MIL-HDBK-3F Volume 3,Chapter 4 Building Block Approach for Composite Structures CHAPTER 4 BUILDING BLOCK APPROACH FOR COMPOSITE STRUCTURES 4.1 INTRODUCTION AND PHILOSOPHY When composites are to be used in structural components,a design development program is gener- ally initiated during which the performance of the structure is assessed prior to use.This process of sub- stantiating the structural performance and durability of composite components generally consists of a complex mix of testing and analysis.Testing alone can be prohibitively expensive because of the number of specimens needed to verify every geometry,loading,environment,and failure mode.Analysis tech- niques alone are usually not sophisticated enough to adequately predict results under every set of condi- tions.By combining testing and analysis,analytical predictions are verified by test,test plans are guided by analysis,and the cost of the overall effort is reduced while reliability is increased. An extension of this synergistic analysis/test approach is to conduct analysis and associated tests at various levels of structural complexity,often beginning with small specimens and progressing through structural elements and details,sub-components,components,and finally the complete full scale product. Each level builds on knowledge gained at previous,less complex levels.This substantiation process, using both testing and analysis in a program of increasingly complex levels,has become known as the "Building Block"approach.The building blocks are integrated with supporting technologies and design considerations as depicted in Figure 4.1(a).One major purpose of employing this approach is to reduce program cost and risk while meeting all technical,regulatory,and customer requirements.The philosophy is to make the design development process more effective in assessing technology risks early in a pro- gram schedule.Cost efficiency is achieved by designing a program in which greater numbers of less ex- pensive small specimens are tested and fewer of the more expensive component and full scale articles are required.Using analyses in place of tests where possible also tends to reduce cost. Building Blocks Full Scale Static Statistical Methods Tests Fatigue Materials Process Component Tests Technologies Subcomponent Tests Test and Analysis Methods 日 Element Tests NDI Acceptance/ Coupon Laminate Rejection Criteria Tests Lamina Supporting Technologies Static Strength Durability Damage Tolerance Design Considerations FIGURE 4.1(a)Building block integration. Although the concept of the Building Block approach is widely acknowledged in the composites indus- try,it is applied with varying degrees of rigor,and details are far from universal.In its simplest form,it represents a method of risk mitigation (both technical and financial)in that testing at the various levels reduces the probability that significant surprises will materialize near the end of a program.In a more 4-1
MIL-HDBK-3F Volume 3, Chapter 4 Building Block Approach for Composite Structures 4-1 CHAPTER 4 BUILDING BLOCK APPROACH FOR COMPOSITE STRUCTURES 4.1 INTRODUCTION AND PHILOSOPHY When composites are to be used in structural components, a design development program is generally initiated during which the performance of the structure is assessed prior to use. This process of substantiating the structural performance and durability of composite components generally consists of a complex mix of testing and analysis. Testing alone can be prohibitively expensive because of the number of specimens needed to verify every geometry, loading, environment, and failure mode. Analysis techniques alone are usually not sophisticated enough to adequately predict results under every set of conditions. By combining testing and analysis, analytical predictions are verified by test, test plans are guided by analysis, and the cost of the overall effort is reduced while reliability is increased. An extension of this synergistic analysis/test approach is to conduct analysis and associated tests at various levels of structural complexity, often beginning with small specimens and progressing through structural elements and details, sub-components, components, and finally the complete full scale product. Each level builds on knowledge gained at previous, less complex levels. This substantiation process, using both testing and analysis in a program of increasingly complex levels, has become known as the “Building Block” approach. The building blocks are integrated with supporting technologies and design considerations as depicted in Figure 4.1(a). One major purpose of employing this approach is to reduce program cost and risk while meeting all technical, regulatory, and customer requirements. The philosophy is to make the design development process more effective in assessing technology risks early in a program schedule. Cost efficiency is achieved by designing a program in which greater numbers of less expensive small specimens are tested and fewer of the more expensive component and full scale articles are required. Using analyses in place of tests where possible also tends to reduce cost. FIGURE 4.1(a) Building block integration. Although the concept of the Building Block approach is widely acknowledged in the composites industry, it is applied with varying degrees of rigor, and details are far from universal. In its simplest form, it represents a method of risk mitigation (both technical and financial) in that testing at the various levels reduces the probability that significant surprises will materialize near the end of a program. In a more
MIL-HDBK-3F Volume 3,Chapter 4 Building Block Approach for Composite Structures elaborate implementation it can be a highly structured and carefully planned effort which addresses many factors in detail,and which may attempt to quantify statistical reliability associated with the process Regardless of the details of a specific Building Block program,each level or block takes the general form idealized in Figure 4.1(b)(except for the lowest level).Knowledge gained from analyses and tests in a previous level is combined with structural requirements and used to define and perform the next level of design and analysis.If an acceptable analytical result is not obtained,a structural redesign and/or analy- sis modification is made until the result is favorable.Once an acceptable analytical result is achieved,it is verified by test.If the test results do not meet the expectations predicted by analysis,the test may be re- designed if an erroneous mode was detected,or the design and/or analytic method may be modified. Additionally,tests or analyses in a previous level may be repeated for verification.The appropriate ac- tions are taken until test results verify an acceptable analytical prediction.When this has been accom- plished,the program has advanced to the next level of complexity.It is important to recognize that,since different programs have varying needs,requirements,and constraints,not all building block approaches use the same number of complexity levels or define these levels in the same way. Next Perform