A Composite Material Qualification Method That Results in Cost, Time ...

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mission is to develop technology that will stimulate the U.S. general aviation industry. Initiated by the ... Figure (2) . AGATE material qualification cost and time comparison. ..... In the case of a B-basis calculation, zB is taken as 1.28155.

A Composite Material Qualification Method That Results in Cost, Time and Risk Reduction John S. Tomblin, Wichita State University/National Institute for Aviation Research, Wichita, KS 67260 John D. Tauriello and Sean P. Doyle, FiberCote Industries, Inc., Waterbury, CT 06708

ABSTRACT One of the largest single regulatory hurdles for an airframe manufacturer, i.e., user, of polymer based advanced composite materials in certified aircraft applications, is to generate design allowables that will satisfy Federal Aviation Regulations (FARs). Due to the lack of a regulatory mechanism that encourages materials users to share data, historically each user has independently executed coupon level test plans and design allowable programs for specific materials - a costly and time consuming process. Design allowables for similar or identical materials have often been generated consecutively by several users as a routine part of their certification efforts, which has resulted in redundant costs to users, materials manufacturers and regulators. A new composite materials qualification methodology has been developed by members the Advanced General Aviation Technology Experiments (AGATE) consortium. Based on Military Handbook 17 (MIL-HBK-17)2 guidelines, the "AGATE Method" describes a "standardized" coupon level material qualification test plan and statistical technique that yields lamina design allowables for a specific material system, such that allowables can be shared among multiple users without each user having to repeat the full qualification procedure. Once the original qualification database is completed and its resultant design allowables are approved for use by the FAA, each user needs only to perform a limited "equivalency" test plan to verify that their process yields properties that are equivalent to the original database. 1. Background and Recent Developments 1.1 Background Properties of traditional "metallic" airframe materials, i.e., aluminum, steel, etc., are well documented in MIL-HDBK-5 and have been long accepted by the FAA for use in airframe design. These allowables enable the airframe manufacturer to substantiate a design using various forms of analysis, such that not every part has to be tested to destruction. Historically, composite lamina and laminate design allowables for FAA certified aircraft structures have been generated independently by their users, i.e., airframe manufacturers. Due to the lack of a regulatory mechanism and company proprietary information restrictions, these "allowables" have generally not been shared among airframe manufacturers. As previously discussed by Hart-Smith3 , this practice has often resulted in redundant sets of material properties and design allowables for similar and identical materials. 1.2 The Advanced General Aviation Technology Experiments (AGATE) Consortium was founded in 1994 and is a cost-sharing industry-university-government partnership whose mission is to develop technology that will stimulate the U.S. general aviation industry. Initiated by the National Aeronautical and Space Administration (NASA), the AGATE consortium has more than 70 members from industry, universities, the FAA, and other

