MIL-HDBK-17-3F Volume 3,Chapter 8 Supportability difficult inspection areas adhesive fillets adhesive layer (a)Closed-hat stiffened configuration rolled noodle (CFRP or adhesive) difficult inspection area adhesive layer (b)Blade stiffened configuration FIGURE 8.2.2.1(a)Difficult to inspect areas on laminate skin stiffened designs. rolled noodle(CFRP or adhesive) difficult inspection area adhesive layer hiO77 FIGURE 8.2.2.1(b)Difficult inspection area of sandwich structural configurations. Most composite structural components will include metal fittings or interfaces with metal parts.It is desirable to ensure that these metal parts can be visually inspected for corrosion and/or fatigue cracking. In addition,if the mating metal parts are aluminum,then it is important to be able to inspect them for po- 8-6
MIL-HDBK-17-3F Volume 3, Chapter 8 Supportability 8-6 FIGURE 8.2.2.1(a) Difficult to inspect areas on laminate skin stiffened designs. FIGURE 8.2.2.1(b) Difficult inspection area of sandwich structural configurations. Most composite structural components will include metal fittings or interfaces with metal parts. It is desirable to ensure that these metal parts can be visually inspected for corrosion and/or fatigue cracking. In addition, if the mating metal parts are aluminum, then it is important to be able to inspect them for po-
MIL-HDBK-17-3F Volume 3,Chapter 8 Supportability tential galvanic corrosion that may be caused by contact with the carbon fibers.This may require removal of fasteners at mating surfaces,so blind fasteners should not be used in these applications.The use of blind titanium fasteners should be kept to a minimum because,when installed,they are literally impossi- ble to inspect to verify correct installation.They are also very difficult to remove when repairing or replac- ing a component. 8.2.2.2 Accessibility for inspection Composite structural components should not be designed such that they must be removed in order for inspections to be made.Some disassembly may be unavoidable,but should be kept to a minimum. This will not only reduce the maintenance burden on the operators,but also reduce airplane out-of- service time. All composite components should be designed to ensure visual accessibility of the external surfaces without detaching any parts,including access panels,from the airplane.In some instances,fairing panels may have to be removed,such as the horizontal stabilizer-to-fuselage fairing for access to the stabilizer skin joints-to-side-of-body rib,or spar-to-center-section attachments. An internal inspection implies that there is visual accessibility that is achieved by removal of detach- able parts,such as access plates or panels.For internal inspection of torque boxes with ribs,spars and stringers,there must be complete visual accessibility through access holes in spars and ribs.These ac- cess holes must be designed such that maintenance technicians can,through the use of flashlights and mirrors,visually inspect all of the internal structure.There must also be accessibility to critical joints or attachment fittings where pins can be removed so that they and the holes can be inspected. 8.2.3 Material selection 8.2.3.1 Introduction Chapter 2 in Volume 3 offers an in-depth review of advanced composite materials.Each one of the composite materials described in Chapter 2 can offer benefits over metallic materials to the designer in terms of performance and costs.However,these benefits will be erased if,when designing a component, the design is focused only on the mechanical and thermal performance of the component and does not take into consideration where the part will be used and how it will be repaired if it is damaged.The goal of the designer must be to design a part that will be both damage tolerant and damage resistant as well as easy to maintain and repair.This section is offered as a guideline for the designer when selecting a mate- rial system. 8.2.3.2 Resins and fibers When selecting a resin,it is important to look at where the resin system will be used,how the resin system has to be processed,what is its shelf life and storage requirements,and is it compatible with sur- rounding materials.Table 8.2.3.2 describes the common resin types,their process conditions and their advantages and disadvantages in terms of repairability.An in-depth review of these materials can be found in Section 2.2. Refer to Section 2.3 for available fibers for composite structures. In terms of supportability,the minimum number of resin systems and material specifications should be chosen.This will reduce the logistic problems of storage,shelf life limitations and inventory control. 8-7
