CA1308581C - Flush head fastener - Google Patents

Abstract

Improvement to Flush Head Fastener Abstract of the Disclosure The invention is a flush head fastener (14) for joining two or more structural elements (10,12) together. The structural elements have a fastener hole (18) therein with one of the structural elements (10) being countersunk to accommodate the head (30) of the fastener. The fastener includes head (30) and shank (40) portions and a layer of material (50) joined to the top of the head of the fastener which can readily be deformed to fill any gaps between the head and the countersink.
Thereafter, the excess material can be removed leaving an aerodynamically smooth surface. The surface also will have no electrical discontinuities. The procedure for installing the fastener includes the steps of 1) providing two or more structural elements (10, 12) having a countersunk fastener hole (18) therein 2) thereafter the fastener is installed in the hole 3) the layer of material (50) is deformed so as to fill any gaps between the head of the fastener and the countersink, and 4) thereafter, excess material is removed leaving a smooth external surface. Preferably, the step of deforming includes the step of heating the layer of material to a point where it is readily deformable.

Description

1 3085~t 1 "Improvement to Flush Head Fastener"

3 Technical Field The invention relates to the field of fa~teners and, 6 in particular, to flush head fasteners which can be 7 blended into the external contour of 2 vehicle 6uch as 8 the aerodynamic surfaces of an aircra~t~

10~ackground Information 12Flush head asteners are used where surface6 must be 13smooth and free of protrusions, for example, the Aero-14dynamic surfaces of aircraft: where, in order to reduce 15drag, flush head fasteners are almost exclusively used.
16~he ~astener hole is countersunk at a total included 17angle of around 100 degrees for metals and 130 degrees 18for composites with the total underhead included angle of 19ths fastener made approximately equal thereto. In order 20to achieve a smooth aerodynamic contour, tha countersink 21overall diameter is usually made slightly largQr than the 22diameter of the fastener h~ad such that upon installa-23tion, the top ~urface of the fa6tener head will be below 24the aerodynamic 6ur~ace. This is also neces6ary because 25the fastener at the ~unction of the upper and underhead 26~ur~ace o~ the head normally includes a small cylindrical 27land to eliminate any sharp edges that would otherwise 28exist at the ~unction thereof. What is usually left upon 29in6tallation i8 a 6mall countersunk ring about the 30fastener head. Of cour8e~ if the fastener is being 31installed on a curved surfacQ, the countersunk hole will 32be elliptical in shape and thus the gap around the 33fastener head becomes uneven. The6e 60rt of gaps are 34commonly called "eyebrows". Another surface discon-35tinuity i~ the typical internal wrenching recess in the 36fastener head, i.e., phillips, slot, tri-wing, allen, 1 ~8581 1 etc. Thus, if it is necessary to have an absolutely 2 6mooth aerodynamic surface or if no electrical discon-3 tinuities can be tolerated, the recess and the exposed 4 portion of the countersunk hole "eyebrows~ ~ust be s covered.

7 Typically, puttie~ are u~ed; conductive ones if 8 electrlcal continuity i6 required. Another method i6 to 9 use covers bonded to the fastener head or retained by means of a protrusion on the cap which i jammed into the 11 internal wrenc~ing reoess in the fastener head. Neither 12 of these methods pr~vide any guarantee of a ~mooth 13 external contour and, of course gaps and discontinuities, 14 while reduced in 6ize, will always be present.
16 The internal wrenching recess can be eliminated if 17 the fastener can be torqued from the shank end. For 18 example, the internal wrenching recess can be provided at 19 the threaded end of the fastener. However, in many applications, access to the opposite end of the fastener 21 i8 restricted so thi6 solution is not often viable.
22 Another approach i5 to use detachable (frangible) 23 external torquing surfaces attached to the head of the 24 fastener designed to break off after the proper torque level is reached. Such a fastener ha6 several disad-26 vantages: there i6 no way of removing it except by 27 mean6 of an easy out (requiring drilling into the 28 fastener head itself) and a rough external head surface 29 remains. Thus, such fasteners are not used on external aerodynamic 6urfaces.

