《纺织复合材料》课程参考文献（Composite Materials Handbook，Volume 3）Chapter 8 Supportability

MIL-HDBK-17-3F Volume 3,Chapter 8 Supportability TABLE 8.2.3.5 Material support issues. Issue Support Impact Autoclave only cure 1.Equipment not available in the field and at small repair facilities 2.Part has to be removed and disassembled for repair Press curing 1.Equipment not available in the field and at small repair facilities.Part has to be removed and disassembled for repair High temperature 1.Damage to surrounding structure in repair on aircraft cure 2.Protective equipment needed to handle high temperatures Transit 1.Dry ice packing requirements may be problematic Freezer storage 1.Equipment not available not available in the field and at small required repair facilities 8.2.4 Damage resistance,damage tolerance,and durability In normal operating condition,components can be expected to be subject to potential damage from sources such as maintenance personnel,tools,runway debris,service equipment,hail,lightning,etc. During initial manufacturing and assembly,these components may be subject to the same or similar con- ditions.To alleviate the effects of the expected damage.most composite components are designed to specific damage resistance,damage tolerance,and durability criteria.How these design criteria affect supportability are discussed in this section.(ldeally,a supportable airframe structure must be able to sus- tain a reasonable level of damaging incidents without costly rework or downtime.Sustainability is being defined as showing no damage after such incidents and having the required residual strength and stiff- ness capability.) 8.2.4.1 Damage resistance Damage resistance is a measure of the relationship between the force or energy associated with a damage event and the resulting damage size and type of damage.A material or structure with high dam- age resistance will incur less physical damage from a given event.For composite airframe structures repair actions are based on visibility,hence,if the damage is not visible,a repair activity is not needed. Therefore,to reduce repair activity,damage resistance levels should be such that at low impact energy levels(4 ft-lb)the damage is not visible and is negligible in high susceptibility areas.This can be accom- plished by zoning the structure based on regions that have high or low susceptibility to damage and its residual strength and stiffness requirements.In defining the requirements,the type of structure(primary or secondary structures),construction method(sandwich or solid laminate),and whether its a removable or non-removable structure are pertinent.In practice,damage resistance is a critical design parameter for supportability,particularly for thin-skinned components.A more detailed discussion of damage resistance can be found in Section 7.5. Damage resistance maybe improved by increasing laminate thicknesses and for sandwich applica- tions by using denser core.However,the decrease in visibility may lead to the increase in nonvisible 8-11

MIL-HDBK-17-3F Volume 3, Chapter 8 Supportability 8-11 TABLE 8.2.3.5 Material support issues. Issue Support Impact Autoclave only cure 1. Equipment not available in the field and at small repair facilities 2. Part has to be removed and disassembled for repair Press curing 1. Equipment not available in the field and at small repair facilities. Part has to be removed and disassembled for repair High temperature cure 1. Damage to surrounding structure in repair on aircraft 2. Protective equipment needed to handle high temperatures Transit 1. Dry ice packing requirements may be problematic Freezer storage required 1. Equipment not available not available in the field and at small repair facilities 8.2.4 Damage resistance, damage tolerance, and durability In normal operating condition, components can be expected to be subject to potential damage from sources such as maintenance personnel, tools, runway debris, service equipment, hail, lightning, etc. During initial manufacturing and assembly, these components may be subject to the same or similar con￾ditions. To alleviate the effects of the expected damage, most composite components are designed to specific damage resistance, damage tolerance, and durability criteria. How these design criteria affect supportability are discussed in this section. (Ideally, a supportable airframe structure must be able to sus￾tain a reasonable level of damaging incidents without costly rework or downtime. Sustainability is being defined as showing no damage after such incidents and having the required residual strength and stiff￾ness capability.) 8.2.4.1 Damage resistance Damage resistance is a measure of the relationship between the force or energy associated with a damage event and the resulting damage size and type of damage. A material or structure with high dam￾age resistance will incur less physical damage from a given event. For composite airframe structures repair actions are based on visibility, hence, if the damage is not visible, a repair activity is not needed. Therefore, to reduce repair activity, damage resistance levels should be such that at low impact energy levels (4 ft-lb) the damage is not visible and is negligible in high susceptibility areas. This can be accom￾plished by zoning the structure based on regions that have high or low susceptibility to damage and its residual strength and stiffness requirements. In defining the requirements, the type of structure (primary or secondary structures), construction method (sandwich or solid laminate), and whether its a removable or non-removable structure are pertinent. In practice, damage resistance is a critical design parameter for supportability, particularly for thin-skinned components. A more detailed discussion of damage resistance can be found in Section 7.5. Damage resistance maybe improved by increasing laminate thicknesses and for sandwich applica￾tions by using denser core. However, the decrease in visibility may lead to the increase in nonvisible