MIL-HDBK-17-3F Volume 3, Chapter 8 Supportability 8-7 tential galvanic corrosion that may be caused by contact with the carbon fibers. This may require removal of fasteners at mating surfaces, so blind fasteners should not be used in these applications. The use of blind titanium fasteners should be kept to a minimum because, when installed, they are literally impossible to inspect to verify correct installation. They are also very difficult to remove when repairing or replacing a component. 8.2.2.2 Accessibility for inspection Composite structural components should not be designed such that they must be removed in order for inspections to be made. Some disassembly may be unavoidable, but should be kept to a minimum. This will not only reduce the maintenance burden on the operators, but also reduce airplane out-ofservice time. All composite components should be designed to ensure visual accessibility of the external surfaces without detaching any parts, including access panels, from the airplane. In some instances, fairing panels may have to be removed, such as the horizontal stabilizer-to-fuselage fairing for access to the stabilizer skin joints-to-side-of-body rib, or spar-to-center-section attachments. An internal inspection implies that there is visual accessibility that is achieved by removal of detachable parts, such as access plates or panels. For internal inspection of torque boxes with ribs, spars and stringers, there must be complete visual accessibility through access holes in spars and ribs. These access holes must be designed such that maintenance technicians can, through the use of flashlights and mirrors, visually inspect all of the internal structure. There must also be accessibility to critical joints or attachment fittings where pins can be removed so that they and the holes can be inspected. 8.2.3 Material selection 8.2.3.1 Introduction Chapter 2 in Volume 3 offers an in-depth review of advanced composite materials. Each one of the composite materials described in Chapter 2 can offer benefits over metallic materials to the designer in terms of performance and costs. However, these benefits will be erased if, when designing a component, the design is focused only on the mechanical and thermal performance of the component and does not take into consideration where the part will be used and how it will be repaired if it is damaged. The goal of the designer must be to design a part that will be both damage tolerant and damage resistant as well as easy to maintain and repair. This section is offered as a guideline for the designer when selecting a material system. 8.2.3.2 Resins and fibers When selecting a resin, it is important to look at where the resin system will be used, how the resin system has to be processed, what is its shelf life and storage requirements, and is it compatible with surrounding materials. Table 8.2.3.2 describes the common resin types, their process conditions and their advantages and disadvantages in terms of repairability. An in-depth review of these materials can be found in Section 2.2. Refer to Section 2.3 for available fibers for composite structures. In terms of supportability, the minimum number of resin systems and material specifications should be chosen. This will reduce the logistic problems of storage, shelf life limitations and inventory control
MIL-HDBK-17-3F Volume 3,Chapter 8 Supportability TABLE 8.2.3.2 Supportability concerns with resin types. Resin Type Cure Temp. Pressure Ranges Processing Options Supportability Ease of Damage Supportability Ranges Advantages Repair Resistance Disadvantages Epoxy Non- RT to 350F Vacuum to 100 psi Autoclave,press, Low level of Good Poor Time limited Toughened (180C) (690kPa) vacuum bag,resin volatiles,low temp storage transfer molding processing,vacuum bageable Epoxy -Toughened RTto350℉ Vacuum to 100 psi Autoclave,press, Low level of Good Good Time limited (180C) (690kPa) vacuum bag and volatiles,low temp storage resin transfer processing,vacuum molding bageable Polyester RTt0350℉ Vacuum Bag to 100 Same as epoxies Ease of processing. Very Good Poor elevated (180℃) psi(690 kPa) quick cure with good temp elevated temp.,low performance, cost health (Styrene) Phenolic 250to350℉(120 Vacuum Bag to 100 Autoclave,press Poor Poor Water off to 180C)with post psi (690 kPa);lower molding gassing,high cure pressure gives high temp cure/post void content cure.high void content Bismaleimides 350F(180C)wth 45to100psi(310 Autoclave,press Lower pressure Poor Poor High (BMI) 400to500°℉(200 to 690 kPa) molding,RTM processing than temperature to 260C)post cure polyimides processing required Polyimides 350to700℉(180 85to200+psi(590 Autoclave and Poor Poor Cost,availability to 370C)post cure to 1400+kPa) press molding of adhesives, required high pressure Structural 500℉+(260C+) Vacuum bag to 200 Autoclave and Reformable Poor Very good High Thermoplastic psi(1400 kPa) press molding temperature processing 8-8