or Test Verifies Yes Building Block Re-evaluate Design/ Level of Verification Analysis Structural Test(s) 7 Complexity No Performance Requirements Yes Redesign Test Yes o Results of Design/ Acceptable Previous Level 、 Tests and Analyses Analysis Result No Redesign Structure and/or Modify Analysis FIGURE 4.1(b)Idealized general building block schematic(one level). Figures 4.1(a)and 4.1(b)and related discussions convey the idea that the Building Block process is a series of steps that progress neatly in order from one block to the next.While this is a convenient way to idealize the Building Block concept,the process is not quite so linear in practice.In reality.program schedules and availability of resources may be such that portions of various blocks overlap in time,and may even occur in parallel.Figure 4.1(c)shows one example of a typical Building Block program flow. The discussion of Building Block levels that follows relates to the idealized model for simplicity. At the lowest Building Block level,small specimen and element tests are most widely used to charac- terize basic unnotched static material properties,generic notch sensitivity,environmental factors,material operational limit (MOL-see Volume 1,Chapter 2,Section 2.2.8),and laminate fatigue response.In the case of this first level,testing is used for starting the Building Block process by providing data for first it- 4-2
MIL-HDBK-3F Volume 3, Chapter 4 Building Block Approach for Composite Structures 4-2 elaborate implementation it can be a highly structured and carefully planned effort which addresses many factors in detail, and which may attempt to quantify statistical reliability associated with the process. Regardless of the details of a specific Building Block program, each level or block takes the general form idealized in Figure 4.1(b) (except for the lowest level). Knowledge gained from analyses and tests in a previous level is combined with structural requirements and used to define and perform the next level of design and analysis. If an acceptable analytical result is not obtained, a structural redesign and/or analysis modification is made until the result is favorable. Once an acceptable analytical result is achieved, it is verified by test. If the test results do not meet the expectations predicted by analysis, the test may be redesigned if an erroneous mode was detected, or the design and/or analytic method may be modified. Additionally, tests or analyses in a previous level may be repeated for verification. The appropriate actions are taken until test results verify an acceptable analytical prediction. When this has been accomplished, the program has advanced to the next level of complexity. It is important to recognize that, since different programs have varying needs, requirements, and constraints, not all building block approaches use the same number of complexity levels or define these levels in the same way. FIGURE 4.1(b) Idealized general building block schematic (one level). Figures 4.1(a) and 4.1(b) and related discussions convey the idea that the Building Block process is a series of steps that progress neatly in order from one block to the next. While this is a convenient way to idealize the Building Block concept, the process is not quite so linear in practice. In reality, program schedules and availability of resources may be such that portions of various blocks overlap in time, and may even occur in parallel. Figure 4.1(c) shows one example of a typical Building Block program flow. The discussion of Building Block levels that follows relates to the idealized model for simplicity. At the lowest Building Block level, small specimen and element tests are most widely used to characterize basic unnotched static material properties, generic notch sensitivity, environmental factors, material operational limit (MOL – see Volume 1, Chapter 2, Section 2.2.8), and laminate fatigue response. In the case of this first level, testing is used for starting the Building Block process by providing data for first it-
MIL-HDBK-3F Volume 3,Chapter 4 Building Block Approach for Composite Structures eration design and analysis.Analysis at this level generally consists of developing material scatter factors and material allowables,evaluation of specimen failure modes,and preliminary laminate analysis.At the same time,external loads for the structure are being defined and initial sizing is being performed. Tests Lamina,Core. Adhesive, Element Fatigue Tests Curve Shapes Analyses & Verify Curve Shapes Generic Environmental Factors Design Laminate Static Fatigue Tests ----Other Statistical Major Component Analysis Material Tests/Strain Verification Allowables Structural Analysis including FEA Full Scale .Loads Design Test Laminate Allowables Analysis Element Tests Structural Structural Prior Design Certification Knowledge 、Rough Sizing Geometry Preliminary Design Basic Data Development Detailed Design (testing with without damage) FIGURE 4.1(c)Typical building block program flow. Analysis in the second level uses the basic information obtained at the first level to calculate internal loads,identify critical areas,and predict critical failure modes.More complex element and sub- component tests are designed to isolate single failure modes and verify analysis predictions.At subse- quent levels,even more complex static and fatigue loadings are analyzed and verified,with particular at- tention directed toward assessing out-of-plane loads and identifying unanticipated failure modes.Vari- abilities introduced by scale-up and response of the structure as a whole are also addressed.The final Building Block level involves full scale static and fatigue testing (as required).This testing validates pre- dicted internal loads.deflections.and failure modes of the entire structure.It also serves to verify that no significant unpredicted secondary loads have appeared. During the entire Building Block process,manufacturing quality is continually monitored to assure that properties developed early in the program remain valid.One aspect of this activity might include process cycle surveys to verify that larger components experience process histories similar to those of smaller elements and specimens.Also,non-destructive inspections,such as ultrasonic testing,are generally used to assess laminate quality with respect to porosity and voids.Destructive tests might also be used to verify fiber volumes,fiber alignment,and the like. As noted earlier,the details of applying the Building Block approach are not standardized.While rela- tionships between numbers of specimens and material basis values are well defined for specimen tests at the lowest level (see Volume 1,Chapter 2,Section 2.2.5),the numbers of specimens used at higher lev- els of complexity are somewhat arbitrary and largely based on historical experience,structural criticality, engineering judgment,and economics.Thus,there is currently no standardized methodology for statisti- cally validating each level of the process,though some attempts have been made to develop models that relate specimen quantities to overall reliability(Reference 4.1).Also,there is no universal approach to 4-3