MIL-HDBK-17-3F Volume 3,Chapter 8 Supportability damage that must be considered for aspects of damage tolerance.The selection of reinforcement fibers that have high strain capability can also have a positive effect.Additionally,the selection of toughened matrix material can greatly enhance damage resistance.The selection of integrally stiffened panels over a honeycomb sandwich construction usually results in a more damage resistant configuration as the skin thicknesses are usually thicker and the impact energy is absorbed by the bending action for the integrally stiffened panel as compared to sandwiches.Possible water ingress into the sandwich panel after impact damage is another supportability drawback of sandwich construction. Other items to improve damage resistance include the use of a layer of fabric as the exterior ply over tape to resist scratches,abrasion,softening of impact,and reduction of fiber breakout during drilling of fastener holes.Laminate edges should not be exposed directly into the air stream that could possibly subject it to delaminations.Avoidance of delaminations is achieved by using non-erosive edge protection, replaceable sacrificial materials or locating the forward edge below the level of the aft edge of the next forward panel. Areas prone to high energy lightning strike should utilize replaceable conductive materials.provide protection at tips and trailing edge surfaces,and make all conductive path attachments easily accessible. 8.2.4.2 Damage tolerance Damage tolerance for structural parts is a measure of the ability of such a part to maintain functional- ity,sufficient residual strength and stiffness,with damage for required loadings.In aircraft design,dam- age tolerance is a safety issue but does affect supportability.A very damage tolerant structure will require large area repair capability,although it may be of low frequency.On the other hand,a structure that can tolerate only small damage sizes will require frequent repair actions. Damage tolerance is achieved by reducing allowable strain levels in damage and strength/stiffness critical areas and/or providing multiple load paths.For civil-aircraft composite parts,it is a requirement that the structure can sustain ultimate load with any damage less or equal to the barely visible size. Therefore,the designer of a structure highly resistant to surface damage has to make sure that the struc- ture is also damage tolerant to hidden damage.Larger damages have lesser load residual strength re- quirements.See Section 5.12.1 for a more detailed discussion. Except in areas where the margin of safety is near zero,a composite structure can tolerate larger than visible damage while still able to sustain ultimate load.A reduction in the number of repair actions would be possible if the part manufacturer provides a map of permissible damage sizes.Such a map would have to include not only the effect of static loads but of durability and functional requirements. 8.2.4.3 Durability Durability of the structure is its ability to maintain strength and stiffness throughout the service life of the structure.In general,structural durability is inversely related to maintenance cost.A durable structure is the one that does not incur excessive maintenance cost during its service life.A composite structure that was designed for damage resistance will have excellent durability as carbon composites have excel- lent corrosion resistance characteristics (assuming no galvanic corrosion)and fatigue characteristics when compared to metals. In composites,fatigue damage due to repeated mechanical loads usually initiates as cracks in the matrix material at laminate edges,notches,and stress discontinuities and then may progress as inter- laminar delaminations.For currently designed structures with low allowable strain levels,in part due to damage tolerance and repair requirements,the fatigue loads are generally below the levels that would cause extensive matrix cracking.One exception is in the vicinity of fastener holes,where,if the bearing stresses are high,hole elongation may cause bolt fatigue failures and other anomalies due to internal load redistribution.Thus,good supportability design should feature low bearing stresses (see Section 5.3.2.3).A general discussion on durability can be found in Section 5.12.2. 8-12