32 Another problem occurs when the aerodynamic surface 33 is made of composite materials, such as graphite Piber 34 reinforced epoxy resins. When used in the wings of 3'5 aircraft which contain fuel, a significant safety problem 1 can exist: for while the structure of the wing may be 2 made of composite materials, the fasteners used are 3 typically metal in order to obtain sufficient structural 4 6trength. The difference in electrical conductivity between the compo~ite material and the fastener causes 6 lightning to become attached to the metallic ~astener 7 head and the fastener can conduct current into the 8 interior of the wing and cause internal arcing in ide the 9 fuel tanX. If fuel vapors are present in the tank, such arcing can cause an explosion. Therefore, it is nQces-ll sary to provide some ~ort of protection for external 12 metal fasteners used in composite structures to prevent 13 such lightning ~trikes attaching thereto.

Typically, a ceat of paint is applied over the 16 serodynamic surfaces. However, the gap between the 17 ~astenQr head and its counter ink hole can cause the 18 paint to thin and crack around the outer edge of the l9 fa~tener. This thinning and cracking of the paint in turn causes a difference in electrical conductivity, 21 which causes electrical streamers to form around the 22 ~dges of the fastener heads. These streamers create an 23 electrical field and increase the chance oS a lightning 24 strike hitting a fastener head. To eliminate this po6sibility various devices to cover the fastener head 26 have been proposed. For example, U.S. Patent No.
27 4,630,16~ "Lightning Protection Fastener" by J. ~unt, 28 disclose6 a fastener having a metallic head and shaft and 29 a dielectric cap covering a top portion of the head.
When the head and cap are installed into the countersunk 31 hole in the skin, the cap being resilient tends to fill 32 the gap between the top portion of the head and the inner 33 side walls of the co~ntersink. The cap forms with the 34 kin an aerodynamic surface which i5 sufficiently 1 continuous to receive and maintain a crack free coat of 2 paint having a uni~orm thic~ne~s.

4 The problem here i6 that the cap encompa6ses a port~on of the underhea~ conical ~ur~ace and when 6 installed is wedged between th~ fastener head and the 7 counter6unk hole. ~his presents a problem in heavily 8 loaded structures in that the resilient material between 9 the fastener head and coun~ersink will tend to extrude under cyclic loading and the fastener ~ay eventually 11 become 1008e fittlng therein. Additionally, since the 12 cap $8 $nstalled w~th a given thickness and tolerance it 13 may not always provide an absolutely smooth external 14 contour. Another approach to providing prote~tion for metal fasteners in composite structures i6 found in U.S.
16 Patent No. 4,681,497 "Encapsulated Fastener" by I.
17 Berecz. Here the head and shank are encapsulated in 18 composite material and thus, it is subject to the same 19 deficiencies as the preceding example. Another approach can be foun~ in U.S. Patent No. 4,502,092 "Integral 21 Lightning Protection System for Compo~ite Aircraft Skins"
22 by E.T. Bannink, Jr. et al. Here a plastic strip is 23 placed over the outer sur~ace and a potting compound is 24 used to fill the space over the fastener. In U.S. Patent 4,628,402 I'Lightning Protection of Fasteners in Composite 26 Material" by J.H. Covey, one embodiment uses a rubber 27 plug over the fastener to fill the space above the 28 fa6tener caused ~y the layers of dielectric material.
29 None of the above fastener systems designed for use with composite materials are suitable for use with metal 31 6urfaces. Thus, there is no available fastener design 32 that can provide a smooth continuous aerodynam$c sur~ace 33 in both composite and metal structures and which does not 34 require a paint be ~pplied over the surface, etc.

Thus, it is a primary object of the subject invention to provide a flush head fas~ener that is capable of producing an aerodynamically smooth external surface.
It is another primary object of the subject invention to provide a flush head fastener that is capable of providiny an aerodynamically smooth external surface when the surface is curved.
It is another object of the subject invention to provide a flush head fastener that produces no electrical discontinuities on the surface after installation.
It is a further object of the subject invention to provide a flush head fastener that can be used with both metal and composite materials.

