Abstract:
An apparatus and method for stiffening a panel provides a composite waffle stiffener and the method for constructing the stiffener. A plurality of fabric plies pre-cut to a selected pattern are laid on a tool having a waffle configuration, the plies overlapping each other to maintain continuity of the structure. The resulting waffle stiffener is then bonded to a panel requiring stiffening. The stiffener is flexible and may be bonded to curved panels. Holes in the stiffener allow access to the volume between the stiffener and the panel for minimizing volume loss within, for example, a wing structure containing fuel and for ventilating air or moisture trapped in the volume.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention generally relates to the field of materials construction and, more specifically, to an apparatus and method for constructing a composite structure.  
           [0003]    2. Description of the Prior Art  
           [0004]    Composite structures are desirable in many industries for many applications. For example, aircraft, space, and land/sea vehicles employ a variety of curved and multiple-contoured surface structures in their fabrication. Composite materials are commonly used for these structures because, among other desirable attributes, composite materials have high strength-to-weight ratios. Even so, composite structures formed from composite materials still need to be stiffened in some instances. Therefore, manufacturers of composite structures are continually searching for better and more economical ways of stiffening composite structures.  
           [0005]    There are various systems and methods of stiffening composite structures. For example, in an aircraft context, hat and blade stiffeners are sometimes utilized. However, a problem with hat and blade stiffeners is that they can have special peel problems at their ends. Furthermore, when utilizing hat and blade stiffeners for strength in perpendicular directions, it becomes very expensive from a manufacturing standpoint because hat and blade stiffeners do not lend themselves well to criss-cross patterns.  
           [0006]    Another common stiffening method is the use of honeycomb structures. However, honeycomb structures are usually manufactured as a sandwich structure, which means that they can trap moisture within them, leading to degradation, and are limited in depth because of volume and weight considerations. In addition, manufacturing honeycomb structures can be very expensive.  
           [0007]    An additional method of stiffening composite structures is the use of waffle structures. Waffle stiffening can achieve the depths of hat and blade stiffeners, without the peeling problems, and can achieve bi-directional strength by nature of their criss-cross patterns. In addition, waffle stiffeners, unlike honeycomb structures, consume less volume and can be made to not trap moisture or fuel, which can be detrimental in aircraft applications. However, waffle stiffening is not without its problems. Prior methods of constructing waffle stiffeners result in sub-standard waffle stiffeners because of folding or kinking problems during lay-up of the composite plies. In addition, prior waffle stiffeners are limited in depth because of strength considerations, which means that the span between supports cannot be extended or increased, which would save a considerable amount of money during manufacture.  
         SUMMARY OF THE INVENTION  
         [0008]    An apparatus and method for stiffening a panel provides a composite waffle stiffener and the method for constructing the stiffener. A plurality of fabric plies pre-cut to a selected pattern are laid on a tool having a waffle configuration, the plies overlapping each other to maintain continuity of the structure. The resulting waffle stiffener is then bonded to a panel requiring stiffening. The stiffener is flexible and maybe bonded to curved panels. Holes in the stiffener allow access to the volume between the stiffener and the panel for minimizing volume loss within, for example, a wing structure containing fuel and for ventilating air or moisture trapped in the volume.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The novel features believed to be characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings.  
         [0010]    [0010]FIG. 1 is a perspective view of an aircraft skin stiffened by a composite waffle structure and constructed in accordance with the present invention.  
         [0011]    [0011]FIG. 2 is a perspective view of a tool for creating a waffle stiffener and in accordance with the present invention  
         [0012]    [0012]FIG. 3 is a cross-sectional view of the tool of FIG. 2 through section  3 - 3  of FIG. 2.  
         [0013]    [0013]FIG. 4 is a cross-sectional view through the tool of FIG. 2 and the stiffener of FIG. 1, the stiffener being formed on the tool according to the present invention.  
         [0014]    [0014]FIG. 5 is a plan view of fabric plies being formed n accordance with the present invention.  
