term service
> exposure, which includes ground time in humid environments from around
> the world. Engineering guidelines for moisture conditioning test
> samples to equilibrium can be found in MIL-HDBK-17.
>
> Relief from assumptions of an equilibrium moisture content condition
> may be possible for thick structures. These structures would not reach
> equilibrium during the specified useful service lifetime of an
> airplane. The surface layers would approach the equilibrium in a
> reasonable time, but the full thickness of the structure would not
> attain equilibrium moisture content prior to retirement.
>
> In conditioning test specimens, elements, sub-components or components
> to evaluate the effects of moisture on material or structural
> properties, it may be desirable to increase the conditioning
> temperature to accelerate the moisture diffusion process. However,
> conditioning temperatures should not be increased to the extent that
> the material degrades or changes due to thermal exposure. This is
> critical for composite materials cured at lower temperatures, where
> high temperature conditioning can either break down the matrix or
> advance the cure.
>
> Elements such as sandwich panels and secondarily bonded structures may
> also have temperature limits for moisture conditioning. Extreme
> environmental conditioning can result in material or structural
> properties that are not characteristic of real service exposures.
>
> When performing mechanical tests with conditioned specimens or
> structures, avoid excessive loss of moisture during the test. This is
> particularly important for hot/wet tests performed with relatively
> thin specimens, where timing is critical to ensure moisture desorption
> is not dominant. In some cases, the test chamber humidity may need to
> be controlled to get the desired data. A traveler coupon may also be
> placed in the test chamber and moisture content determined after the
> test.

4.5 Analyses and Tests

> Static strength substantiation for environmental exposures and
> repeated loads may use building block tests, analyses supported by
> test evidence, relevant existing data or some combination of the
> three. As discussed previously, perform component ultimate load tests
> unless sufficient experience exists to rely on analysis supported by
> sub-component tests or component tests taken to appropriate lower load
> levels. For component test articles without prior exposure, some
> options exist when considering the effects of environment and repeated
> loads in the structural substantiation.

4.6 Full Scale Test Considerations

> For critical loads testing, use one of two main approaches to account
> for either prior repeated loading or environmental exposure, or both.
>
> In the first approach, conduct the static test on structure with prior
> repeated loading or with damage that simulates prior repeated loading.
> Furthermore, perform the static test with structure representative of
> the minimum accepted manufacturing quality and impact damage at the
> threshold of detectability. These will be discussed later in this
> document. Condition the test article to simulate the environmental
> exposure and then test in that environment.
>
> The second approach relies upon coupon, element, and sub-component
> test data to assess the possible degradation of static strength after
> application of repeated loading and environmental exposure. Account
> for degradation characterized by these tests in the full-scale, static
> test or in the analysis of results of the static test. An example of
> the former is to use a load enhancement factor in the full-scale,
> static test. An example of the latter is to analytically show a
> positive margin of safety with allowables that include the degrading
> effects of environment and repeated load. In either case, the
> component static test may be performed in an ambient atmosphere.
>
> In practice, the two approaches may be combined to get the desired
> result. For example, a large-scale static test may be performed at
> temperature with a load enhancement factor to account for moisture
> absorbed over the airplane structure's life. In developing load
> enhancement factors to account for the environment, more statistical
> confidence is gained from a difference in the means between critical
> and test environments rather than between basis values.

