MIL-HDBK-17-1F Volume 1,Chapter 2 Guidelines for Property Testing of Composites CHAPTER 2 GUIDELINES FOR PROPERTY TESTING OF COMPOSITES 2.1 INTRODUCTION This chapter provides guidelines for the experimental characterization of polymer matrix composites and documents the requirements for publishing material property data in MIL-HDBK-17.Recommended test matrices for a number of uses are presented and discussed.Potential problem areas in testing and test matrix planning are highlighted and helpful options are provided.The chapter sections cover the fol- lowing: Section 2.1 introduces the chapter and presents an approach to categorizing testing needs Section 2.2 discusses a wide variety of factors that affect test results and basis values,focusing on issues of particular importance during test planning,whether for a single test or for a large testing program requiring the evaluation of hundreds or thousands of test specimens. Section 2.3 presents a number of preplanned test matrices organized by the key categories intro- duced in Section 2.1,covering the characterization of specific sets of properties at recommended test environments,and including requirements for batch and specimen quantities. Section 2.4 describes procedures for normalizing,reducing,and reporting test data Section 2.5 describes detailed test population sampling requirements,and specific test data nor- malization and documentation requirements for inclusion of data into MIL-HDBK-17 Volume 2. 2.1.1 Building-block approach to substantiation of composite structures Analysis alone is generally not considered adequate for substantiation of composite structural de- signs.Instead,the "building-block approach"to design development testing is used in concert with analy- sis.This approach is often considered essential to the qualification/certification'of composite structures due to the sensitivity of composites to out-of-plane loads,the multiplicity of composite failure modes and the lack of standard analytical methods. The building-block approach is also used to establish environmental compensation values applied to full-scale tests at room-temperature ambient environment,as it is often impractical to conduct these tests under the actual moisture and temperature environment.Lower-level tests justify these environmental compensation factors.Similarly,other building-block tests determine truncation approaches for fatigue spectra and compensation for fatigue scatter at the full-scale level. The building-block approach is shown schematically in Figure 2.1.1 and discussed in detail in Refer- ences 2.1.1(b)and(c).The approach can be summarized in the following steps: 1.Generate material basis values and preliminary design allowables. 2.Based on the design/analysis of the structure,select critical areas for subsequent test verification. 3.Determine the most strength-critical failure mode for each design feature. 4. Select the test environment that will produce the strength-critical failure mode.Special attention should be given to matrix-sensitive failure modes (such as compression,out-of-plane shear,and bondlines)and potential "hot-spots"caused by out-of-plane loads or stiffness tailored designs. 5. Design and test a series of test specimens,each one of which simulates a single selected failure mode and loading condition,compare to analytical predictions,and adjust analysis models or de- sign allowables as necessary. 6. Design and conduct increasingly more complicated tests that evaluate more complicated loading situations with the possibility of failure from several potential failure modes.Compare to analyti- cal predictions and adjust analysis models as necessary. Design substantiation is often called"qualification"in U.S.DOD applications and "certification"in civilian applications involving the U.S.FAA.All three terms describe a similar process,but "substantiation"can be considered the more generic temm,with "qualifica- tion"and "certification"often limited to the foregoing more restricted senses. 2-1
MIL-HDBK-17-1F Volume 1, Chapter 2 Guidelines for Property Testing of Composites 2-1 CHAPTER 2 GUIDELINES FOR PROPERTY TESTING OF COMPOSITES 2.1 INTRODUCTION This chapter provides guidelines for the experimental characterization of polymer matrix composites and documents the requirements for publishing material property data in MIL-HDBK-17. Recommended test matrices for a number of uses are presented and discussed. Potential problem areas in testing and test matrix planning are highlighted and helpful options are provided. The chapter sections cover the following: • Section 2.1 introduces the chapter and presents an approach to categorizing testing needs. • Section 2.2 discusses a wide variety of factors that affect test results and basis values, focusing on issues of particular importance during test planning, whether for a single test or for a large testing program requiring the evaluation of hundreds or thousands of test specimens. • Section 2.3 presents a number of preplanned test matrices organized by the key categories introduced in Section 2.1, covering the characterization of specific sets of properties at recommended test environments, and including requirements for batch and specimen quantities. • Section 2.4 describes procedures for normalizing, reducing, and reporting test data. • Section 2.5 describes detailed test population sampling requirements, and specific test data normalization and documentation requirements for inclusion of data into MIL-HDBK-17 Volume 2. 2.1.1 Building-block approach to substantiation of composite structures Analysis alone is generally not considered adequate for substantiation of composite structural designs. Instead, the "building-block approach" to design development testing is used in concert with analysis. This approach is often considered essential to the qualification/certification1 of composite structures due to the sensitivity of composites to out-of-plane loads, the multiplicity of composite failure modes and the lack of standard analytical methods. The building-block approach is also used to establish environmental compensation values applied to full-scale tests at room-temperature ambient environment, as it is often impractical to conduct these tests under the actual moisture and temperature environment. Lower-level tests justify these environmental compensation factors. Similarly, other building-block tests determine truncation approaches for fatigue spectra and compensation for fatigue scatter at the full-scale level. The building-block approach is shown schematically in Figure 2.1.1 and discussed in detail in References 2.1.1(b) and (c). The approach can be summarized in the following steps: 1. Generate material basis values and preliminary design allowables. 2. Based on the design/analysis of the structure, select critical areas for subsequent test verification. 3. Determine the most strength-critical failure mode for each design feature. 4. Select the test environment that will produce the strength-critical failure mode. Special attention should be given to matrix-sensitive failure modes (such as compression, out-of-plane shear, and bondlines) and potential "hot-spots" caused by out-of-plane loads or stiffness tailored designs. 5. Design and test a series of test specimens, each one of which simulates a single selected failure mode and loading condition, compare to analytical predictions, and adjust analysis models or design allowables as necessary. 6. Design and conduct increasingly more complicated tests that evaluate more complicated loading situations with the possibility of failure from several potential failure modes. Compare to analytical predictions and adjust analysis models as necessary. 1 Design substantiation is often called "qualification" in U.S. DOD applications and "certification" in civilian applications involving the U.S. FAA. All three terms describe a similar process, but "substantiation" can be considered the more generic term, with "qualification" and "certification" often limited to the foregoing more restricted senses