Disclosure of the Invention ~ ccordlng to one aspect, the present inventlon is a flush head fastener for joining two or more structural elements together, the structural elements havlng a fastener hole therein wlth a recess for receiving the head of the fastener ln communication with one surface of the structural elements, the fastener comprising: a shank; a head ~oined to said shank; and a layer of non-resilient material permanently joined to and generally coverlng the entire top surface of said head of the fastener, said layer of materlal readily reconfigurable relative to said head and the structural elements; such that upon installation of the fastener in the hole said layer of material can be reshaped by deformation filling any gap between said head, said layer of material of the fastener and the recessed end of the hole, and the excess material above that needed to blend said layer of material into substantial conformity with the one surface of the structural elements can be readily removed.
Preferably, the layer of material has a melting point substantially lower than the mel~ing point of the flush head fastener and the structural elements such that after installation of the fastener, the layer of material can be heated to a temperature wherein it is readily deformable relative to the fastener and the structural elements.
If the external surface is made of titanium and a steel fastener is used then the layer of material could be made of a soft non-heat treated titanium alloy or the pure metal itself.
This layer can be friction welded to the head of the fastener.
Other means that can be used are brazing, soldering, and bonding, etc. If the external surface i~ aluminum and a titanium or steel fastener is used, then the layer of material can be pure aluminum or a compatible solder. Whether or not the layer of material would be heated or not would depend on the combination of materials selected which wlll be obvious to those skilled in the art. However, if the external surface is a composite material having an organic matrix, then, ideally, the layer of material is selected from the large group of presently available thermoplastic resins. Here the thermoplastic resin layer can easily be heated to a point where it is deformable. In most applications the heating of the layer of material need only raise the temperature to a point wherein it is easily deformable. It must be understood that the layer of material could be heated to a point wherein it melts.
While the above description has been directed at a fastener havlng a metal head and shank, it must be understood that they could be made of a plastic materlal or an organic matrix composite material as long as the plastic or matrix materials have a higher service temperature ~higher melting point) and~or are harder than the layer of material. Furthermore, the fastener could be deslgned for use in a spherical countersink, dimpled, or counterbored hole and the like.
Having thus described the fastener it is now necessary to describe the method of installing. The steps are basically as follows:
forming a countersink fastener hole in said structural elements, said countersink located on an external surface of one of said structural elements; providing a fastener with a shank and head portions and a layer of material permanently joined to the top surface of said head, said layer of material readil~
reconfigurable relative to said head and shank and structural elements; positioning said fastener in said hole with said head of said fastener in contact with said countersink; deforming said layer of material such tha~ any gaps between said countersink and said head are filled; and removing excess materi.al of said layer of material such that the upper surface of said deformed layer of material is continuous with the external surface contiguous to said fastener.

-` 1 30858 1 1 In one preferred embodiment, the head of the fastener 2 incorporates an internal wrenching recess and the layer of 3 material includes a wrenching tool access hole therethrough 4 in communication with the internal wrenching recess in the fastener head. A plug is provided, made of the same 6 material as the layer of material or other compatible 7 materials, which can be inserted ~nto the hole and/or 8 recess a~ter torquing the fa6tener in place and which will 9 become part of the fa8tener after the 6tep of deforming.
11 In another preferred embodiment the layer of material 12 incorporates frangible external wrenching surfaces ~oined 13 to the layer of material which can be broken off when the 14 proper torque level is reached.
16 ln still another preferred embodiment of the invention the 17 underhead surface of the fastener is conical in shape having 18 a total included angle substantially equal to the total 19 included angle of the countersink hole. The layer of material extends out from the head of the fasténer having a conical 21 shaped underslde contiguous with the conical underslde of 22 the head and having a total included angle substantially equal23 thereto. Thus, when this fastener i8 ln~talled lnto the 24 countersunk fastener hole, no gap will exist between the materiel layer and counter~ink, and, little or no deformation 26 of the layer of material need t8ke place, although some 27 deformation may be required to insure line-to-line contact.
28 Thereafter, the excess material can be removed in the manner 29 previously described.
31 The novel features which are believed to be charac-32 teristic of the invention, both as to its organization 33 and method of operation, together with further ob~ects --" 1 30858 1 1 nnd advantages thereof, will be better understood from 2 th~ ~ollowing de~cription in connect~on with the accom-3 pany~ng drawing~ in which pre~ently preferred embodimQnt~
4 of the invention are ~llu~trated by way of exampleg. It is to be expre~sly understood, however, that the drawings 6 are for purposes of illustration and description only and 7 are not intended as a definition of the limits of the 8 invention.

10Brief Description of the Drawinqs 12Illustrated in Figure 1 is a partial cross-6ectional 13view of the fastener installed in structural elements;
14the right ~ide of the figure showing ths fa~tener a8 15provided while on the left side the ~astener is Rhown 16after lnstallation is complete.

