Patent Abstract:
A technique and tool are provided for repairing damaged areas of honeycomb structures. The tool includes a plurality of rods axially translatable into the damaged area. Foam material is inserted about the rods and allowed to cure. The rods are later withdrawn leaving a porous core of material. Sleeves may be provided to receive and support the rods as they translate into the damaged area. The sleeves may remain in the damaged area after the rods are withdrawn into the sleeves.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     (Not Applicable) 
     STATEMENT RE: FEDERAL SPONSORED RESEARCH/DEVELOPMENT 
     (Not Applicable) 
     FIELD OF INVENTION 
     The present invention relates to an apparatus and technique useful to repair honeycomb core structures, such as used in unmanned aircraft, small submersibles and the like. 
     BACKGROUND OF THE APPLICATION 
     Many contemporary aircraft have honeycomb sandwich structures formed in the wing leading edges. In the course of use the leading edges may sustain damage, e.g. from bird strikes and accidents during maintenance. 
     In some cases a honeycomb core may be crushed proximate the point of impact, but the outer skin may not show any visual damage at that location. The resulting core shear failure may extend over a larger area than the original damaged area. Such unseen core damage may result in the outer skin buckling and delaminating under stress. 
     Repair of the damage may require skin and core repair operations, either in the field or in a repair depot. A replacement core specific to that wing must be matched to correctly restore the aircraft&#39;s integrity. However, maintaining a stock of cores specific to each aircraft is logistically challenging, especially during field operations. 
     In accordance with contemporary repair techniques damaged skin and the adjacent honeycomb core are completely removed to the opposing skin. A replacement core may be shaped and sanded to fit snugly into the area being repaired. Replacement material may be selected to substantially match the original case material. However, conformity of the materials does not insure conformity to the structural and mechanical properties of the honeycomb shaped core. One type of replacement material is syntactic foam, an epoxy resin material that contains glass micro-balloons. This material can be tailored to different densities and properties. However the material may be too heavy for certain applications. Moreover, the material is isotropic in nature, i.e. the strength is equal in all directions, which may be undesirable or at least differ from the honeycomb properties of the adjacent structure. 
     For reference, several prior art methods to repair aircraft laminates are described below. U.S. Pat. No. 6,149,749 discloses a patch used to cover the damaged area, without any replacement of the original structure, including the lightweight structure between the two outer skins. The patch has apertures that allow air to be removed from below the patch thereby improving the adhesion. 
     U.S. Pat. No. 4,961,799 discloses a method for repairing damaged areas by bolting or bolt bonding a repair patch on the outer surfaces of the damaged material. This method does not provide a matching replacement honeycomb structure. 
     U.S. Pat. No. 5,741,574 discloses a truss reinforced sandwich structure that uses fiber bundles or tows cured in very small diameters; the preferred method of the attachment being stitching the foam core within the face sheets or laminates. U.S. Pat. No. 3,328,218 discloses using rigid filaments to manufacture the core structure. In both references the filaments must be fastened to the outer plies. Moreover, the references require the foam to be under compressive pressures of 50 to 90 psi. 
     U.S. Pat. No. 5,612,117 discloses an anchoring means comprising an insert introduced into a hole in the skins and held in place by the core to provide an anchoring of the entire structure. 
     U.S. Pat. No. 5,773,121 describes a syntactic form core structure that is produced using a powdered resin instead of a liquid resin. In order to improve or modify characteristics of the resultant structure, chopped fibers or pre-built honeycomb structure is added to the powered resin, before curing. 
     U.S. Pat. No. 5,547,629 discloses a method using rubber mandrels to fabricate hollow structures such as a wing. After the two skins are compressed together, with the mandrels in the hollow cavities providing support, the mandrels are pulled from one end. As they stretch, they contract and can be removed from the molded part. No foam honeycomb structure is introduced during the molding process. 