MIL-HDBK-17-1F Volume 1,Chapter 2 Guidelines for Property Testing of Composites 7.Design (including compensation factors)and conduct,as required,full-scale component static and fatigue testing for final validation of internal loads and structural integrity.Compare to analy- SiS. COMPONENTS SUB-COMPONENTS STRUCTURAL FEATURES DETAILS ELEMENTS 88☐ COUPONS 卧理 DATA BASE 0 ⅢⅢ22☒ FIGURE 2.1.1 The pyramid of tests(Reference 2.1.1(a)). 2.1.2 Test levels and data uses Testing activities can be defined in two basic ways,Structural Complexity Level and Data Application Category.The classes within each are discussed in more detail in the sections that follow,and can be used to map large-scale testing programs as an aid to test planning,as illustrated in Section 2.1.2.3. 2.1.2.1 Structural complexity levels The five Structural Complexity Levels'are each geometry or form-based:constituent,lamina,lami- nate,structural element,and structural subcomponent.The material form(s)to be tested,and the relative emphasis placed on each level,should be determined early in the material data development planning process,and will likely depend upon many factors,including:manufacturing process,structural applica- tion,corporate/organizational practices,and/or the procurement or certification agency.While a single level may suffice in rare instances,most applications will require at least two levels,and it is common to use all five in a complete implementation of the building-block approach.Regardless of the Structural Complexity Level selected,physical and chemical property characterization of the prepreg(or the matrix, Due to the popularity of lamina-level testing and analysis,discussions in this handbook often emphasize development of a lamina- level database;however,this is not intended to inhibit use of any of the other Structural Complexity Levels,either singly or in combi- nation.Also,this handbook does not emphasize the structural subcomponent category since it is so strongly application dependent: however,many of the test planning and data documentation concepts for coupon testing contained herein can be extended to struc- tural subcomponent(or higher)testing. 2-2
MIL-HDBK-17-1F Volume 1, Chapter 2 Guidelines for Property Testing of Composites 2-2 7. Design (including compensation factors) and conduct, as required, full-scale component static and fatigue testing for final validation of internal loads and structural integrity. Compare to analysis. FIGURE 2.1.1 The pyramid of tests (Reference 2.1.1(a)). 2.1.2 Test levels and data uses Testing activities can be defined in two basic ways, Structural Complexity Level and Data Application Category. The classes within each are discussed in more detail in the sections that follow, and can be used to map large-scale testing programs as an aid to test planning, as illustrated in Section 2.1.2.3. 2.1.2.1 Structural complexity levels The five Structural Complexity Levels1 are each geometry or form-based: constituent, lamina, laminate, structural element, and structural subcomponent. The material form(s) to be tested, and the relative emphasis placed on each level, should be determined early in the material data development planning process, and will likely depend upon many factors, including: manufacturing process, structural application, corporate/organizational practices, and/or the procurement or certification agency. While a single level may suffice in rare instances, most applications will require at least two levels, and it is common to use all five in a complete implementation of the building-block approach. Regardless of the Structural Complexity Level selected, physical and chemical property characterization of the prepreg (or the matrix, 1 Due to the popularity of lamina-level testing and analysis, discussions in this handbook often emphasize development of a laminalevel database; however, this is not intended to inhibit use of any of the other Structural Complexity Levels, either singly or in combination. Also, this handbook does not emphasize the structural subcomponent category since it is so strongly application dependent; however, many of the test planning and data documentation concepts for coupon testing contained herein can be extended to structural subcomponent (or higher) testing
MIL-HDBK-17-1F Volume 1,Chapter 2 Guidelines for Property Testing of Composites if it is added as part of the process,as with resin transfer molding)is necessary to support physical and mechanical property test results.Each procurement or certification agency has specific minimum re- quirements and guidelines for use of data.Users of MIL-HDBK-17 are advised to coordinate with the procuring or certifying agency before planning and conducting any testing that supports structural qualifi- cation or certification. The five Structural Complexity Levels cover the following areas: Constituent Testing: This evaluates the individual properties of fibers,fiber forms,matrix materials,and fiber-matrix pre- forms.Key properties,for example,include fiber and matrix density,and fiber tensile strength and tensile modulus. Lamina Testing: This evaluates the properties of the fiber and matrix together in the composite material form.For the purpose of this discussion prepreg properties are included in this level,although they are sometimes bro- ken-out into a separate level.Key properties include fiber areal weight,matrix content,void content, cured ply thickness,lamina tensile strengths and moduli,lamina compressive strengths and moduli,and lamina shear strengths and moduli. Laminate Testing: Laminate testing characterizes the response of the composite material in a given laminate design. Key properties include tensile strengths and moduli,compressive strengths and moduli,shear strengths and moduli,interlaminar fracture toughness,and fatigue resistance. Structural Element Testing: This evaluates the ability of the material to tolerate common laminate discontinuities.Key properties include open and filled hole tensile strengths,open and filled hole compressive strengths,compression after impact strength,and joint bearing and bearing bypass strengths. Structural Subcomponent(or higher)Testing: This testing evaluates the behavior and failure mode of increasingly more complex structural assem- blies.These are application specific and not specifically covered by MIL-HDBK-17. 2.1.2.2 Data application categories Material property testing can also be grouped by data application into one or more of the following five categories:screening,'qualification,acceptance,equivalence,and structural substantiation.The starting point for testing most material systems is usually material screening.Material systems intended for use in engineering hardware are subjected to further testing to obtain additional data.While structural substantiation requirements,the last category,are not specifically addressed by MIL-HDBK-17 data gen- erated in accordance with MIL-HDBK-17 guidelines may form part of these requirements.The five Data Application Categories cover the following areas: Screening Testing. This is the assessment of material candidates for a given application,often with a given application in mind.The purpose of screening testing is initial evaluation of new material systems under worst-case environmental and loading test conditions.This handbook provides guidelines for screening new material systems based on key properties for aerospace structural applications.The MIL-HDBK-17 screening test matrix provides average values for various strength,moduli,and physical properties,includes both lamina A more limited form of screening testing for the characteristic response of a limited number of specific properties(often only one property)is not explicitly named as a testing category,but is commonly performed.Such limited testing usually consists of small test populations of three to six,usually from a single material batch,and often focuses on a specific environmental condition.As each instance of testing of this type has a specific but widely varying purpose MIL-HDBK-17 does not provide explicit test matrix recommendations;however,the guidance provided for the remaining testing categories remains a useful reference for test planning. 2-3
