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"O" Ring Fasteners at Seal Groove

"O" Ring Fasteners at Seal Groove

Variable Fastener Torque*

Countersink Radius too Small*

Countersink Radius too Small*

Fastener Misalignment*

Fastener Misalignment*

* In Conjunction with Unshimmed Gap Conditions.

Fig. 11.27. Fastener Installation Defects5

substructure (or both) as the fastener is being installed and pulls the two pieces together. In fuel tanks, channel seal grooves are often used to help prevent fuel leakage. In addition, fasteners with O-ring seals can be used to further prevent leakage. It has been found through experience that these are potential areas for interlaminar cracking. While good clamp-up of the fastener is certainly desirable, over-torquing fasteners can also result in cracking. If the countersink radius is too small and does not match that of the fastener head-to-shank radius, the fastener can apply a concentrated point load and cause matrix cracking. Likewise, fastener misalignment, where the hole and the countersink are not properly aligned, can result in point loading and cracking. In addition, fastener cocking during loading (Fig. 11.28) can result in point loading and lead to progressive damage during fatigue cycling.

In any mechanically fastened joint, high clamp-up forces are beneficial to both static and fatigue strength. High clamp-up produces friction in the joint, delays fastener cocking and reduces joint movement, or ratcheting, during fatigue loading. Most holes eventually fail in bearing caused by fastener cocking and localized high bearing stresses.12 To allow the maximum clamp-up in composites without locally crushing the surface, special fasteners have been designed for composites that have large footprints (large heads and nut areas that bear against the composite) to help spread the fastener clamp-up loads over as large an area as possible. Washers are also frequently used under the nut, or collar, to help spread the clamp-up loads. In general, the larger the bearing area, the greater the clamp-up that can be applied to the composite, resulting in improved joint strength.13 In addition, tension head, rather than shear head, fasteners are normally used in composites, because they are not as susceptible to bolt bending during fatigue or fastener pull-through during installation in thin structure.

11.4.2 Solid Rivets

Traditionally, riveting has been the most prevalent method of building aerospace structure. However, with greater amounts of composite and titanium structure replacing aluminum structure on newer aircraft, the amount of riveted structure will decrease in the future, since rivets are not normally used in titanium or composite structure. However, rivets are, and will remain, an important fastening method for aerospace structure. Rivets should be restricted to joints that are primarily loaded in shear with some secondary tension loading allowed, but should not be used where the primary loads are in tension.

Solid rivets are one piece fasteners made from malleable metal installed by squeezing or vibration driving. They are used in permanent applications where the lowest cost, reliable fasteners are desired. Aluminum rivets are by far the most widely used, with 2117-T4 aluminum ("AD" rivet) the most prevalent. Where higher strengths are required, 7050-T73 or 2024-T4 aluminum rivets can

"zu v

Fig. 11.28. Fastener Cocking in Single Lap Shear12

be used instead of the AD rivet. However, 2024-T4 rivets require refrigeration before driving, and they have somewhat lower stress corrosion resistance.

Universal and countersunk rivet heads are most common rivets used in aircraft construction. Universal, or protruding, head rivets are used internally in structures not exposed to the air stream. Since a universal head does not require countersinking the skin, the joint is capable of higher bearing loads than a countersunk joint. The standard countersunk rivet has a 100° included angle. Since the countersunk joint is inherently weaker, a larger number of rivets are usually required to compensate for the reduced bearing and shear strength.

During rivet installation, several physical changes take place: (1) the rivet diameter expands to fill the hole, (2) the hardness of the rivet increases due to work hardening, and (3) the manufactured head is formed through plastic deformation. One advantage of rivets is that they do expand during installation to give a tight fit in the hole.

Pneumatic rivet guns are driven by compressed air and are classified as light, medium, and heavy hitting. Light hitting guns are used to install 0.09-0.125 in. diameter rivets, while medium hitting guns are used for 0.15-0.19 in. diameter rivets. The heavy hitting guns are used for larger diameters. There are two types of gun sets, one for the universal head rivets and one for countersunk rivets. During manual riveting, a bucking, or backing, bar is used to form the manufactured head during the riveting process. Bucking bars come in different shapes, sizes, and weights. The weight of the bucking bar should be proportional to the size of the rivet being installed. A number of rivet defects are shown in Fig. 11.29. Perhaps the most serious is the clinched rivet due to improper bucking during installation. The rivet forms to one side which can later trap moisture and potentially cause a corrosion problem.

