Abstract:
A fabrication method for providing a relatively smooth surface associated with lightning strike protection for composite materials joined with fasteners recessed below the surface is described. The method includes coating at least one of a head of the fastener and a preformed dielectric insert with an adhesive, inserting the preformed dielectric insert into the recess such that the adhesive engages both contours of the recess and the preformed dielectric insert, and removing any excess adhesive from the surface area of the composite material and a top surface of the insert.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to lightning strike protection, and more specifically, to fabrication and installation of preformed dielectric inserts. 
         [0002]    There are thousands of recessed fasteners in a typical aircraft surface. In one aircraft wing example, these fasteners penetrate from the skin exterior and extend into the fuel tank. One currently contemplated fabrication plan is application of an uncured sealant over the fasteners as part of an integrated lightning strike protection system. The sealant is cured in place, then shaved with a cutter to flushness with the surrounding wing skin. In composite aircraft structures, the sealant functions as a dielectric insulating layer, insulating the fasteners from lightning strikes. 
         [0003]    Mechanical devices with reciprocating metal blades are known, and utilized to shave an upper portion of a cured in place dielectric sealant material such that it is substantially flush with a surface. However, these devices cannot be utilized in certain production environments. In one example, the number of places to be shaved in a given area may preclude use of the mechanical device. In another example, the surface where the sealant has been placed may be contoured, which also may not be easily compatible with the described shaving process. 
         [0004]    The above described process is labor intensive, and the in-place curing and shaving of dielectric sealant may effect aerodynamic dimensional tolerances. Another issue is that sealant application is an integral step in the assembly process rather than prior to or concurrent with the assembly process (i.e., more labor, time, and interference with other production steps). Accordingly, there is continued effort in the art for improvement in regard to installation of dielectric materials in certain applications. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    In one aspect, a fabrication method for providing a relatively smooth surface, the smooth surface associated with lightning strike protection for composite materials joined with fasteners recessed below the surface is provided. The method includes coating at least one of a head of the fastener and a preformed dielectric insert with an adhesive, inserting the preformed dielectric insert into the recess such that the adhesive engages both contours of the recess and the preformed dielectric insert, and removing any excess adhesive from the surface area of the composite material and a top surface of the insert. 
         [0006]    In another aspect, a structure is provided that comprises a first mechanical member comprising a surface having at least one bore therein, at least one fastener configured to attach the first mechanical member to a second mechanical member, the fastener configured for insertion into the bore, an area between a top surface of the fastener and the surface of said first mechanical member defining a recess, a preformed insert having substantially complimentary dimensions to the recess, and an adhesive. The adhesive is applied to at least one of the fastener and the preformed insert which is inserted into the recess such that the adhesive engages both the recess and the insert. 
         [0007]    In still another aspect an assembly is provided that is associated with lightning strike protection for joined composite materials, where the composite materials are joined with fasteners recessed below the surface of the composite materials. The assembly comprises a preformed dielectric insert comprising at least one layer of dielectric material and a top surface configured to be substantially planar with the surface of the composite materials, and an adhesive. The preformed dielectric insert is configured for insertion into a recess associated with one of the fasteners and has a shape complimentary to the recess. The adhesive is configured to cause the preformed dielectric insert to adhere to the recess. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIGS. 1A and 1B  are side and top views respectively of a recessed fastener with respect to a top surface of the item being fastened. 
           [0009]      FIGS. 2A and 2B  are side and top views respectively of the assembly of  FIG. 1 , with a layer of adhesive deposited on a top surface of the fastener. 
           [0010]      FIGS. 3A and 3B  are side and top views respectively of a preformed dielectric insert with removable backing. 
           [0011]      FIGS. 4A and 4B  are side and top views respectively of the preformed dielectric insert inserted into the recess and contacting the adhesive shown in  FIG. 2 . 
           [0012]      FIGS. 5A and 5B  are side and top views respectively of the removable backing being removed from the preformed dielectric insert. 
