Abstract:
Lightweight structural elements are fabricated by machining a serpentine groove in a sheet of structural material such as honeycomb composite material, filling the groove with adhesive, and folding the sheet along the groove to achieve a desired configuration. The groove defines one or more tabs and one or more corresponding recesses. The tabs enter corresponding recesses when the sheet is folded and force excess adhesive from the groove which then can be removed to reduce weight of the structural element. In some applications, the use of adhesive may be eliminated entirely. This technique of building lightweight structural elements is particularly attractive in the aerospace industry where weight reduction is of paramount concern.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to the field of fabricating structural elements, such as those made of lightweight honeycomb composite materials found on modern aircraft. More particularly, this disclosure relates to a finger joint formed in foldable panels used to make structural elements that uses less glue than that used in prior joints. 
       BACKGROUND 
       [0002]    Flexible, lightweight sheet goods, such as composite honeycomb panels, are widely used in a variety of applications in the aerospace industry to create enclosures, containers, boxes, and other structural elements useful on modern aircraft. For example, these panels can be used to create the overhead luggage bins found on today&#39;s passenger aircraft. 
         [0003]    Single unitary panels made of such materials are usually bent into a desired shape. Typically, composite honeycomb panels are routed with a groove and then bent along the groove. Before bending the panel, the groove is filled with adhesive which forms a structural joint when the panel is bent into the desired shape. This technique is known as “ditch and pot” or “bend and fold.” The groove that allows the panel to bend, however, creates a large void for adhesive to reside. The size of this void is more than necessary to create a strong joint. For example, on average, the glue in a 48 inch long 90° joint in a ½ inch panel will weigh 0.38 pounds. This technique of creating joints can thus add up to substantial excess weight across an entire airplane. 
       SUMMARY 
       [0004]    This problem has been solved by a tab/slot geometry in a ditch/pot joint. Alternating tabs formed in one side of the groove enter corresponding recesses formed in the opposite side of the groove of a ditch/pot joint formed in the surface of a sheet of structural material. When the sheet of material is folded along the fold line defined by the groove, the tabs enter the corresponding recesses on the other side of the groove and displace the volume normally occupied by adhesive. The disclosed joint geometry thus will limit the amount adhesive the joint will accept. The adhesive will collect at the inner seam of the joint at the intersection between the two face sheets, thus bonding the two face sheets together to form a structural joint with a minimal amount of adhesive. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  shows a flat unfolded sheet of structural material into which an illustrative finger joint in accordance with the invention has been fabricated. 
           [0006]      FIG. 2  shows the sheet of structural material of  FIG. 1  partially folded along a fold line defined by the finger joint. 
           [0007]      FIG. 3  shows the sheet of structural material of  FIG. 1  completely folded into a desired shape along a fold line defined by the finger joint. 
           [0008]      FIG. 4  shows an edge view of the joint of  FIG. 1 . 
           [0009]      FIG. 5  is a top view of the sheet of  FIG. 1  showing more detail of the joint. 
           [0010]      FIG. 6  is a detailed view of a part of the groove located in the dotted box in  FIG. 5 . 
           [0011]      FIG. 7  is an unfolded sheet of structural material that can be folded to create the luggage bin. 
           [0012]      FIG. 8  shows an aircraft overhead luggage bin illustratively utilizing finger joints in accordance with the invention created by folding the sheet of material shown in  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION 
       [0013]      FIGS. 1 through 6  show a sheet  10  of foldable material of thickness t that can be used to make a structural element such as a container, box, compartment, covering, or the like. The thickness t of the sheet  10  may, for example, be 0.375 inches to 0.5 inches. The invention is not limited to sheet of any particular thickness, however. 
         [0014]    The material may be a piece of honeycomb composite comprising a core of Nomex, Kevlar, or other paper-like material shaped into a matrix of cells resembling a honeycomb. The core is sandwiched between two face sheets that illustratively can be made of Kevlar, fiber glass, carbon fiber, aluminum, or other material. The principles of the invention may also be applied to sheets  10  made of materials other than honeycomb composite, for example, sheets  10  made of rigid foam materials. 
         [0015]    Sheets of structural material like the sheet  10  in  FIG. 1  are illustratively useful in making lightweight structural elements used on aircraft such as overhead luggage bins and the like. The invention is not limited, however, to any particular sheet material or application. The invention may be used to create any item that can be made by folding a sheet of material to a desired shape or configuration. The material can be any foldable sheet material that can provide the required structural integrity for the finished product. 