18Illustrated in Figure 2 is an enlarged partial 19cross-6ectional view of the fastener shown in Figure 1.
21Illustrated in Figure 3 is a partial cro~s-sectional 22view of a second embodiment of the fastener installed in 23~tructural elements; with the right side of the figure 24showing the fastener as provided while on the left side 25the fastener iB shown after installation i~ completed.

27Illustrated in Figure 4, is a partial cross-section-28al view of a third embodiment o~ the fastener installed 29in structural elements; with the right side of the figure 30showing the fastener in the as provided and the left side 31of the fiqure the rastener i~ shown a~ter installatlon is 32complQted.

34Illustrated in Figure 5, 6, 7, and 8 are partial 35cros~-sectional views of the fastener illustrating the 1 process by whlch the fa~tener i6 ~ nstalled ~n the 2 structural elements.
4 Best Mode for carryi~q Out the I~vention 6 Illustrated in Figure 1 i~ a partial cross-sectional 7 view of two structural elements, indicated by numerals lo 8 and 12, respectively, joined together by the improved g flush head fas~ener~ generally indicated by numeral 14.
lo The fastener 14 is primarily designed for use on air~
11 craft, and, in particular, to She fastening of external 12 skins or other external structural elements to interior 13 structures. Thus, ~or purposes of illustration, struc-14 tural element 10 can be considered the outer skin of an aircraft having an exterior surface 16 exposed to 16 aerodynamic forces and lightning strikes as well as 17 illumination by thrsat radars. The fastener 14 is shown 18 installed as provided on the right side of the figure and 19 after install~tion i8 compl~ted on the left.
21 The fastener 14 i6 mounted in a countersunk fastener 22 hole 18 with the counter~unk portion 20 in communication 23 with the surface 16 and having a total included angle 24 indicated by numeral 22 ~typically lOo degrees for metal~
~nd 130 degrees for composites). Fastener 14 includes a 26 head portion 30 having a top surface 32 and a conical 27 ~haped underhead sur~ace 34 having a total included angle 28 substantially equal to the an~le 22 of the countersink 29 portion 20. The fastener further includes a small cylindrical land 36 at the ~unction of the top and 31 underhead 6urfaces 32 and 34, respectively. Thi~ land 36 32 is machined to ~liminate any sharp edges that would 33 normally exist at the junction of the surfaces 32 and 34.
34 However, this land 36 is extremely small and is ~hown out of proportion for illustration purposes only. Thus, a ~ 1 3nsssl 1 volume indicated by numeral 38 i~ created between the 2 fastener head and countersink. Also the counter~ink 3 portion 20 ~, typically, larger than the overall 4 diameter of the land 36, thus, the volume 38 is ring ~haped. On a contoured ~urface the volume 38 takes on a 6 more crescent shape.

8The fastener 14 further inclu~es a shank portion 40 9having a threaded end 42 with an internal wrenching 10recess 43 and is held in place by means of a nut and 11washer, indicated by numeral6 44 and 46, respectively.
12Thus, a fastener can be locked in place by use of tools 13applied from the shank end of the fastener.

15Still referring to Figure 1 and additionally to 16Flgure 2, which is an enlarged view of a portion of the 17left side of Figure 1, a layer of material 50 having a 18thickness 51 (right sidQ of Figure 1) i6 ~oined to the 19top surface 32 of the fastener 14. If the structural 20element 10 is steel, or aluminum, the fastener is 21typically high strength heat treated steel or titanium.
22The layer 50 would be made of a soft aluminum or titanium 23material or a ~older such as those covered by Federal ~4Standard QQ-S-571, Tin Alloy, Tin-lead Alloy and Lead 25Alloy Solders. If ~older is used for the layer of 26material it can be heated to a very soft, readily 27deformable state (having a melting point well below that 2Rof the aluminum or titanium structural element 10 or the 29head and 6hank of the fastener) such that the structural 30element or fastener are not effected at this tempera-31ture. By a process to be subsequently di~cussed, the 32layer of material 50 is deformed 80 as to ~ill the 33volume 38 about the head portion 30 (between the land 36 34and counter6in~ portion 20) and, thereafter, the excess 35material, indicated by dotted lines and numeral 52 on the -" 1 3Q858 1 1 left side of F$gure 1, is removed. The remaining materlal 2 layer 54 and surface 16 are blended into one smooth and contlnuous 3 surface. The overall thickness 51 of the layer of material 50 is 4 therefore selected eO provide sufficient material to fill the volume 38 and the remaining unfllled portion of the counterslnk 20 6 above the surface 32 of the head portion 30. This thlckness will 7 depend upon the allowable (selected) tolerances of the various 8 dimensions of the countersink portion 20 and the head portion 30.
9 The end result ls that no discontinuities will exist ehat could reflect incoming radar slgnals, the surface is aerodynamically 11 smooth and there are no electrical discontinuities.