MIL-HDBK-17-3F Volume 3, Chapter 8 Supportability 8-8 TABLE 8.2.3.2 Supportability concerns with resin types. Resin Type Cure Temp. Ranges Pressure Ranges Processing Options Supportability Advantages Ease of Repair Damage Resistance Supportability Disadvantages Epoxy NonToughened RT to 350°F (180°C) Vacuum to 100 psi (690 kPa) Autoclave, press, vacuum bag, resin transfer molding Low level of volatiles, low temp processing, vacuum bageable Good Poor Time limited storage Epoxy -Toughened RT to 350°F (180°C) Vacuum to 100 psi (690 kPa) Autoclave, press, vacuum bag and resin transfer molding Low level of volatiles, low temp processing, vacuum bageable Good Good Time limited storage Polyester RT to 350°F (180°C) Vacuum Bag to 100 psi (690 kPa) Same as epoxies Ease of processing, quick cure with elevated temp., low cost Very good Good Poor elevated temp performance, health (Styrene) Phenolic 250 to 350°F (120 to 180°C) with post cure Vacuum Bag to 100 psi (690 kPa); lower pressure gives high void content Autoclave, press molding Poor Poor Water off gassing, high temp cure/post cure, high void content Bismaleimides (BMI) 350F (180°C) with 400 to 500°F (200 to 260°C) post cure required 45 to 100 psi (310 to 690 kPa) Autoclave, press molding, RTM Lower pressure processing than polyimides Poor Poor High temperature processing Polyimides 350 to 700°F (180 to 370°C) post cure required 85 to 200+ psi (590 to 1400+ kPa) Autoclave and press molding Poor Poor Cost, availability of adhesives, high pressure Structural Thermoplastic 500°F+ (260°C+) Vacuum bag to 200 psi (1400 kPa) Autoclave and press molding Reformable Poor Very good High temperature processing
MIL-HDBK-17-3F Volume 3,Chapter 8 Supportability 8.2.3.3 Product forms A detailed description of available composite product forms can be found in Section 2.5. The goal when repairing a composite part is to return it to its original performance capability while in- curring the least cost and weight gain.Therefore,the ease of repairing different product forms should be taken into consideration when selecting the material system.Figure 8.2.3.3 shows the relative ease of repairing various product forms. 8.2.3.4 Adhesives Table 8.2.3.4 provides descriptions of issues for use of adhesives in repairs. 8.2.3.5 Supportability issues Table 8.2.3.5 offers a list of Material Support issues for your consideration. 8.2.3.6 Environmental concerns Health and safety:There are recognized hazards that go with advanced composite materials. Knowing about these hazards,one can protect oneself and others from exposure to them.It is important to read and understand the Material Safety Data Sheets (MSDS)and handle all chemicals,resins and fibers correctly.Refer to SACMA publication "Safe Handling of Advanced Composite Materials"for addi- tional information(Reference 8.2.3.6). Disposal of scrap and waste:When selecting materials,consideration must be given to the dis- posal of scrap and waste.Disposal of scrap and waste should be specified under federal,state and local laws.See Section 8.2.5.6 on how to dispose of uncured materials. Most Difficult Tape Woven Fabric Stiched Fabric Knitted Fabric (2D) (3D) Product Form Complexity FIGURE 8.2.3.3 Difficulty of repairing product forms. 8-9