government agencies. 1.3 The AGATE Advanced Materials Program has directed the creation of composite material allowables that have been approved and witnessed by the FAA for use in next generation single pilot, four place, near all-weather light airplanes - the first two being Cirrus' SR20 and Lancair's Columbia 300 [see photograph]. AGATE me mbers - A&P Technology, Cessna Aircraft, Cirrus Design, FiberCote Industries, Global Aircraft, The Lancair Company, Raytheon Aircraft, Stoddard-Hamilton, Simula Technologies and Toray Composites America - have been contributing industrial members to the Advanced Materials Program. A major goal of the AGATE Advanced Materials Program has been to produce a "standard" FAA approved composite material qualification methodology for use within the general aviation community. In addition, a recently published AGATE document entitled "Material Qualification and Equivalency for Polymer Matrix Composite Material Systems"1 has been approved by the FAA as a reference for Part 23 certifications. The AGATE document describes a program to substantiate that the materials and processes employed will meet FAA requirements for a selected material system. This is the first FAA public document that "standardizes" the procedure to qualify a composite material system that follows guidelines set forth by the MIL-HDBK-17 committee. It should also be noted that subject to FAA approval and oversight, use of the AGATE Method has also been extended to certain Part 25 certified structures. 1.3.1 The AGATE Method gives specific information about the qualification procedure for epoxy-based pre-impregnated carbon and fiberglass unidirectional tapes and woven fabrics (prepregs) that are cured and processed at or above 240°F. Currently, the qualification procedure applies to the original material qualification only. Once certified, changes to the material, process, tooling and/or facility require a review, and repetition of some (or all) of the qualification tests may be required. The AGATE Method describes a "standardized" coupon level qualification test plan and statistical techniques that that yield lamina design allowables for a specific prepreg material system, such that the allowables can be shared among multiple users without each user having to repeat the full qualification procedure [see Figure (1)]. Once the original qualification database is completed and the FAA approves its resultant design allowables for use, each user needs only to perform a limited "equivalency" test plan to verify that their process yields properties that are equivalent to the original database. Using the AGATE Method, the overall savings to an individual user can result in an "order of magnitude" savings in cost and over a factor of four savings in qualification time [see Figure (2)]. In addition, due to time constraints, material allowables are often used to verify design assumptions after the fact rather than optimize design decisions up-front. Using the AGATE Method, optimally the designer can evaluate materials with existing design allowables prior to the design phase. Finally, time and cost savings may also be realized by regulators due the fact that the AGATE Method promotes the use of common qualification databases and data sharing among manufacturers.

Lancair Columbia 300 all composite aircraft

Figure (1). AGATE composite material certificat ion impact.

Figure (2) . AGATE material qualification cost and time comparison.

2. AGATE QUALIFICATION METHODOLOGY 2.1 The General Methodology is presented in the AGATE Document1 and describes the tests required to substantiate a statistically based design allowable for each material property. The AGATE Method and its specific test procedures borrow greatly from MIL-HDBK-17, and were approved in advance by the FAA. Several separate and distinct batches of material must be characterized to establish a statistically based material design allowable for a given material. The definition of a batch of prepreg material in this plan is defined as a quantity of a single homogenous resin formula (i.e., resin, catalyst, etc.) that is prepared in one operation and is traceable to unique and single individual component batches of resin and fiber raw materials. In order to account for process variables, each batch of prepreg material is processed into panels using a minimum of two independent process/cure cycles [see Figure (3)].

Figure (3). Example of specimen selection and batch/process traceability.

In order to characterize environmental effects on a specific material, several extreme conditions are defined and tested. The test conditions defined in this qualification plan are as follows: Cold Temperature Dry (CTD)

-65°F; "as fabricated" moisture content

Room Temperature Dry

Ambient lab conditions; "as fabricated"

(RTD)

moisture content

Elevated Temperature Dry (ETD)

180°F; "as fabricated" moisture content

Elevated Temperature Wet (ETW)

180°F; equilibrium moisture gain; 85% RH environment

2.2 Uncured Material Testing Table 1 describes the physical tests that are required for, and must be traceable to each batch of material. These test methods assure control of the prepregging process and are used in addition to specific values used to normalize material data (described in Section 3). Some physical tests should be repeated during incoming receiving inspection, the purpose of which is to verify that the material did not change between the time of manufacture and the time of receipt by the user. Note that physical properties generally influence the producibility of a material, but do not commonly influence the resultant mechanical properties. Test methods listed in Table 1 were taken from suggestions in MIL-HDBK-17-1E and include American Society of Tests Methods (ASTM) and Suppliers of Advanced Composite Materials Association (SACMA) test methods. These chemical and physical tests represent prepreg, i.e. combined resin and fiber, properties. In addition, the prepreg manufacturer's quality control procedures should be reviewed to ensure that quality control procedures are in place to address incoming fiber, woven fabric and resin raw materials, and the resin mix procedure. The prepreg manufacturer's quality procedures should be kept on file as part of the original qualification and as part of the user's quality assurance documentation. Table 1. Physical and chemical property tests performed by prepreg manufacturer

No.