4.7 Other Exposures

> The ground temperature exposure of composite airplane structure is
> often higher than is possible when subjected to operating conditions.
> For example, the cooling effects of taxi and takeoff tend to reduce
> the temperature of structure. The structure is not required to meet
> critical loads at the peak temperatures possible in ground exposure.
> However, account for any time-related changes in material properties
> due to such exposure. This issue is usually dealt with by selecting
> materials with properties that are invariant to temperatures possible
> in ground exposure.
>
> Other exposures that can degrade the static strength of composite
> structure include exposure to aviation fluids. The former is typically
> addressed in material screening and qualification by evaluating
> strength after exposure to fluids in accordance with test standards
> (for example, American Society for Testing and Materials). During
> material qualification, it is often advisable to consider even those
> fluids that the airplane may not have exposure to in order to support
> future applications of the material.
>
> Ultraviolet rays from the sun can degrade the polymer matrix of many
> composites, starting with surface layers, which are directly exposed.
> Paints and other surface coatings that contain appropriate ultraviolet
> blocking compounds can eliminate this issue for composite airplane
> structure. When using this approach, conduct tests or obtain
> authenticated published data to ensure the blocking compounds are
> effective within the paint thickness specification limits. Maintenance
> practices must also be developed to ensure long-term exposure of the
> composite is not possible as the surface layers degrade over time.

**5.0 Static Strength Analysis - "A" and "B" Basis Allowables**

5.1 General

> For static strength analysis, use "A" or "B" basis allowables unless
> lower design values are required (see Section 5.3). If lower design
> values are required, conduct further engineering analysis to minimize
> the probability of structural failure. The conditions for static
> strength analysis using "A" or "B" basis allowables are as follows:
>
> \(A\) The part should be designed, analyzed, and tested using "A"
> basis allowables whenever its failure would result in the loss of the
> structural integrity of the component involved (i.e., inability of the
> airplane structure to carry limit load). Such structure is possible
> when applied loads are distributed within an assembly through a single
> load path or a single member whose failure is catastrophic.
>
> \(B\) Damage tolerant or fail-safe structures, in which the failure of
> individual elements would result in applied loads being safely
> redistributed to other load carrying members without exceeding the
> limit load capability of the airplane structure, may be designed,
> analyzed, and tested using "B" basis allowables.
>
> Material "A" and "B" basis allowable strength values and other basic
> material properties are typically determined by small-scale tests,
> such as coupon tests.
>
> Property characterization requirements of all material systems (for
> example, pre-pregs, adhesives) and constituents (for example, fibers,
> resins) should be identified, documented, and approved. Include the
> approved requirements in all appropriate procedures and specifications
> that control the raw materials and the processes used to advance them
> to the stable state characteristic of the airplane structure. As a
> result, the statistical bases for composite property variations are
> adequately linked to the documents used to ensure repeatable material
> and process quality.
>
> In developing a statistical basis in coupon tests, consider the design
> and manufacturing characteristics of the airframe structure. For
> example, some processes may produce structure with an average bondline
> thickness of 0.06 inch and a maximum permissible bond up to 0.10 inch
> thick. Coupon testing should include bonded joints with similar
> characteristics. If sufficient data exists to determine the bondline
> thickness yielding the lowest strength, most of the coupon testing
> could assume the worst case permitted.

5.2 Policy on Material Qualification and Equivalency

> Engineering protocol to generate base composite material allowables
> and the equivalency testing to share such databases has been
> documented previously (see references (1) and (2) under 10.0). The
> test methods, data reduction, and statistical procedures are based on
> those documented in MIL-HDBK-17, which provides details on commonly
> accepted engineering practices. Typically, environmental effects are
> also considered as part of the program to generate "A" or "B"
> allowables.

5.3 Detail Design and Notch Sensitivity

> Some factors that lead to design values, which are lower than base
> material properties, include the following:
>
> \- Point design considerations (stress risers, joints, and so forth);
>
> \- Stiffness requirements (flutter or vibration margins);
>
> \- Fatigue;
>
> \- Manufacturing defects and damage; and
>
> \- Other overriding issues.
>
> Often, such considerations cannot be addressed in simple coupon or
> element testing. As a result, higher levels of the building block
> tests are needed to assess the strength that can be achieved at the
> structural scale. Engineering judgment is then needed to apply a
> statistical basis generated with coupons to the smaller sample used in
> static strength tests performed with structural details characteristic
> of the design. When evaluating sub-component tests, which have no
> repetition, it is generally assumed that the results represent the
> mean of the population.