MIL-HDBK-17-1F Volume 1, Chapter 2 Guidelines for Property Testing of Composites 2-3 if it is added as part of the process, as with resin transfer molding) is necessary to support physical and mechanical property test results. Each procurement or certification agency has specific minimum requirements and guidelines for use of data. Users of MIL-HDBK-17 are advised to coordinate with the procuring or certifying agency before planning and conducting any testing that supports structural qualification or certification. The five Structural Complexity Levels cover the following areas: Constituent Testing: This evaluates the individual properties of fibers, fiber forms, matrix materials, and fiber-matrix preforms. Key properties, for example, include fiber and matrix density, and fiber tensile strength and tensile modulus. Lamina Testing: This evaluates the properties of the fiber and matrix together in the composite material form. For the purpose of this discussion prepreg properties are included in this level, although they are sometimes broken-out into a separate level. Key properties include fiber areal weight, matrix content, void content, cured ply thickness, lamina tensile strengths and moduli, lamina compressive strengths and moduli, and lamina shear strengths and moduli. Laminate Testing: Laminate testing characterizes the response of the composite material in a given laminate design. Key properties include tensile strengths and moduli, compressive strengths and moduli, shear strengths and moduli, interlaminar fracture toughness, and fatigue resistance. Structural Element Testing: This evaluates the ability of the material to tolerate common laminate discontinuities. Key properties include open and filled hole tensile strengths, open and filled hole compressive strengths, compression after impact strength, and joint bearing and bearing bypass strengths. Structural Subcomponent (or higher) Testing: This testing evaluates the behavior and failure mode of increasingly more complex structural assemblies. These are application specific and not specifically covered by MIL-HDBK-17. 2.1.2.2 Data application categories Material property testing can also be grouped by data application into one or more of the following five categories: screening,1 qualification, acceptance, equivalence, and structural substantiation. The starting point for testing most material systems is usually material screening. Material systems intended for use in engineering hardware are subjected to further testing to obtain additional data. While structural substantiation requirements, the last category, are not specifically addressed by MIL-HDBK-17 data generated in accordance with MIL-HDBK-17 guidelines may form part of these requirements. The five Data Application Categories cover the following areas: Screening Testing: This is the assessment of material candidates for a given application, often with a given application in mind. The purpose of screening testing is initial evaluation of new material systems under worst-case environmental and loading test conditions. This handbook provides guidelines for screening new material systems based on key properties for aerospace structural applications. The MIL-HDBK-17 screening test matrix provides average values for various strength, moduli, and physical properties, includes both lamina 1 A more limited form of screening testing for the characteristic response of a limited number of specific properties (often only one property) is not explicitly named as a testing category, but is commonly performed. Such limited testing usually consists of small test populations of three to six, usually from a single material batch, and often focuses on a specific environmental condition. As each instance of testing of this type has a specific but widely varying purpose MIL-HDBK-17 does not provide explicit test matrix recommendations; however, the guidance provided for the remaining testing categories remains a useful reference for test planning
MIL-HDBK-17-1F Volume 1,Chapter 2 Guidelines for Property Testing of Composites and laminate level testing,and is designed both to eliminate deficient material systems from the material selection process and to reveal promising new material systems before planning subsequent,more in- depth,evaluations. Material Qualification Testing: This step proves the ability of a given material/process to meet the requirements of a material specifi- cation;it is also the process of establishing the original specification requirement values.Rigorous mate- rial qualification testing considers the statistics of the data and is ideally a subset of,or directly related to. the design allowables testing performed to satisfy structural substantiation requirements.(However,while a material may be qualified to a given specification,it still must be approved for use in each specific appli- cation.)The objective is quantitative assessment of the variability of key material properties,leading to various statistics that are used to establish material acceptance,equivalence,quality control,and design basis values.Since there are various sampling and statistical approaches used within the industry,the approach used must be explicitly defined.While a generic basis value can be obtained many ways,a MIL-HDBK-17 basis value carries with it well-defined sampling requirements and a specific statistical de- termination process,and emphasizes additional considerations like test methodology,failure mode,and data documentation. Acceptance Testing: This is the task of verifying material consistency through periodic sampling of material product and evaluation of key material properties.Test results from small sample sizes are statistically compared with control values established from prior testing to determine whether or not the material production process has changed significantly. Equivalence Testing: This task assesses the equivalence of an alternate material to a previously characterized material often for the purpose of utilizing an existing material property database.The objective is evaluation of key properties for test populations large enough to provide a definitive conclusion,but small enough to pro- vide significant cost savings as compared to generating an entirely new database.A significant use in- cludes evaluation of possible second-sources of supply for a previously qualified material.However,the most common uses for this process are:1)evaluation of minor constituent,constituent processing,or fab- rication processing changes for a qualified material system,and 2)substantiation of previously estab- lished MIL-HDBK-17 basis values. Structural Substantiation Testing: This is the process of assessing the ability of a given structure to meet the requirements of a specific application.The development of design allowables,ideally derived or related to material basis values obtained during a material qualification task,is considered a part of this effort.When performed for the U.S.DOD this task is called structural qualification,and when the U.S.FAA is the certifying agency it is called structural certification 2.1.2.3 Test program definition A matrix is shown in Table 2.1.2.3 that can be used in test planning for large-scale testing programs. The material property tests from the Structural Complexity Levels and Data Application Categories are listed on the axes of an array,with each intersecting cell describing a distinct testing activity(though cer- tain combinations will rarely be used).Groups of cells can be used to summarize the scope of entire building-block testing programs.The array shown in Table 2.1.2.3 illustrates a common(but by no means universal)testing sequence in the substantiation of a composite-based aerospace structural application. The sequence begins with the hatched cells at the upper left of the array and proceeds,with time,toward the cells at the lower right,with the numbered notes indicating the approximate order in the sequence. (The structural substantiation category and structural subcomponent level are shaded to indicated that they are not specifically addressed by MIL-HDBK-17). 2-4