MIL-HDBK-3F Volume 3, Chapter 4 Building Block Approach for Composite Structures 4-3 eration design and analysis. Analysis at this level generally consists of developing material scatter factors and material allowables, evaluation of specimen failure modes, and preliminary laminate analysis. At the same time, external loads for the structure are being defined and initial sizing is being performed. FIGURE 4.1(c) Typical building block program flow. Analysis in the second level uses the basic information obtained at the first level to calculate internal loads, identify critical areas, and predict critical failure modes. More complex element and subcomponent tests are designed to isolate single failure modes and verify analysis predictions. At subsequent levels, even more complex static and fatigue loadings are analyzed and verified, with particular attention directed toward assessing out-of-plane loads and identifying unanticipated failure modes. Variabilities introduced by scale-up and response of the structure as a whole are also addressed. The final Building Block level involves full scale static and fatigue testing (as required). This testing validates predicted internal loads, deflections, and failure modes of the entire structure. It also serves to verify that no significant unpredicted secondary loads have appeared. During the entire Building Block process, manufacturing quality is continually monitored to assure that properties developed early in the program remain valid. One aspect of this activity might include process cycle surveys to verify that larger components experience process histories similar to those of smaller elements and specimens. Also, non-destructive inspections, such as ultrasonic testing, are generally used to assess laminate quality with respect to porosity and voids. Destructive tests might also be used to verify fiber volumes, fiber alignment, and the like. As noted earlier, the details of applying the Building Block approach are not standardized. While relationships between numbers of specimens and material basis values are well defined for specimen tests at the lowest level (see Volume 1, Chapter 2, Section 2.2.5), the numbers of specimens used at higher levels of complexity are somewhat arbitrary and largely based on historical experience, structural criticality, engineering judgment, and economics. Thus, there is currently no standardized methodology for statistically validating each level of the process, though some attempts have been made to develop models that relate specimen quantities to overall reliability (Reference 4.1). Also, there is no universal approach to
MIL-HDBK-3F Volume 3,Chapter 4 Building Block Approach for Composite Structures the types of analyses or tests,as these may be highly dependent upon particular design details,loadings, and structural criticality. While it is certainly desirable to standardize the Building Block approach and to develop methods for assigning statistical reliability to the process,these goals are viewed as fairly long term,given the current body of work and diversity of individual approaches.The purpose of this chapter is to summarize the most prevalent and widely accepted methodology,and to present examples of Building Block programs for various applications and material forms. This section has presented an introduction to the concept of a building block approach.The rationale and assumptions required in developing a building block approach are described in Section 4.2.The general methodology of such an approach is described in Section 4.3.An example describing the use of the building block approach for EMD and production aircraft,processed using autoclave cure of prepreg. is presented in detail in Section 4.4.Section 4.5 includes the use of the building block approach for other applications with general descriptions and references to the more detailed example.The implications of using other types of processing and material forms are discussed in the final section. 4.2 RATIONALE AND ASSUMPTIONS The Building Block approach has been used in aircraft structures development programs long before the application of composites.However,this approach is more crucial for the certification of composite structures because of issues such as sensitivity to out-of-plane loads,their multiplicity of potential failure modes,and their sensitivity to operating environment.The combination of these issues and an inherent defect sensitivity of the composites,which are best classified as quasi-brittle,has resulted in a lack of analytical tools to predict the behavior of full-scale structure from the lowest level material properties. The multiplicity of potential failure modes is perhaps the main reason that the Building Block ap- proach is essential in the development of composite structural substantiation.The many failure modes in composite structures are mainly due to the defect,environmental and out-of-plane sensitivities of the materials. The low interlaminar strength of composites makes them sensitive to out-of-plane loads.Out-of- plane loads can arise directly or be induced from in-plane loads.The most difficult loads to design and analyze for are those loads which arise insidiously in full-scale built-up structures.Analysis tools currently available for structural engineers often assume these loads as secondary loads and they are usually simulated with lower degrees of accuracy.Therefore,it is very important to simulate all potential out-of- plane failure modes and obtain experimental data through a well planned Building Block testing program. Simulation of the correct failure modes plays an important role in a Building Block testing program. Since failure modes are frequently dependent on the test environment and defects present(manufactur- ing,bad design detail,or accidental damage),it is important to carefully select the correct test specimens that will simulate the desired failure modes.Special attention should be given to matrix sensitive failure modes.Following selection of the critical failure modes,a series of specimens is designed,each one to simulate a single failure mode.These specimens will generally be lower complexity specimens. Ideally,if structural analysis tools are fully developed and the failure criteria fully established,the structural behavior would be predictable from the constituent properties.Unfortunately,the capability of the state-of-the-art analysis methods are limited.Thus,lower level test data can not always be used to accurately predict the behavior of structural elements and components with higher levels of complexity. The accuracy of the analytical results are further complicated by the material property variability,the in- clusion of defects,and the structural scale-up effects.Therefore,step-by-step building block testings are required to: Uncover failure modes which do not occur at a lower level tests 4-4