MIL-HDBK-17-3F Volume 3, Chapter 8 Supportability 8-9 8.2.3.3 Product forms A detailed description of available composite product forms can be found in Section 2.5. The goal when repairing a composite part is to return it to its original performance capability while incurring the least cost and weight gain. Therefore, the ease of repairing different product forms should be taken into consideration when selecting the material system. Figure 8.2.3.3 shows the relative ease of repairing various product forms. 8.2.3.4 Adhesives Table 8.2.3.4 provides descriptions of issues for use of adhesives in repairs. 8.2.3.5 Supportability issues Table 8.2.3.5 offers a list of Material Support issues for your consideration. 8.2.3.6 Environmental concerns Health and safety: There are recognized hazards that go with advanced composite materials. Knowing about these hazards, one can protect oneself and others from exposure to them. It is important to read and understand the Material Safety Data Sheets (MSDS) and handle all chemicals, resins and fibers correctly. Refer to SACMA publication "Safe Handling of Advanced Composite Materials" for additional information (Reference 8.2.3.6). Disposal of scrap and waste: When selecting materials, consideration must be given to the disposal of scrap and waste. Disposal of scrap and waste should be specified under federal, state and local laws. See Section 8.2.5.6 on how to dispose of uncured materials. FIGURE 8.2.3.3 Difficulty of repairing product forms
MIL-HDBK-17-3F Volume 3,Chapter 8 Supportability TABLE 8.2.3.4 Repair adhesive considerations. Consideration Response Performance The adhesive system must be capable of transferring structural,thermal,acoustic properties loads through a patch material and back into the parent structure.The adhesive system must also be capable of transferring those loads while operating within the vehicles environmental envelope (i.e.,presence of hydraulic fluid,fuel,and dirt, and vibro-acoustic conditions). Service temperature The maximum surface temperature a structure will operate over the vehicle life. Exhaust sections and leading edges typically will operate at 50-500%higher temperatures than surrounding areas.The surface preparation method,adhesive primer,cure profile,heat sinks,and coatings and treatments can all influence the maximum temperature of the structure and associated repair. Compatibility with Surface preparation can be anything from nothing to an electrochemically etched surface preparation surface containing a commingled primer system.In addition the surface could be technique dirty,contain oxidation,hydrocarbons or moisture,or not lend itself well to chemically bonding with the adhesive. Wetability The ability of an adhesive to flow within all areas of the repair.Improvements in wetability reduce resin-starved areas and associated porosity,maintain bondline tolerances,and in general produce more reliable bonds. Porosity of bondline Curing without external pressure (i.e..vacuum bags)increases the potential of trapping volatiles created during the cure process.Application of heat and vacuum/pressure in the correct sequence will minimize porosity and,therefore, provide better bonds. Tolerance of All repair areas have varying thermal densities (substructure,patch ply drop-offs) temperature deltas which create a wide range of temperature deltas during adhesive cure.Adhesives across repair area that can cure well over a broad temperature range are more suited for repair applications.In addition,during repair only a small area of the structure is heated while the remaining structure is at ambient temperature which could be as low as- 10°℉or as high as180°F. Outtime at ambient Repairs can take a long time to assemble before the cure starts.Adhesives that temperature are stable and fully thawed for several hours at ambient temperature will produce better and more reliable repairs. Tolerance of bondline Uniform bondlines produce the best load transfer medium.Maintaining a uniform thickness bondline thickness is difficult on structures that are wavy and have ply discontinuities.Adhesives that perform well with bondlines from 3-15 mils will produce the best repair performance. Cure time Ideally,cure time should be as short as possible to reduce vehicle downtime. Adhesives that can be heated at 5-7F/min and dwelled at the cure temperature for less than 2 hrs.are optimum. Cure pressure In repair applications the only patch compaction force available is from atmospheric or mechanical pressure.Since autoclaves and associated tooling are not readily available and components are difficult to remove,vacuum bags or mechanical clamps will be the pressure devices of choice. Cure temperature A rule of thumb for repair applications is to use an adhesive with the lowest cure temperature that meets all the performance constraints.As temperatures increase,the tolerance of acceptable cure decreases.In addition,most hot bond control units manage the upper temperature limit,therefore,the cure temperature variance should be +0 and -40F. Storability at ambient Since many materials must be cold stored to minimize the effects of crosslinking, temperature an adhesive that is tolerant of sustained outtime at ambient temperature is more suited for the repair environment.In addition,some repair facilities lack the cold storage equipment necessary and must rely on temporary cold storage methods such as iced coolers or just in time delivery of repair materials from distribution centers. 8-10