MIL-HDBK-17-1F Volume 1, Chapter 2 Guidelines for Property Testing of Composites 2-4 and laminate level testing, and is designed both to eliminate deficient material systems from the material selection process and to reveal promising new material systems before planning subsequent, more indepth, evaluations. Material Qualification Testing: This step proves the ability of a given material/process to meet the requirements of a material specification; it is also the process of establishing the original specification requirement values. Rigorous material qualification testing considers the statistics of the data and is ideally a subset of, or directly related to, the design allowables testing performed to satisfy structural substantiation requirements. (However, while a material may be qualified to a given specification, it still must be approved for use in each specific application.) The objective is quantitative assessment of the variability of key material properties, leading to various statistics that are used to establish material acceptance, equivalence, quality control, and design basis values. Since there are various sampling and statistical approaches used within the industry, the approach used must be explicitly defined. While a generic basis value can be obtained many ways, a MIL-HDBK-17 basis value carries with it well-defined sampling requirements and a specific statistical determination process, and emphasizes additional considerations like test methodology, failure mode, and data documentation. Acceptance Testing: This is the task of verifying material consistency through periodic sampling of material product and evaluation of key material properties. Test results from small sample sizes are statistically compared with control values established from prior testing to determine whether or not the material production process has changed significantly. Equivalence Testing: This task assesses the equivalence of an alternate material to a previously characterized material, often for the purpose of utilizing an existing material property database. The objective is evaluation of key properties for test populations large enough to provide a definitive conclusion, but small enough to provide significant cost savings as compared to generating an entirely new database. A significant use includes evaluation of possible second-sources of supply for a previously qualified material. However, the most common uses for this process are: 1) evaluation of minor constituent, constituent processing, or fabrication processing changes for a qualified material system, and 2) substantiation of previously established MIL-HDBK-17 basis values. Structural Substantiation Testing: This is the process of assessing the ability of a given structure to meet the requirements of a specific application. The development of design allowables, ideally derived or related to material basis values obtained during a material qualification task, is considered a part of this effort. When performed for the U.S. DOD this task is called structural qualification, and when the U.S. FAA is the certifying agency it is called structural certification. 2.1.2.3 Test program definition A matrix is shown in Table 2.1.2.3 that can be used in test planning for large-scale testing programs. The material property tests from the Structural Complexity Levels and Data Application Categories are listed on the axes of an array, with each intersecting cell describing a distinct testing activity (though certain combinations will rarely be used). Groups of cells can be used to summarize the scope of entire building-block testing programs. The array shown in Table 2.1.2.3 illustrates a common (but by no means universal) testing sequence in the substantiation of a composite-based aerospace structural application. The sequence begins with the hatched cells at the upper left of the array and proceeds, with time, toward the cells at the lower right, with the numbered notes indicating the approximate order in the sequence. (The structural substantiation category and structural subcomponent level are shaded to indicated that they are not specifically addressed by MIL-HDBK-17)
MIL-HDBK-17-1F Volume 1,Chapter 2 Guidelines for Property Testing of Composites TABLE 2.1.2.3 Test program definition. STRUCTURAL COMPLEXITY DATA APPLICATION CATEGORIES LEVEL Material Material Material Material Structural Screening Qualification Acceptance Equivalence Substantiation Constituent 1 Lamina 4 Laminate Structural 3 Element Structural 9 Subcomponent This handbook defines a number of recommended test matrices in Section 2.3.organized by Data Application Category. 2.2 TEST PROGRAM PLANNING 2.2.1 Overview Section 2.2 discusses a number of testing objectives that affect the execution of testing programs. The next section,2.3 on Recommended Test Matrices,completes these items by providing recommended test matrices(types of tests and test quantities at various environments)for a number of composite mate- rial forms and objectives.These pre-defined test matrices may hove to be customized for use with a spe- cific application. Characterization of composite material properties is distinctly different than for either metals or unrein- forced plastics.Section 2.2 provides information on many of the critical differences that affect testing and test planning,including: ·testing matrices. material sampling and pooling issues, statistical calculations, ● test method selection. material and processing variation. conditioning and non-ambient testing issues, alternative coupon confiqurations, data normalization and documentation,and application-specific testing. All significant testing programs should begin with preparation of a detailed test plan document.A test plan specifies material properties to be evaluated,selects tests methods,eliminates options offered by 2-5
MIL-HDBK-17-1F Volume 1, Chapter 2 Guidelines for Property Testing of Composites 2-5 TABLE 2.1.2.3 Test program definition. STRUCTURAL COMPLEXITY LEVEL DATA APPLICATION CATEGORIES Material Screening Material Qualification Material Acceptance Material Equivalence Structural Substantiation Constituent 1 - - - - Lamina 2 4 - - Laminate - 5 - 7 Structural Element 3 6 - 8 Structural Subcomponent - - - - 9 This handbook defines a number of recommended test matrices in Section 2.3, organized by Data Application Category. 2.2 TEST PROGRAM PLANNING 2.2.1 Overview Section 2.2 discusses a number of testing objectives that affect the execution of testing programs. The next section, 2.3 on Recommended Test Matrices, completes these items by providing recommended test matrices (types of tests and test quantities at various environments) for a number of composite material forms and objectives. These pre-defined test matrices may hove to be customized for use with a specific application. Characterization of composite material properties is distinctly different than for either metals or unreinforced plastics. Section 2.2 provides information on many of the critical differences that affect testing and test planning, including: • testing matrices, • material sampling and pooling issues, • statistical calculations, • test method selection, • material and processing variation, • conditioning and non-ambient testing issues, • alternative coupon configurations, • data normalization and documentation, and • application-specific testing. All significant testing programs should begin with preparation of a detailed test plan document. A test plan specifies material properties to be evaluated, selects tests methods, eliminates options offered by