MIL-HDBK-3F Volume 3, Chapter 4 Building Block Approach for Composite Structures 4-4 the types of analyses or tests, as these may be highly dependent upon particular design details, loadings, and structural criticality. While it is certainly desirable to standardize the Building Block approach and to develop methods for assigning statistical reliability to the process, these goals are viewed as fairly long term, given the current body of work and diversity of individual approaches. The purpose of this chapter is to summarize the most prevalent and widely accepted methodology, and to present examples of Building Block programs for various applications and material forms. This section has presented an introduction to the concept of a building block approach. The rationale and assumptions required in developing a building block approach are described in Section 4.2. The general methodology of such an approach is described in Section 4.3. An example describing the use of the building block approach for EMD and production aircraft, processed using autoclave cure of prepreg, is presented in detail in Section 4.4. Section 4.5 includes the use of the building block approach for other applications with general descriptions and references to the more detailed example. The implications of using other types of processing and material forms are discussed in the final section. 4.2 RATIONALE AND ASSUMPTIONS The Building Block approach has been used in aircraft structures development programs long before the application of composites. However, this approach is more crucial for the certification of composite structures because of issues such as sensitivity to out-of-plane loads, their multiplicity of potential failure modes, and their sensitivity to operating environment. The combination of these issues and an inherent defect sensitivity of the composites, which are best classified as quasi-brittle, has resulted in a lack of analytical tools to predict the behavior of full-scale structure from the lowest level material properties. The multiplicity of potential failure modes is perhaps the main reason that the Building Block approach is essential in the development of composite structural substantiation. The many failure modes in composite structures are mainly due to the defect, environmental and out-of-plane sensitivities of the materials. The low interlaminar strength of composites makes them sensitive to out-of-plane loads. Out-ofplane loads can arise directly or be induced from in-plane loads. The most difficult loads to design and analyze for are those loads which arise insidiously in full-scale built-up structures. Analysis tools currently available for structural engineers often assume these loads as secondary loads and they are usually simulated with lower degrees of accuracy. Therefore, it is very important to simulate all potential out-ofplane failure modes and obtain experimental data through a well planned Building Block testing program. Simulation of the correct failure modes plays an important role in a Building Block testing program. Since failure modes are frequently dependent on the test environment and defects present (manufacturing, bad design detail, or accidental damage), it is important to carefully select the correct test specimens that will simulate the desired failure modes. Special attention should be given to matrix sensitive failure modes. Following selection of the critical failure modes, a series of specimens is designed, each one to simulate a single failure mode. These specimens will generally be lower complexity specimens. Ideally, if structural analysis tools are fully developed and the failure criteria fully established, the structural behavior would be predictable from the constituent properties. Unfortunately, the capability of the state-of-the-art analysis methods are limited. Thus, lower level test data can not always be used to accurately predict the behavior of structural elements and components with higher levels of complexity. The accuracy of the analytical results are further complicated by the material property variability, the inclusion of defects, and the structural scale-up effects. Therefore, step-by-step building block testings are required to: 1 Uncover failure modes which do not occur at a lower level tests
MIL-HDBK-3F Volume 3,Chapter 4 Building Block Approach for Composite Structures 2 Verify or modify analysis methods which has been already verified at a lower level. 3 Allow inclusion of the defects in configured structure,which often do not take the same form in speci- mens and elements (e.g.,accidental damage caused by impact). This approach is based on the assumption that the structural/material response to applied load in test specimens with lower levels of complexity is directly transferable to specimens at higher levels of com- plexity.For example,fiber strength at the specimen level is the same as the fiber strength in the compo- nent.It is also implied that their variability is transferable upward.Thus,a statistical knockdown deter- mined from coupon tests (allowables)provides the same level of confidence at the structural component level. In a successful Building Block testing program,therefore,specimens can be designed so that failure modes at the lower level of structural complexity would be eliminated at the more complex specimens,by using verified design/analysis methods.Thus,the new failure modes at the next higher level of structural complexity can be isolated.The results of the more complex tests would be used to further modify/verify the analysis methods.Finally,an adequate analysis of methodology is verified and final design can be achieved. 4.3 METHODOLOGY In Section 4.1,Introduction and Philosophy,the Building Block Approach is introduced and the phi- losophical framework behind it are discussed,whereas,the Rationale and Assumptions in Section 4.2 provide a logical framework to guide the use of this approach while providing the key assumptions used. However,the Methodology used in performing a building block composite structure development program can spell success or failure in the effort.This section will discuss such Methodology,providing guidelines for its selection and use.The following discussion will present and discuss the methodology used in "building block composite structures development"for various vehicle applications.While there are some differences in methodology among these vehicle types,much of it is similar. 4.3.1 General approach The methodology used is shown in a generally logical,chronological order,but,during an actual vehi- cle "building block composite structure development"program,the start and completion of the methodol- ogy stages may overlap or not be in the order discussed herein.In such development programs in the real world,preliminary design/analysis of parts and elements and subcomponents may be accomplished using preliminary or estimated allowables.Element and/or subcomponent testing may be started or com- pleted before "design-to"allowables are available.But,"design-to"allowables should be completed be- fore full-scale component testing starts. The first step is to plan and initiate a suitable composite materials design allowables specimen test program on each composite material to be used.The number of material lots and the number of repli- cates required per type and environment will depend on whether the vehicle being developed is a proto- type,intermediate development(EMD),or production.In addition,the vehicle's structure criticality within its vehicle category (for instance,Aircraft,Spacecraft,Helicopter,Ground Vehicle,etc.)will affect the number of material lots and specimen replicates per test type and environment. The materials receiving inspection and acceptance requirements and the Materials Processes specification requirements will be a function of the structure criticality of the various parts of the selected vehicle.The number and kind of physical,mechanical,thermal,chemical,electrical and process proper- ties tests on the composite material will be a function of this structure criticality. The amount and level of quality assurance required on the test elements and subcomponents,as well as on the actual parts for the vehicle,is a function of the structure criticality of those parts and defect con- 4-5