MIL-HDBK-17-3F Volume 3, Chapter 8 Supportability 8-10 TABLE 8.2.3.4 Repair adhesive considerations. Consideration Response Performance properties The adhesive system must be capable of transferring structural, thermal, acoustic loads through a patch material and back into the parent structure. The adhesive system must also be capable of transferring those loads while operating within the vehicles environmental envelope (i.e., presence of hydraulic fluid, fuel, and dirt, and vibro-acoustic conditions). Service temperature The maximum surface temperature a structure will operate over the vehicle life. Exhaust sections and leading edges typically will operate at 50 - 500% higher temperatures than surrounding areas. The surface preparation method, adhesive primer, cure profile, heat sinks, and coatings and treatments can all influence the maximum temperature of the structure and associated repair. Compatibility with surface preparation technique Surface preparation can be anything from nothing to an electrochemically etched surface containing a commingled primer system. In addition the surface could be dirty, contain oxidation, hydrocarbons or moisture, or not lend itself well to chemically bonding with the adhesive. Wetability The ability of an adhesive to flow within all areas of the repair. Improvements in wetability reduce resin-starved areas and associated porosity, maintain bondline tolerances, and in general produce more reliable bonds. Porosity of bondline Curing without external pressure (i.e., vacuum bags) increases the potential of trapping volatiles created during the cure process. Application of heat and vacuum/pressure in the correct sequence will minimize porosity and, therefore, provide better bonds. Tolerance of temperature deltas across repair area All repair areas have varying thermal densities (substructure, patch ply drop-offs) which create a wide range of temperature deltas during adhesive cure. Adhesives that can cure well over a broad temperature range are more suited for repair applications. In addition, during repair only a small area of the structure is heated while the remaining structure is at ambient temperature which could be as low as - 10°F or as high as 180°F. Outtime at ambient temperature Repairs can take a long time to assemble before the cure starts. Adhesives that are stable and fully thawed for several hours at ambient temperature will produce better and more reliable repairs. Tolerance of bondline thickness Uniform bondlines produce the best load transfer medium. Maintaining a uniform bondline thickness is difficult on structures that are wavy and have ply discontinuities. Adhesives that perform well with bondlines from 3-15 mils will produce the best repair performance. Cure time Ideally, cure time should be as short as possible to reduce vehicle downtime. Adhesives that can be heated at 5-7°F/min and dwelled at the cure temperature for less than 2 hrs. are optimum. Cure pressure In repair applications the only patch compaction force available is from atmospheric or mechanical pressure. Since autoclaves and associated tooling are not readily available and components are difficult to remove, vacuum bags or mechanical clamps will be the pressure devices of choice. Cure temperature A rule of thumb for repair applications is to use an adhesive with the lowest cure temperature that meets all the performance constraints. As temperatures increase, the tolerance of acceptable cure decreases. In addition, most hot bond control units manage the upper temperature limit, therefore, the cure temperature variance should be +0 and -40°F. Storability at ambient temperature Since many materials must be cold stored to minimize the effects of crosslinking, an adhesive that is tolerant of sustained outtime at ambient temperature is more suited for the repair environment. In addition, some repair facilities lack the cold storage equipment necessary and must rely on temporary cold storage methods such as iced coolers or just in time delivery of repair materials from distribution centers