Q

Fastener Type

Fig. 11.25. Typical Fighter Aircraft Fastener Usage

Table 11.1 Fastener Material Compatibility

Structural Materials Being Joined

Fastener Material

Preferred

Acceptable

Prohibited

Aluminum to Aluminum

Titanium to Titanium Austenitic Stainless Steel Nickel Base Alloys

Titanium to Aluminum Carbon/Epoxy

Anodized

Aluminum

Titanium

Titanium

Titanium

Titanium Cadmium Plated Steel A286

A286 Alloy Steel

Inconel 718 Aluminum

Aluminum Coated Fasteners

A286 Aluminum

Inconel 718 Aluminum Coated Fasteners

Inconel 718 Aluminum

A286 Aluminum Coated Fasteners fastener is long enough, so that the threads are never loaded in bearing or shear, i.e. no threads should be allowed in the hole. In addition, no more than three, and no less than one thread, should be showing when the nut is installed and tightened to the proper torque. Fasteners exposed to the outer moldline of the aircraft are normally installed "wet" by dipping the end of the fastener in polysulfide sealant prior to installation. This helps in preventing moisture that can cause corrosion from intruding into the structure.

11.4.1 Special Considerations for Composite Joints

When a hole is placed in a composite laminate, it creates a stress concentration and the overall load-bearing capability of the laminate is severely reduced. Even a properly designed mechanically fastened joint exhibits only 20-50% of the basic laminate tensile strength.11 The various failure modes for composite joints are shown in Fig. 11.26. The only acceptable failure mode is when the joint fails

Fig. 11.26. Composite Joint Failure Modes5

Fig. 11.26. Composite Joint Failure Modes5

in bearing, since the joined members do not separate catastrophically. Bearing failures are characterized by localized damage, such as delaminations and matrix crazing around the hole. Potential causes for the other failure modes shown include:

1 Shearout: Insufficient edge distance, or too many plies oriented in the load direction.

2 Tension: Insufficient width, or too few plies oriented in the load direction.

3 Cleavage-Tension: Insufficient edge distance and width, or not enough cross-plies (e.g., +45° and -45°).

4 Fastener Pull-through: Countersink too deep, or use of a shear head fastener.

5 Fastener Failure: Fastener too small for laminate thickness, unshimmed gaps or excessive shimmed gaps in joint, or insufficient fastener clamp-up.

Like hole drilling, fastener installation in composites is more difficult and damage prone than for metallic structure. Some of the potential problems with fastener installation are shown in Fig. 11.27. As previously discussed, unshimmed gaps can cause cracking of either the composite skin or the composite

Skin

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