Rivets are rarely used in composites for two reasons: (1) aluminum rivets will galvanically corrode when in contact with carbon fibers, and (2) the vibration and expansion of the rivet during the driving process can cause delaminations. If rivets are used, they are usually a bimetallic rivet consisting of a Ti-6Al-4V pin with a softer titanium-niobium tail, which are installed by squeezing rather than

Proper Head above Bucktail Clinched Spread

Installation Moldline too Flat Bucktail Head

Proper Head above Bucktail Clinched Spread

Installation Moldline too Flat Bucktail Head

Shave Over Improper Material

Head Driven Bucking Bar too Hard

Shave Over Improper Material

Head Driven Bucking Bar too Hard

Cracked Slanted Scarred Head

Head Bucktail Manufacturing Head Not Flat

Cracked Slanted Scarred Head

Head Bucktail Manufacturing Head Not Flat

Cold Working Misaligned Gun Removed Misaligned During Driving Bucking too Quickly Gun

Fig. 11.29. Installed Rivet Defects

Cold Working Misaligned Gun Removed Misaligned During Driving Bucking too Quickly Gun

Fig. 11.29. Installed Rivet Defects vibration driving. In addition, the head that is upset must be against metallic structure and not composite. There are also hollow end solid rivets designed to allow flaring of the ends without damaging expansion, when used in double countersunk holes in composites.13

11.4.3 Pin and Collar Fasteners

Pin and collar fasteners are the most commonly used fasteners for permanent installations, where there is no requirement to remove the fastener. The pin, similar to a bolt, is used with a self-locking or swaged-on-collar, which cannot be removed with typical tools without destroying the collar or pin. Pin and collar fasteners are high strength fasteners made from either Ti-6Al-4V, A286 iron-nickel, or Inconel 718. Typical head designs include protruding tension head, protruding shear head, 100° full countersink, and 100° reduced countersink.

A typical pin and collar fastener is the Hi-Lok fastener shown in Fig. 11.30. A Hi-Lok fastener consists of a threaded pin and a collar. The threaded pin is essentially a modified bolt, while the collar is basically a nut with a breakaway groove, which controls the amount of torque and preload on the pin. Hi-Lok fasteners are available in both flush and protruding heads, and as shear and tension-shear fasteners. Hi-Lok fasteners can be installed either clearance-fit or interference-fit. The fastener pin is usually made from Ti-6Al-4V with an A286 nut. Titanium nuts are occasionally used, but the threads tend to gall if they are not coated with an anti-galling lubricant, which then adversely affects long-term clamp-up. A hex key is inserted into the fastener stem to react the torque applied to the nut. The nut is tightened down until a predetermined torque level is achieved, and the top portion of the nut fractures. Washers can be used under the head to help spread the bearing load on the surface.

Lockbolts are another common pin and collar fastener that can be installed by either pulling or swaging the collar from the backside. A typical pull type Lockbolt installation sequence is shown in Fig. 11.31. Lockbolts differ from Hi-Loks in that Hi-Loks have true threads that the nut is threaded onto, while Lockbolts have a series of annular grooves that the collar is swaged into. Once swaged in place, they cannot back off (loosen) and have superior vibration resistance.13 Lockbolts are available with flush or protruding heads, as shear or tension pull types, and as shear stump types. Lockbolts are normally lighter and cost less to install than bolt-nut combinations of the same diameter. There are two precautions that need to be followed when installing Lockbolts in composite structure: (1) pull type Lockbolts exert quite a bit of force on the composite, due to the pulling action necessary to swage the collar onto the pin. If the composite is thin, fastener pull-through is a real possibility, and if there are any unshimmed gaps, cracking and delaminations can occur when the fastener pin fractures; and (2) if backside Lockbolts (called stump Lockbolts) are installed in

Installation Tool Contains Hex Key to Prevent Fastener from Spinning

Installation Tool Engages Nut and Applies Torque Thread Nut onto Fastener

Top Portion of Nut Fractures When Predetermined Clamp-up Achieved

Installation Tool Contains Hex Key to Prevent Fastener from Spinning

Installation Tool Engages Nut and Applies Torque Thread Nut onto Fastener

Top Portion of Nut Fractures When Predetermined Clamp-up Achieved

Fig. 11.30. Installation of Hi-Lok Fastener12

composites, they should be installed by a piece of automated equipment, where careful control of the swaging operation can be exercised.

A third type of pin and collar fastener is the Eddie bolt shown in Fig. 11.32. As shown in the installation sequence, the collar initially threads onto the pin, but then is swaged into flutes on the pin to provide a positive lock. The advantage of Eddie bolts is that they do provide a positive lock and will retain clamp-up loads better than Hi-Loks that rely on torque only. However, the fasteners and installation tools are expensive, the sockets are subject to wear, and the installation procedure is more difficult. They are often specified in inlet duct areas, where there is the potential for a fastener pin coming loose and flying into the engine blades and damaging the engine.