MIL-HDBK-3F Volume 3, Chapter 4 Building Block Approach for Composite Structures 4-5 2 Verify or modify analysis methods which has been already verified at a lower level. 3 Allow inclusion of the defects in configured structure, which often do not take the same form in specimens and elements (e.g., accidental damage caused by impact). This approach is based on the assumption that the structural/material response to applied load in test specimens with lower levels of complexity is directly transferable to specimens at higher levels of complexity. For example, fiber strength at the specimen level is the same as the fiber strength in the component. It is also implied that their variability is transferable upward. Thus, a statistical knockdown determined from coupon tests (allowables) provides the same level of confidence at the structural component level. In a successful Building Block testing program, therefore, specimens can be designed so that failure modes at the lower level of structural complexity would be eliminated at the more complex specimens, by using verified design/analysis methods. Thus, the new failure modes at the next higher level of structural complexity can be isolated. The results of the more complex tests would be used to further modify/verify the analysis methods. Finally, an adequate analysis of methodology is verified and final design can be achieved. 4.3 METHODOLOGY In Section 4.1, Introduction and Philosophy, the Building Block Approach is introduced and the philosophical framework behind it are discussed, whereas, the Rationale and Assumptions in Section 4.2 provide a logical framework to guide the use of this approach while providing the key assumptions used. However, the Methodology used in performing a building block composite structure development program can spell success or failure in the effort. This section will discuss such Methodology, providing guidelines for its selection and use. The following discussion will present and discuss the methodology used in “building block composite structures development” for various vehicle applications. While there are some differences in methodology among these vehicle types, much of it is similar. 4.3.1 General approach The methodology used is shown in a generally logical, chronological order, but, during an actual vehicle “building block composite structure development” program, the start and completion of the methodology stages may overlap or not be in the order discussed herein. In such development programs in the real world, preliminary design/analysis of parts and elements and subcomponents may be accomplished using preliminary or estimated allowables. Element and/or subcomponent testing may be started or completed before “design-to” allowables are available. But, “design-to” allowables should be completed before full-scale component testing starts. The first step is to plan and initiate a suitable composite materials design allowables specimen test program on each composite material to be used. The number of material lots and the number of replicates required per type and environment will depend on whether the vehicle being developed is a prototype, intermediate development (EMD), or production. In addition, the vehicle’s structure criticality within its vehicle category (for instance, Aircraft, Spacecraft, Helicopter, Ground Vehicle, etc.) will affect the number of material lots and specimen replicates per test type and environment. The materials receiving inspection and acceptance requirements and the Materials & Processes specification requirements will be a function of the structure criticality of the various parts of the selected vehicle. The number and kind of physical, mechanical, thermal, chemical, electrical and process properties tests on the composite material will be a function of this structure criticality. The amount and level of quality assurance required on the test elements and subcomponents, as well as on the actual parts for the vehicle, is a function of the structure criticality of those parts and defect con-
MIL-HDBK-3F Volume 3,Chapter 4 Building Block Approach for Composite Structures siderations for structural substantiation and maintenance.In addition the type of tests selected,the num- ber of replicates,and instrumentation needed is a function of the part's structure criticality. Customer requirements and costs as well as safety and durability concerns may dictate the full scale testing requirements in addition to analytical prediction verification.Such full-scale testing could be proof loading to critical design limit load at RTD conditions,proof loading at various environmental conditions, static test to Design Limit Load (DLL)and Design Ultimate Load(DUL)at RT with or without load en- hancement factors to simulate elevated temperatures,and of course static loading to failure,in some cases.In addition,damage tolerance testing is often required to ensure safety for flight critical structure. Durability(fatigue)testing is sometimes required in severe environments and may be required to prove- out long term acceptable economic lifetimes. The individual methodologies discussed above are,in many cases,available within the companies doing the development work,or,are readily available at a specialty subcontractor.It is usually a matter of organizing such methodologies in a rational manner to achieve an acceptable vehicle composite structure building block development program.Such methodologies are defined and organized in more detail in the individual vehicle type subsections listed below. 4.4 CONSIDERATIONS FOR SPECIFIC APPLICATIONS 4.4.1 Aircraft for prototypes A detailed description of the allowables and building block test effort needed for acceptable risk and cost effective DOD/NASA prototype composite aircraft structure is presented in the following sections Section 4.4.1.1 presents the PMC composite allowables generation for DOD/NASA prototype aircraft structure.In Section 4.4.1.2,the PMC composites building block structural development for DOD/NASA prototype aircraft is detailed.And,finally,a summary of allowables and building block test efforts for DOD/NASA prototype composite aircraft structure is given in Section 4.4.1.3. 