     U.S. Pat. No. 5,868,886 discloses the use of Z-pins to provide a mechanical link between the patch/parent structures. It suggests removal of the pins, leaving holes that are filled when the patch material is introduced into the repair area. Small pin sizes are used in order to reduce associated structural degradation caused by fiber breakage due to concentrated stress. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of this invention to provide an improved method for repairing structures having outer skins and an inner honeycomb core structure. 
     It is another object of this invention to provide a repair method that can be implemented in-site or at a facility to separate from the aircraft. 
     It is an additional object of this invention to reduce the weight of the syntactic foam by displacing a part of the volume of foam with air tunnels. 
     It is a further object of this invention to regulate the structural or mechanical characteristics of the replacement core by selection of the core angle(s), number of layers and choice of materials. It is a further object of this invention to reduce the inventory of core specific parts that must be matched with a particular aircraft, which creates a logistic nightmare, especially during field operations. 
     In order to reduce the inventory of specific foam cores, a small number of tools can be kept that will allow syntactic tunnel cores to be fabricated as needed. The tooling can be used to produce these syntactic tunnel cores both off or on the vehicle. The tooling utilizes metal rods that are removable and are held in place with at least 2 rod guide plates. The mechanisms, to hold the tooling in place for repair work both off and on the vehicle, may be the same depending on the area to be reworked. 
     If the repair is to be accomplished off aircraft, then the first step is typically to remove the damaged skin and inner honeycomb core. This damaged area is normally removed to the opposite skin. If both skins are damaged, one skin is typically repaired first. The tooling that holds the removable rods may be positioned above a container or tray. This tray is typically deeper and larger than the scraped out area. 
     The removable rods are placed into the aligning holes in the rod guide plates and positioned in the cavity created in the tray. The syntactic foam may be introduced into the cavity, surrounding the removable rods. Once the foam has cured, the removable rods may be removed and the tooling is extracted. The top and bottom surface of the replacement syntactic tunnel core is now shaped to match the final surface shapes and the original core shape. The shaping of the replacement tunnel core include matching the physical shape of the scraped outer area. The outer skin can now be repaired using contemporary skin replacement techniques. 
     The core tunnels made from the space taken up by the removable rods may all be the same geometric shape or vary across the replacement tunnel core. The most common shapes are circular or hexagonal cylindrical tunnels. 
     The shape of the tunnels may be a series of graduated sectional profiles narrowing in the direction of the tunnel depth. Preferably there are no undercuts that may damage the tunnel shapes upon the extraction of the removable rods. 
     The angle of the tunnels may typically be perpendicular to the direction of the load of the structure or outer surface or may be offset from the being perpendicular. Alternatively, to implement selective structural or mechanical features, the tunnels may be bored at other angle/gradients. A 5-degree offset, for example, will increase the shear characteristics. 
     During the process, the metal rods may be heated or cooled in accordance with predetermined temperature probes, to regulate the final structural characteristics or speed up the curing process. 
     The technique and tool used to implement repaired damaged honeycomb structures is as follows. The tool includes a plurality of rods axially translatable into the damaged area. Foam material is inserted about the rods and allowed to cure. The rods are later withdrawn, leaving a porous core of material. Sleeves may be provided to receive and support the rods as they translate into the damaged area. The sleeves may remain in the damaged area after the rods are withdrawn into the sleeves. 
     Rods may be formed to have various cross-sectional areas, such as circular, hexagonal, or other geometric shapes. Rods may be inserted into the area at different angles relative to the load flow in the damaged area. The particular angle at which the rods are inserted into the damaged area may be selected to increase the sheer strength or other properties of the honeycomb structure, or to match the properties of the particular remaining structure. 
     In one embodiment the rods may be heated, or the temperature otherwise regulated in order to facilitate curing of the foam material to produce desired properties. 
     In another embodiment the foam may be inserted in sequential applications to define a series of separate layers, wherein each layer may be formed of different material, and/or cured in accordance with different profiles, in order to achieve a desired set of core/structure properties. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         FIG. 1  is an illustration of the parts of an aircraft structure. 
         FIG. 2  contains views of two types of honeycomb structures. 