MIL-HDBK-17-3F Volume 3, Chapter 8 Supportability 8-12 damage that must be considered for aspects of damage tolerance. The selection of reinforcement fibers that have high strain capability can also have a positive effect. Additionally, the selection of toughened matrix material can greatly enhance damage resistance. The selection of integrally stiffened panels over a honeycomb sandwich construction usually results in a more damage resistant configuration as the skin thicknesses are usually thicker and the impact energy is absorbed by the bending action for the integrally stiffened panel as compared to sandwiches. Possible water ingress into the sandwich panel after impact damage is another supportability drawback of sandwich construction. Other items to improve damage resistance include the use of a layer of fabric as the exterior ply over tape to resist scratches, abrasion, softening of impact, and reduction of fiber breakout during drilling of fastener holes. Laminate edges should not be exposed directly into the air stream that could possibly subject it to delaminations. Avoidance of delaminations is achieved by using non-erosive edge protection, replaceable sacrificial materials or locating the forward edge below the level of the aft edge of the next forward panel. Areas prone to high energy lightning strike should utilize replaceable conductive materials, provide protection at tips and trailing edge surfaces, and make all conductive path attachments easily accessible. 8.2.4.2 Damage tolerance Damage tolerance for structural parts is a measure of the ability of such a part to maintain functional￾ity, sufficient residual strength and stiffness, with damage for required loadings. In aircraft design, dam￾age tolerance is a safety issue but does affect supportability. A very damage tolerant structure will require large area repair capability, although it may be of low frequency. On the other hand, a structure that can tolerate only small damage sizes will require frequent repair actions. Damage tolerance is achieved by reducing allowable strain levels in damage and strength/stiffness critical areas and/or providing multiple load paths. For civil-aircraft composite parts, it is a requirement that the structure can sustain ultimate load with any damage less or equal to the barely visible size. Therefore, the designer of a structure highly resistant to surface damage has to make sure that the struc￾ture is also damage tolerant to hidden damage. Larger damages have lesser load residual strength re￾quirements. See Section 5.12.1 for a more detailed discussion. Except in areas where the margin of safety is near zero, a composite structure can tolerate larger than visible damage while still able to sustain ultimate load. A reduction in the number of repair actions would be possible if the part manufacturer provides a map of permissible damage sizes. Such a map would have to include not only the effect of static loads but of durability and functional requirements. 8.2.4.3 Durability Durability of the structure is its ability to maintain strength and stiffness throughout the service life of the structure. In general, structural durability is inversely related to maintenance cost. A durable structure is the one that does not incur excessive maintenance cost during its service life. A composite structure that was designed for damage resistance will have excellent durability as carbon composites have excel￾lent corrosion resistance characteristics (assuming no galvanic corrosion) and fatigue characteristics when compared to metals. In composites, fatigue damage due to repeated mechanical loads usually initiates as cracks in the matrix material at laminate edges, notches, and stress discontinuities and then may progress as inter￾laminar delaminations. For currently designed structures with low allowable strain levels, in part due to damage tolerance and repair requirements, the fatigue loads are generally below the levels that would cause extensive matrix cracking. One exception is in the vicinity of fastener holes, where, if the bearing stresses are high, hole elongation may cause bolt fatigue failures and other anomalies due to internal load redistribution. Thus, good supportability design should feature low bearing stresses (see Section 5.3.2.3). A general discussion on durability can be found in Section 5.12.2

MIL-HDBK-17-3F Volume 3,Chapter 8 Supportability 8.2.5 Environmental compliance Many aspects of the design,repair and maintenance of polymer matrix composites are impacted by environmental rules and regulations.Many people associate environmental compliance with the correct disposal of hazardous wastes.This is certainly an important factor,but is by no means the only factor to consider.In fact,by the time we are concerned with the disposal of hazardous waste,we have missed a tremendous number of opportunities to reduce the generation of waste in the first place.The concept of reduction of hazardous waste before it is generated,known as pollution prevention,can begin as early as the initial design phase.It can greatly reduce labor,cost,and paperwork associated with the disposal of hazardous wastes generated by repair and maintenance of the component throughout its life cycle.This section will identify factors to consider during the design and repair design phase to facilitate true life cy- cle pollution prevention. 8.2.5.1 Elimination/reduction of heavy metals The requirement for heavy-metal containing coatings and treatments not only presents environmental compliance difficulties during manufacture,but presents additional challenges every time the coating needs to be removed,repaired or replaced.Traditional requirements for chromic acid anodizing or alodine processes impact mostly metal components,however,we encounter similar issues with polymer matrix composites as well.Typical culprits include cadmium plated fasteners,and chromated sealants and primers.When designers consider environmental compliance along with cost and quality when specifying design materials,we may be able to eliminate the specification of these materials in the first place. Non-chromated sealants and primers are currently available and research and development initiatives are underway to evaluate their suitability for long term use on military aircraft.The specification of a non- chromated primer or sealant in the design of a component will create benefits throughout its life cycle by reducing hazardous waste and personnel exposure to hazardous materials. 