MIL-HDBK-17-1F Volume 1,Chapter 2 Guidelines for Property Testing of Composites standard test methods by selecting specific specimen and test configurations,and defines success crite- ria.It is prepared by the contractor,approved by the certifying agency,and is the focal point for under- standing between the contractor and certifying agency.A clearly written,well-prepared test plan is also a primary management tool to define the scope of the work,degree of success,and progress toward com- pletion. 2.2.2 Baseline and alternate approaches for statistically-based properties Much of MIL-HDBK-17 focuses on guidelines for establishing basis values for strength and strain-to- failure properties A specific statistical methodology for calculating basis values from test results,illus- trated in Figure 8.3.1,has been developed by this handbook,is recommended for general use in reducing data,and is required for evaluation of data published in Volume 2. Additional requirements imposed on data published within this handbook include:specific population sampling methods and reporting of supporting data.For the purposes of obtaining a reasonable evalua- tion of material variation,basis values published in this handbook are based on a minimum of thirty specimens from at least five batches of a material per environment and direction as discussed in Sections 2.2.5 and 2.5.3.These data are normalized (where appropriate)as discussed in Sections 2.2.11 and 2.4.3,statistically evaluated in accordance with the process described by Figure 8.3.1 and discussed in Section 8.3,and reported in accordance with Volume 2,Section 1.4.2. This same statistical procedure can be used on populations of fewer batches and/or replicates,but,if data from such populations are submitted to the handbook for publication,the published data summary will not include a basis value. Depending on both the application and the procuring or certifying agency.modifications to the base- line MIL-HDBK-17 approach may be justified when developing new material data.In such cases the handbook guidelines remain useful for support and reference.Alternate sampling and statistical ap- proaches to development of basis values may be justified in certain instances,though they are less com- monly used.These alternate approaches directly affect test matrix development and generally require a relatively sophisticated knowledge of both statistics and of the material behavior of the specific material system.An introduction to one type of alternate approach is provided in Section 2.3.6.1,with the related statistical background summarized in Section 8.3.5.3.When using such alternate approaches,advance approval of the procurement or certification agency is strongly recommended. 2.2.3 Issues of data equivalence Evaluation for data pooling(whether data from two possibly different subpopulations are enough alike to be combined)and material equivalence (whether a material with common characteristics to another is sufficiently alike to use its data for design)are similar issues of data equivalence.Both require statistical procedures to assess the similarities and differences between two subpopulations of data.These,and other related issues,are covered in more detail in Sections 2.3.4.1,2.3.7,and 2.5.3.4.Assessment of the equivalence of data begins by examining key properties for various within-batch and between-batch sta- tistics (see Section 8.3.2). The ability to pool different subpopulations of test data is highly desirable,if for no other reason than to obtain larger populations that are more representative of the universe(see Section 2.2.5 for a summary discussion of sample size effects).Equally desirable is the ability to show one material without basis val- ues equivalent to another that already has established basis values (see Sections 2.3.4.1,2.3.7,and A B-basis value,as defined in Section 1.7,is the value above which at least 90 percent of the population of values is expected to fall,with a confidence of 95 percent.Statistical estimates of basis values for material properties are considered by the handbook to be material properties unto themselves. 2If some properties are found similar and others not,engineering judgment must assess the criticality for the given application of the dissimilar properties before the altemate material can be deemed equivalent.The equivalence then only applies to that application and must be reassessed for a different application. 2-6
MIL-HDBK-17-1F Volume 1, Chapter 2 Guidelines for Property Testing of Composites 2-6 standard test methods by selecting specific specimen and test configurations, and defines success criteria. It is prepared by the contractor, approved by the certifying agency, and is the focal point for understanding between the contractor and certifying agency. A clearly written, well-prepared test plan is also a primary management tool to define the scope of the work, degree of success, and progress toward completion. 2.2.2 Baseline and alternate approaches for statistically-based properties Much of MIL-HDBK-17 focuses on guidelines for establishing basis values for strength and strain-tofailure properties1 . A specific statistical methodology for calculating basis values from test results, illustrated in Figure 8.3.1, has been developed by this handbook, is recommended for general use in reducing data, and is required for evaluation of data published in Volume 2. Additional requirements imposed on data published within this handbook include: specific population sampling methods and reporting of supporting data. For the purposes of obtaining a reasonable evaluation of material variation, basis values published in this handbook are based on a minimum of thirty specimens from at least five batches of a material per environment and direction as discussed in Sections 2.2.5 and 2.5.3. These data are normalized (where appropriate) as discussed in Sections 2.2.11 and 2.4.3, statistically evaluated in accordance with the process described by Figure 8.3.1 and discussed in Section 8.3, and reported in accordance with Volume 2, Section 1.4.2. This same statistical procedure can be used on populations of fewer batches and/or replicates, but, if data from such populations are submitted to the handbook for publication, the published data summary will not include a basis value. Depending on both the application and the procuring or certifying agency, modifications to the baseline MIL-HDBK-17 approach may be justified when developing new material data. In such cases the handbook guidelines remain useful for support and reference. Alternate sampling and statistical approaches to development of basis values may be justified in certain instances, though they are less commonly used. These alternate approaches directly affect test matrix development and generally require a relatively sophisticated knowledge of both statistics and of the material behavior of the specific material system. An introduction to one type of alternate approach is provided in Section 2.3.6.1, with the related statistical background summarized in Section 8.3.5.3. When using such alternate approaches, advance approval of the procurement or certification agency is strongly recommended. 