           [0013]      FIGS. 6A and 6B  are side and top views respectively of the completed assembly including the fastener, adhesive, and preformed dielectric insert. 
           [0014]      FIG. 7  is a side view of a layer of adhesive deposited on exposed surfaces of a preformed dielectric insert. 
           [0015]      FIG. 8  is a flowchart illustrating a method associated with the utilization of preformed dielectric inserts. 
           [0016]      FIG. 9  is a flowchart describing fabrication of dielectric inserts. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    The methods and apparatus described herein are related to a preformed dielectric insert that is applied over recessed fasteners, such as those utilized in aircraft fuselage and wing fabrication. Utilization of the preformed dielectric inserts, rather than cured in place sealant, reduce installation and inspection time, waste, chances of damaging the aircraft surface material, for example, composite material. The preformed dielectric inserts also increase precision in meeting dimensional tolerances to provide better lightning strike protection. By covering fasteners utilized in aircraft and aircraft wing fabrication, essentially burying the fasteners under dielectric material, it is thought that lightning will tend to be attracted to areas that are better configured to dissipate the electrical charge that is present in a lightning strike. 
         [0018]      FIGS. 1A and 1B  are side and top views respectively of a recessed fastener  10  with respect to a top surface  12  of the item  14  being fastened. Item  14 , in a particular application, is a portion of an airframe assembly, but is representative of any item that might be utilized in conjunction with one or more recessed fasteners  10 . In a particular embodiment, fastener  10  has a top portion  16  that is configured to engage a complimentary shaped portion of a bore  20  in the item  14 , causing at least a part of the engagement between the fastener  10  and item  14 . Top portion  16  of fastener  10  includes a fastener top surface  22 . In  FIG. 1 , the portion of bore  20  and the top portion  16  of fastener  10  are both frusta-conical in shape, but other geometries for fasteners and bores are known. In various embodiments, fastener  10  is a rivet, a bolt, screw, or other fastening device. 
         [0019]      FIGS. 2A and 2B  are side and top views respectively of the assembly of  FIG. 1 . A layer of adhesive  30  has been deposited on the top surface  22  of the fastener  10 . In various embodiments, adhesive  30  is a thermoset, or thermoplastic elastomer, or a pressure sensitive adhesive. Examples of thermoset adhesives includes a polysulfide-based or a polythioether-based polymeric material based on polymer resin system. In a specific embodiment, the polysulfide has a backbone structure containing a —(CH 2 CH 2 —O—CH 2 —O—CH 2 CH 2 )— functionality where C, H, and O represent the presence of carbon, hydrogen, and oxygen atoms, respectively. 
         [0020]      FIGS. 3A and 3B  are side and top views respectively of an insert  50  and an optional removable backing material  52 . In a particular embodiment, insert  50  is a preformed dielectric insert. In the side view of  FIG. 3A , the insert  50  (and removable backing material  52 ) is shown in a position above the fastener  10  and adhesive  30  combination. As is evident from the illustration, insert  50  is formed to have a shape substantially complimentary to the portion of bore  20  that is not occupied by fastener  10 . It should be noted that a thickness of adhesive  30  is dependent on the type of insert utilized. In the illustrated embodiment, insert  50  has a substantially frusta-conical shape though both the bore  20  and insert  50  could have other complimentary shapes. In particular embodiments, backing material is colored, or color coded, for identification. It is contemplated that color coding could be utilized to identify different dielectric insert configurations. Embodiments of inserts  50  that do not utilize a removable backing material are also contemplated. In any of the embodiments, the inserts  50  may be color coded as well. 
         [0021]    The inserts  50  are described below as being fabricated from a moldable dielectric material. However, other embodiments are contemplated, including, but not limited to, ceramic materials and composite materials. In a specific embodiment, the insert  50  is fabricated from the same composite material as item  14 . 