8.2.5.2 Consideration of paint removal requirements During the design of polymer matrix composite components,consideration must be given to removal of coatings.Chemical paint removers are not acceptable for most polymer matrix composites because the active ingredients that attack the organic coating also attack the matrix.Abrasive paint removal tech- niques,such as plastic media blasting.have proven successful on polymer matrix composites but their use can be limited by substrate thickness and specialized surface treatments or coatings.The considera- tion of paint removal techniques during manufacture may highlight minor changes in design that can af- fect major savings in maintenance over the life cycle of the component. 8.2.5.3 Shelf life and storage stability of repair materials A significant portion of a waste stream is made up of materials that cannot be used within their useful life.In its worst case,this involves materials that are purchased,sit on the shelf and are then disposed of as hazardous waste without ever making it to the work center.At best,it represents containers that have been opened but not finished before the shelf life expires.The following are some of the ways this waste stream can be minimized by design decisions. a)Specify common materials.Maintainers have difficulty "using up"materials if they are specific to a single aircraft or component.In many cases material manufacturers establish"minimum buys" of their product dictating the minimum purchase of several gallons when only a pint is required. The excess material often simply sits on the shelf until it is no longer useable and is then sent to disposal.This issue can be alleviated through the specification of materials that are already in the inventory,or that are used on a wide variety of components. b)Specify long shelf life,and/or room temperature storable materials.Obviously,the longer the shelf life of a product,and the less restrictive the storage,handling and transportation require- 8-13

MIL-HDBK-17-3F Volume 3, Chapter 8 Supportability 8-13 8.2.5 Environmental compliance Many aspects of the design, repair and maintenance of polymer matrix composites are impacted by environmental rules and regulations. Many people associate environmental compliance with the correct disposal of hazardous wastes. This is certainly an important factor, but is by no means the only factor to consider. In fact, by the time we are concerned with the disposal of hazardous waste, we have missed a tremendous number of opportunities to reduce the generation of waste in the first place. The concept of reduction of hazardous waste before it is generated, known as pollution prevention, can begin as early as the initial design phase. It can greatly reduce labor, cost, and paperwork associated with the disposal of hazardous wastes generated by repair and maintenance of the component throughout its life cycle. This section will identify factors to consider during the design and repair design phase to facilitate true life cy￾cle pollution prevention. 8.2.5.1 Elimination/reduction of heavy metals The requirement for heavy-metal containing coatings and treatments not only presents environmental compliance difficulties during manufacture, but presents additional challenges every time the coating needs to be removed, repaired or replaced. Traditional requirements for chromic acid anodizing or alodine processes impact mostly metal components, however, we encounter similar issues with polymer matrix composites as well. Typical culprits include cadmium plated fasteners, and chromated sealants and primers. When designers consider environmental compliance along with cost and quality when specifying design materials, we may be able to eliminate the specification of these materials in the first place. Non-chromated sealants and primers are currently available and research and development initiatives are underway to evaluate their suitability for long term use on military aircraft. The specification of a non￾chromated primer or sealant in the design of a component will create benefits throughout its life cycle by reducing hazardous waste and personnel exposure to hazardous materials. 8.2.5.2 Consideration of paint removal requirements During the design of polymer matrix composite components, consideration must be given to removal of coatings. Chemical paint removers are not acceptable for most polymer matrix composites because the active ingredients that attack the organic coating also attack the matrix. Abrasive paint removal tech￾niques, such as plastic media blasting, have proven successful on polymer matrix composites but their use can be limited by substrate thickness and specialized surface treatments or coatings. The considera￾tion of paint removal techniques during manufacture may highlight minor changes in design that can af￾fect major savings in maintenance over the life cycle of the component. 