13 If the structural element 10 is a composite material 14 having an organic matrix, such as a thermoplastic or thermosetting resin matrix with fiberglass or Kevlar filamentary reinforcements, 16 then the layer 50 could be a thermoplastic material such as a 17 polycarbonate; one ehat melts upon heatlng and does not cure 18 (with a melting point well below the melting point of the matrix-19 material). Another suitable material is Nylon 6 manufactured by the Dupont Corporation, Wllmington, Delaware. In either case 21 the metal fa6tener i8 protected from lightning strike sttachment.
22 If the structural element 10 were made of graphite reinforced 23 PEEK (polyetherether~etone) manufactured by ICI, Americas, 24 Inc., Wilmington, Delaware, then the layer of material could be carbon flber reinforced Nylon 6 to eliminate electrical 26 discontinuities. As can be seen, whether the layer of material is 27 a metal, dielectric material or a dielectric material loaded with 28 conductive fillers will depend upon the particular application.

Illustrated ln Flgure 3 is 8 partial cross-sectional 31 view of a second embodlment of the fastener indlcated by 32 numesal 14A. ~he fastener 14A differs only in havlng an 33 internal wrenching recess 60 in the head 30A and the 335 * 1~ 4~k )~

1elimination of the internal wrenching recess 43 (see 2Figure 1) from the shank 40A. All of the other pre-3viou~ly me~tioned dimensions of the fastener are identi-4cal, and thus, have the same identifying number. The 5layer of material, indicated by numeral 50A, having a 6thickness 51A, incorporates external wrenching ~urfaces 762 ~onnected by means of a necked down frangible portion 864. Thu8, the nut 44 cDn be prevented from rotation AS
9the fa6tener is torqued. However, in most applications lothe fastener would be engaqin~ a plate nut ~not shown).
11When the proper torque level is reached the wrenching 12surfaces 62 can be "torqued off" 6eparating at the 13nec~ed down portion 64. Therea~ter, the same procedure 14i8 used, which again will be subsequently discussed, to 15produce the completely installed fastener with excess 16material 52A removed leaving remaining layer 54A, as 17illustrated on the left side of Figure 3.

19Illustrated in Figure 4 i8 a th~rd embodiment of the 20fa6tener. The ~astener 14B includes a wrenching recess 21 60B in the head 30B and ~he ...... layer of material 50B
22includes an aperture 70 in commun~cation with the rece6s 2360i3. Thus, a8 in the embodiment illustrated in Figure 3, 24the fastener 14B can be prevented from rotation when the 25nut 44 is torqued ~or vica versa). A plug 71, preferably 26made of the same materi~l ~8 the 12yer of material 50B
27or another suitabl~ ~aterial iB provided which can be 28inserted lnto the recess 60B after the fastener 14B has 29been locked in place ~left side of Figure 4). The layer 3050B, having a thickness indicated by 51B, include~ an 31underhead surface 72 with an included angle 74 egual to 32angle 22 of the countersin~ hole and extends upward and 33outward so as to almost completely fill the countersink 34(leaving only the small volume 38' between the land 36 35~nd countersink 20). $his makes the filling of the ~ `

-` 1 30858 ~

1 countersink by the process to be ~ubsequently discussed 2 much eas~ er. Once the fa~tener ~5 installed, the ~nsert 3 . or plug is installed (driven in place by a hammer or the 4 l~ke) prior to rsmoving excess layer material S2B, s leaving remaining l~yer 54B. If the fastener needs to be 6 removed, the plug can be "dug cut", preferably after 7 heating to soften it.

9Having thus described three embodiments of the 10improved fastener, it is readily apparent that t~e 11features of each can be interchanged. For example, the 12concept of a conically shaped underhead of the layer of 13materials 50B in the embodiment illustrated in Figure 4, 14can be used in the embodiments illustrated in Figures 1 15and 3. Since the discussion of the physical embodiments 16of the fasteners prior to installation and after final 17installation has been completed, it is nacessary to now 18discu6s the procedure for installing fastener.

