Info

■ Delaminations

Installation of Conventional Interference Fit Fasteners Can Cause Delaminations in Composite Structures

■ Delaminations

Expandable Sleeve

Installation of Interference Fit Fasteners with Sleeves Spreads Forces to Eliminate Delaminations

Fig. 11.39. Interference Fit Fasteners in Composites12

Interference Fit Sleeve Type Lockbolt

Interference Fit Threaded Corebolt Blind Fastener

Fig. 11.40. Installation of Sleeve Type Interference Fit Fasteners12

Interference Fit Threaded Corebolt Blind Fastener

Fig. 11.40. Installation of Sleeve Type Interference Fit Fasteners12

prevent the delamination problem. Interferences as high as 0.006 in. have been obtained without damaging the composite. Both pin and collar (Lockbolts), and threaded core blind bolt fasteners (Fig. 11.40), are available with sleeves. There are several potential advantages to using interference fit fasteners in composite structure:12 lower joint deflection; reduction in fastener cocking that leads to high localized bearing stresses; locks up structure to prevent ratcheting during fatigue; and reduced assembly costs when interference fits are required in metallic structure (no disassembly and ream operation for the composite).

11.5 Sealing

Many structures require sealing for (1) corrosion protection, (2) to keep water out of the structure, or (3) to keep fuel in the structure. The typical wing fuel tank configuration shown in Fig. 11.41 will be used to explain the different sealing and corrosion protection methods. For joints between carbon/epoxy and aluminum parts, it is common practice to cocure, or bond, a thin layer of glass cloth to the surface of the carbon/epoxy part, which acts as an electrical isolation barrier to prevent galvanic corrosion of the aluminum.

A good sealant must have good adhesion properties, high elongation, and be resistant to both temperature and chemicals. Sealing is usually accomplished using polysulfide sealants, which are available in a variety of product forms, with a range of viscosities and cure times. Polysulfide sealants are usable in the temperature range of -65 to 250° F, with short-term capabilities to 350° F. They also contain leachable corrosion resistant compounds, which aid in preventing corrosion of aluminum structure. If higher temperatures are required, then silicone sealants can be used that have temperature capabilities as high as 500° F.14 Moldline fasteners are usually installed "wet" by applying sealant to the fastener before installation. The nuts are often over coated after installation. During assembly, the faying surfaces are sealed, and then fillet seals are placed around

Channel Seal Groove Packed with Sealant

Fay Surface Sealant

Aluminum Closure Spar

Liquid Shim

Fuel Tank Area

Aluminum Closure Spar

Fuel Tank Area

Fasteners Overcoated with Polysulfide Sealant

Channel Seal Groove Packed with Sealant

Fay Surface Sealant

Liquid Shim

Fillet Seal

Titanium Fasteners Installed "Wet" Carbon/Epoxy Skin with Polysulfide Sealant

Fasteners Overcoated with Polysulfide Sealant

Fillet Seal

Titanium Fasteners Installed "Wet" Carbon/Epoxy Skin with Polysulfide Sealant

Fig. 11.41. Typical Wing Fuel Tank Sealing the periphery. Because the fay seal is extremely thin and may be separated due to structural deflections, it cannot be considered to be the primary seal. The fillet seal, applied after assembly, must be the primary seal. All potential leak paths must be fillet sealed. Fillet beads must be dressed with a filleting tool, to work out air bubbles and voids and provide the finished fillet shape. Adequate fillet size is important in preventing leakage. Selecting the right sealant working life is important, or the sealant may set up prior to finishing the job, or it may take too long to cure, affecting the production schedule.

Fuel tanks often have a channel groove that is packed with a fluorosilicone sealant, which contains around 10% of small microspheres that are graduated from 0.002 to 0.030 in. in diameter, for more effective gap filling. They assist in keeping the compound in the groove and are effective in helping to seal gaps, up to 0.010 in. wide. When fuel comes in contact with the sealant, it swells to help further seal the structure. Normally the channel is prepacked with sealant and then injected under pressure (up to 4000 psi) after assembly. Injection points, usually spaced at 4-6 in. apart, can be at fastener holes, or through specially designed fasteners that contain internal injection ports.

11.6 Painting

Metallic parts are usually chemically treated prior to painting. Aluminum, in particular, requires careful surface preparation prior to painting to prevent corrosion. Anodizing, as covered in Chapter 2 on Aluminum, and conversion coatings are the two most common surface treatments. After treatment, the detail part is usually primed with one or two layers of corrosion resistant epoxy primer. Paint adhesion to composite structure is actually not as difficult as it is with metallic structure. The surface should be cleaned of all dirt and grease. If the part contains a peel ply, it should be removed. Surface preparation can be accomplished by either scuff sanding with 150-180 grit sandpaper, or by lightly grit blasting.