         [0015]    [0015]FIG. 6 is a perspective view of an alternate embodiment of the fabric plies of FIG. 5 and being in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    Aircraft skin  11  comprises waffle stiffener  13  and panel  15 . Panel  15  is preferably a generally-planar, composite component, though panel  15  may be somewhat curved. Panel  15  is preferably formed from materials such as, for example, carbon fibers, Kevlar fibers, or glass fibers, though panel  15  may also be formed from other rigid materials, such as aluminum. The thickness of panel  15  may be insufficient to provide the required bending and torsional stiffness for a particular application, thus requiring a stiffener to be attached to panel  15  to increase the stiffness thereof.  
         [0017]    Stiffener  13  is a waffle structure that is preferably formed from composite fabrics and unidirectional fibers of the same composite materials described above, the particular material being selected to optimize various characteristics, such as strength and coefficient of thermal expansion. The waffle structure is defined by the formation of an array of pockets  17  by two sets of intersecting webs  19 ,  21 , each web  19 ,  21  being formed from a fabric of woven fibers. Webs  19 ,  21  are shown as generally perpendicular to each other, though webs  19 ,  21  may be formed to be at other angles relative to each other. Each pocket  17  is formed as a base  23  surrounded by tapered portions of webs  19 ,  21 . The upper portions of webs  19 ,  21  form cap regions, or crests,  25 ,  27 , respectively. Stiffener  13  can be produced in various shapes and sizes, for example, each pocket  17  may have webs  19 ,  21  that may have horizontal lengths of 4 inches and a vertical depth of ½ inch. Though shown as a flat plane, stiffener  13  can also be formed with a curvature to match that of a curved outer panel that requires stiffening.  
         [0018]    Stiffener  13  is bonded to panel  15  using an adhesive or other means between the inner side of each base  23  and the inner surface  29  of panel  15 . Additionally, stiffener  13  is bonded to inner surface  29  at outer edge  31 . The amount of bonding surface area can be increased by increasing the size of bases  23 . Because of the three-dimensional shape of stiffener  13 , a hollow volume  33  exists between stiffener  13  and panel  15 . Holes  33 ,  35  located throughout stiffener  13  may provide for venting of moisture that accumulates in volume  33  between skin  15  and stiffener  13  or for access to volume  33  for fuel storage and recovery in a wing structure. By providing holes  33 ,  35 , stiffener  13  minimizes the loss of available volume for fuel storage within the wing, since fuel would be able to flow into and out of volume  33  through holes  33 ,  35 . By forming a portion of holes  33 ,  35  in webs  19 ,  21  near crests  25 ,  27  and a portion near outer bases  23 , fuel may be substantially recovered from within volume  33  whether stiffener  13  is located above a lower wing surfaces or below an upper wing surfaces.  
         [0019]    Tool  37 , shown in FIG. 2, is used as a die to form stiffener  13  of FIG. 1. Tool  37  has a similar waffle structure as stiffener  13  and is preferably formed from a solid block of metal through any suitable process, such as machining. Tool  37  has a base  39  from which intersecting webs  41 ,  43  extend upward. Webs  41 ,  43  are shown as perpendicular to each other, though this is not required. Webs  41  extend for the desired height of tool  37  and terminate in crests  45 , whereas crests  47  of webs  43  are located at a lower height than crests  45 . FIG. 3 illustrates a cross-section of tool  37  along the section line  3 - 3  of FIG. 2 and shows crests  47  extending through web  41  and forming U-shaped channels  48 . Referring again to FIG. 2, pockets  49  are formed between sidewalls  51 ,  53  of webs  41  and between sidewalls  55 ,  57 , each pocket  49  having a base  59 . As described below, layers of composite materials are laid over webs  41 ,  43  and within pockets  49  to form stiffener  13  (FIG. 1).  
         [0020]    [0020]FIG. 4 is a cross-section through the middle of a row of pockets  49  parallel to webs  41 . Stiffener  13  is located on tool  37 , the cross-section revealing fabric plies  61 ,  63  that form webs  19 ,  21  of stiffener  13 . Precision-cut fabric plies  61 ,  63  are strips that are laid on tool  37  with their lengths being parallel to webs  43 . Multiple layers of overlapping fabric plies  61 ,  63  form the thickness of stiffener  13 , which is preferably two to six layers thick. Fabric plies  61 ,  63  are cut from flat composite fabric, preferably made from woven carbon fibers, and are impregnated with a laminating resin that is cured before stiffener  13  is removed from tool  37  and bonded to a panel. Fabric plies  61 ,  63  encapsulate cap plies  65 ,  67  for tying together webs  19 ,  21  and for further cross-stiffening of stiffener  13 . Cap plies  65 ,  67  are thin, rectangular strips and are preferably formed from unidirectional carbon fibers, extending for the length of webs  41 ,  43 . Cap plies  65 ,  67  are equal in width to crests  47  and  45 , respectively, and the ends of adjacent cap plies  65 ,  67  maybe staggered at the ends of webs  19 ,  21  to ease load introduction to webs  19 ,  21 . When installed, it is preferable for the fibers of fabric plies  61 ,  63  to be +/−45° from the unidirectional fibers of cap plies  65 ,  67 .  