**6.0 Static Test Article Fabrication and Assembly**

6.1 General

> Fabricate and assemble the static test articles in accordance with
> production specifications and processes such that they are
> representative of the production structure. Without such practice,
> technical issues will arise before and after certification as
> production tools are developed and manufacturing processes evolve.
>
> It is important to ensure the test hardware fabricated for type
> certification does not use tools and process steps that are likely to
> change in production. Production process changes and improvements
> often require additional testing to ensure they have not led to
> differences in structural performance. To help minimize such issues,
> integrate design and manufacturing development during building block
> tests and analyses, culminating in final structural tests with
> hardware processed using production tools, fabrication, and assembly
> steps. A structural substantiation approach, which includes analysis,
> can also help minimize the numbers of tests that need to be repeated
> when the inevitable process changes are sought.

6.2 Design and Manufacturing Detail Definition

> During the course of composite structural design and manufacturing
> detail definition, create sufficient records to ensure that parts and
> assemblies remain invariant with time. Although such records are not
> unique to composites, there are issues that go beyond basic material
> type and form call-outs and part geometry definition. These include
> definition of part contour, cure process parameters, and ply lay-up
> details for laminated composites.
>
> There are two common approaches for defining the contour of a
> composite part. One uses engineering drawings or data sets that define
> loft lines for the mold. The other relies on either a master model or
> a mold to define the contour.
>
> When a master model or mold is used to define the part contour,
> include enough provisions in drawings to check for warpage and contour
> changes in the master model or mold throughout their lives. Known
> locations for fixed reference points are often used at reasonable
> spacing when combined with a contour board. The approach chosen by an
> applicant must detect geometric changes in both the parts and the
> tools used. When tools are changed or modified, possibly to
> accommodate derivative models, keep records of the changes so that it
> is possible to determine the contour of previously delivered aircraft
> and to support the production of spare parts.
>
> During type design definition, establish tolerances for the parameters
> affecting strength and durability characteristics of the structure.
> Examples of these parameters include ply orientation, stacking
> sequence, cure time, temperature, pressure, and ply drop off
> locations. There is no fixed standard for the magnitude of these
> tolerances, but they should be selected based on sound engineering
> judgment and criteria that ensures the parts manufactured within
> tolerance limits will be capable of meeting the certification
> requirements.
>
> Where tolerances are chosen in excess of those that were demonstrated
> to provide adequate control in the past, additional testing may be
> required to show that the structure can sustain the design loads and
> is safe from flutter within the tolerance envelope. For example, an
> applicant wishing to make ply orientations optional for a structural
> component should demonstrate, either by test or by analysis, or both,
> that the structure is capable of sustaining the design loads and is
> free of flutter at the most adverse orientation possible.
>
> The shape of each ply used in the airplane should be established
> through the drawings and specifications. When this is accomplished
> through the use of templates or electronic data sets, these templates
> or data sets then become part of the type design and must be
> controlled as such.

6.3 Conformity Inspections

> Due to the nature of many composite materials and processes, it is
> often necessary to conduct conformity inspections during the
> fabrication of components to a greater extent than in the case of
> metallic components where inspection can be conducted after assembly.
> For example, type certificate applicants should plan on conducting
> conformity inspections of the lay-up processes and bonded assemblies
> early in the program.
>
> Once a fabrication inspection system has been established, it is
> possible to rely on that system to verify the orientation of hidden
> plies as well as other processing steps, so that conformity
> inspections can be done on subsequent components after lay-up and cure
> is complete. It is usually in the applicant's best interest to
> establish this system and the appropriate process specifications as
> early in the type certification process as possible.
>
> During development of the drawings and process specifications, when
> major changes are made to the type design, it is often necessary to
> re-conform the test articles. The extent to which changes need to be
> re-conformed is left to the Aircraft Certification Office.