Test Property

Test Methods ASTM

SACMA

No. of Replicates per Batch

1

Resin Content

D3529, C613, D 5300

RM23, RM24

3

2

Volatile Content

D 3530

-

3

3

Gel Time

D 3532

RM19

3

4

Resin Flow

D 3531

RM22

3

5

Fiber Areal Weight

D 3776

RM23, RM24

3

6

IR (Infrared Spectroscopy)

E 1252, E 168

-

3

7

HPLC (High Performance Liquid Chromatography)a

-

RM20

3

8

DSC (Differential Scanning Calorimetry)

E 1356

RM25

3

aSection 5.5.1 & 5.5.2 of MIL-HDBK-17-1E describes procedures to extract resin from prepreg and perform HPLC tests. 2.3 Material Qualification Program for Cured Lamina Properties The required number of batches and replicates per batch are presented in Table 3. The "# x #" format in Table 3 represents the number of batches and replicates per batch, where the first # represents the number of batches and the second # represents the number of replicates per batch. For example, "3 x 6" refers to three batches and six specimens per batch - for a total of 18 test specimens. Table 2 shows the cured lamina physical properties required to support the maximum operational temperature limit of the material system and specific data to be used in the statistical design allowable generation. Typically, the maximum operating temperature for a material should be at least 50°F below the wet glass transition temperature (Tg). The fiber, resin and void fraction of each panel are measured to verify laminate consistency, to establish acceptable production ranges, to develop prepreg procurement specifications and to develop acceptable limits for incoming receiving inspection. Table 2. Cured lamina physical property tests Physical Property

Test Procedure

No. of Replicates per Batch

Fiber Volume

ASTM D3171-90a or D2584-94b

c

Resin Volume

ASTM D3171-90a or D2584-94b

c

Void Content

ASTM D2734-94d

c

Cured Neat Resin Density

ASTM D792-91

e

Glass Transition Temperature (dry f)

SACMA RM 18-94

c

Glass Transition Temperature (wet g )

SACMA RM 18-94

c

2.4 Reduced Sampling Requirements for A-basis and B-basis Allowables Table 3 describes the number of tests required for each environmental condition and the relevant test method. Table 3. Reduced sampling requirements for cured lamina properties TEST

METHOD (REF.)

No. OF SPECIMENS PER TEST CONDITION

CTDh

RTDi

ETWj

ETDk

0° (warp) Tensile Strenght

ASTM D3039-95

1X4

3X4

3X4

1X4

0° (warp) Tensile Modulus, Strenght and Poisson's Ratio*

ASTM D3039-95

1X2

3X2

3X2

1X2

90° (fill) Tensile Strenght

ASTM D3039-95

1X4

3X4

3X4

1X4

90° (fill) Tensile Mod. and Strenght*

ASTM D3039-95

1X2

3X2

3X2

1X2

0° (warp) Compressive Strenght

SACMA SRM 1-94

1X6

3X6

3X6

1X6

0° (warp) Compressive Modulus*

SACMA SRM 1-94

1X2

3X2

3X2

1X2

90° (fill) Compressive Strenght

SACMA SRM 1-94

1X6

3X6

3X6

1X6

90° (fill) Compressive Modulus*

SACMA SRM 1-94

1X2

3X2

3X2

1X2

In-Plane Shear Strenght

ASTM D5379-93

1X4

3X4

3X4

1X4

In-Plane Shear Mod. and Strenght*

ASTM D5379-93

1X2

3X2

3X2

1X2

Short Beam Shear

ASTM D2344-89

-

3X6

-

-

*strain gages used during testing a Test method used for carbon or graphite materials. b Test method used for fiberglass materials. c At least one test shall be performed on each panel manufactured for qualification. d Test method may also be applied to carbon or graphite materials. e Data should be provided by material supplier for each batch of material. f Dry specimens - "as fabricated" and maintained at ambient conditions in an environmentally controlled laboratory. g Wet specimens are humidity aged until an equilibrium moisture weight gain is achieved. h Only one batch of material is required (test temperature = -65 ( 5° F, moisture content = as fabricatede) i Three batches of material are required (test temperature = 70 ( 10° F, moisture content = as fabricatede) j Three batches of material are required (test temperature = 180 ( 5° F, moisture content = equilibrium) k Three batches of material are required (test temperature = 180 ( 5° F, moisture content = as fabricatede) 3. DESIGN ALLOWABLE GENERATION