         [0021]    As shown in FIG. 5, fabric plies  61 ,  63  are strips, each comprising two portions that alternate in a repeating pattern for the length of fabric plies  61 ,  63  and have fibers oriented +/−45° from the length of plies  61 ,  63 . Fabric ply  61  comprises rectangular base portions  69  and diamond-shaped web portions  71 . Each base portion  69  is rectangular and sized to fit on base  59  of pockets  49 , partially extending up sidewalls  55 ,  57  of webs  43 . Web portion  71  is sized to wrap over sidewalls  51 ,  53  and crests  45  of webs  41 . Each web portion  71  has two lateral extensions  73  that give web portion  71  a width slightly larger than the width between channels  48 , providing for full coverage of sidewalls  51 ,  53  when web portion  71  is laid over webs  41 .  
         [0022]    Fabric ply  63  is a strip comprising hexagonal web portions  73  and rectangular channel portions  75 . Each web portion  73  has two lateral extensions  77  for partially extending down sidewalls  55 ,  57  when fabric ply  63  is placed on crest  47 . Channel portions  75  are narrow strips sized to fit within U-shaped channels  48  on webs  41 . FIG. 5 also shows the amount of overlap of fabric ply  63  over fabric ply  61  when plies  61 ,  63  are laid on tool  37 , the overlapped edge of fabric ply  61  being shown as a dotted line.  
         [0023]    Referring again to FIG. 4, stiffener  13  is formed by laying fabric plies  61 ,  63  parallel to each other on tool  37 . To form each row of pockets, the row being parallel to webs  43 , a first fabric ply  61  is laid on tool  37  with base portions  69  centered in pockets  49 . In each pocket  49 , base portion  69  rests on base  59 , the lateral edges of base portion  69  partially extending up sidewalls  55 ,  57  of webs  43 . Web portions  71  extend over each sidewall  51  and (though not shown in this view) each sidewall  53  of adjacent webs  41 . The angular shape of the edges of extensions  73  of web portions  71  corresponds to the shape of sidewalls  55 ,  57 . Web portions  71  then extend over crest  45  of each web  41  and continue down into the next adjacent pocket, repeating this sequence for the length of the row of pockets  49 .  
         [0024]    After the first fabric ply  61  is installed, fabric ply  63  is laid on tool  37 . Fabric ply  63  is laid along crest  47  of web  43 , channel portions  75  aligning with channels  48  in webs  41  and extensions  77  aligning with sidewalls  55 ,  57 . Extensions  77  extend downward on sidewalls  55 ,  57 , the lower ends of extensions  77  overlapping the upper ends of the sides of base portion  69 . The angular shape of the edges of extensions  77  of web portions  73  corresponds to the shape of sidewalls  55 ,  57 . A press, having a waffle configuration and preferably made from silicone, is used in a hot-debulking process to compact fabric plies  61 ,  63  on tool  37  after the installation of each layer of stiffener  13 .  
         [0025]    To form the next layer of stiffener  13 , a second fabric ply  61  is installed, the edges of fabric ply  61  overlapping the edges of the first fabric ply  63 . Before the second fabric ply  63  is installed, though, a cap ply  65  is laid along crest  47  above and parallel to fabric ply  63 . The second fabric ply  63  is now installed over cap ply  65 , encapsulating cap ply  65 . For the first layers of stiffener  13 , only cap plies  65 , which are parallel to crests  47 , are used. After a number of layers of plies  63 ,  65  have been laid, the thickness above crests  47  is such that the upper surface of the next cap ply  65  is even with the upper surface of fabric plies  61  over crest  45 . A cap ply  67  is laid above and parallel to crest  45  of web  41  for cross-stiffening of stiffener  13  in a direction parallel to webs  41  and perpendicular to that of cap plies  65  and fabric plies  61 ,  63 . The thickness of stiffener  13  is increased as further layers are constructed by alternately stacking fabric plies  61  and  63  and placing cap plies  67  between layers.  