         [0022]      FIGS. 4A and 4B  are side and top views respectively of the insert  50  inserted into the above described bore  20  and contacting the adhesive  30  described above with respect to  FIG. 2 . In one application, a user applies pressure to backing material  52  (or the insert  50  itself in embodiments that do not incorporate backing material  52 ), forcing insert  50  into the adhesive  30  and causing a portion of the adhesive  30  to be displaced. The adhesive  30  then surrounds the frusta-conical surface of the insert  50 , and a portion of the adhesive  30  may escape and come into contact with the top surface  12  of the item  14  being fastened.  FIG. 4  illustrates the displacement of a portion of adhesive  30 . 
         [0023]      FIGS. 5A and 5B  are side and top views respectively of the removable backing  52  being removed from the insert  50  illustrating that the removable backing  52 , in one embodiment, is a peel off backing. The top surface  12  is wiped to remove excessive adhesive from a top area of the composite material and a top surface of the insert before the adhesive dries. In an alternative embodiment, removable backing  52  is attached to insert utilizing a pressure sensitive adhesive. 
         [0024]      FIGS. 6A and 6B  are side and top views respectively of the completed assembly including the fastener  10 , adhesive  30 , and insert  50 , which as described above is a preformed dielectric insert, configured for lightning strike protection. 
         [0025]    In an embodiment, insert  50  is fabricated from one or more dielectric materials, and may include single or multiple functional layers. More specifically, the insert  50  may include single or multiple functional layers that are fabricated from dielectric materials. In certain embodiments, insert  50  is fabricated from two or more laminar functional layers with different conductivities, and in specific embodiments, the different functional layers alternate one or more times. In these embodiments, at least one of the functional layers is a dielectric material that is in contact with the adhesive  30  which is in contact with the top surface  22  of the fastener  10  (shown in  FIG. 1 ), and one of the functional layers is a conductive material. 
         [0026]    In various embodiments, the dielectric material is a ceramic, a thermosetting material, or a thermoplastic material that can be filled with organic or inert particulate or fiber fillers. The thermosetting material may be an epoxy composite or an elastomer. Elastomer embodiments include polysulfide-based polymeric material and polythioether-based polymeric material. In a specific embodiment, the polysulfide-based material has a backbone structure containing a —(CH 2 CH 2 —O—CH 2 —O—CH 2 CH 2 )— functionality where C, H, and O represent the presence of carbon, hydrogen, and oxygen atoms, respectively. In an alternative embodiment, the thermosetting material is formulated so that the material is extrudable and/or injectable. 
         [0027]    The above described configurations for insert  50  all include dielectric properties and provide a certain amount of protection against lightning strikes when properly fabricated and installed. 
         [0028]      FIG. 7  is a side view illustrating an embodiment where adhesive  30  has been deposited on exposed surfaces of a preformed dielectric insert  50  configured to engage an item  14  being fastened.  FIG. 7  illustrates that it is not always feasible to coat the top surface  22  of the fastener  10 . This is particularly true when installing inserts  50  on an underside of a device, for example, the underside of an aircraft wing or when the viscosity of the adhesive is too thin for application to the top surface  22  when it is oriented in an upside down configuration as shown in  FIG. 7 . Additionally, uncured sealant may be placed on an insert  50  and the combination of the two frozen until the insert  50  is to be used. Of course, the installation represented in  FIG. 7  is applicable to embodiments of insert  50  that do not incorporate the backing material  52  and the thickness of the adhesive  30  depends on the adhesive type and the composition of the dielectric inserts  50  being utilized. 
         [0029]    The insert  50  of  FIG. 7  is adhered to both the fastener head and walls of the recess with the above described adhesive. Prior to installation, insert  50  can be inspected for structural inconsistencies and dimensional requirements prior to installation, or alternatively (and additionally), prior to shipment from a supplier. 
         [0030]    As can be discerned from the above description, inserts  50  are fabricated to conform to the shape of a recess prior to installation due to their complementary shape. In various embodiments, these inserts are configured with an outer surface, with reference to an installed insert, that has a specific contour, tailored to provide aerodynamic advantages. 