4.4.1.1 PMC composite allowables generation for DOD/NASA prototype aircraft structure Allowables generation is needed to support the building block test program depicted in Figure 4.4.1.1, Part A consists of five steps: 1.Experimentally generate ply level static strength and stiffness properties including the testing of 0 or 1-axis tension and compression,90 or 2-axis tension and compression and 0or 12-axis in-plane shear specimens with stress/strain curves utilizing,to the extent possible,ASTM D 3039, D3410,andD3518. 2.Experimentally generate quasi-isotropic laminate level,static strength and stiffness properties in- cluding the testing of x-axis plain and open hole tension,compression,and in-plane shear speci- mens and tension and compression loaded double shear bearing specimens per ASTM D 3039 for tension and compression and bearing specimens per other standards,respectively,that are currently under development in the ASTM D-30 Committee. 3.The test data generated will be reduced,statistically,to obtain allowable type values using the B-basis value(90%probability,95%confidence)approach or the 85%of mean value approach if the test scatter is too high.The higher of the two values should be used.This approach was first presented by Grimes in Reference 4.4.1.1. 4.Develop input ply allowables for use in analytical methods that are used in design/analysis.In general the lower of the ultimate or 1.5 x yield strength reduced value should be used for tension, compression,and in-plane shear strength critical allowables.When in-plane shear strength is not critical the reduced ultimate shear strength(a high value)should be used. 4-6
MIL-HDBK-3F Volume 3, Chapter 4 Building Block Approach for Composite Structures 4-6 siderations for structural substantiation and maintenance. In addition the type of tests selected, the number of replicates, and instrumentation needed is a function of the part’s structure criticality. Customer requirements and costs as well as safety and durability concerns may dictate the full scale testing requirements in addition to analytical prediction verification. Such full-scale testing could be proof loading to critical design limit load at RTD conditions, proof loading at various environmental conditions, static test to Design Limit Load (DLL) and Design Ultimate Load (DUL) at RT with or without load enhancement factors to simulate elevated temperatures, and of course static loading to failure, in some cases. In addition, damage tolerance testing is often required to ensure safety for flight critical structure. Durability (fatigue) testing is sometimes required in severe environments and may be required to proveout long term acceptable economic lifetimes. The individual methodologies discussed above are, in many cases, available within the companies doing the development work, or, are readily available at a specialty subcontractor. It is usually a matter of organizing such methodologies in a rational manner to achieve an acceptable vehicle composite structure building block development program. Such methodologies are defined and organized in more detail in the individual vehicle type subsections listed below. 4.4 CONSIDERATIONS FOR SPECIFIC APPLICATIONS 4.4.1 Aircraft for prototypes A detailed description of the allowables and building block test effort needed for acceptable risk and cost effective DOD/NASA prototype composite aircraft structure is presented in the following sections. Section 4.4.1.1 presents the PMC composite allowables generation for DOD/NASA prototype aircraft structure. In Section 4.4.1.2, the PMC composites building block structural development for DOD/NASA prototype aircraft is detailed. And, finally, a summary of allowables and building block test efforts for DOD/NASA prototype composite aircraft structure is given in Section 4.4.1.3. 4.4.1.1 PMC composite allowables generation for DOD/NASA prototype aircraft structure Allowables generation is needed to support the building block test program depicted in Figure 4.4.1.1, Part A consists of five steps: 1. Experimentally generate ply level static strength and stiffness properties including the testing of 0° or 1-axis tension and compression, 90° or 2-axis tension and compression and 0° or 12-axis in-plane shear specimens with stress/strain curves utilizing, to the extent possible, ASTM D 3039, D 3410, and D 3518. 2. Experimentally generate quasi-isotropic laminate level, static strength and stiffness properties including the testing of x-axis plain and open hole tension, compression, and in-plane shear specimens and tension and compression loaded double shear bearing specimens per ASTM D 3039 for tension and compression and bearing specimens per other standards, respectively, that are currently under development in the ASTM D-30 Committee. 3. The test data generated will be reduced, statistically, to obtain allowable type values using the B-basis value (90% probability, 95% confidence) approach or the 85% of mean value approach if the test scatter is too high. The higher of the two values should be used. This approach was first presented by Grimes in Reference 4.4.1.1. 4. Develop input ply allowables for use in analytical methods that are used in design/analysis. In general the lower of the ultimate or 1.5 x yield strength reduced value should be used for tension, compression, and in-plane shear strength critical allowables. When in-plane shear strength is not critical the reduced ultimate shear strength (a high value) should be used
MIL-HDBK-3F Volume 3,Chapter 4 Building Block Approach for Composite Structures 5.Laminate design should be fiber-dominated by definition,i.e.,a minimum of 10%of the plies should be in each of the0°，+45°，-45°，and90°directions.For tape and fabric laminates,always input the 0 or 1-axis strength allowable values in both the 1-and 2-axis slots in the analytical methods for tensile and compressive loads.Shear inputs will be as described above.This ap- proach will ensure fiber dominated failure and was first presented by Grimes in Reference 4.4.1.1.All laminates should be balanced and symmetric. NOT PART OF BBA 7777ZZZ22Q7 ALLOWABLES PartA ALLOW BLES COUPONS GENERATION BUILDING BLOCK KAPPROACH (BBA) ELEMENTS-SNGLE LOAD PATH B玉cTo2 TCAL SE OET ANO SUBCOMPONENTS-MULTPLE LOAD PATH COMPONENTS-CONTOURED,MULTIPLE LOAD PATH FULL-SCALE ARCRAFT STRUCTURE FIGURE 4.4.1.1 Aircraft structural development goals using building block approach(BBA). A structure classification/allowables chart which defines the relationship between aircraft structure criticality and the allowables requirements for prototypes is presented in Table 4.4.1.1(a).In Table 4.4.1.1(b)structural classification vs.physical defect maximum requirements are given so that the ac- ceptable physical defect size parameter varies indirectly with the aircraft structure criticality.Thus,aircraft structure criticality controls the reliability of the data(allowables)and the material and parts quality that are necessary to support it. 4-7