8.2.5.3 Shelf life and storage stability of repair materials A significant portion of a waste stream is made up of materials that cannot be used within their useful life. In its worst case, this involves materials that are purchased, sit on the shelf and are then disposed of as hazardous waste without ever making it to the work center. At best, it represents containers that have been opened but not finished before the shelf life expires. The following are some of the ways this waste stream can be minimized by design decisions. a) Specify common materials. Maintainers have difficulty “using up” materials if they are specific to a single aircraft or component. In many cases material manufacturers establish “minimum buys” of their product dictating the minimum purchase of several gallons when only a pint is required. The excess material often simply sits on the shelf until it is no longer useable and is then sent to disposal. This issue can be alleviated through the specification of materials that are already in the inventory, or that are used on a wide variety of components. b) Specify long shelf life, and/or room temperature storable materials. Obviously, the longer the shelf life of a product, and the less restrictive the storage, handling and transportation require-

MIL-HDBK-17-3F Volume 3,Chapter 8 Supportability ments,the better the chances that the material can be consumed before its shelf life expires. Designers should be aware that even though these materials may be slightly more expensive,or may not be the material of choice during the manufacture of the product-they may well be suit- able for repair and/or maintenance. 8.2.5.4 Cleaning requirements Cleaning is one of the primary maintenance processes that create hazardous waste.The construc- tion of the component often dictates the cleaning options available for that part.Many of the cleaning processes that previously utilized ozone depleting solvents and other hazardous chemicals are being re- placed with aqueous cleaning processes.If a component is constructed such that water intrusion is a concern,then aqueous cleaning of the part may also be a problem.The requirement for solvent cleaning places a heavy burden on the maintainer-which will continue to worsen as environmental restrictions tighten.Designing components so they can tolerate aqueous cleaning will facilitate maintenance re- quirements throughout the life cycle. 8.2.5.5 Non-destructive inspection requirements The requirement to perform non-destructive inspection on a component often requires cleaning and paint removal(resulting in hazardous waste generation)that would not otherwise be necessary.Often, non-destructive inspection requirements set during the design phase are maintained throughout the life cycle regardless of whether defects are ever found during the inspection.Periodic reviews of inspection requirements will present the opportunity to eliminate non-value added requirements thereby saving money,time and hazardous waste generation. 8.2.5.6 End of life disposal considerations Unlike the situation for metals,there is not a widespread market waiting to buy composite materials from scrap aircraft.There are several initiatives underway to find uses for these materials.Designers need to stay abreast of these initiatives so that if a market is identified for certain polymer composites,this can be given consideration when selecting design materials. Machining of carbon fiber laminates during cutting and trimming operations produces particulates that are nominally considered a nuisance dust by bio-environmental engineers.TLV(Threshold Limit Values) limits were updated in 1997 by the American Conference of Government and Industrial Hygienists (ACGIH)to define loose composite fiber/dust exposure limits for composite workers.Excessive exposure may require the use of NIOSH-certified respirators with HEPA filters.Resins used in composite materials and adhesives may cause dermal sensitization in some workers,thus silicon-free/lint-free gloves should be mandated for use.This will also ensure a contaminant-free laminate. Uncured prepregs and resins are treated as hazardous materials in waste stream analyses.Scrap materials should be cured prior to disposal to inert the resins and reduce the HazMat disposal costs.It is important to ensure that scrap materials containing carbon fibers are sent to non-burning landfills;pyrol- ized carbon fibers freed by resin burn-off can represent a respiratory and electrical hazard. 8.2.6 Reliability and maintainability The maintainability of a structure is achieved by developing schemes for methods of inspection and maintenance during the design phase.The designer with the overall knowledge of the performance and operational characteristics of the structure should access,based on the construction method,configura- tion,material selection,etc.,whether the structure is maintainable.Such factors in assessment would include development of cradle-to-grave inspection methodology,techniques,protection schemes and de- fined inspection intervals for maintenance. 8-14

