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Fig. 11.22. Polycrystalline Diamond Drills5

PCD drills are very expensive, the number of holes obtained per drill and the fewer changes required make them cost effective. It should be noted that PCD drills cannot be used with free hand or non-rigid setups; the point will immediately chip and break if any vibration or chatter is present during drilling.

11.3.6 Reaming

Although it is desirable to drill the final hole size in one pass, it is often necessary to ream the hole to final diameter. Reaming is done with HSS or carbide reamers at about one half the drilling speed (e.g., 500-1000rpm). In some composite-to-metal structures, fasteners are installed clearance fit in the composite and interference fit in the metallic structure for metallic fatigue life enhancement. In this situation, the final hole diameter would be drilled in the composite-to-metal stack-up, the composite skin would then be taken down, and the holes reamed to provide a clearance fit for the fastener. When the stack-up is reassembled, the fasteners would be installed clearance fit in the composite and interference fit in the metal.

11.3.7 Countersinking

Countersinking should only be used where protruding head fasteners will not satisfy design requirements. In general, countersinking reduces joint strength and fatigue life. During countersinking for flush head fasteners, it is important not to countersink too deep and create a knife-edge condition in the countersunk member. A knife-edge creates a significant stress riser, as well as allowing the fastener to tilt and rise up on the countersink surface, resulting in low joint yield strength and reduced fatigue life. As a general rule, at least 0.025 in. or no more than 80% of the sheet thickness, whichever is less, should not be countersunk, as shown in Fig. 11.23. Piloted countersinks are helpful in centering the countersink tool in the hole and depth control can be obtained with microstop cages.

0.020 in. Minimum for Metals

0.025 in. Minimum, or 80% Thickness Maximum, for Composites

Fig. 11.23. Countersinking

Radius Required to Match Head-to-Shank Radius on Fastener

Liquid Shim

0.020 in. Minimum for Metals

0.025 in. Minimum, or 80% Thickness Maximum, for Composites

Fig. 11.23. Countersinking

Countersinking of composite structures is similar to that done in metals with one additional caution - the area where the countersink transitions into the hole must have the same radius as the fastener head-to-shank radius. Again, due to the low interlaminar shear strength of composites, this condition can result in cracks and delaminations under the force of fastener installation. Countersinking cutters for composites are normally made of solid carbide, steel bodies with carbide inserts, or steel bodies with PCD inserts.

11.4 Fastener Selection and Installation

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, with examples shown in Fig. 11.24. There are also many other miscellaneous fasteners such as quick release multiple piece fasteners, latches, straight pins, headed pins, lock pins, cotter pins, threaded inserts, retaining rings and washers. A typical structural fastener usage for a fighter aircraft is shown in Fig. 11.25.

Since the number and types of fasteners used in aircraft construction is large and complex, aerospace companies have developed fastener usage policies for their various programs, which establish the policies and criteria for the selection and application of fasteners. Typical contents include usage limitations, selection criteria, hole size/callout information, strength allowables, material compatibility and protection, and lists of approved fasteners. Minimum edge distances and fastener spacing requirements are specified in the fastener usage policy or on the individual engineering drawings. Typical edge distances are 2-3D with typical fastener spacings of 4-6D, where D is the hole diameter.

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. Some examples of the compatibility of fastener materials with structural materials are shown in Table 11.1. It should also be noted that fasteners, especially steel fasteners, are often coated for corrosion protection (e.g., cadmium) and the compatibility of the coating with materials being joined needs to be considered. For example, cadmium-plated fasteners should not be used with titanium due to the potential of stress corrosion cracking.

Mechanical fastener material selection for composites is important in preventing potential corrosion problems. Aluminum and cadmium coated steel fasteners will galvanically corrode when in contact with carbon fibers. Titanium (Ti-6Al-4V) is usually the best fastener material for carbon fiber composites, based on its high strength-to-weight ratio and corrosion resistance. When higher strength is required, cold worked A286 iron-nickel or the iron-nickel based

Flush Universal

Counterbore

Installed Rivets

Flush Protruding

Flush Protruding

Protruding Bolts and Nuts

Flush

Flush

Hi-Lok Collar

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