         [0016]      FIG. 1  shows the sheet  10  in a completely unfolded state.  FIG. 2  shows the sheet  10  in a partially folded, intermediate state.  FIG. 3  shows the sheet  10  in its final folded condition. The sheet  10  has an internal joint  12  in accordance with this invention formed in one of the flat surfaces of the sheet  10 . The joint  12  divides the sheet  10  into two panels or sections  16  and  18  joined together at a fold line  14 . The joint  12  is a serpentine groove of predetermined width machined to a predetermined depth into the surface of sheet  10 . The width and depth of the groove is such that the sheet  10  can be folded into a desired configuration and there is enough material joining the two panels  16  and  18  to maintain the structural integrity of the finished structural element. 
         [0017]    The joint  12  substantially defines a fold line  14  along which the sheet  10  is folded. As noted above, the joint  12  separates the sheet  10  into two flat panels  16  and  18  at the fold line  14 . When the sheet  10  is folded, the two panels  16  and  18  are connected at the joint  12  and form an interior angle θ, as shown in most clearly  FIG. 4 . The angle θ can be, for example, 30° to 135°, or any other desired angle. Adhesive may be introduced into part or all of the groove  12  before the sheet is folded to strengthen the joint between the panels  16  and  18 . 
         [0018]    As shown most clearly in  FIGS. 5 and 6 , the meandering of the serpentine groove  12  in the sheet  10  results in the creation of a row of tabs  20 ,  22 ,  24 ,  26 , and  28  in one of the two sidewalls of the groove  10 . A row of recesses  21 ,  23 ,  25 , and  27  in that one sidewall are located between adjacent tabs  20 ,  22 ,  24 ,  26 , and  28 . The other sidewall of the groove  12  contains similar tabs  30 ,  32 ,  34 , and  36  and recesses  29 ,  31 ,  33 ,  35 , and  37  so that each tab on one side of the groove  12  is opposite a recess on the other side of the groove  12 . The rows of tabs and recess in groove  12  extend between two edge regions  38  and  40  of groove  12 . Illustratively, the width of the edge regions  38  and  40  (dimension A in  FIG. 6 ) can be about 0.2945 inches to 1.3090 inches. Dimension B may be 1.5 inches, dimension C may be 1.75 inches, dimension D may be 0.25 inches, dimension E may be 0.0982 inches to 0.4363 inches, and dimension F may be 0.1963 inches to 0.8727 inches. None of these illustrative dimensions is meant to be in any way limiting, however. 
         [0019]    In some applications, prior to folding the sheet  10 , a bead of adhesive may be run in part or all of the groove to reinforce the joint  12 . In prior ditch and pot joints, excess adhesive had a tendency to collect in the joint which increased the weight of the finished article. This weight increase is important in many industries, particularly in the aerospace industry, where weight reduction is of paramount concern. The shape and configuration of the groove in accordance with this invention, however, is such that excess adhesive is forced out of the joint  12  by the entry of the tabs into the recesses when the sheet  10  is folded into its final configuration. This excess adhesive can then be removed which will result in a meaningful weight reduction. In some low stress applications, the use of adhesive may even be dispensed with altogether and the structural elements may be assembled dry, thus further increasing the weight savings achieved by this invention. 
         [0020]      FIGS. 7 and 8  show an illustrative aircraft overhead storage bin constructed in accordance with this invention.  FIG. 7  shows a unitary sheet  42  of structural material, such as a suitable honeycomb composite, comprising a top panel  44  joined to two side panels  46  and  48  and a back panel  50 . The panels  46 ,  48 , and  50  are joined to panel  44  by means of joints  52 ,  54 , and  56 , respectively, each of which is configured like the joint  12  described above. The joints  52 ,  54 , and  56  each comprise a serpentine groove formed in the sheet  42  that defines a series of tabs and recesses that come together as described above when the sheet  42  in  FIG. 7  is folded along the joints  52 ,  54 , and  56  to create the overhead bin of  FIG. 8 . Adhesive may be introduced into all or part of any of the joints  52 ,  54 , and  56  for reinforcement depending on the stresses expected on the overhead bin. Edge  58  of back panel  50  and edge  60  of side panel  46  are joined together using conventional mortise and tenon joints such as those disclosed in U.S. Pat. Nos. 6,164,477 and 6,325,568. Edges  66 ,  68 , and  70  are joined to corresponding edges of a suitably curved bottom panel of the stowage bin not shown in  FIGS. 7 and 8  using similar mortise and tenon joints. A hinged and latchable door not shown in  FIGS. 7 and 8  is attached in conventional fashion to the front edge  74  of panel  44 . The door secures the contents of the storage bin for flight by closing the storage bin along edges  72 ,  74 , and  76  shown in  FIGS. 7 and 8  and along a front edge of the bottom panel not shown in  FIGS. 7 and 8 . 
         [0021]    The Title, Technical Field, Background, Summary, Brief Description of the Drawings, Detailed Description, and Abstract are meant to illustrate the preferred embodiments of the invention and are not in any way intended to limit the scope of the invention. The scope of the invention is solely defined and limited in the claims set forth below.