6.4 Defects and Damage

> The static test article should consider manufacturing defects, field
> damages and repairs likely to occur in production and service. From a
> safety perspective, include in critical structure areas the defects
> and field damages that will likely not be detected by the inspections
> specified in manufacturing or maintenance. From the economic
> perspective of airplane manufacturers and users, include in testing
> any defects and field damages that can be detected by inspection, but
> are allowed because the required levels of strength and durability are
> maintained. The level of allowed manufacturing defects should be
> included in either specifications or drawings, or both. Finally,
> validate a complete range of repair scenarios that are anticipated to
> fix manufacturing defects and service damage.
>
> Note: Damage repaired in the field by secondary bonding is generally
> limited in size. The damaged structure should have limit load
> capability, without the bonded repair, just as other bonded structural
> details are expected to be redundant[^2]. Although this issue is one
> that is more commonly considered under damage tolerance
> substantiation, repair capabilities are often demonstrated in static
> strength testing to ultimate loads.

**7.0 Effects of Impact Damage and Minor Discrete Source Damage**

7.1 Damage at the Threshold of Detectability and Allowed Damages

> Impact damage (or other minor discrete source damage) can
> significantly reduce the static strength of composite structures by
> causing matrix cracking, delamination, and fiber failure, which
> locally alters the load path. Depending on the impact event and
> inspection procedure, specified inspection methods may not detect the
> resulting damage. In the case of the lower levels of impact damage,
> which are not detectable, demonstrate sufficient static strength
> capability.
>
> Impact damage that can be realistically expected from manufacturing
> and service, but not more than the established threshold of
> detectability for the selected inspection procedure, should not reduce
> the structural strength below ultimate load capability. The load level
> differs for damage that is detectable (limit load) because maintenance
> inspection and repair practices can be relied on to return the
> structure to sufficient strength.
>
> As discussed previously, include in test and analysis substantiation
> any detectable damage that does not reduce the strength of a
> particular composite structural design below ultimate loads. This data
> can benefit manufacturing and maintenance activities for a
> certificated product. In the case of most past composite aircraft
> structure, detectable damage implies clearly visible damage because
> the inspections used in service rely on a mechanic finding the damage
> without sophisticated equipment.