Collar Placed over Pin

Installation Tool

Pulls on Pin Drawing Parts Together

Installation Tool Swages Collar into Fastener Grooves Forming Permanent Lock

Top of Fastener Pin Fractures at Predetermined Clamp-up

Collar Placed over Pin

Installation Tool

Pulls on Pin Drawing Parts Together

Installation Tool Swages Collar into Fastener Grooves Forming Permanent Lock

Top of Fastener Pin Fractures at Predetermined Clamp-up

Fig. 11.31. Installation of Pull Type Lockbolt12

3 Lobes

Swage-locking Collar Washer

5 Flutes

5 Flutes

3 Lobes

Swage-locking Collar Washer

Collar

Collar

Hex Pin Recess

Installation Socket

Collar Material Swaged into Flutes to Provide Positive Swage-lock

Hex Pin Recess

Installation Socket

Collar Material Swaged into Flutes to Provide Positive Swage-lock

Fig. 11.32. Eddie Bolt Positive Lock Fastener12

11.4.4 Bolts and Nuts

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 with wrenches, may be used when there is access to both sides. Nut plates and gang channels are used when one-sided access is applicable. The shanks of all structural bolts must be long enough to insure that there are no threads bearing in the joint. Extra washers may be used to adjust the grip length. However, the use of lock washers is often prohibited because they can damage the protective finish on the structure being joined. A washer should be used under both the bolt head of protruding bolts and the nut, to help distribute the load and prevent damaging the surface finish.

There are many head styles including protruding tension flange head, protruding shear head, 100° full countersink, and 100° reduced countersink. For structural bolts, the threads are rolled and the heads are forged for additional strength. Since both of these operations induced residual compression stresses in the fastener, they also improve their resistance to stress corrosion cracking. Bolts smaller than 0.190 in. in diameter are considered to be non-structural; they are used to attach brackets and other miscellaneous hardware. The threads on non-structural bolts can be either machined or rolled. Some nuts are self-locking while others are not. Cotter pins and safety wire are used to secure those without self-locking features. Self-aligning nuts (up to 8°) are also available for situations in which the structural members are not parallel. There are also nuts designed specifically for tension or shear loading applications.

When installing structural bolts, a high preload (i.e., high torque) is desirable from the standpoint of fatigue and vibration. However, since a high preload places the bolt under tension stress, too high a preload can increase the susceptibility of certain fastener materials to stress corrosion cracking. Therefore, bolt preload is usually restricted to around 50-60% of the bolt yield strength. It is also important that all of the fasteners in the joint are preloaded to close to the same torque, so that the fasteners share the load equally. If there is variable torque on the fasteners, those that are preloaded to higher values will carry a substantially higher portion of the load than those with lower torque. Low bolt preloads can result in joint rotation, misalignment, loosening, and the formation of gaps between mating parts. Low bolt preloads also reduce the fatigue life of the installed fastener. For highly loaded bolted joints, torque values are often specified on the engineering drawing.

High strength bolts, along with nut plates or gang channels, are used when there is a requirement for skin removal. A typical nut plate and gang channel are shown in Fig. 11.33. Note that three holes are required for each nut plate: two small holes for rivets to attach the nut plate to the structure and then the main fastener hole, which is threaded to accept the threads in the screw. There are a wide variety of nut plate configurations available for different installations, including self-aligning nut plates. Because they do not require two rivet holes per fastener, gang channels are frequently used where there are long rows of fasteners to be installed. They are attached at periodic points along the channel, thus saving installation labor. Bolts with nut plates or gang channels do not perform as well as blind fasteners, either in static or fatigue loading, due to increased joint deflection and compliance of the fastening system.12

Nut Plate

Rivet Hole (One of Two)

Nut Plate

Channel

Clip

Gang Channel

Fig. 11.33. Nut Plate and Gang Channel14

Channel

Clip

Gang Channel

Fig. 11.33. Nut Plate and Gang Channel14

11.4.5 Blind Fasteners

Blind fasteners are used in areas where there is limited, or no access, to the backside of the structure. However, the solid pin and collar fasteners previously discussed are usually preferred, because they are stronger, provide better clamp-up, and have better fatigue resistance. Two types of blind fasteners are the threaded core bolt type and the pull type shown in Fig. 11.34. The threaded core bolt (Fig. 11.35) relies on an internal screw mechanism to deform the head and pull it up tight against the structure, while the pull type blind fastener uses a pure pulling action to form the backside head. Higher clamp-up forces and larger

Threaded Core Bolt Blind Fastener

Pull Type Blind Fastener

Fig. 11.35. Installation of Blind Fasteners12

Pull Type Blind Fastener

Fig. 11.35. Installation of Blind Fasteners12

footprints are obtainable with the threaded core bolt, leading to longer fatigue life. However, the pull types install quicker, are lighter, and are less expensive. The use of blind fasteners in composite materials is not recommended unless the upset portion of the fastener bears against metal structure. This being said, there are some special blind fasteners with large foot prints that can be placed directly against composite surfaces.