3.1 Introduction to Design Allowables Upon completion of the mechanical test program and associated data reduction (MIL-HDBK-17, Vol. 2 provides data reduction guidelines), the next step is to calculate design allowables for each property. Due to the inherent mechanical property variability in composite materials, this variability should be acknowledged when design values are assigned. Although the statistical procedures presented account for most common types of variability, note that these procedures might not account for all sources of variability. A-basis and B-basis allowables are determined for each strength property using the statistical procedures outlined in the following sections. For modulus and Poisson's ratio design values, the average value of all corresponding tests for each environmental condition should be utilized. If strain design allowables are required, simple one-dimensional linear stress-strain relationships may be used to obtain corresponding strain design values. However, note that this process should approximate tensile and compressive strain behavior relatively well but may produce conservative shear strain values due to nonlinear behavior. A more viable approach for shear strain allowables is to use a maximum strain value of 5% (reference MIL-HDBK- 17-1E, Section 5.7.6). 3.2 Statistical Analysis Compared to metallic materials, fiber reinforced composite materials exhibit a high degree of material property variability. This variability is due to many factors, including raw material and prepreg manufacture, material handling, part fabrication techniques, ply stacking sequence, environmental conditions, and test tec hniques, and thus increases testing costs and tends to render smaller data sets than are used for metallic materials. The use of statistical techniques to determine reasonable composite material allowables becomes necessary. 3.2.1 Methodology The statistical analyses and A & B-basis allowable generation is performed using the methodology presented by Shyprykevich4. In this data reduction method, the data from all environments, batches and panels are utilized together to generate statistical information about the corresponding test. This approach utilizes essentially small data sets to generate test condition statistics such as population variability and corresponding basis values to pool results for a specific failure mode across all environments. This section describes the methodology applied to a design allowable generated using the test procedures presented in the qualification plan. For additional information the reader is referred to Shyprykevich4 or MIL-HDBK- 17-1E, Chapter 82. The data reduction methodology presented in this section requires several underlying assumptions in order to generate a valid design allowable. By pooling data sets, the variability across environments should be comparable and the failure modes for each environment should not significantly change. The methodology also uses a normal distribution to analyze the data. If the assumption of normality is not acceptable, the Weibull distribution generally produces the most conservative basis values. If the variability or failure modes significantly change or if the assumption of normality is violated, traditional MIL-HDBK- 17 methods should be utilized. The methodology assumes that all test data for each condition and testing environment has been reduced and is in terms of failure stress. Normality is assumed and used to model the behavior. The step-wise process determines a basis design allowable value (A or B) as follows: 1. Normalize all relevant fiber dominated data via the procedures presented in MIL- HDBK17-1E, Section 2.4.3. This normalization procedure will account for variations in the fiber volume fraction between individual specimens, panels and/or batches of material.

2. For a single test condition (such as 0° compression strength), collect the data for each environment being tested. The number of observations in each environmental condition is njwhere the subscript j represents the total number of environments beings pooled. Calculate the sample mean and sample standard deviation s for each environment via