         [0031]      FIG. 8  is a flowchart  100  illustrating a method associated with the utilization of the above described preformed dielectric inserts. Referring specifically to flowchart  100 , molded dielectric inserts are selected  102  based on the specific application, for example, based on at least one of diameter, and countersink dimensions such as depth and an angle associated with the recess where the fastener is located. The selected dielectric inserts may then be inspected  104  for structural inconsistencies. It should be noted that an inspection process for the dielectric inserts is likely also implemented as part of the insert fabrication process. The acceptable dielectric inserts, those that pass the inspection process, are set aside. The fastener head or the applicable portion of the insert is then coated  106  with a dielectric adhesive. 
         [0032]    Dielectric inserts that have passed the inspection process are placed  108  into the recesses such that adhesive is in contact with both the fastener and the insert. In an embodiment, pressure is applied to the dielectric insert, through the backing material or directly, so that an amount of adhesive is squeezed out of the recess, thereby insuring that adhesive is also in contact with side walls (e.g., the complimentary surface of the dielectric insert. The optional backing material, which in an embodiment described above is color coded, is removed  110  from the dielectric inserts and any excess adhesive that is around the top surface of the dielectric insert and/or the surface area of part (e.g., aircraft wing) being fabricated is removed  112 . 
         [0033]    Fabrication of the dielectric inserts is also contemplated.  FIG. 9  is a flowchart  150  describing fabrication of the dielectric inserts described above. Referring to flowchart  150 , various molds with specific countersink depths, diameters, and angled walls are provided  152 , consistent with the various recessed fasteners utilized in a fabrication application. The examples utilized above, include aircraft fuselage and wing fabrication, but the embodiments described herein are not limited to the listed applications. Once the molds are provided  152 , a dielectric is injected  154  into the molds and optionally a removable backing material, color coded in one embodiment, is applied  156  to the dielectric inserts. Backing material is typically applied before or after the curing process insert so as not to interfere with the curing process. In specific embodiment, backing material is typically placed in the mold and brought into contact with the dielectric material during the injection process. Alternatively, the backing material is applied after the insert is removed from the mold, so as not to interfere with the curing process. 
         [0034]    Once cured, the dielectric inserts are removed  158  from the mold, along with the attached backing material (applied before or after curing) and inspected  160  for structural inconsistencies, for example, any pinholes that might occur as a result of the curing process. Particularly, preformed single functional layer inserts are fabricated in a two-faced mold with cavities conforming to a shape of a recess associated with a fastener. In a particular application, the cavity conforms to the shape of the recess as the cavity is machined out of one of the two sides of the mold. In one embodiment of mold, the side of the mold coinciding with the top surface of the insert  50  is machined to produce a specific contour that has been tailored to provide an aerodynamic advantage, for example, the contour is consistent with a contour of the area surrounding the recess into which the insert  50  is to be placed. 
         [0035]    In alternative embodiments, the mold is fabricated out of Teflon, PTFE, other fluorinated polymeric materials, the mold is fabricated out of non-fluorinated polymeric materials, or the mold is fabricated utilizing one or more metals that have been subsequently sprayed or coated with a release agent. 
         [0036]    In the thermosetting material embodiment, the material is injected into the mold and allowed to cure in the mold until dimensionally stable, then the release film or removable backing material is applied, the release film being chosen so that the sealant being utilized does not permanently adhere to the release film. 
         [0037]    In the various embodiments described herein, the installation of preformed dielectric inserts replaces installation of an uncured dielectric material to improve lightning strike protection by providing uniformity in their formation. A higher confidence will result in users and inspectors, based on the inspection of all surface areas of the inserts, which is difficult and time consuming with inserts that are cured in the recess they are intended to fill. While this higher confidence may result due to an assumption that no structural inconsistencies present on any of the surfaces likely indicates that there are no inconsistencies in the interior of the insert, X-ray inspection may also be utilized. X-ray inspection, which cannot be utilized in the present seal cure and shave process, or other inspection processes helps to ensure no voids are present, resulting in an even more effective lightning strike protection system. In a particular application, alluded to several times herein, the dielectric inserts are used by airplane producers seeking to use it as a part of a lightning strike protection system in primary composite structures, and airlines using the insert in maintenance operations. 