         FIG. 3  is an illustration of a removable pin with multiple geometric shapes. 
         FIG. 4  is a view of a damaged honeycomb section after removal of the damaged honeycomb section 
         FIG. 5A  is a view of the tooling that is utilized to fabricate the syntactic tunnel core off of the aircraft. 
         FIG. 5B  is a view of the tooling that is utilized to fabricate the syntactic tunnel core on the aircraft. 
         FIG. 6  is an illustration of a removable plug and insert. 
         FIG. 7  is a view of an off-aircraft repair plug. 
         FIG. 8  is an illustration of the placement of the plug into the removed damaged honeycomb area. 
         FIG. 9  is a top view of the replacement honeycomb plugs in the aircraft structure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The aircraft or vehicle structure  10  is shown in  FIG. 1 . The structure  10  is made up of two exterior skins  11  and  13 . Sandwiched between the skins  11  and  13  is a honeycomb structure  12 . Skin  11  is illustrated in a translucent rendering so that the honeycomb structure  12  can be seen. This honeycomb structure  12  is used to not only reduce the weight of the overall structure  10 , but also to provide the required characteristics for a specific application. The tunnels or air pockets of the honeycomb structures can be of any geometric shape. 
       FIG. 2  shows two different honeycomb structure geometric shapes from two viewing angles  20 . The honeycomb views,  23  and  24  are from different elevations, based on a circular, cylindrical rod. The honeycomb views,  21  and  22 , are based on a hexagonal cylindrical rod. 
     The characteristics of the honeycomb structure can be made directional, whether mechanical, electrical or dielectric, by controlling the amount of holes, their sizes, their geometric shape, their sectional views, their direction, syntactic resin wall thickness and strength, composition of the syntactic material and/or temperature/pressure curing profiles. The shape of the tunnels, and their angles to the direction of the load of the honeycomb structure can further be varied across the face of the repaired area. 
       FIG. 3  illustrates a removable rod  25 , that includes multiple geometric shapes on a single rod. It shows a rod with three geometric shapes, a larger circle  26 , a smaller circle  27  just below the larger circle  26 , and a hexagonal shape  28  just below the smaller circle  27 . Draft may be added to any of the geometric shapes. The only design preference is that the removable rod be capable of being pulled out of the cured honeycomb core. Undercuts could damage the core, by scraping out part of the core. 
     It is also within the scope of this invention to use layers of syntactic foam, with each having different properties, to introduce structural gradients or otherwise improve matching to the surrounding honeycomb structure. 
       FIG. 4  shows an area of the aircraft structure  30  that was damaged, and the repair has been started. Though hitting an object, such as by a bird or a projectile, could damage both skins  31  and  32 ,  FIG. 4  assumes that only the top skin  31  is damaged. First, the damaged portion of the skin  31  is removed as show as a circle of material missing from skin  31 . The honeycomb internal structure  33  is thereby exposed. The damaged area is normally removed down to the undamaged skin  32  and the removed area  34  is hexagonal in the shape for clarity. It is also possible that the damaged area of the honeycomb structure  33  will not require the entire removal of the honeycomb structure down the other skin  32  surface. 
     Now that the damaged honeycomb area  34  has been removed, a replacement must be fabricated. This can be accomplished offsite or directly on the vehicle. The decision is based on several factors, including the location of the structure being repaired. In a repair depot, when room safety is more readily insured, the fabrication of the syntactic tunnel core will most likely be performed on the vehicle. In the field, especially in war theater, the fabrication of the syntactic tunnel core would more likely be accomplished off the vehicle. 
       FIG. 5A  shows the fabrication tooling  40  for a syntactic tunnel core that is fabricated offsite or off the aircraft. The fabrication tool  40  basically consists of a housing including at least guide plates  42  and  43  and a number of extendable rods  41 . The rods  41  in  FIG. 5A  are typically of circular, cylindrical shape, and adapted for axial movement within the guide plates. As will be apparent to those in the field, the rod shapes may alternately be of other geometric shapes as shown in  FIG. 2  and  FIG. 3 . The outer mechanism used to position the plates above the work area may be any of a number of different mechanisms that are well known in the industry. 