MIL-HDBK-3F Volume 3, Chapter 4 Building Block Approach for Composite Structures 4-7 5. Laminate design should be fiber-dominated by definition, i.e., a minimum of 10% of the plies should be in each of the 0°, +45°, -45°, and 90° directions. For tape and fabric laminates, always input the 0° or 1-axis strength allowable values in both the 1- and 2-axis slots in the analytical methods for tensile and compressive loads. Shear inputs will be as described above. This approach will ensure fiber dominated failure and was first presented by Grimes in Reference 4.4.1.1. All laminates should be balanced and symmetric. FIGURE 4.4.1.1 Aircraft structural development goals using building block approach (BBA). A structure classification/allowables chart which defines the relationship between aircraft structure criticality and the allowables requirements for prototypes is presented in Table 4.4.1.1(a). In Table 4.4.1.1(b) structural classification vs. physical defect maximum requirements are given so that the acceptable physical defect size parameter varies indirectly with the aircraft structure criticality. Thus, aircraft structure criticality controls the reliability of the data (allowables) and the material and parts quality that are necessary to support it
MIL-HDBK-3F Volume 3,Chapter 4 Building Block Approach for Composite Structures TABLE 4.4.1.1(a)DOD/NASA aircraft structure classification vs.PMC allowables data requirements for prototypes(Reference 4.4.1.1). PART A (From Figure 4.4.1.1) Aircraft Structure Classification Allowable Data Requirements for Prototype Design Classification Description Preliminary(Tape/Fabric) Final (Tape/Fabric) PRIMARY CARRIES PRIMARY AIR LOADS Based on ·Fracture critical Failure will cause loss of 1. Estimates using data on 1-lot materials testing:5 to 8 (F1C) vehicle similar materials and replicates per test type(static) experienc8..… Noncritical(N/C) Failure will not cause loss of 2 Vendor Data 1-lot materials testing:4 to 6 vehicle 3. Journals,magazines and replicates per test type(static) books SECONDARY CARRIES SECONDARY AIR Based on OTHER LOADS ·Fatigue critical Failure will not cause loss of 1. Estimates using data on same 1-lot materials testing:3 to 4 (FA/C)&economic vehicle but may cause cost or similar materials replicates per test type (static) life critical (EC) critical parts replacements plus fatigue testing ·Noncritical(N/C) Failure will not cause loss of 2.Vendor data Use legitimate,verified data vehicle 3. Journals,magazines and bases No cost or fatigue critical parts books NONSTRUCTURAL NON-OR MINOR LOAD Based on BEARING ·Noncritical(N/C) Failure replacement of parts 1.Estimates using data on Estimates using data on similar causing minor inconvenience, similar materials,or materials,or not cost critical 2. Vendor data,or Vendor data,or 3. Journals,magazines and Journals,magazines and books books 4-8
MIL-HDBK-3F Volume 3, Chapter 4 Building Block Approach for Composite Structures 4-8 TABLE 4.4.1.1(a) DOD/NASA aircraft structure classification vs. PMC allowables data requirements for prototypes (Reference 4.4.1.1). PART A (From Figure 4.4.1.1) Aircraft Structure Classification Allowable Data Requirements for Prototype Design Classification Description Preliminary (Tape/Fabric) Final (Tape/Fabric) PRIMARY CARRIES PRIMARY AIR LOADS Based on • Fracture critical (F/C) • Failure will cause loss of vehicle 1. Estimates using data on similar materials and experience 1 - lot materials testing: 5 to 8 replicates per test type (static) • Noncritical (N/C) • Failure will not cause loss of vehicle 2. Vendor Data 3. Journals, magazines and books 1 - lot materials testing: 4 to 6 replicates per test type (static) SECONDARY CARRIES SECONDARY AIR & OTHER LOADS Based on • Fatigue critical (FA/C) & economic life critical (EL/C) • Failure will not cause loss of vehicle but may cause cost critical parts replacements 1. Estimates using data on same or similar materials 1 - lot materials testing: 3 to 4 replicates per test type (static) plus fatigue testing • Noncritical (N/C) • Failure will not cause loss of vehicle • No cost or fatigue critical parts 2. Vendor data 3. Journals, magazines and books Use legitimate, verified data bases NONSTRUCTURAL NON- OR MINOR LOAD BEARING Based on • Noncritical (N/C) • Failure replacement of parts causing minor inconvenience, 1. Estimates using data on similar materials, or Estimates using data on similar materials, or not cost critical 2. Vendor data, or Vendor data, or 3. Journals, magazines and books Journals, magazines and books
MIL-HDBK-3F Volume 3,Chapter 4 Building Block Approach for Composite Structures TABLE 4.4.1.1(b)DOD/NASA aircraft structure classification vs.PMC physical defect minimum requirements for prototypes (Reference 4.4.1.1). PART AAND B (From Figure 4.4.1.1) Aircraft Structure Physical Defect Maximum Requirements for Parts:Carbon or Glass Reinforced PMC Example Classification Description Tape Fabric PRIMARY CARRIES PRIMARY AIR LOADS s3%porosity over s10%of area. ≤5%porosity over≤10%of area. ·Fracture critical Failure will cause loss of vehicle Delaminations over s1%of area. Delaminations over s1%of area. (F/C) No edge delaminations allowed No edge delaminations allowed (including holes). (including holes). ·Noncritical(N/C) Failure will not cause loss of vehicle SECONDARY CARRIES SECONDARY AIR ≤3%porosity over≤15%of area. <5%porosity over s15%of area. OTHER LOADS Delaminations over s2%of area. Delaminations over s2%of area. Fatigue critical Failure will not cause loss of No edge delaminations allowed No edge delaminations allowed (FA/C)& vehicle but may cause cost (including holes). (including holes). economic life critical parts replacements critical(EL/C) Noncritical(N/C) Failure will not cause loss of vehicle No cost or fatigue critical parts NONSTRUCTURAL NON-OR MINOR LOAD s4%porosity over s20%of area. s4%porosity over s20%of area. BEARING Noncritical (N/C) Failure replacement of parts Delaminations over s3%of area. Delaminations over s3%of area. causing minor inconvenience. Repaired edge delaminations Repaired edge delaminations not cost critical s10%of edge length or hole <10%of edge length or hole circumference are allowed. circumference are allowed. 4-9
MIL-HDBK-3F Volume 3, Chapter 4 Building Block Approach for Composite Structures 4-9 TABLE 4.4.1.1(b) DOD/NASA aircraft structure classification vs. PMC physical defect minimum requirements for prototypes (Reference 4.4.1.1). PART A AND B (From Figure 4.4.1.1) Aircraft Structure Physical Defect Maximum Requirements for Parts: Carbon or Glass Reinforced PMC Example Classification Description Tape Fabric PRIMARY CARRIES PRIMARY AIR LOADS ≤3% porosity over ≤10% of area. ≤5% porosity over ≤10% of area. • Fracture critical (F/C) • Failure will cause loss of vehicle Delaminations over ≤1% of area. No edge delaminations allowed (including holes). Delaminations over ≤1% of area. No edge delaminations allowed (including holes). • Noncritical (N/C) • Failure will not cause loss of vehicle SECONDARY CARRIES SECONDARY AIR & OTHER LOADS ≤3% porosity over ≤15% of area. Delaminations over ≤2% of area. ≤5% porosity over ≤15% of area. Delaminations over ≤2% of area. • Fatigue critical (FA/C) & economic life critical (EL/C) • Failure will not cause loss of vehicle but may cause cost critical parts replacements No edge delaminations allowed (including holes). No edge delaminations allowed (including holes). • Noncritical (N/C) • Failure will not cause loss of vehicle • No cost or fatigue critical parts NONSTRUCTURAL NON- OR MINOR LOAD BEARING ≤4% porosity over ≤20% of area. ≤4% porosity over ≤20% of area. • Noncritical (N/C) • Failure replacement of parts causing minor inconvenience, not cost critical Delaminations over ≤3% of area. Repaired edge delaminations ≤10% of edge length or hole circumference are allowed. Delaminations over ≤3% of area. Repaired edge delaminations ≤10% of edge length or hole circumference are allowed