2.2.3 Issues of data equivalence Evaluation for data pooling (whether data from two possibly different subpopulations are enough alike to be combined) and material equivalence (whether a material with common characteristics to another is sufficiently alike to use its data for design) are similar issues of data equivalence. Both require statistical procedures to assess the similarities and differences between two subpopulations of data2 . These, and other related issues, are covered in more detail in Sections 2.3.4.1, 2.3.7, and 2.5.3.4. Assessment of the equivalence of data begins by examining key properties for various within-batch and between-batch statistics (see Section 8.3.2). The ability to pool different subpopulations of test data is highly desirable, if for no other reason than to obtain larger populations that are more representative of the universe (see Section 2.2.5 for a summary discussion of sample size effects). Equally desirable is the ability to show one material without basis values equivalent to another that already has established basis values (see Sections 2.3.4.1, 2.3.7, and 1 A B-basis value, as defined in Section 1.7, is the value above which at least 90 percent of the population of values is expected to fall, with a confidence of 95 percent. Statistical estimates of basis values for material properties are considered by the handbook to be material properties unto themselves. 2 If some properties are found similar and others not, engineering judgment must assess the criticality for the given application of the dissimilar properties before the alternate material can be deemed equivalent. The equivalence then only applies to that application and must be reassessed for a different application
MIL-HDBK-17-1F Volume 1,Chapter 2 Guidelines for Property Testing of Composites 2.5.3.4).Requirements for the use of pooled data or equivalent materials are normally established for each application during discussions with the certifying agency or,for data being considered for publication in MIL-HDBK-17,by the MIL-HDBK-17 Data Review Working Group. Before determining statistical degree of equivalence,basic engineering considerations should be sat- isfied;the two materials should be of the same chemical,microstructural,and material form families.To some extent the criteria for this may be application dependent.For example,property data from two composite systems with the same matrix and similar fibers may not warrant pooling if the fiber/matrix in- terface is distinctly different,even if the fibers have similar modulus and tensile strength.Data equiva- lence is typically evaluated for data sets that differ due only to relatively minor changes in precursor manufacturing or material processing,such as: minor changes in constituents or constituent manufacturing processes, identical materials processed by different component manufacturers, identical materials processed at different locations of the same manufacturer, slight changes in processing parameters,or some combination of the above. Statistical data equivalence methods currently assume that between-and within-laboratory test method variation is negligible.When this assumption is violated this test method-induced artificial varia- tion severely weakens the ability of the statistical methods to meaningfully compare two different detests. This is discussed further in Sections 2.2.4 and 2.2.5. 2.2.4 Test method selection Test results in an empirical determination of either an intrinsic material property(like material com- pressive modulus or tensile strength)or a generic structural response (like quasi-isotropic laminate open hole tensile strength)from a small and relatively simple specimen are often used as input to a simulation of the response of a larger and more complicated specific structure.Test methods historically developed for metals or plastics,in most cases,cannot be directly applied to advanced composite materials.While the basic physics of test methods for composites may be similar to their unreinforced counterparts,the heterogeneity,orthotropy,moisture sensitivity,and low ductility of typical composites often lead to signifi- cant differences in testing requirements,particularly with the mechanical tests,including: the strong influence of constituent content on material response,creating a need to measure the material response of every specimen, a need to evaluate properties in multiple directions, a need to condition specimens to quantify and control moisture absorption and desorption, increased importance of specimen alignment and load introduction method,and a need to assume consistency of failure modes. Other distinguishing characteristics of many composite materials also contribute to testing differences, including: compressive strength often lower than tensile strength(though specific material systems like bo- ron/epoxy may behave counter to this), operating temperatures relatively closer to material property transition temperatures(compared to metals), shear stress response uncoupled from normal stress response,and heightened sensitivity to specimen preparation practices. One measure of a test method is the theoretical ability of a perfect test to produce a desired result, such as a uniform uniaxial stress state throughout the conduct of the test.However,the above factors tend to increase the sensitivity of composites to a wider variety of testing parameters than is seen with conventional materials.Therefore test method robustness,or relative insensitivity to minor variations in 2-7
MIL-HDBK-17-1F Volume 1, Chapter 2 Guidelines for Property Testing of Composites 2-7 2.5.3.4). Requirements for the use of pooled data or equivalent materials are normally established for each application during discussions with the certifying agency or, for data being considered for publication in MIL-HDBK-17, by the MIL-HDBK-17 Data Review Working Group. Before determining statistical degree of equivalence, basic engineering considerations should be satisfied; the two materials should be of the same chemical, microstructural, and material form families. To some extent the criteria for this may be application dependent. For example, property data from two composite systems with the same matrix and similar fibers may not warrant pooling if the fiber/matrix interface is distinctly different, even if the fibers have similar modulus and tensile strength. Data equivalence is typically evaluated for data sets that differ due only to relatively minor changes in precursor manufacturing or material processing, such as: • minor changes in constituents or constituent manufacturing processes, • identical materials processed by different component manufacturers, • identical materials processed at different locations of the same manufacturer, • slight changes in processing parameters, or • some combination of the above. Statistical data equivalence methods currently assume that between- and within-laboratory test method variation is negligible. When this assumption is violated this test method-induced artificial variation severely weakens the ability of the statistical methods to meaningfully compare two different detests. This is discussed further in Sections 2.2.4 and 2.2.5. 2.2.4 Test method selection Test results in an empirical determination of either an intrinsic material property (like material compressive modulus or tensile strength) or a generic structural response (like quasi-isotropic laminate open hole tensile strength) from a small and relatively simple specimen are often used as input to a simulation of the response of a larger and more complicated specific structure. Test methods historically developed for metals or plastics, in most cases, cannot be directly applied to advanced composite materials. While the basic physics of test methods for composites may be similar to their unreinforced counterparts, the heterogeneity, orthotropy, moisture sensitivity, and low ductility of typical composites often lead to significant differences in testing requirements, particularly with the mechanical tests, including: • the strong influence of constituent content on material response, creating a need to measure the material response of every specimen, • a need to evaluate properties in multiple directions, • a need to condition specimens to quantify and control moisture absorption and desorption, • increased importance of specimen alignment and load introduction method, and • a need to assume consistency of failure modes. Other distinguishing characteristics of many composite materials also contribute to testing differences, including: • compressive strength often lower than tensile strength (though specific material systems like boron/epoxy may behave counter to this), • operating temperatures relatively closer to material property transition temperatures (compared to metals), • shear stress response uncoupled from normal stress response, and • heightened sensitivity to specimen preparation practices. One measure of a test method is the theoretical ability of a perfect test to produce a desired result, such as a uniform uniaxial stress state throughout the conduct of the test. However, the above factors tend to increase the sensitivity of composites to a wider variety of testing parameters than is seen with conventional materials. Therefore test method robustness, or relative insensitivity to minor variations in
