《纺织复合材料》课程参考文献（Composite Materials Handbook，Volume 3）Chapter 4 Building Block Approach for Composite Structures

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 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. 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

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 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. 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-

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 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

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 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: 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 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-