11.4.6 Fatigue Improvement and Interference Fit Fasteners

Since fatigue cracks often initiate at fastener holes in metallic structure, methods such as cold working fastener holes and interference fit fasteners have been developed to improve fatigue life. Both cold working and interference fit fasteners set up residual compressive stress fields in the metal immediately adjacent to the hole, as shown in Fig. 11.36. The applied tension stress during fatigue loading must then overcome the residual compressive stress field, before the hole becomes loaded in tension. The fatigue improvement due to cold working in 2024-T851 aluminum is shown in Fig. 11.37. Somewhat surprisingly, the fatigue strength of the material with a tight fastener in a coldworked hole is actually somewhat better than the base material.

Zone of Compressive Residual Stress Extends One Radius from Edge of Hole

Tension Zone Beyond Compression Zone

Surface Surrounding Hole is Yielded Beyond Elastic Limit

Fig. 11.36. Residual Stress State Around Coldworked Hole Source: Fatigue Technology, Inc.

Cold working of holes is usually conducted using either the split-sleeve or split-mandrel method. Both methods involve pulling a mandrel through the hole that expands the hole diameter, creating plastic deformation of material around the hole and a resulting residual compressive stress field. The residual stress field, depending upon the material and the amount of expansion, will extend approximately one radius from the edge of the hole.15 In the split-sleeve process (Fig. 11.38), a stainless steel split sleeve is placed over a tapered mandrel and inserted into the hole. The hole is coldworked when the largest part of the mandrel is drawn back through the sleeve. After cold working, the sleeve is removed and discarded. In the split-mandrel process, a collapsible mandrel is placed in the hole, and as the mandrel is withdrawn, it expands to coldwork the hole.

Interference fit fasteners are also frequently used in metallic structure to improve the fatigue life. When the interference fit fastener is installed in metal, it also plastically deforms a small zone around the hole, setting up a compressive stress field, which again is beneficial when fatigue loading is primarily in tension. The amount of interference can vary, depending on structural requirements, but it is usually in the range of 0.003-0.004 in. In some highly loaded holes, both

2024-T851 Al

2024-T851 Al

104 105 106 107

Cycles to Failure

Fig. 11.37. Fatigue Life Improvement with Cold working

104 105 106 107

Cycles to Failure

Fig. 11.37. Fatigue Life Improvement with Cold working

Sleeve is Slipped onto the Mandrel Which is Attached to a Hydraulic Puller Unit.

Mandrel

Split Sleeve

Mandrel and Sleeve are Inserted into Hole.

Mandrel is Pulled Through Sleeve and Hole. The Combined Thickness of the Sleeve and Mandrel Expands the Hole.

Fig. 11.38. Split-Sleeve Cold working15

cold working and interference fit fasteners are used. While both cold working and the use of interference fit fasteners are proven methods of improving fatigue resistance, both increase assembly costs and should only be specified when they are really needed.

In composite-to-metal assemblies, it is possible to have interference fit in the metallic structure and clearance fit in the composite. This is normally done by drilling the hole through the stack-up. The composite skin is then removed and opened up to a larger diameter with a reamer. The composite skin is then re-assembled to the metallic structure, and the fastener is installed clearance fit through the composite skin but interference fit in the metallic substructure. Even then, it is important to be careful when installing the interference fit fastener, i.e. excessive vibration from a rivet gun has been known to produce delaminations around the composite hole even though it is a clearance fit.

Since composites do not plastically deform, there is no fatigue life improvement with using interference fit fasteners. However, a potential benefit to having some interference fit fasteners in composite structure is that they will help "lock up" the structure and prevent any movement at the joint (called ratcheting) during fatigue loading. Previous work has shown that installing standard interference fit fasteners in composites, with as little as 0.000 7 in. of interference, can lead to cracking and interlaminar delaminations.12 To eliminate this problem, special sleeve type interference fit fasteners (Fig. 11.39) have been designed so that the sleeve spreads the load evenly during installation to

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