MIL-HDBK-17-3F Volume 3, Chapter 8 Supportability 8-14 ments, the better the chances that the material can be consumed before its shelf life expires. Designers should be aware that even though these materials may be slightly more expensive, or may not be the material of choice during the manufacture of the product - they may well be suit￾able for repair and/or maintenance. 8.2.5.4 Cleaning requirements Cleaning is one of the primary maintenance processes that create hazardous waste. The construc￾tion of the component often dictates the cleaning options available for that part. Many of the cleaning processes that previously utilized ozone depleting solvents and other hazardous chemicals are being re￾placed with aqueous cleaning processes. If a component is constructed such that water intrusion is a concern, then aqueous cleaning of the part may also be a problem. The requirement for solvent cleaning places a heavy burden on the maintainer - which will continue to worsen as environmental restrictions tighten. Designing components so they can tolerate aqueous cleaning will facilitate maintenance re￾quirements throughout the life cycle. 8.2.5.5 Non-destructive inspection requirements The requirement to perform non-destructive inspection on a component often requires cleaning and paint removal (resulting in hazardous waste generation) that would not otherwise be necessary. Often, non-destructive inspection requirements set during the design phase are maintained throughout the life cycle regardless of whether defects are ever found during the inspection. Periodic reviews of inspection requirements will present the opportunity to eliminate non-value added requirements thereby saving money, time and hazardous waste generation. 8.2.5.6 End of life disposal considerations Unlike the situation for metals, there is not a widespread market waiting to buy composite materials from scrap aircraft. There are several initiatives underway to find uses for these materials. Designers need to stay abreast of these initiatives so that if a market is identified for certain polymer composites, this can be given consideration when selecting design materials. Machining of carbon fiber laminates during cutting and trimming operations produces particulates that are nominally considered a nuisance dust by bio-environmental engineers. TLV (Threshold Limit Values) limits were updated in 1997 by the American Conference of Government and Industrial Hygienists (ACGIH) to define loose composite fiber/dust exposure limits for composite workers. Excessive exposure may require the use of NIOSH-certified respirators with HEPA filters. Resins used in composite materials and adhesives may cause dermal sensitization in some workers, thus silicon-free/lint-free gloves should be mandated for use. This will also ensure a contaminant-free laminate. Uncured prepregs and resins are treated as hazardous materials in waste stream analyses. Scrap materials should be cured prior to disposal to inert the resins and reduce the HazMat disposal costs. It is important to ensure that scrap materials containing carbon fibers are sent to non-burning landfills; pyrol￾ized carbon fibers freed by resin burn-off can represent a respiratory and electrical hazard. 8.2.6 Reliability and maintainability The maintainability of a structure is achieved by developing schemes for methods of inspection and maintenance during the design phase. The designer with the overall knowledge of the performance and operational characteristics of the structure should access, based on the construction method, configura￾tion, material selection, etc., whether the structure is maintainable. Such factors in assessment would include development of cradle-to-grave inspection methodology, techniques, protection schemes and de￾fined inspection intervals for maintenance

MIL-HDBK-17-3F Volume 3,Chapter 8 Supportability 8.2.7 Interchangeability and replaceability A composite structure can be maintained using a variety of methods,each dependent on the support plan and maintenance concept selected.One of the first design considerations that must be determined is the ease with which a damaged piece of structure can be repaired.Large integral structural elements, such as a wing skin panel,cannot be readily removed from the aircraft and,therefore,must be repaired in-place.Many panels,however,can be removed and the damaged panel replaced with a new panel. Ease of maintenance can have a direct impact on the design and surrounding structure.By develop- ing the maintenance concept early in the design process,tradeoffs can be made before the design is finalized that will provide the aircraft operator with more maintenance options and provide aircraft that are potentially more available to perform their design function. The design of removable panels can have significant impact on the ease of maintenance and the as- sociated maintenance and downtime cost.There are two commonly used types of panels used for struc- tural maintenance-interchangeable and replaceable. The interchangeable panel is one that can be installed onto the aircraft without any trimming,drill- ing.or other customizing.Interchangeable parts are designed through a selection of materials,toler- ances,and fastening techniques to fit a production run of aircraft within the same model series. Replaceable panels may or may not fit between different aircraft and usually require trimming and drilling on installation. Figure 8.2.7 shows the differences between interchangeable and replaceable panels. Interchangeability and replaceability(1&R)requirements for non-repairable,high-unit cost,frequently damaged,or highly loaded components need to be assessed early in the design process to ensure cost and operational effectiveness.Typically,I&R components are more costly to manufacture due to the close tolerances,materials and design attributes.Designers must be assured that form,fit,and function are fully realized with removable parts and realize that thermal and material mismatches,part number changes,and different manufacturing techniques,may alter a component's ability to be replaceable or interchangeable.In some cases,1&R are design requirements and can easily be met using loose toler- ances and numerically controlled master tooling during the manufacturing process. Components that must be removed frequently (<1000 flight hours)to facilitate other maintenance ac- tions are typically good candidates for interchangeable panels.Components that are large and contain a variety of inner mold line geometries and fastener configurations (fuselage skins and components that have attachment fittings,i.e.,landing gear doors)are good candidates for replaceable panels.Mil-1-8500, The Use of Interchangeable Components,provides requirements and guidance. 