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