         [0026]    A second embodiment of stiffener  13  uses base ply  79  and fabric ply  81 , shown in FIG.6. To form base  23  of each pocket  17 , base ply  79  is cut from flat, composite fabric to have multiple rows of base portions  83 , each base portion having four corner portions  85 . Base portions are sized to fit within pockets  49  of tool  37  with the outer edges of base portions partially extending up sidewalls  51 ,  53 ,  55 ,  57 . Corner portions  85  are sized and shaped to fit in the corners formed at the intersections of sidewalls  51 ,  53 ,  55 ,  57 . Fabric ply  81  is also cut from flat, composite fabric and provides for multiple rows of web portions  87 ,  89  sized to wrap over webs  41 ,  43  of tool  37 . Each web portion  87 ,  89  has two lateral extensions  91  sized to lay on sidewalls  51 ,  53  and  55 ,  57  within each pocket  49  for forming the walls of pockets  17  of stiffener  13 . Holes  93  are cut in fabric ply  81  for overlaying base portions. The edges of extensions  91  are shaped to follow the angles created at the intersections of sidewalls  51 ,  53 ,  55 ,  57 . Because plies  79 ,  81  are symmetrical, each can be oriented so that rows of portions  83 ,  87 ,  89  are placed parallel to either web  41 ,  43 .  
         [0027]    To form stiffener  13  using plies  79 ,  81 , base ply  79  is placed on tool  37 , base portions  83  laying on and being centered in pockets  49 . Comer portions  85  are positioned to align with the corners formed by sidewalls  51 ,  53 ,  55 ,  57 . Next, fabric ply  81  is placed on tool  37 , the rows of web portions  87 ,  89  being aligned with webs  41 ,  43  and being laid on crests  45 ,  47 . Extensions  91  are laid on sidewalls  51 ,  53 ,  55 ,  57  to form the walls of pockets  17 , extensions  91  overlapping the edges of base portions  83  and corner portions  85  of base ply  79 . A hot-debulking process follows the installation of each layer.  
         [0028]    The second layer is begun by placing a second base ply  79  on tool  37 , the edges of the second base ply  81  overlapping the edges of the first fabric ply. As described in the previous embodiment, a unidirectional cap ply (not shown), like cap plies  65 ,  67  in FIG. 4, is placed above fabric ply  81  and along each crest  47  of web  43  prior to the next fabric ply  81  being laid on tool  37 . This sequence repeats for approximately two layers, cap plies  65 ,  67  then being laid along web  41  in each successive layer until the desired thickness of stiffener  13  is produced.  
         [0029]    A third embodiment of the invention, though not shown in the figures, comprises forming stiffener  13  using a fabric made of short, discontinuous fibers. The fabric is shaped to tool  37  using heat and an elastomeric press, the press being forced toward tool  37  with a vacuum. Cap plies are laid between layers of the fabric, first in a direction parallel to one of the crests  45 ,  47 , then between other layers in a direction perpendicular to the first cap ply and parallel to the other of the crests  45 ,  47 . The fabric and cap plies are impregnated with a resin that is cured before stiffener  13  is removed from tool  37 .  
         [0030]    The advantages of the present invention are many. The stiffeners formed using this process are strong, lightweight, and flexible, being able to flex for bonding to contoured surfaces. Together with holes at selected locations, the three-dimensional structure provides for a minimal loss of volume within a wing structure or for ventilation of trapped air and moisture. The method of forming the stiffeners is quick and relatively inexpensive, providing for cost and labor efficiency in manufacturing. An additional advantage is that peeling between a waffle-stiffening structure and an aircraft skin, for example, is significantly reduced. The quality of waffle stiffening structures is improved by substantially reducing or eliminating any folds, kinks, bumps, or other imperfections when laying composite plies on a tool. Waviness in cap plies is minimized or eliminated by placing each cap ply in a different plane.  
         [0031]    While the invention has been shown in only some of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit thereof.