For each environment, the environmental groupings must be checked for any outliers as well as for the assumption of normality. In addition, the variances of each environmental grouping should be checked for equality. If any outliers exist within each environmental grouping, the disposition of each outlier should be investigated [see Ref (1) for further details]. For the check of population normality, "engineering judgment" should be applied to verify that the assumption of normality is not significantly violated. If the assumption of normality is significantly violated, other statistical models should be investigated to fit the data. As stated above, the Weibull distribution provides the most conservative basis values. If the variance of each environmental grouping is significantly different, traditional statistical methods of MIL- HDBK-172 should be utilized. 3. Normalize the data in each environment by dividing the individual strength by the mean strength for the corresponding environment. Normalizing will result in all data being close to a magnitude of 1.0. Pool all the normalized data together from each environment into one data set. 4. For the pooled, normalized data set, calculate the number of samples N, the sample mean X and sample standard deviation s via eqn. (1) and (2). For the pooled data set, check for any outliers as well as a visual comparison of the best normal fit and actual data. If any outliers exist within the pooled data set, the disposition of each outlier should be investigated. For the distributional check of normality, "engineering judgment" should be applied to verify that the assumption of normality is not significantly violated. If the assumption of normality is significantly violated, the other statistical models should be investigated to fit the data. In general, the Weibull distribution provides the most conservative basis value. 5. Calculate the one-sided B-basis and A-basis tolerance factors for the normal distribution for each environment j that is based upon the number of samples in the pooled data set N and the number of samples in each environment nj. The B-basis tolerance factor (number of standard deviations), (kB)j may be approximated by

where nj is the number of observations of the selected environment (a subset of N, the number of total pooled observations) and zB is the 0.1 quantile of the standard normal

distribution. In the case of a B-basis calculation, zB is taken as 1.28155. The coefficients bB and cB are given by the following relationships

where ƒ = N-2 is the degrees of freedom for the variance. In the case f ) 3, Q may be approximated by

For ƒ = 2, the exact value of Q may be used as Q = 0.05129. The above approximations are accurate within 1.2% of the tabulated values. The A-basis tolerance factor, kA may be approximated by

where nj is the number of observations of the selected environment (a subset of N, the number of total pooled observations) and zA is the 0.01 quantile of the standard normal distribution. In the case of an A-basis calculation, zA is taken as 2.32634. The coefficients bA and cA are given by the following relationships

where ƒ = N-2 is the degrees of freedom for the variance. In the case f ( 3, Q may be approximated by

For ƒ = 2, the exact value of Q may be used as Q = 0.05129. The above approximations are accurate within 0.9% of the tabulated values. 6. Calculate the normal distribution B-basis and A-basis allowable using the pooled mean, standard deviation and tolerance factors for each environment j via the equation

This number should essentially be a "knockdown" factor less than 1. The A-basis value for each environment may be obtained similarly by

7. Multiply the pooled basis values obtained in step 6 by the mean strength calculated for each environment obtained in step 2. These values then become the basis values (A and B) for each individual environmental condition. A flow chart depicting this step-wise procedure is shown in Figure (4).

Figure (4). Step-wise data reduction procedure for design allowable generation 4. IMPLEMENTATION AND CASE STUDY 4.1 Background Since late 1996, FiberCote Industries has been a supporting member of AGATE and has participated in a comprehensive program to validate the AGATE Method. As a result, FiberCote has developed the first family of FAA approved, publicly available lamina design allowables databases generated by the AGATE Method. The databases are based on