For aerospace applications, the standard finishing system is epoxy primer, followed by one or two layers of polyurethane topcoat. Epoxy primers are addition curing polyamides that contain: (1) strontium chromate that is an exceptional corrosion inhibitor for aluminum; (2) titanium dioxide for enhanced durability and chemical resistance; and (3) fillers, such as silica, to control viscosity and reduce cost. After sanding, the part should be primed within 36 h. The primer is applied to a dry film thickness of 0.000 8-0.0014 in. and then room temperature cured for a minimum of 6 h. Polyurethane topcoats are aliphatic ester based polyurethanes that exhibit good weathering, chemical resistance, durability, and flexibility. They are applied to a dry film thickness of around 0.002 in., with an initial cure within 2-8 h, and a full cure within 7-14 days.16 Environmentally more friendly paint systems are being developed that are free of solvents, or low in solvents, called low volatile organic compounds (VOC) coatings. Also, toxic heavy metals (e.g., chromium) are being replaced with self-priming topcoats that are non-chromated high solids polyurethane coatings, which replace both the epoxy primer and traditional polyurethane topcoat.16

Summary

Due to the labor intensity associated with assembly operations and the resultant high costs, structures should be designed to eliminate as much assembly as possible. However, the requirement to mechanically fasten structures will not disappear in the foreseeable future.

Prior to hole drilling and fastener installation, it is critical to locate any gaps between structural members and shim them appropriately, before starting the actual assembly operations. Failure to properly address gaps will lead to excessive preloads and possibly cracks and delaminations in composites during fastener installation and clamp-up. In addition, preloaded metallic parts could be subject to fatigue or stress corrosion cracking.

There are many types of drill motors and units that can be used to drill structures, but they can be broadly classified as either hand, power feed, automated drilling units or automated riveting equipment. Free hand drilling of structures should be avoided if possible. Power feed or automated drilling equipment gives much better and more consistent hole quality. Although free hand drilling is obviously not the best method, it is frequently used because it requires no investment in tooling (i.e., drill templates) and in many applications, where access is limited, it may be the only viable method. Power feed drilling is much preferred to hand drilling. In power feed drilling, the drill unit is locked into a drill template that establishes both hole location and maintains drill normality. In addition, once the drilling operation starts, the unit is programmed to drill at a given speed and feed.

For high volume hole generation, automated drilling equipment can be designed and built for specific applications. Being large and sophisticated machine tools, these units are expensive, so the number of holes drilled and the number of units produced needs to be large enough to justify the equipment investment. Automated riveting equipment is also available that will drill the hole, inspect the hole, install sealant on the rivet (if required), and then install the rivet by squeezing. This equipment is available in a wide variety of sizes and price ranges, ranging from small units to very large units, capable of installing stringers on full size commercial aircraft wing skins.

Composite materials require more care when machining and drilling than comparable metals like aluminum. Their relative brittleness, low interlaminar shear and peel strengths, and low heat tolerance calls for special care in all machining and drilling operations. Special drill geometries are available for composites that reduce the occurrence of defects, such as hole splintering and fiber pull-out.

There are many types of fasteners used in aerospace structural assembly, the most prevalent being solid rivets, pins with collars, bolts with nuts, and blind fasteners.The selection of a specific fastener depends on its ability to satisfactorily transmit the expected design loads, be environmentally compatible with the materials it joins, and be amenable to installation in the intended joint. Environmental or corrosion compatibility depends on both the fastener material and the materials in the joint. For composite structures, fasteners with large footprints should be selected to spread the clamp-up loads across the composite surfaces. Fastener installation processes that induce vibration or sudden impact loads on the parts should never be used in fastening composites.

Solid rivets are one piece permanent fasteners made from malleable metal installed by vibration driving or squeezing. They are used primarily in aluminum structure as permanent fasteners, where low cost reliable fasteners are desired. Pin and collar fasteners are the most commonly used fasteners for permanent installations, where there is no requirement to remove the fastener. Typical pin and collar fasteners include Hi-Loks, Lockbolts, and Eddie Bolts. Pin and collar fasteners require two sided access.

Bolts, along with nuts and washers, are used to join highly loaded structural members which must be removable for service access. They are also used as permanent attachments for structure. Structural bolts are used in fatigue, shear, and tension critical joints. Nuts, which are tightened by wrenches, may be used when there is access to both sides. Nut plates and channel nuts are used when one sided access is applicable.