20Referring to Figure 5, it can be seen that the 21fastenQr which was ~llustrated in Figure 4 i8 used a~ an 22example. After the fastener has been locked in place, ~n 23curved structural elements 10A and 12A, the plug 71 is 24driven into place as illustrated. Thereafter, as 25illustrated in Figure 6, heat is applied to the material 26layer 50~ and plug 71 until, the materialS are 80ft and 27ea5ily deformable (as illustrated, a heatlamp 75 is 28uced). Illustrated in Figure 7, the layer 50B and plug 2971 are tapped down to firmly drive the layer of material 3050B into contact with the countersink 20 and filling 31volume 38'. This step typically deforms the layer of 32materials 50~ 60 that it has an irregular surface, 33indicated by numeral 76. This can easily be accomplished 34by use of a rivet gun, partially illustrated and indi-35cated by numeral 77. In many ca es, little or no impact -` 1 308581 1 will be requixed and the ~tep can be eliminated. However, 2 with the Qmbodiments illustrated in ~ and 3, cons~d~rable 3 impact may be neces~ary to achieve filling of the volume 4 38'. It must also be noted that if the layer of the material and plug is ~oft at room temperature heat may 6 not be neCes~ary. Furthermore, if heat ~6 applied, it 7 may be po~sible to soften the layer of material 50B
8 sufficiently 60 that little tapping ox driving is 9 neces~ary. Regardless, of whether the layer of material 50B raquires heatin~ and drivin~, heatin~ alone or 11 driving alone, the next step as illustrated in Figure 8 12 is to remove the excess material 52BI leaving remaining 13 layer 54B~. This is easily accomplished by use of a 14 chisel 80 having a knife edge 82 conforming to the contour of the curved surface 16'. It is important to 16 note that if the structural element lo is a compo~ite 17 material with an organic matrix, the layer of the 18 material 50B and plug 71, ideally would be a thermoplas-19 tic material, which would require heating to make it deformable.

22 In some applications it may be desirable to heat the 23 layer o~ material to A temperature wherein it melts and 24 i~ easily flowable provided the 6tructural elements can withstand the temperature. This would eliminate the need 26 for impact. The main advantage of melting is the 27 elimination of any gaps between the ~astener head portion 28 and the countersin~ improv~nq electrical continuity, etc.
29 However, ~ome difficulty may be encountered in removinq excess material and providing a smooth surface. Melting 31 could easily be accomplished when using a solder, 32 thermoplastic or even thermosetting materials such as an 33 epoxy.

1 3085~1 1 Thus, the main advantages of the fastener are 2 readily apparent: 1) it can provide a ~mooth aero-3 dynamic 6urface. 2) can provide a 6urface with no 4 electrical discontinuities. 3) can be used with both metal and composite ctructures. 4) can provide lightn-6 ing strike protection. Other advantages are a reduced 7 tendency to rotate and loosen, in some applications the 8 tolerances on the countersink end of the hole can be 9 relaxed and it may be possible to eliminate aorrosion resistant coatings or sealants on the fastener or hole 11 where such coatings or 6ealants are presently required.
12 Furthermore, while primarily de~igned for u~e on aircraft 13 the fastener has application on external surfaces of land 14 vehicles and ships, interna; fluid ducts in general or on any surface for cosmetic purposes. In this latter 16 application it ls apparent that the concept is applicable 17 to fnsteners having plastic heads and shanks as long as 18 the plastic has a higher strength and~or hi~her melting 19 point than the layer of material.
21 While the invention ha6 been described with reference 22 to particular embodiments, it should be under6tood that 23 the embodiments are merely illustrative as there are 24 numerous variations and modifications which may be made by tho6e skilled in the art. Thus, the invention is to 26 be con6trued as bein~ limited only by the spirit and 27 scope o~ the appended cla$ms.
2~
29 Industrial Applicability 31 The invention has applicability to vehicles and 32 ~tructures and, in particular, to aircraft.

Claims (23)

1. A flush head fastener for joining two or more structural elements together, the structural elements having a fastener hole therein with a recess for receiving the head of the fastener in communication with one surface of the structural elements, the fastener comprising: a shank a head joined to said shank; and a layer of non-resilient material permanently joined to and generally covering the entire top surface of said head of the fastener said layer of material readily reconfigurable relative to said head and the structural elements; such that upon installation of the fastener in the hole said layer of material can be reshaped by deformation filling any gap between said head, said layer of material of the fastener and the recessed end of the hole, and the excess material above that needed to blend said layer of material into substantial conformity with the one surface of the structural elements can be readily removed.