8.2.8 Accessibility Accessibility is an important factor when designing structures for repair.Sufficient access should al- ways be provided to properly inspect,prepare the damage structure,fit and install the repair parts and use repair tools and bonding equipment.Limited access may dictate the repair approach,i.e.,use of pre- cured patches,use of mechanical fasteners in lieu of cocuring,etc.If feasible,two-sided access is pre- ferred. 8.2.9 Repairability Designing for repairability is an essential element in the effective use of composite materials in aircraft structures.Selecting a repair approach during the design phase will influence the choice of lay-up pat- terns and design strain levels.It is important that the repair philosophy be set during the conceptual de- sign stage and that the repair designs be developed along with the component design.Candidate repair designs should be tested as part of the development test program.Repair concepts and materials should 8-15

MIL-HDBK-17-3F Volume 3, Chapter 8 Supportability 8-15 8.2.7 Interchangeability and replaceability A composite structure can be maintained using a variety of methods, each dependent on the support plan and maintenance concept selected. One of the first design considerations that must be determined is the ease with which a damaged piece of structure can be repaired. Large integral structural elements, such as a wing skin panel, cannot be readily removed from the aircraft and, therefore, must be repaired in-place. Many panels, however, can be removed and the damaged panel replaced with a new panel. Ease of maintenance can have a direct impact on the design and surrounding structure. By develop￾ing the maintenance concept early in the design process, tradeoffs can be made before the design is finalized that will provide the aircraft operator with more maintenance options and provide aircraft that are potentially more available to perform their design function. The design of removable panels can have significant impact on the ease of maintenance and the as￾sociated maintenance and downtime cost. There are two commonly used types of panels used for struc￾tural maintenance - interchangeable and replaceable. The interchangeable panel is one that can be installed onto the aircraft without any trimming, drill￾ing, or other customizing. Interchangeable parts are designed through a selection of materials, toler￾ances, and fastening techniques to fit a production run of aircraft within the same model series. Replaceable panels may or may not fit between different aircraft and usually require trimming and drilling on installation. Figure 8.2.7 shows the differences between interchangeable and replaceable panels. Interchangeability and replaceability (I&R) requirements for non-repairable, high-unit cost, frequently damaged, or highly loaded components need to be assessed early in the design process to ensure cost and operational effectiveness. Typically, I&R components are more costly to manufacture due to the close tolerances, materials and design attributes. Designers must be assured that form, fit, and function are fully realized with removable parts and realize that thermal and material mismatches, part number changes, and different manufacturing techniques, may alter a component’s ability to be replaceable or interchangeable. In some cases, I&R are design requirements and can easily be met using loose toler￾ances and numerically controlled master tooling during the manufacturing process. Components that must be removed frequently (<1000 flight hours) to facilitate other maintenance ac￾tions are typically good candidates for interchangeable panels. Components that are large and contain a variety of inner mold line geometries and fastener configurations (fuselage skins and components that have attachment fittings, i.e., landing gear doors) are good candidates for replaceable panels. Mil-I-8500, The Use of Interchangeable Components, provides requirements and guidance. 8.2.8 Accessibility Accessibility is an important factor when designing structures for repair. Sufficient access should al￾ways be provided to properly inspect, prepare the damage structure, fit and install the repair parts and use repair tools and bonding equipment. Limited access may dictate the repair approach, i.e., use of pre￾cured patches, use of mechanical fasteners in lieu of cocuring, etc. If feasible, two-sided access is pre￾ferred. 8.2.9 Repairability Designing for repairability is an essential element in the effective use of composite materials in aircraft structures. Selecting a repair approach during the design phase will influence the choice of lay-up pat￾terns and design strain levels. It is important that the repair philosophy be set during the conceptual de￾sign stage and that the repair designs be developed along with the component design. Candidate repair designs should be tested as part of the development test program. Repair concepts and materials should