Abstract:
An internal stiffening member of varying configurations in which the stiffening elements support the skin using a compression-only load path is disclosed. A rib can be inserted into an assembled structural box beam, and a filler material can be used to fill any gaps between the slip-in rib and the interior surface of the structural box beam. The filler material is preferably an expandable material, such as an expandable foam-type material. However, in situations where a slip-in rib forms a primary structural rib, the filler material is preferably a structural adhesive or liquid shim material. A solid adhesive or filler would not crush under the clamping forces from fasteners or bolts at localized fitting attachments.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Divisional of U.S. application Ser. No. 10/546,490, having a filing date of 12 Jun. 2006, now U.S. Pat. No. 8,156,711, titled “CONTACT STIFFENERS FOR STRUCTURAL SKINS”, which was the National Stage of International Application No. PCT/US2004/005585, filed on 24 Feb. 2004, which claims the benefit of U.S. Provisional Application No. 60/450,096, filed 24 Feb. 2003, now abandoned all of which are hereby incorporated by reference for all purposes as if fully set forth herein. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present invention relates to structural skin. In particular, the present invention relates to methods and apparatuses for stiffening structural skin. 
     2. Description of Related Art 
     Structural skin is often used in manufacturing large parts, such as aircraft wing torque boxes, fuselages or control surface structures. This type of structure utilizes thin skins that would not be stable under bending and torsion loads that produce significant shear or compression in the walls. This type of construction is typical of most aerospace structures including wings, fuselages, control surfaces, tail booms, etc. Structural skins can be made thinner, and are, therefore, more weight efficient, when internal stiffening elements are used. A rib, for example, is a structural stiffening element that is disposed perpendicular to the longitudinal axis of a box beam, i.e., the rib lies in a cross-sectional plane of the beam structure. Ribs serve a variety of purposes in thin-skinned structure, including: (1) to provide support for the skin/skin stringer or spar panels against catastrophic buckling; (2) to maintain shape and contour of the skin; (3) to provide stiffness at major load introduction points; (4) to distribute concentrated loads into surrounding thinner structure; (5) to provide a shear redistribution path in the case of failure of any structural elements; and (6) to distribute pressure into the skin. These ribs are typically located at major load introduction points. In most instances, the entire rib is used to react loads; however, in some instances only certain regions of the rib is used to react loads. In addition, some ribs do not have any load introduction points, but react internal pressure loads. 
     Assembly of these structural box beams can be very complex, often with very tight tolerances required. As the number of parts is reduced, the manufacturing tolerances become even more critical, because there are fewer joints where variances can be accommodated. The installation of fasteners into these box beams presents additional difficulties, including limited access to small interior spaces and complicated sealing requirements. 
     One-piece closed cell structures can economically be produced with a variety of methods, including filament winding, automated tape placement, resin transfer molding, and others. However, these assembly-tolerance issues often preclude the use of one-piece closed-cell torque box structures with secondarily attached internal ribs, i.e., slipped-in ribs. Because of the reduction in part count and assembly labor associated with consolidating the torque box skins into a single part, a substantial cost savings could be realized if the assembly tolerance issues could be overcome. Several composite fabrication technologies are available to economically produce such a joint-free torque-box structure, including filament winding, automated tape placement, resin transfer molding, and others. Practical application of one-piece, jointless torque box structures has been limited because of the difficulty of installing the internal stiffening ribs. A rib installation design that allowed for large assembly tolerances and the resulting gaps between the rib and the torque box skins would enable more widespread application of these cost-saving technologies. 
     SUMMARY OF THE INVENTION 
     There is a need for a skin stabilization system that allows for a contact-only element in one of the two primary directions for skin stability. There is also a need for a stabilization element installation design that allows for large assembly tolerances and the resulting gaps between skin and the skin stabilization element, thereby reducing the fabrication cost of assembled structures. 
     Therefore, it is an object of the present invention to provide a structural stiffened skin in which the stiffening elements support the skin using a compression-only load path. 
     This object is achieved by providing a slip-in rib, or other stiffening member, of varying configurations in which the stiffening elements support the skin using a compression-only load path. In the preferred embodiment, the stiffening element has a peripheral edge that is adapted to be press fit into contact with the skin. The stiffening member may be held in place by various retention devices. Another configuration is a slip-in rib having a flange with a peripheral channel in which a filler material is disposed. The rib is inserted into an assembled structural box beam, and the filler material is used to fill any gaps between the slip-in rib and the interior surface of the structural box beam. The filler material is preferably an expandable material, such as an expandable foam-type material. However, in situations where a slip-in rib forms a primary structural rib, the filler material is preferably a structural adhesive or liquid shim material. A solid adhesive or filler would not crush under the clamping forces from fasteners or bolts at localized fitting attachments. 
     The present invention provides significant advantages, including: (1) costs associated with manufacturing closed-box structures are reduced due to relaxed tolerances and the ability to reduce part count; (2) failure of the rib/box bond is not a significant structural concern, because shear transfer through the rib/box bond is a secondary load path; (3) tolerance build-up is accommodated, because the slip-in ribs can be bonded in place as one-piece ribs; and (4) manufacturing labor is reduced, because fastener installation is reduced or eliminated. 