MIL-HDBK-17-1F Volume 1,Chapter 2 Guidelines for Property Testing of Composites specimen and test procedure.is just as important as theoretical perfection.Robustness,or lack thereof. is assessed by interlaboratory testing,and is measured by precision(variation in the sample population) and bias(variation of the sample mean from the true average).'The precision and bias of test methods are evaluated by comparison testing (often called "round-robin"testing)both within-laboratory and be- tween laboratories.The obvious ideal is high precision(low variation)and low bias(sample mean close to true average)both within-laboratory and between laboratories.Such a test method would repeatedly produce reproducible results without regard to material,operator,or test laboratory.However,quantifica- tion of bias requires a material standard for each test;none of which are currently available for compos- ites.As a result,bias of composite test methods can currently only be qualitatively assessed. Somewhat separate from the precision and bias of a test method (for a given specimen)is the effect on precision and bias of variation in test specimen size and geometry.For heterogeneous materials, physically larger specimens can be expected to contain within the coupon a more representative sample of the material microstructure.While desirable,a larger specimen is more apt to contain a greater num- ber of micro-or macro-structural defects than a smaller specimen,and thus can be expected to produce somewhat lower strengths (though possibly also with lower variation).Variations in specimen geometry can also create differing results.Size and geometry effects can produce statistical differences in results independent of the "degree of perfection"of the remaining aspects of a test method or its conduct;such effects should be expected.Therefore,even though the specimen response may not (and probably won't)be identical to that of the structure,the "ideal"test method will incorporate a specimen geometry that can be consistently correlated with structural response As the criticality of various test parameters are still being researched and understood(even for rela- tively common tests)and as "standard laboratory practices,"upon close examination,are actually found to vary from laboratory to laboratory,it is critical to control or document as many of these practices and parameters as possible.ASTM Committee D-30,responsible for standardization of advanced composite material test methods,tries to consider all of these factors when improving existing and developing new standard test methods (see Reference 2.2.4).Due to both their completeness and their status as full- consensus standards,ASTM D-30 test methods,where applicable,are emphasized by this handbook. Failure to minimize test method sensitivities,whatever the cause,can cause the statistical methods contained within MIL-HDBK-17 to break-down,as all variation in data is implicitly assumed by the statisti- cal methods to be due to material or process variation.Any additional variation due to specimen prepara- tion or testing procedure is added to the material/process variation,which can result in extraordinarily conservative.or even meaningless.basis value results. Test methods,with emphasis on ASTM standards for advanced composites,are discussed in Chap- ters 3 through 7.The advantages and disadvantages of the various test methods for composites are dis- cussed,including.for completeness,non-standard but often referenced methods that have appeared in the literature.Chapters 3 and 4 cover constituent testing.Chapter 5 covers prepreg test methods. Chapter 6 covers lamina and laminate testing.Chapter 7 covers structural element test methods.Data produced by the following test methods (Table 2.2.4)are currently being accepted by MIL-HDBK-17 for consideration for inclusion in Volume 2. The term"accuracy"is often used as a generic combination of aspects of both precision and bias.The terms "precision"and "bias",being more specific,are preferred for use where appropriate 2-8
MIL-HDBK-17-1F Volume 1, Chapter 2 Guidelines for Property Testing of Composites 2-8 specimen and test procedure, is just as important as theoretical perfection. Robustness, or lack thereof, is assessed by interlaboratory testing, and is measured by precision (variation in the sample population) and bias (variation of the sample mean from the true average).1 The precision and bias of test methods are evaluated by comparison testing (often called "round-robin" testing) both within-laboratory and between laboratories. The obvious ideal is high precision (low variation) and low bias (sample mean close to true average) both within-laboratory and between laboratories. Such a test method would repeatedly produce reproducible results without regard to material, operator, or test laboratory. However, quantification of bias requires a material standard for each test; none of which are currently available for composites. As a result, bias of composite test methods can currently only be qualitatively assessed. Somewhat separate from the precision and bias of a test method (for a given specimen) is the effect on precision and bias of variation in test specimen size and geometry. For heterogeneous materials, physically larger specimens can be expected to contain within the coupon a more representative sample of the material microstructure. While desirable, a larger specimen is more apt to contain a greater number of micro- or macro-structural defects than a smaller specimen, and thus can be expected to produce somewhat lower strengths (though possibly also with lower variation). Variations in specimen geometry can also create differing results. Size and geometry effects can produce statistical differences in results independent of the "degree of perfection" of the remaining aspects of a test method or its conduct; such effects should be expected. Therefore, even though the specimen response may not (and probably won't) be identical to that of the structure, the "ideal" test method will incorporate a specimen geometry that can be consistently correlated with structural response. As the criticality of various test parameters are still being researched and understood (even for relatively common tests) and as "standard laboratory practices," upon close examination, are actually found to vary from laboratory to laboratory, it is critical to control or document as many of these practices and parameters as possible. ASTM Committee D-30, responsible for standardization of advanced composite material test methods, tries to consider all of these factors when improving existing and developing new standard test methods (see Reference 2.2.4). Due to both their completeness and their status as