Blind fasteners are used in areas where there is limited or no access to the backside of the structure. However, the solid core pin and collar fasteners are usually preferred, because they are stronger and have better fatigue resistance. Two types of blind fasteners are the threaded core bolt type and the pull type.

Since fatigue cracks often initiate at fastener holes in metallic structure, methods such as cold working of fastener holes and interference fit fasteners have been developed to improve fatigue life. Both cold working and interference fit fasteners set up a residual compressive stress field in the metal immediately adjacent to the hole. The applied tension stress during fatigue loading must then overcome the residual compressive stress field before the hole becomes loaded in tension.

Aerospace structure is usually sealed to keep water out using polysulfide sealants. For corrosion protection, metallic structure will often be treated, such as anodizing or conversion coating treatments for aluminum. Paint primers with corrosion resistant compounds are used to provide additional protection. Outer moldline structure is then coated with polyurethane topcoats. Sealing and painting of composites is very similar to the processes used for metallic structure. In fact, since composites themselves are not prone to corrosion, in some cases, the job is far less complex, since special corrosion inhibiting compounds are not required. Likewise, paint adhesion to composites is as good, or better, than to metals, provided that proper surface preparation techniques are followed.

Recommended Reading

[1] Paleen, M.J., Kilwin, J.J., "Hole Drilling in Polymer-Matrix Composites", in ASM Handbook Vol. 21 Composites, ASM International, 2001, pp. 646-650.

[2] Parker, R.T., "Mechanical Fastener Selection", in ASM Handbook Vol. 21 Composites, ASM International, 2001, pp. 651-658.

References

[1] Taylor, A., "RTM Material Developments for Improved Processability and Performance", SAMPE Journal, Vol. 36, No. 4, July/August 2000, pp. 1-24.

[2] Fraccihia, C.A., Bohlmann, R.E., "The Effects of Assembly Induced Delaminations at Fastener Holes on the Mechanical Behavior of Advanced Composite Materials", 39th International SAMPE Symposium, 11-14 April 1994, pp. 2665-2678.

[3] Paleen, M.J., Kilwin, J.J., "Hole Drilling in Polymer-Matrix Composites", in ASM Handbook Vol. 21 Composites, ASM International, 2001, pp. 646-650.

[4] Born, G.C., "Single-Pass Drilling of Composite/Metallic Stacks", 2001 Aerospace Congress, SAE Aerospace Manufacturing Technology Conference, 10-14 September 2001.

[5] Campbell, F.C., "Assembly", in Manufacturing Processes for Advanced Composites, Elsevier Ltd, 2004, pp. 440-469.

[6] Astrom, B.T., Manufacturing of Polymer Composites, Chapman & Hall, 1997.

[7] Bolt, J.A., Chanani, J.P., "Solid Tool Machining and Drilling", in Engineered Materials Handbook Vol. 1 Composites, ASM International, 1987, pp. 667-672.

[8] Bohanan, E.L., "F/A-18 Composite Wing Automated Drilling System", 30th National SAMPE Symposium, 19-21 March 1985, pp. 579-585.

[9] McGahey, J.D., Schaut, A.J., Chalupa, E., Thompson, P., Williams, G., "An Investigation into the Use of Small, Flexible, Machine Tools to Support the Lean Manufacturing Environment", 2001 Aerospace Congress, SAE Aerospace Manufacturing Technology Conference, 10-14 September 2001.

[10] Jones, J., Buhr, M., "F/A-18 E/F Outer Wing Lean Production System", 2001 Aerospace Congress, SAE Aerospace Manufacturing Technology Conference, 10-14 September 2001.

[11] Niu, M.C.Y., Composite Airframe Structures: Practical Design Information and Data, Conmilit Press, Hong Kong, 1992.

[12] Parker, R.T., "Mechanical Fastener Selection", in ASM Handbook Vol. 21 Composites, ASM International, 2001, pp. 651-658.

[13] Armstrong, K.B., Barrett, R.T., Care and Repair of Advanced Composites, SAE International, 1998.

[14] Hoeckelman, L.A., "Environmental Protection and Sealing", in ASM Handbook Vol. 21 Composites, ASM International, 2001, pp. 659-665.

[15] Leon, A., "Developments in Advanced Cold working", SAE 982145, 1998.

[16] Spadafora, S.J., Eng, A.T., Kovalseki, K.J., Rice, C.E., Pulley, D.F., Dumsha, D.A., "Aerospace Finishing Systems for Naval Aviation", 42nd International SAMPE Symposium, 4-8 May 1997, pp. 662-676.

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Appendix A

Metric Conversions

To Convert From

To

Multiply By

To Convert From

To

Multiply By

Area

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