7.2 Impact Considerations

> Show the static strength capability of structure with impact damage by
> analysis supported by test evidence, or by a combination of tests at
> the coupon, element, sub-component, and component levels. The complex
> nature of impact damage in composite structures is such that there is
> a general dependence on tests to evaluate its effect on static
> strength.
>
> Typical impact threats include runway debris, dropped tools, hail,
> ground vehicle or equipment collisions, and handling damage. Much of
> this impact damage can be considered in the scope of composite damage
> tolerance (detectable damage, specified inspection, no significant
> growth, and a limit residual strength requirement). However, impact
> damage up to the threshold of detectability that can be realistically
> expected from manufacturing and service has a static strength
> requirement for ultimate loads (§ 23.573(a)(1)).
>
> The impact-damaged composite requirement for static strength at
> ultimate loads evolved from initial applications where the specific
> laminated composite materials had brittle matrices. For such
> composites, relatively small levels of impact caused large areas of
> delamination, without visible surface indications. Local instability
> of this damage led to a drop in residual strength under compression or
> shear loads. The resulting ultimate load requirement is intended to
> ensure sufficient static strength for impact damage generated by
> relatively low-energy impacts.
>
> Industry has adopted some standards for ensuring ultimate load
> capability for the case of impact damage up to the threshold of
> detectability that can be realistically expected in service. These
> standards have helped bound the structural substantiation for this
> requirement to practical limits.
>
> Standard impactor types and geometry include weighted,
> spherical-shaped, metal impactors with diameters between 0.5 in. (12.7
> mm) and 1.0 in. (25.4 mm). Typically, such weights are dropped to
> impact the structure at a perpendicular angle. Varying the height of
> dropped weight can attain different impact energies, which yield
> different levels of visibility.
>
> Often, special towers or stands with an impacting tube are constructed
> and a support fixture is needed to tilt the structure for dropped
> weight impact. This facilitates impacting components in selected
> locations and at a perpendicular angle. Industry has sometimes used
> more sophisticated impacting devices (for example, air guns with
> controlled impact velocity) as an alternative to simple dropped weight
> impacts. These devices create normal impact to structure that is not
> oriented in a horizontal position; however, they generally result in
> higher test cost.
>
> Typically, a test survey of varying impact energies is needed to
> establish the threshold of detectability for a given structural
> location. Alternatively, clearly visible damage can be applied for the
> ultimate load requirement, providing a robust design criteria, which
> could benefit subsequent manufacturing and maintenance activities.
>
> In the case of thick composite structures, high energy levels may be
> needed to establish the threshold of detectability. When such a level
> is thought to exceed what is realistically expected in service, then a
> cut-off energy may be applied for the static strength requirement of
> ultimate loads. Consider those impacts that exceed the cut-off level
> within damage tolerance substantiation, with a residual strength
> requirement of limit load.
>
> In the past, the conservative energy cut off applied as an upper level
> limit to the ultimate load requirement for thick structures was 1,000
> inch-pounds. A value of 500 inch-pounds is a more realistic energy
> cutoff; however, lower impact energy levels have been justified using
> service data and probabilistic analyses. In most composite shell
> structures that are characteristic of small airplanes and business
> jets, the cut-off level is not applied because damage visibility is
> typically achieved at impact energies of 300 inch-pounds or less.
>
> Impact test boundary conditions can significantly effect the resultant
> impact damage to the test specimen (test coupons, elements,
> sub-components, and components). This issue usually relates to the
> energy level needed to create some level of detectability for a given
> impactor geometry. As a result, some structural substantiation of the
> residual strength with impact damage at the component level is
> generally needed to adequately demonstrate compliance with the
> requirement.

7.3 Damage Tolerance

> The purpose of § 23.573 (a)(1) is to ensure sufficient ultimate load
> capability with impact damage up to the threshold of detectability
> that can be realistically expected from manufacturing and service.
> This is not to be confused with damage tolerance requirements that
> ensure sufficient residual strength at limit loads. In addressing the
> damage tolerance issues for detectable damage, a range of impact
> variables that cause more severe damage should be considered. Some of
> the same test fixtures as those used for the ultimate load requirement
> can be used for damage tolerance substantiation, although additional
> impactor geometry should be considered to get a more complete range of
> potential threats.
>
> There is another way of demonstrating the capability of composite
> structure and repairs subjected to repeated loads. Following damage
> tolerance substantiation, repair the detectable damage and apply
> additional repeated loads before demonstrating ultimate load
> capability of a component.

**8.0 Material and Process Variability**

8.1 General

> Consider the material and process variability of the composite
> structure in the static strength substantiation. This can be achieved
> in two basic ways:
>
> \(1\) Analytical Substantiation Supported by Tests: The first is
> achieved by establishing sufficient process and quality controls to
> manufacture structure and reliably substantiate the required strength
> in tests and analyses, which support a building block approach. The
> allowables and design values, which quantify material and process
> variability, are then applied in the analysis of the results of
> structural substantiation tests. This approach, which is similar to
> that typically applied for metal structure, is based on sufficient
> validation of the analyses at appropriate structural scales.
>
> \(2\) Substantiation Primarily by Tests with Minimal Analyses: The
> second approach relies more heavily on test results. When sufficient
> process and quality controls cannot be achieved, it may be necessary
> to account for greater variability with special factors (similar to
> those discussed in § 23.619, but with values derived for a given
> composite application) applied to the design. Account for these
> factors in the component static tests or analyses.