         [0038]    The following once again compares the current method associated with lightning strike protection and the embodiments described herein. In the current practice, an application, cure and shave process is utilized in assembly which is laborious, messy, time consuming, and waste generating. The embodiments herein describe a preformed dielectric insert that is fabricated prior to assembly, possibly by a supplier, which is installed into an assembly, and any excess adhesive is simply wiped off. The current process also dictates cold storage of uncured sealant and/or on site mixing processes as opposed to ambient on site storage of preformed dielectric inserts. Therefore, the current method often requires a sealant thawing step before it can be applied onto the recessed fasteners. Further, as the current method is performed by persons, there is a variation in the formation of the top surface of the cured in place dielectric material, as opposed to a possible automated formation of the dielectric inserts described herein. 
         [0039]    There are no labeling requirements with the current method, however, with the described embodiments, labeling is likely to be utilized, although labeling might be accomplished through the use of different colored backing materials and/or different colored dielectric inserts, to identify various dimensions and/or depths associated with a dielectric insert. Trimming and/or adjustment is likely, for cured in place dielectric materials used in the current processes. Preformed dielectric inserts may also require some trimming, for example, to compensate for any irregularities in recessed fastener hole dimensions. However, trimming of the inserts is less likely to affect the surrounding material as the trimming may be done before the insert is installed. 
         [0040]    There is a non-destructive inspection for structural inconsistencies under the current apply, cure, and shave process, however, it is limited to the upper surface of the dielectric material. With a preformed dielectric insert, the non-destructive inspection process can be performed on all surfaces before it is inserted into the fastener recess. When surface inconsistencies are found on an insert that is cured in place, a repair process must be conducted, typically involved removal of cured material, whereas in the described embodiments, a preformed dielectric insert that is less than desired may be discarded prior to delivery or installation, reducing repair time. 
         [0041]    The existing methods generate a large amount of waste sealant due to the shaving operation. In addition, there may be a problem meeting aerodynamic dimensional tolerances due to sealant shrinkage following installation. The dielectric application, cure, and shave process also is an integral step in the assembly process, rather than prior to or concurrent with the assembly process resulting in more labor, time, and interference with other assembly process steps as a heat source may be utilized in curing. Further, there has been a propensity for effecting surrounding structure during the shaving operation. 
         [0042]    In summary, the described embodiments provide an improvement over existing lightning strike protective measures because there is a complete surface inspection of the dielectric inserts for inconsistencies. Additionally, the dielectric inserts are formed prior to installation and therefore any less than ideal inserts can be discarded prior to delivery. Another improvement is that no metal blades touch composite materials (airframe surface damage control), and a separate dielectric insert fabrication step lends itself to automation, thereby eliminating human variability. Waste is reduced (material costs), it is easier to meet aerodynamic dimensional tolerances, and a cure time and/or temperature is reduced as compared to existing methods (curing is eliminated in the production environment). 
         [0043]    The described embodiments reduce installation and inspection time, waste, chances of damaging the composite, and increase precision in meeting dimensional tolerances to provide better lightning strike protection. Users have better uniformity while the suppliers have new value added products in their inventory. Cost savings that are believed to result from the application of the herein described embodiments include flow savings (less time to install), cost avoidance associated with the price of material (minimize material usage/waste), labor savings (less people to certify to perform the sealing task), and rework time savings (no need for additional heat cure/ambient cure of less than ideal tops). Additionally, repair by users will be faster than with the current method as less time is needed for re-installation. 
         [0044]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.