FASTENING DEVICE AND METHOD FOR COMPOSITE STRUCTURES

Abstract: Fastener, e.g., stud (16), installation in a composite panel (12), such as a graphite epoxy structure, having a longitudinal conductivity substantially greater than the transverse conductivity is achieved in a manner providing increased conductivity between fastener assembly and the panel to which it is attached. Through panel countersink and fastener configuration, e.g., by use of grounding inserts (20 & 22), increased electrical contact is made to conductive fibers in the composite material which run at right angles to the central axis of the fastener.

Description: FASTENI.NG DEVICE AND METHOD FOR COMPOSITE STRUCTURES This invention relates to fastener arrangements for composite structures ^ d more particularly to methods and associated fastener arrangements for establishing electri¬ cal continuity between fasteners and composite structures. Electrical conductivity through graphite epoxy com¬ posite structural joints which would utilize current fastening techniques- would tend to crea,te CD potential structural damage, and C2L possible hazardous fuel ignition from lightning currents- in certain applications. This potential damage and/or ignition hazard potential is the result of reduced conductivity of composites such as graphite epoxy laminates when fastened together. It will be recognized that the resistivity of graphite epoxy 3 6 composites varies from 10 to 10 that of an equivalent aluminum sheet. Besides the aforementioned problem of utilizing current fasteners for composite structural joints, the utilization of current fasteners of the ground stud type for electrical bonding in fuel hazard areas becomes more critical due to the increased resistivity of such composite structures, viz., graphite epoxy laminates which are utilized to replace aluminum structure in air- frames. Static charge dissipation and fault current loads from system failures must satisfy the requirements of an electrical bonding and grounding system for composite structured aircraft, As a consequence, current fasteners of the ground stud type for use in composite structure electrical system current return paths, as well as bonding for RF potential and static charge applications, would not appear as satisfactory. The patent literature, e.g., U, S. Patents 3,105,950 and 3,145,362, are noted for their showing in Figure 4 of bolt fastening devices employing dual countersink con¬ figurations, A further similarly configured fastening construction is shown in U. S, Patent No. 2,030,088. It is accordingly an object of the present invention 05 to provide the means in combination with a fastener for providing increased electrical conductivity between fastener and fastened composite structures. It is a further object of the present invention to provide a method of assembling fasteners and composite 10 structures in a manner providing increased conductivity between structures and fastener. .t is yet another object of the present invention to provide means for increasing electrical conductivity between fasteners and graphite epoxy structures by increase- is ing the number of electrical connections between the fastener and the graphite filaments of the epoxy structures. A full understanding of the invention, and of its further objects and advantages and the several unique aspects thereof, will be had from the following description ,20 when taken in conjunction with the accompanying drawings in whichJ FIGURE 1 is illustrative of prior art fastener connec¬ tion installed in a composite panel structure; FIGURE 2 is a sectional view illustrative of an 25 apparatus and method in accordance with a preferred embodi¬ ment of the present invention for providing increased conductivity between fastener and composite structure; FIGURE 3 is a view of the assembly of Figure 2 blown apart to show the manner of assembly of the fastener arrange 30 ment of Figure 2; and, FIGURE 4 is an expanded and enlarged view of the cross- section of the present fastener assembly shown in Figure 2 deemed helpful in understanding the manner in which increased conductivity is- achieved, 35 Turning now to Figure i, it will be seen that in the prior art stud assembly attachment to a composite panel OMPI the stud assembly indicated generally by the numeral 10 includes a bolt having a head portion 14 and a shank portion 16 passing through the composite panel structure 12, The usual washer ' s and locking nut 18 complete the 05 assembly of grounding stud 10 to composite panel structure 12, Turning now briefly to Figure 2, it will be noted that a pair of countersunk insert deyices 20 and 22 are utilized on opposite sides of composite panel member 12 to provide 10 . increased and improved conductivity-between the shank 16 of fastener 10 and composite panel structure 12, Conductiv¬ ity to tϋe shank 16' is .Jζrom two directions, viz,: II from composite panel member 12 to countersunk insert device 22 through the lock washer and 5 through, to head portion 14 which is integral to shank 16; and, 21 from composite panel member 12 to countersunk insert device 20 and through locking nut 18 which, is threaded to shank 16. 0 t can be seen in Figure 3 that countersunk insert device 22 comprises a generally tubular shaped body of corrosion resistant stainless steel or titanium having two continuous integral coaxial portions 30 and 32 disposed about the central axis 40 of the fastener 10 shown in place 5 Figure 4. Countersunk insert device 22, being of tubular shape about coaxial hole 31 comprises an upper frustro conical shaped portion 30 and a lower cylindrically shaped flange-like portion 32. Graphite epoxy structures such as panel member 12 0 shown in Figure 4 have much better conductivity in a longi¬ tudinal direction Cthe current .flow stays in the plies), than in the transverse direction, such as in the direction 40. along the central axis of a fastener inserted in the graphite epoxy structure. The reason for this is that the 5 currents ust cross, the epoxy boundary between plies and run transverse to the graphite filaments which lie along OMPI YA the longitudinal direction 42 of current flow within the graphijte epox structure 12. This difference in conductiv¬ ity can be as high as 200;1, The countersunk insert devices ' 20. and 22 in accordance with a preferred embodiment of the present invention provide the increased conductiv¬ ity between shank, of a fastener, such as the grounding stud 20 shown in Figure 4, and. the longitudinal fibers of the conductive structure running in the direction of current flow represented by the numerals 42 in Figure 4, The ratio of transverse Csurface- ^ to-surfacel to longitudinal G≥dge-tα-r-edgeL resistivity is approximately 2Q0;1 for a graphite epoxy* type composite structure 12, Further, it should be noted that the resistivity of graphite epoxy 3 6 composite structures varies from 1Q to 10 that of an equivalent aluminum sheet. Therefore, the optimum bonding and ground resistance for a fastener such as a ground stud 2Q shown in Figure 4 in a composite panel structure 12 can be successfully realized by utilization of countersunk insert devices 2Q and 22 which provide electrical contact : between shaft portion 16 of a fastener and an increased number of the longitudinal fibers of the composite structure 12. This is due to the frustro conical shaped portion 30 . of countersunk insert 22. Transverse conductivity is also improved between the faying surface of countersunk insert device 22 and the ' outer plies of composite structure 12. Countersunk insert device 20 is shaped identical to countersunk insert device 22 hereinbefore described, and is located in a complementary-shaped countersunk hole on the other side of composite structure 12, The hereinabove described embodiment of the present invention teaches a metho and structures including the utilization of countersunk insert devices which, will achieve the ■ maximum transfer of currents between the shanks of fastening deyices and transversely extending fibers in composite panel structures in which, fasteners are utilized. The longitudinal conductivity property of these advanced composites* such as graphite epoxy composites may be OMPI utilized in accordance with further embodiments of the invention b those skilled in the art, recognizing that the aforementioned countersunk insert devices may be utilized where, for example, fasteners such as self-locking nuts, .shear collars, fixed nut plates or floating nut plates are utilized. The present countersunk insert devices! * may* be utilized on either side of a pair of composite panel members being secured together by standard types- of fasteners, having shank portions 16 which must be connected to longitudinally extending fibers in the composite panel members utilizing the increase conductivity teachings of the e e ed embodiment of the present inven¬ tion.