8.2 Analytical Substantiation Supported by Tests

> A number of engineering guidelines can be used to decide whether it is
> sufficient to use allowables, design values, and analysis to account
> for material and process variability in structural substantiation.
> First, materials and fabrication processes used to make test samples,
> which establish a statistical basis for structural analysis, must be
> sufficiently controlled to ensure the allowables and design values
> that are derived represent those achieved in the structural
> components. Second, all analyses must be validated through the course
> of testing at different structural scales, including predictions of
> deflections, load paths, and overall strength.
>
> Predictions of deflections and load paths are typically achieved
> through correlation with test results from strain gauges and other
> test instrumentation. In the case where a finite element model (FEM)
> is being used for analysis, the FEM is also being validated by test to
> meet the requirement of § 23.307. Model validation is required to
> establish confidence that model and modeling techniques are
> satisfactory to allow them to be used to show compliance for static
> strength conditions that are not tested. It is also necessary for
> model revisions needed in other analyses (for example, rotor burst,
> fire, and other discrete source failure conditions).
>
> Validation of the strength prediction requires that critical locations
> are identified and the anticipated failure loads and modes are
> confirmed by tests. Such substantiation should include methods to
> account for the reduction in static strength associated with design
> details (for example, attachments, joints and cutouts), environmental
> effects, manufacturing defects and damage.
>
> Although the numbers of tests and analyses increase with this approach
> to structural substantiation, generally more efficient structure and
> lighter weights are possible. This approach also yields a test and
> analysis database that tends to prove useful for engineers involved in
> solving subsequent manufacturing and maintenance problems.

8.3 Substantiation Primarily by Tests with Minimal Analyses

> A number of engineering guidelines can be used to decide whether it is
> appropriate to use overload factors for structural substantiation in
> component tests. If the desired materials and manufacturing processes
> are not sufficiently mature to rely on accurate determination of
> structural strength margins, then it may be appropriate to overload
> test articles to account for the effects of material and process
> variability when this variability is greater than that in conventional
> metallic structure. This greater manufacturing and process variability
> is accounted for through the use of an overload factor.
>
> Component test overload factors may also be appropriate when the
> structure being certificated is known to have sufficiently higher
> strengths than needed. In such a case, an overload testing approach
> may be used to avoid the more rigorous analysis validation, which is
> needed to substantiate the use of low margins of safety. Under such
> circumstances, some savings in development costs may be possible with
> an overload approach to structural substantiation, as opposed to an
> investment in the rigorous analysis and test correlation of a building
> block approach.
>
> Applicants who use an overload approach to static strength test
> substantiation are relying on the structure to be sufficiently
> over-designed to ensure failure is not likely to occur below ultimate
> load levels due to material and process variations. Since these
> overload factors are in addition to those used to account for the
> environmental effects or repeated loads, the component hardware may be
> loaded well above ultimate loads.
>
> An overload approach must also include the effects of design details,
> manufacturing defects, and damage as part of the component test
> hardware. Unfortunately, the types, location, and extent of defects
> covered in component tests, without validated analyses, may prove
> limited as applied to subsequent manufacturing and maintenance
> problems.
>
> The conservatism applied to structure that is not strength critical is
> another concern in using overload factors. For example, structure
> failing in stability modes, which are not strongly affected by
> material and process variations in strength, would fit in this
> category. If analysis is able to predict stability failure, then an
> overload factor based on variation in material stiffness may be more
> appropriate.

