Fastening assembly including washer for sealing the assembly for lightning strike protection in composite structures
A washer, a fastening assembly including the washer and a composite structure including the washer sealing the assembly for internal lightning strike protection. The washer includes one or more concentric ribs that are dielectric rings on both sides. When used (e.g., with a nut and bolt) internal to a structure, especially a composite structure, the washer seals the fastener hole (i.e., that the bolt passes through) and contains any sparking and hot gasses that may arise in the fastener hole from entering the structure.
1. A composite structure comprising:
a composite skin;
an internal substructure, said composite skin being attached to said internal substructure, said internal substructure including metal substructure;
a plurality of holes through said composite skin; and
a fastening assembly attaching said composite skin to said metal substructure through ones of said holes, said fastening assembly providing lightning strike protection internal to said composite structure, said fastening assembly comprising:
a fastener passing through one said hole to said metal substructure,
a washer with concentric grooves in both sides, said fastener further passing through said washer,
a sleeve, said fastener further passing through said sleeve, said sleeve fixed to said fastener against said washer and holding said fastener in place, and
a soft dielectric material on both sides of said washer, said soft dielectric material filling each concentric groove and radiating outward and inward from said each concentric groove, wherein said hole is sealed internally by said washer and said soft dielectric material.
2. A composite structure as in claim 1 , wherein said concentric grooves are midway between a central orifice and, the perimeter of, said washer.
3. A composite structure as in claim 2 , wherein said concentric grooves have a depth and width at least one tenth (10%) of the washer thickness.
4. A composite structure as in claim 2 , further comprising a lubricating dielectric material coating said washer.
5. A composite structure as in claim 4 , wherein said lubricating dielectric material coating is a ceramic coating.
6. A composite structure as in claim 4 , wherein said lubricating dielectric material coating is a resin.
7. A composite structure as in claim 4 , wherein said structure is an aircraft, said fastener is a screw, said sleeve is a nut and said metal substructure is in a wing fuel tank.