a robust 250°F cure/180°F wet service system. Several E-765 applications that use FiberCote's FAA approved databases have entered production, or are in the process of being approved by the FAA. This section will review FiberCote's E-765 materials and database development program, and an E-765 materials and database application case study for an FAR Part 25 Supplemental Type Certified (STC'd) Boeing DC9/Airbus A320 Radome Skirt Fairing. 4.2 FiberCote's E-765 Materials Development Program After a careful study of potential aircraft applications and FAA regulatory requirements, FiberCote undertook a resin development program, the objective of which was to create a tough, controlled flow, 250°F cure resin that would provide good translation of fiber properties from -65°F dry to 180°F wet and be competitive with the properties and costs of first generation 350°F cure epoxy systems. Since a major investment would be required to generate several design allowables databases, the system had to meet several process requirements such that it would fit a diverse array of applications. 4.2.1 The E-765 Resin System E-765 chemistry is based on a semi-interpenetrating network that includes a multifunctional epoxy backbone, reactive thermoplastic modifiers for toughness and flow control, and a duplex curative system to allow generation of maximum glass transition temperatures and mechanical strengths at a 250°F cure temperature. It is important to note that all of the raw materials used in E-765 are standard "commercial" grades that are readily available. In addition, E-765 can be prepregged by solution or hotmelt methods, can be cured in low pressure, i.e., vacuum, or high pressure, i.e., autoclave or press, environments, and exhibits good shop handling characteristics that can be tailored to meet customers' process requirements. 4.3 FiberCote's Design Allowable Database Program Based on cost and risk issues, it was decided that the initial qualification effort would yield lamina material allowables, as this is the first level of the MIL-HDBK-17 "building block" qualification approach. FiberCote has since initiated a laminate allowables program, i.e., the second "building block" [see 4.3.1]. A vacuum process specification was selected based on the needs of several general aviation manufacturers, and because this would yield a conservative database that could also likely be conformed for use in certain autoclave applications. After an extensive process development and scale-up effort, three batches of each of the qualification materials were produced to FiberCote material specifications and corresponding test panels were fabricated by FiberCote to FiberCote process specifications, all in accordance with the AGATE Method. Critical elements of the FiberCote's material specifications include physical and chemical property test c riteria as detailed in Table 2, and critical mechanical test criteria as detailed in Table 3. Critical elements of the FiberCote's process specifications include material outlife, working environment, cutting and lay-up methods, bagging, and cure cycles details. Test panel fabrication was witnessed and conformed by an FAA Designated Airworthiness Representative (DAR). The test results, including raw data and reduced data, were submitted to, and reviewed and approved by the FAA. 4.3.1 Additional Material and Database Developments In addition to the first three material forms, and based on the success of the program to date, FiberCote has several material and database developments in progress, including a lamina allowables program for T300 6K 5 Harness Satin 370gsm woven carbon processed via autoclave (expected completion dateDecember 2000), T700 12K Plain Weave 193 and 370gsm woven carbon, a surface film, lightning strike materials and unidirectional boron materials. FiberCote is also developing an extensive set of laminate allowables for T300 3KPW/E-765 and T700 24K Unidirectional/E-

765 and T300 6K5HS/E-765 in cooperation with AGATE, Rotorcraft Industry Technology Association (RITA) and Bell Helicopter Textron. (expected completion date-January 2001). 4.4 Demonstration of Equivalence to the Original Database Given user access, ready availability of FAA approved design allowables generated per the AGATE Method can reduce or eliminate two significant hurdles associated with a material qualification, (1) generation and execution of a costly and time consuming coupon test plan and, (2) data reduction, design allowable generation and regulatory approval of design allowables. Alternately, it should not be assumed that allowables generated by the AGATE Method are necessarily suitable for use in all certified aircraft applications. Suitability of the material and process method must be reviewed by the manufacturer and be reviewed and approved by the FAA for use in a specific application. If a material and its FAA approved database are considered to be suitable for use in an application, the next step is for the user to generate and execute an FAA approved "site" equivalence test plan that proves conformance to the material supplier's original database. As part of the test plan, the user is required to submit material and process specifications, but these specifications must in fact be identical in all critical aspects to the material supplier's material specifications and similar in all critical aspects to material supplier's process specifications. FiberCote has a detailed site equivalence test plan template and detailed material and process specifications available to assist users to develop a site equivalence test plan that will comply with FAA requirements. The flow chart in Figure (5) describes the steps to determine material and process suitability and to obtain site equivalence and conformance to the original database.

Figure (5). Steps to obtain site equivalence and conformance to the original database An FAA Aircraft Certification Office (ACO) must review and approve the airframe manufacturer's site equivalence test plan prior to execution. Although the FAA has the final authority to set the requirements for the site equivalence test plan, Table (4) shows a typical set of test requirements to demonstrate site equivalence. Note that the test methods and laminate schedules in the site equivalence test plan and the original database test plan are identical. The critical changes to the user's site equivalence test plan versus the original database are: (1) the "site" where the test panels are fabricated, i.e. personnel, location, equipment and any changes to the process specification and, (2) the limited test matrix, i.e., one batch of material, two cure cycles, and two environmental conditions for the site equivalence plan versus three batches, six cure cycles and four environmental conditions for the original database test plan. Table (4). Typical set of test requirements to demonstrate site equivalence.