MIL-HDBK-3F Volume 3,Chapter 4 Building Block Approach for Composite Structures 4.4.1.2 PMC composites building block structural development for DOD/NASA prototype aircraft Part B of the flowchart in Figure 4.4.1.1 defines the building block test effort in the general categories of: 1.Trade studies and concept development(element-single load path), 2.Selection,proof of concept,and analytical methods verification (sub-component-multiple load paths), 3. Structural verification and analytical methods improvement(contoured composite-multiple load path),and 4.Structural integrity and FEM validation(full-scale aircraft structure testing). The allowables shown in Figure 4.4.1.1 Part A and in Table 4.4.1.1 (a)logically flow into Part B,building block testing.Table 4.4.1.1(b)on physical defect requirements applies to both Parts A and B.The Part B building block test effort is delineated in Table 4.4.1.2(a)in accordance with the part's structural classifica- tion.The four categories,above,are defined in detail for each structural classification,with the higher the structural classification,the more testing and analysis required.The key point here is that these are guidelines for structural development testing.The actual structural testing needed for a specific classifica- tion of structure could be more or less,depending on the vehicle's mission and whether it is manned or unmanned.Knowing the structural part classification,the aircraft's purpose and mission,risk analysis can be applied to minimize testing cost and risk.FEM and closed form composite analysis methods utilizing proper mechanical and physical properties and allowables input data will be necessary every step of the way.Failure modes and loads(stresses)as well as strain and deflection readings must be monitored and correlated with predictions to assure low risk.The use of FEM or other analysis methods alone (without testing)or with inadequate testing that does not properly interrogate failure modes,stresses(strains),and deflections for comparison with predictions can create high risk situations that should not be tolerated. Another risk issue for composite structure is quality assurance (QA),a subject that applies to both Parts A and B.Table 4.4.1.2(b)presents the nominal QA requirements for the categories of 1.Material and process selection,screening,and material specification qualification, 2.Receiving inspection/acceptance testing, 3. In-process inspection, 4. Non-destructive inspection(NDI), 5. Destructive testing (DT),and 6. Traceability The QA requirements in each of these categories vary with the structural classification,with the higher the classification,the more quality assurance required.By following the procedure outlined in this table,the amount of QA necessary to keep risk at an acceptable level can be ascertained.Again the amount of QA needed and the risk taken will be a function of the aircraft type and mission and whether it is manned or unmanned.Risk and cost are inversely proportional to each other for composite structural parts in each classification,so the determination of acceptable risk is necessary to this building block test program for prototypes. 4-10
MIL-HDBK-3F Volume 3, Chapter 4 Building Block Approach for Composite Structures 4-10 4.4.1.2 PMC composites building block structural development for DOD/NASA prototype aircraft Part B of the flowchart in Figure 4.4.1.1 defines the building block test effort in the general categories of: 1. Trade studies and concept development (element-single load path), 2. Selection, proof of concept, and analytical methods verification (sub-component-multiple load paths), 3. Structural verification and analytical methods improvement (contoured composite-multiple load path), and 4. Structural integrity and FEM validation (full-scale aircraft structure testing). The allowables shown in Figure 4.4.1.1 Part A and in Table 4.4.1.1 (a) logically flow into Part B, building block testing. Table 4.4.1.1(b) on physical defect requirements applies to both Parts A and B. The Part B building block test effort is delineated in Table 4.4.1.2(a) in accordance with the part's structural classification. The four categories, above, are defined in detail for each structural classification, with the higher the structural classification, the more testing and analysis required. The key point here is that these are guidelines for structural development testing. The actual structural testing needed for a specific classification of structure could be more or less, depending on the vehicle's mission and whether it is manned or unmanned. Knowing the structural part classification, the aircraft's purpose and mission, risk analysis can be applied to minimize testing cost and risk. FEM and closed form composite analysis methods utilizing proper mechanical and physical properties and allowables input data will be necessary every step of the way. Failure modes and loads (stresses) as well as strain and deflection readings must be monitored and correlated with predictions to assure low risk. The use of FEM or other analysis methods alone (without testing) or with inadequate testing that does not properly interrogate failure modes, stresses (strains), and deflections for comparison with predictions can create high risk situations that should not be tolerated. Another risk issue for composite structure is quality assurance (QA), a subject that applies to both Parts A and B. Table 4.4.1.2(b) presents the nominal QA requirements for the categories of 1. Material and process selection, screening, and material specification qualification, 2. Receiving inspection/acceptance testing, 3. In-process inspection, 4. Non-destructive inspection (NDI), 5. Destructive testing (DT), and 6. Traceability The QA requirements in each of these categories vary with the structural classification, with the higher the classification, the more quality assurance required. By following the procedure outlined in this table, the amount of QA necessary to keep risk at an acceptable level can be ascertained. Again the amount of QA needed and the risk taken will be a function of the aircraft type and mission and whether it is manned or unmanned. Risk and cost are inversely proportional to each other for composite structural parts in each classification, so the determination of acceptable risk is necessary to this building block test program for prototypes