8.4 Overload Factors

> Derivation of the overload factors used to account for the effects of
> material and process variability in static strength substantiation
> tests should come from a rational engineering basis, which includes a
> statistically significant number of tests. Data from coupon tests of
> base material properties for multiple material batches and processing
> cycles generally provides such a basis.
>
> Experience with composite materials and structures suggests that such
> coupon data should prove conservative in quantifying variations at
> structural scales, unless there is reason to believe processes used in
> manufacturing airplane components or structural failure modes yield
> greater variation. Conformity of the certification test hardware
> should provide the necessary data to make such engineering judgments.
> The overload factor should be sufficiently high to ensure it is
> extremely unlikely that material and process variations could yield
> structural components that fail below ultimate load.
>
> Typically, overload factors for material and process variability may
> be based on the ratio between the mean and statistical "A" or "B"
> basis values of key material properties for single load path or damage
> tolerant structures, respectively. Such values indicate minimal
> attempts to validate analyses and associated structural margins of
> safety.
>
> Lower overload factors may be appropriate in cases where some analysis
> validation is achieved in building block testing. For example, some
> analysis validation of the stiffness and internal loads could be used
> to reduce the overload factor such that the material variability
> characteristic of metal structures is subtracted from the difference
> between the mean and basis values. Overload factors discussed in this
> paragraph may not be sufficient if processes used to manufacture
> airplane structure are found to vary more than those used to
> manufacture panels for coupon tests.

8.5 Other Considerations

> Using the overload approach to account for material and process
> variability in composite structural substantiation is more difficult
> when metal structure is part of the component design. Since overload
> factors are derived from composite properties, the capability of metal
> parts may not be sufficient unless a conservative design practice is
> adopted. In these cases, additional metal reinforcement may be used
> for test purposes to allow the overall structure to reach the higher
> loads selected for composite structural substantiation.
>
> Any metal reinforcement used for test purposes must not change the
> load share between metal and composite parts. The reinforcement may
> also have to occur in steps during the sequence of testing if the
> metal part must first be included in static strength tests that
> substantiate their capability at lower load levels.

**9.0 Conclusion**

> Several means of compliance in the static strength substantiation of
> composite airplane structure have been described in this document. The
> importance of performing large-scale tests to validate sufficient
> ultimate strength in the presence of complex load paths and failure
> mechanisms has been highlighted. The use of lower-level tests and
> validated analyses to address the critical issues, which includes the
> effects of environment, variability, manufacturing defects, service
> damage, and repair has also been discussed.
>
> Building block tests and analyses at coupon, element, and
> sub-component levels also help minimize the number of large-scale
> tests. These engineering practices, which include rigorous structural
> analysis validation, more complete testing, and thorough quality
> control practices, may be used to avoid overloading large-scale
> structures to levels that force conservative design practices or cause
> failures in any integrated metal parts. On the other hand, overload
> factors may be used to successfully expedite static strength
> substantiation, provided the composite structure has sufficient
> strength when overloaded to levels accepted by the appropriate
> Aircraft Certification Office.
>
> Analysis and test technologies as well as material and manufacturing
> quality controls are likely to advance, providing more confidence in
> scaling results from lower levels of study to configured airplane
> structure. As a result, the joint efforts of regulatory agencies,
> other government groups, industry, and academia will no doubt extend
> the state-of-the-art, reducing the burden in composite static strength
> substantiation in the future. When sufficient advancement in the field
> is achieved, this policy document will be updated accordingly.

**10.0 References**

> \(1\) Memorandum entitled "Material Qualification and Equivalency for
> Polymer Matrix Composite Material Systems,\" ACE-100 Proposed Policy,
> May 2000.
>
> \(2\) \"Material Qualification and Equivalency for Polymer Matrix
> Composite Material Systems,\" FAA Technical Report - DOT/FAA/AR-00/47,
> April 2001.

If you have any questions or comments, please contact Mr. Lester Cheng,
Regulations and Policy Section either by telephone at 316-946-4111, by
fax at 316-946-4407, or by email at lester.cheng@faa.gov.

S/ Scott Sedgwick

for

Michael Gallagher

Manager, Small Airplane Directorate

Aircraft Certification Service

[^1]: ^1^ Other sources of heat and temperature limitations (for
    example, flying temperature limits for avionics equipment or other
    limitations specified in the flight manual) may lead to higher or
    lower structural temperatures, respectively.

[^2]:
    > ^2^ Per § 23.573, there are other means of compliance for bonded
    > structural details (for example, NDE and proof testing); however,
    > neither are believed to be appropriate for field repairs.