No

Test

Method

Color Color

Color

Color Color

Color

Color RTD1

ETW2

Panel 1

Panel 2

Panel 1

Panel 2

1

0° (warp) Tens. Str.

ASTM D3039-95

1x3

1x3

1x3

1x3

2

0° (warp) Tens. Mod.

ASTM D3039-95

3

3

3

3

3

0° (warp) Poisson's Ratio

ASTM D3039-95

3

3

3

3

4

90° (fill) Tens. Str.

ASTM D3039-95

1x3

1x3

1x3

1x3

5

90° (fill) Tens. Mod.

ASTM D3039-95

4

4

4

4

6

0° (warp) Comp. Str.

SACMA SRM 1-94

1x3

1x3

1x3

1x3

7

0° (warp) Comp. Mod.

SACMA SRM 1-94

1x3

1x3

1x3

1x3

8

90° (fill) Comp. Str.

SACMA SRM 1-94

1x3

1x3

1x3

1x3

9

90° (fill) Comp. Mod.

SACMA SRM 1-94

1x3

1x3

1x3

1x3

10

In Plane Shear Str.

ASTM D5379-93

1x3

1x3

1x3

1x3

11

In Plane Shear Mod.

ASTM D5379-93

5

5

5

5

12

Short Beam Shear

ASTM D2344-89

1x3

1x3

1x3

1x3

13

Fiber Volume

ASTM D3171-90 or D2584-94

Two samples per panel

14

Resin Volume

ASTM D3171-90 or D2584-94

Two samples per panel

15

Void Content

ASTM D2734-94

Two samples per panel

16

Glass Transition Temp.

SACMA RM18-94

Two samples per panel

17

Cured Ply Thickness

SACMA RM180-94

Two samples per panel

1

RTD as defined in FiberCote Industries Document Number E765QP1000, Section 3.2 2 ETW as defined in FiberCote Industries Document Number E765QP1000, Section 3.2 3 Data obtained from same test specimens as used for No. 1 4 Data obtained from same test specimens as used for No. 4 5 Data obtained from same test specimens as used for No. 10

4.5 Case Study - DC-9/A-320 Radome Skirt Fairing In Fall 1999, Flight Structures, Inc. div. of BE Aerospace approached FiberCote regarding the potential use of FiberCote E-765 products and FAA approved databases to build a Radome Skirt Fairing that is used to mount a radome that receives live television signals via satellite, and that fits on Boeing DC-9 and Airbus A-320 transport aircrafts [see photograph]. Due to intense competition among satellite TV signal providers, the progra m was driven primarily by "speed to market" and originally, less an a ninety-day window existed from material selection to installation of a prototype fairing on a customer aircraft. Flight Structures did not have access to any existing FAA approved design allowable databases for a material that would be suitable for this application. In addition, it was clearly not possible to complete a full material qualification in the time constraints that were placed on Flight Structures by their customer. Finally, this program provided an additional challenge because the approval was to be accomplished under FAR Part 25 for Transport Category Airplanes while the shared database concept had been developed under FAR Part 23 for Normal, Utility, Acrobatic, and Commuter Category Airplanes.