     The removable rods  41  may be positioned into a container or tray  44  of an appropriate depth, typically at least as deep as the removed area  34 , as shown in  FIG. 4 . Syntactic foam is then placed into the remaining cavity of the tray  44  and allowed to cure. If required, the removable rods  46  can be heated or cooled to reduce the cure time or to maintain a temperature to achieve selected properties in the cured foam. The rods  46 , are then removed leaving a replacement syntactic tunnel core tray-shaped structure. This syntactic tunnel core structure may be shaped to fit into the cavity  34  of  FIG. 4 . The final shape of the syntactic tunnel core  55  mates with the cavity  65  is shown in  FIG. 8 . The mated replacement honey tunnel core and original honeycomb core is shown in  FIG. 9 . 
       FIG. 5B  shows the fabrication tooling  44  for a syntactic tunnel core that is fabricated on aircraft. The fabrication tooling  45  is basically consisting of at least 2-rod guide plates  47  and  48  and a number of removable rods  46 . The rods  46  in  FIG. 5B  are a circular, cylindrical shape. The shape may be other geometric shapes as shown in  FIG. 2 . The rods or rod guide plates may, depending on the honeycomb design, be made up of several sections that are mechanically fastened together. The plate sections may contain aperture patterns that are different from the other plate sections, so that the properties of the final syntactic tunnel core replacement is better matched to the original honeycomb core. Once the fabrication tooling  45  is in place, the syntactic foam material can then be poured into the remaining cavity of the removed area  34 . The syntactic foam is allowed to cure. If required, the removable rods  46  can be heated or cooled at selected thermal profiles to reduce the cure time, or to regulate the temperature to achieve described properties in the cured foam. The rods  46  are removed, leaving a replacement syntactic tunnel core structure  73  as shown in  FIG. 9 . 
       FIG. 6  illustrates a rod set  51 . Sleeve  53  is placed on the first end of the rod  52 , which will have syntactic foam material placed around it. The sleeve  53  may remain in the tunnel core after removal of rod  52 , any may be thermally or otherwise controlled to provide additional desired core properties. The sleeve  53  may be made of Teflon or any material that will provide desired characteristics. 
       FIG. 7  illustrates a completed off aircraft fabricated syntactic tunnel core  56 . The shape of the edges  57  are fabricated to fit properly within the cavity in the repair area  65  as shown in  FIG. 8 . 
     The off aircraft fabricated replacement syntactic tunnel core  56  may be placed into the cavity  65  of the original honeycomb core  63 . As shown in  FIG. 8  the shape defined by the edges of core  61  mate with the shape defined by the edges of the cavity  65 . The procedure to fasten the core  61  to the undamaged skin  64  may then proceed in accordance with standard repair procedures. 
       FIG. 9  shows the top view of a repaired honeycomb area  70 . The repaired area may have been fabricated off aircraft or on aircraft. Within the repaired area  70 , the dark hexagonal shaped area is the replacement syntactic tunnel core  73  of the present invention. The lighter colored syntactic tunnel core  72  is the original undamaged tunnel core. The bottom skin  74  supports the bottom of the replacement tunnel core  73 . The upper surface of the replacement core  73  must be shaped to match the upper surface shape of the original core  72 . Once the surfaces are uniform between the two cores  72  and  73 , then a patch can be added to close the opening hole  75  in the damaged skin  71 . The patch is not shown. 
     The shear strength of the core with tunnels perpendicular to the skins is low. If desired, the rods  41  or  45  can be positioned at an angle that is not perpendicular to the load path direction. Even a  5 -degree offset will increase the load angle. The angle selected is based on the desired characteristics. 
     It is understood that although the above represents several embodiments of the invention, the invention may take a wider variety of embodiments intended to effect alternate designs or additional features. Such embodiments are within the scope and spirit of the present invention.

Technology Classification (CPC): 8