2. The flush head fastener as set forth in claim 1, wherein said layer of material has a melting point substantially lower than the melting point of said head and shank of the fastener and the structural elements such that after installation said layer of material can be raised to a temperature wherein its reconfigurable properties are enhanced.

3. The flush head fastener as set forth in claim 1, wherein said recess is a countersink.

4. The flush head fastener as set forth in claim 3, wherein the countersink is conical in shape having a specific total included angle, the fastener further comprising: said head having a conically shaped underhead having a total included angle generally equal to the total included angle of the countersunk hole; and said layer of material extending outward and upward from the top surface of said head, said layer of material having a conically shaped underhead contiguous and aligned with and having a total included angle generally equal to said conical underside of said head.

5. The flush head fastener as set forth in claim 4, wherein said layer of material is a conductive material.

6. The flush head fastener as set forth in claim 5, wherein said layer of material is a metal.

7. The flush head fastener as set forth in claim 5, wherein said layer of material is a dielectric material loaded with conductive fillers.

8. The flush head fastener as set forth in claim 4, wherein said layer of material is a dielectric material.

9. The flush head fastener as set forth in claim 4, wherein said layer of material is selected from the group consisting of a thermoplastic and thermosetting materials.

10. The flush head fastener as set forth in claims 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9 wherein frangible external wrenching surfaces are attached to the top of said layer of material.

11. The flush head fastener as set forth in claim 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9 wherein: said head incorporates an internal wrenching recess; said layer of material incorporates a wrenching tool access hole therethrough in communication with the said internal wrenching recess; and a plug made of a compatible material for filling said recess in said head and said hole in said layer of material.

12. The flush head fastener as set forth in claim 11, wherein said plug is made of a material generally identical to said layer of material.

13. A method of joining two or more structural elements together comprising the steps of: forming a countersink fastener hole in said structural elements, said countersink located on an external surface of one of said structural elements, providing a fastener with a shank and head portions and a layer of material permanently joined to the top surface of said head, said layer of material readily reconfigurable relative to said head and shank and structural elements; positioning said fastener in said hole with said head of said fastener in contact with said countersink;
deforming said layer of material such that any gaps between said countersink and said head are filled; and removing excess material of said layer of material such that the upper surface of said deformed layer of material is continuous with the external surface contiguous to said fastener.

14. The method as set forth in claim 13, wherein the countersink is conical in shape having a specific total included angle, the fastener further comprising. said head having a conically shaped underhead having a total included angle generally equal to the total included angle of the countersunk hole; and said layer of material extending outward and upward from said head having a conically shaped underhead contiguous with and having a total included angle generally equal to the conical underside of said head.

15. The method as set forth in claim 14, wherein, prior to the step of deforming said layer of material, said layer of material is heated to a temperature wherein it is readily reconfigurable.

16. The method as set forth in claim 15 wherein during said step of heating, said layer of material is heated to a point wherein it melts.

17. The method as set forth in claim 14 or 15 or 16 wherein said fastener includes frangible external wrenching surfaces attached to the top of said layer of material, the method including the additional step of breaking off said frangible external wrenching surfaces prior to said step of deforming said layer of material.

18. The method as set forth in claim 14 or 15 or 16 wherein said head of said fastener incorporates an internal wrenching recess and said layer of material incorporates a wrenching tool access hole therethrough in communication with said internal wrenching recess, the method further including the steps, prior to said step of deforming said layer of material, of: providing a plug made of a material generally identical to said layer of material for filling said recess in said head, and said hole in said layer of material; and installing said plug in said hole and said recess.

19. The method as set forth in claim 13 or 14 or 15 or 16, wherein said layer of material is a conductive material.

20. The method as set forth in claim 19, wherein said layer of material is a metal.

21. The method as set forth in claim 19, wherein said layer of material is a dielectric material loaded with conductive fillers.

22. The method as set forth in claim 13 or 14 or 15 or 16, wherein said layer of material is a dielectric material.

. .

23. The method as set forth in claim 22, wherein said layer of material is selected from the group consisting of thermoplastic and thermosetting resins.