fullconsensus standards, ASTM D-30 test methods, where applicable, are emphasized by this handbook. Failure to minimize test method sensitivities, whatever the cause, can cause the statistical methods contained within MIL-HDBK-17 to break-down, as all variation in data is implicitly assumed by the statistical methods to be due to material or process variation. Any additional variation due to specimen preparation or testing procedure is added to the material/process variation, which can result in extraordinarily conservative, or even meaningless, basis value results. Test methods, with emphasis on ASTM standards for advanced composites, are discussed in Chapters 3 through 7. The advantages and disadvantages of the various test methods for composites are discussed, including, for completeness, non-standard but often referenced methods that have appeared in the literature. Chapters 3 and 4 cover constituent testing. Chapter 5 covers prepreg test methods. Chapter 6 covers lamina and laminate testing. Chapter 7 covers structural element test methods. Data produced by the following test methods (Table 2.2.4) are currently being accepted by MIL-HDBK-17 for consideration for inclusion in Volume 2. 1 The term "accuracy" is often used as a generic combination of aspects of both precision and bias. The terms "precision" and "bias", being more specific, are preferred for use where appropriate
MIL-HDBK-17-1F Volume 1,Chapter 2 Guidelines for Property Testing of Composites TABLE 2.2.4 Summary of test methods for MIL-HDBK-17 data submittal (continued on next page). Test Category Source of Test Method ASTM SACMA Prepreg Tests Resin Content D3529,C613,D5300 RM 23,RM 24 Volatiles Content D3530 Resin Flow D3531 RM 22 Resin Gel Time D3532 RM19 Fiber Areal Weight D3776 RM 23,RM 24 Moisture Content D4019 Tack HPLC RM 20 IR E1252,E168 DMA(RDS) D4065,D4473 RM19 DSC E1356 RM 25 Lamina Physical Tests Moisture Conditioning D5229 RM 11 Fiber Volume D3171,D2734 RM10 Resin Content D3171,D2734 RM10 Void Content D2584 Density D792,D1505 Cured Ply Thickness(CPT) RM10 Glass Transition Temperature,dry D4065 RM18 Glass Transition Temperature,wet RM 18 CTE,out-of-plane E831 CTE,in-plane D696,E228 Equilibrium Moisture Content D5229 RM 11 Moisture Diffusivity D5229 Thermal Diffusivity E1461 Specific Heat E1269 2-9
MIL-HDBK-17-1F Volume 1, Chapter 2 Guidelines for Property Testing of Composites 2-9 TABLE 2.2.4 Summary of test methods for MIL-HDBK-17 data submittal (continued on next page). Test Category Source of Test Method ASTM SACMA Prepreg Tests Resin Content D 3529, C 613, D 5300 RM 23, RM 24 Volatiles Content D 3530 --- Resin Flow D 3531 RM 22 Resin Gel Time D 3532 RM 19 Fiber Areal Weight D 3776 RM 23, RM 24 Moisture Content D 4019 --- Tack --- --- HPLC --- RM 20 IR E 1252, E 168 --- DMA (RDS) D 4065, D 4473 RM 19 DSC E 1356 RM 25 Lamina Physical Tests Moisture Conditioning D 5229 RM 11 Fiber Volume D 3171, D 2734 RM 10 Resin Content D 3171, D 2734 RM 10 Void Content D 2584 --- Density D 792, D 1505 --- Cured Ply Thickness (CPT) --- RM 10 Glass Transition Temperature, dry D 4065 RM 18 Glass Transition Temperature, wet --- RM 18 CTE, out-of-plane E 831 --- CTE, in-plane D 696, E 228 --- Equilibrium Moisture Content D 5229 RM 11 Moisture Diffusivity D 5229 --- Thermal Diffusivity E 1461 --- Specific Heat E 1269 ---
MIL-HDBK-17-1F Volume 1,Chapter 2 Guidelines for Property Testing of Composites TABLE 2.2.4 Summary of test methods for MIL-HDBK-17 data submittal,concluded. Test Category Source of Test Method ASTM SACMA Lamina/Laminate Mechanical Tests 0Marp Tension D3039 RM 4,RM9 90°/Fill Tension D3039,D5450 RM 4,RM9 0°Warp Compression D3410,D5467 RM 1,RM6 90/Fill Compression D3410,D5449 RM1,RM6 In-Plane Shear(1) D3518.D5448,D5379 RM7 Interlaminar Shear D5379 Short Beam Strength D2344 RM8 Flexure(7) Open-Hole Compression (draft) RM3 Open-Hole Tension D5766 RM5 Single-Shear Bearing(2) (draft) Double-Shear Bearing(2) (draft) Compression after Impact (draft) RM2 Mode I Fracture Toughness D5528 Mode ll Fracture Toughness (draft) Tension/Tension Fatigue D3479 Tension/Compression Fatigue Notes: 1)ASTM D 4255 will also be accepted for in-plane shear modulus of flat panels. 2)Bearing test procedures are presented in Chapter 7 until the draft ASTM test method that is based on them are released.These Chapter 7 test methods will also be accepted. 3)Certain material forms or processes(like filament winding)may,for a specific material property,be restricted to a single test method.See the detailed test method descriptions in Chapters 3 through 7,or the test methods them- selves,for a more complete explanation. 4) SACMA test methods,in many cases,are subsets or supersets of the referenced ASTM test methods,and in other cases have either a different scope or use a different testing methodology.For cases where a SACMA test method exists,and either there is no ASTM test method covering the same property or the existing ASTM test method uses a different methodology.ASTM is considering adopting a form of the SACMA test method.Where ASTM and SACMA test methods overlap,ASTM and SACMA are working to consolidate the test methods into the next release of the ASTM standard. 5)For properties where there are more than one test method listed for either ASTM or SACMA,the different test methods either apply to different material forms or use different testing methodologies. 6)Data from other test methods not listed may be considered by the Testing and Data Review Working Groups, following the guidelines described in Section 2.5.5. 7)See Section 6.7.7. 2-10
MIL-HDBK-17-1F Volume 1, Chapter 2 Guidelines for Property Testing of Composites 2-10 TABLE 2.2.4 Summary of test methods for MIL-HDBK-17 data submittal, concluded. Test Category Source of Test Method ASTM SACMA Lamina/Laminate Mechanical Tests 0°/Warp Tension D 3039 RM 4, RM 9 90°/Fill Tension D 3039, D 5450 RM 4, RM 9 0°/Warp Compression D 3410, D 5467 RM 1, RM 6 90°/Fill Compression D 3410, D 5449 RM 1, RM 6 In-Plane Shear (1) D 3518, D 5448, D 5379 RM 7 Interlaminar Shear D 5379 --- Short Beam Strength D 2344 RM 8 Flexure (7) --- --- Open-Hole Compression (draft) RM 3 Open-Hole Tension D 5766 RM 5 Single-Shear Bearing (2) (draft) --- Double-Shear Bearing (2) (draft) --- Compression after Impact (draft) RM 2 Mode I Fracture Toughness D 5528 --- Mode II Fracture Toughness (draft) --- Tension/Tension Fatigue D 3479 --- Tension/Compression Fatigue --- --- Notes: 1) ASTM D 4255 will also be accepted for in-plane shear modulus of flat panels. 2) Bearing test procedures are presented in Chapter 7 until the draft ASTM test method that is based on them are released. These Chapter 7 test methods will also be accepted. 3) Certain material forms or processes (like filament winding) may, for a specific material property, be restricted to a single test method. See the detailed test method descriptions in Chapters 3 through 7, or the test methods themselves, for a more complete explanation. 4) SACMA test methods, in many cases, are subsets or supersets of the referenced ASTM test methods, and in other cases have either a different scope or use a different testing methodology. For cases where a SACMA test method exists, and either there is no ASTM test method covering the same property or the existing ASTM test method uses a different methodology, ASTM is considering adopting a form of the SACMA test method. Where ASTM and SACMA test methods overlap, ASTM and SACMA are working to consolidate the test methods into the next release of the ASTM standard. 5) For properties where there are more than one test method listed for either ASTM or SACMA, the different test methods either apply to different material forms or use different testing methodologies. 6) Data from other test methods not listed may be considered by the Testing and Data Review Working Groups, following the guidelines described in Section 2.5.5. 7) See Section 6.7.7