Claims: What is claimed is: 1. In combination, an electrical connection between the shank portion of a fastener having a central axis extending in a first direction and conductive fibers of a composite structure extending in a plane perpendicular to said central axis, said electrical connection comprising: a generally tubular shaped body of electrically conductive material coaxially disposed about the ahank portion of said fastener; said generally shaped tubular body of electrically conductive material having two continuous coaxial portions, a first coaxial frustro conically shaped portion having an outer surface abutting said conductive fibers of said conductive structure, and a second cylindri- cally shaped flange-like portion having an inner surface abutting an outer surface of said composite structure. 2. The invention according to claim 1 wherein said outer surface of said frustro conically shaped portion has an angle of inclination of about 50 degrees with respect to the central axis of said frustro conically shaped portion. 3. The method of securing an electrical connection between the shank portion of a fastener and a composite structure comprising the steps of: providing a countersink electrically conductive insert device having a . frustro conically shaped outer wall portion; countersinking a hole in said composite structure in a manner providing an inner wall surface and have an angle complementary with respect to the frustro conically shaped outer wall portion of said countersink electrically conductive insert device thereby exposing electrically conductive fibers in said composite structure; inserting said countersink electrically conductive insert device in said countersunk hole with said frustro conically shaped outer wall portion abutting said exposed electrically conductive fibers; . inserting said shank portion through the central axis of said countersunk electrically conductive insert device; and then, securing said fastener about said composite structure. *-^ϋRE OMPI

Inventors: OLSON GLENN OLIVER, THOMSON LELAND EDWARD, OLSON, GLENN, OLIVER, THOMSON, LELAND, EDWARD
Applicants: BOEING CO, THE BOEING COMPANY
Application year: 1981
Main classification: H01R4
IPC codes: H01R4, H01R9