Boeing DC9/Airbus A320 Radome Skirt Fairing Manufactured by Flight Structures Initially, Flight Structures reviewed data for FiberCote's E-765/7781 system, however a change was made to T300 3KPW/E765 due to critical design considerations. FiberCote's lamina allowables were used, however laminate mechanical properties, particularly open hole compression, filled hole tension, bolt bearing, and fastener pull through were also required. The FAA required allowables for the aforementioned laminate mechanical properties in addition to the demonstration of site equivalence. In addition, material and process specifications developed by FiberCote for the original material qualification were deemed sufficient and were incorporated into Flight Structure's existing quality procedures without change. The material equivalence test plan was similar to the plan outlined in Table 4, but included the addition of the laminate allowables testing mentioned above. Test panels were fabricated in December 1999. While only one batch of material was required to demonstrate of site equivalence, three batches were required for the creation of the new laminate property allowables. Unlike the original qualification, which required four environmental conditions, the laminate allowables required only two environmental conditions, i.e., RTD and ETW. First article parts were fabricated concurrently with the test panels. Two changes were made to the original process specification during fabrication of the first article parts to accommodate the thick cross section areas of the design: (1) To reduce cured laminate void content, warm debulk cycles were substituted for the RT debulks in the original process specification and, (2) an extended cure cycle was developed to eliminate potential exotherms. Ideally, these process changes should have been incorporated into the site equivalence test plan, however the need for these changes was not realized until after test panels had been fabricated, therefore, follow-on testing substantiated the process changes. 5. CONCLUSIONS Based on industry accepted MIL-HDBK-17 guidelines, the AGATE Method is an effective program to substantiate specific materials and processes that will meet FAR Part 23 and certain Part 25 application requirements. The AGATE Method describes and encourages the

use of a "standardized" coupon level material qualification test plan and statistical techniques to derive design allowables for specific materials and processes, that can be shared among multiple users such that each user does not have to repeat the full qualification plan. Rather, each user performs a limited site equivalence test plan to verify that their process will yield properties that conform to the original material database properties. Us ing the AGATE Method, substantial cost and time savings can be realized by airframe manufacturers, as well as regulators and material suppliers. In addition, airframe manufacturers can reduce their design risks due to the fact that design allowables are known up front. Design allowables generated by the AGATE Method may also be suitable for certain applications on defense aircraft and non-aerospace applications. 6. REFERENCES 1. J. S. Tomblin, C.Y. Ng and K.S. Raju, "Material Qualification and Equivalency for Polymer Matrix Material Systems," FAA Technical Report, DOT/FAA/AR-xx/xx, NTIS, Springfield, VA. In Press. 2. L.J. Hart-Smith, "Division of Responsibilities Between Prepreggers and Their Customers for Composite Material Characterization," 44th International SAMPE Symposium and Exhibition, Anaheim, CA (1999). 3. Military Handbook 17 Volumes 1,2 & 3. Technomic Publishing Company, Lancaster, PA. (1999). 4. P. Shyprykevich, "The Role of Statistical Data Reduction in the Development of Design Allowables for Composites, Test Methods for Design Allowables for Fibrous Composites," 2nd Volume, ASTM STP 1003, C.C. Chamis, Ed., American Society for Testing and Materials, Philadelphia, PA, 1989, pp. 111-135. 5. F. Scholz, "Approximation for Allowable k-Factors," Boeing Computer Services, The Boeing Company, Seattle, WA (internal paper).

Biographies Dr. John S. Tomblin, Associate Professor, Wichita State University Dr. Tomblin has worked on material qualification issues for the past 5 years primarily as part of the AGATE program. He is chairman of the advanced materials working group for AGATE and the FAA Airworthiness Assurance Center of Excellence (AACE). He also serves as co-chairman for the Military Handbook 17 Data Review work group. Dr. Tomblin has a Ph.D. in Mechanical Engineering from West Virginia University. Mr. John D. Tauriello, Director of Marketing and Sales, FiberCote Industries, Inc. Mr. Tauriello has worked in technical marketing and sales, business development and management capacities in the advanced composites industry for the past 17 years. Mr. Tauriello has a B.A., cum Laude, in Psychology and Chemistry from Saint John Fisher College. Mr. Sean Doyle, Technical Sales Manager-Western Region, FiberCote Industries, Inc.

Mr. Doyle has been actively involved in the design, fabrication, and certification of composite structures for general aviation aircraft for the past seven years in a variety of academic and professional functions. He is an FAA certified single and multi-engine airline transport pilot and flight instructor. Mr. Doyle has a B.S., cum Laude, in Mechanical Engineering from Rensselaer Polytechnic Institute.

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