     Additional objectives, features, and advantages will be apparent in the written description that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features believed characteristic of the invention are set forth in the appended claims. However, the invention itself, as well as, a preferred mode of use, and further objectives and advantages thereof, will best be understood by reference to the following detailed description when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a perspective view of a conical tail boom of an aircraft having stiffening members according to the present invention; 
         FIG. 2  is a longitudinal cross-sectional view of the tail boom of  FIG. 1  showing several types of stiffening members according to the present invention installed therein; 
         FIG. 3  is an exploded perspective view of a box beam structure having a slip-in rib according to the present invention; 
         FIG. 4  is an exploded top view of the box beam structure having a slip-in rib of  FIG. 3  showing the installation location of the slip-in rib; 
         FIG. 5  is a cross-sectional view of the box beam structure having a slip-in rib of  FIG. 4  taken at V-V; 
         FIG. 6  is a cross-sectional view of the box beam structure having a slip-in rib of  FIG. 5  taken at VI-VI; 
         FIGS. 7A and 7B  are alternate embodiments of the box beam structure having a slip-in rib according to the present invention; 
         FIG. 8  is another alternate embodiment of the box beam structure having a slip-in rib according to the present invention; 
         FIG. 9  is another alternate embodiment of the box beam structure having a slip-in rib according to the present invention; and 
         FIG. 10  is a perspective view of the slip-in ribs of  FIGS. 8 and 9 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A curved structural skin can be stabilized with circumferential stiffening elements, such as frames or ribs, or longitudinal stiffening elements, such as longerons, stringers, or caps. The present invention represents the basic discovery that the internal support mechanism for reacting buckling and retaining shape in structural skin can perform functionally through compressive load transfer only. In other words, the structural attachment of internal stiffening elements to the skin and other frame elements is redundant and unnecessary. As such, either type of stiffening element can be allowed to float relative with the skin using compression-only “contact” support, provided the stiffening element is trapped or supported by the attached or integral stiffening elements in the other primary direction. 
     The present invention is useful over a wide range of industries, and provides valuable benefits and advantages in any application in which it is desirable to internally support a structural skin. Although the present invention is described herein with reference to certain aircraft applications, it will be appreciated that the methods and apparatuses of the present can be used in many different applications. 
     Non-linear finite element analysis of box beam structures has shown that ribs with only enough structural connection to hold them in place are quite effective in supporting skins against initial buckling, and in allowing load redistribution to occur in post-buckled skins. The reason for this is that in order for the beam cross-section to deform when a panel buckles, for example, the members supporting the edges of that panel must move toward one another. This collapsing deformation is resisted by internal ribs, which prevent the edge members from moving toward one another. The ribs react collapsing deformations in compression, so no shear attachment to the skins or spars is required. The rib perimeter bond can then be considered very damage tolerant, because local discontinuities and damage do not impact the performance of the rib. 
     The more general case of a large aircraft fuselage skin panel stiffened using snapped-in contact sticks between frames has also been demonstrated using nonlinear finite element analysis. The slip-in sticks function adequately as stringers to prevent global buckling and fuselage collapse under shear and compression loading. In this application, the frames, which are analogous to the ribs in a torque box structure, are fixed while the stringers are not directly attached to the skins. Thus, like the slip-in rib example, this demonstrates the basic premise that stiffening elements using contact or compression-only load paths are structurally adequate for thin-skinned structures when used in one of the two orthogonal stiffening orientations. 
     In the aircraft applications described herein, the present invention is a means of supporting the skin/skin stringer or spar panels against catastrophic buckling, and for maintaining the shape and contour of a structural box beam, such as aircraft wing torque boxes or control surface structures. In its broadest sense, the subject invention covers two main concepts: (1) that structural stiffening elements can be installed into assembled thin-skinned structures without fasteners or shims; and (2) that the structural attachment of these stiffening elements to the skins of the structural box beam and spars is redundant and not necessary. 
     Referring to  FIGS. 1 and 2  in the drawings, the preferred embodiment of a contact stiffener for structural skin according to the present invention is illustrated. This preferred embodiment will be described with reference to a conical tail boom  11  for an aircraft. Tail boom  11  includes a skin  13  that is supported longitudinally by a plurality of elongated spaced apart longerons  15  and circumferentially by a plurality of slip-in ribs  17 . Skin  13  is made with using a number of methods, including filament winding, braiding, fiber placement, or hand lay-up with either prepreg, thermoset or thermoplastic, or resin infusion methods, such as resin transfer molding, or variations, such as pultrusion, extrusion, or roll forming from metallic or composite sheets or panels of fiber composite laminates. Longerons  15  are co-cured or integral with skin  13 . Slip-in ribs  17  include castellations  18  to fit around longerons  15 . It will be appreciated that in some applications it may be desirable that a clearance exist between the top and sides of castellations  18  and longerons  15 , as the primary load transfer is in compression between the interior surface of skin  13  and the peripheral edges of slip-in rib  17  that are in contact therewith. 
     Slip-in ribs  17  are simply inserted through the large end opening of conical tail boom  11  and pressed into place. The peripheral edges of slip-in ribs  17  are press fit into contact with the interior surface of skin  13 . Thus, there is no need for any filler between the peripheral edges of slip-in ribs  17  and the interior surface of skin  13  to account for tolerances. In most applications, slip-in ribs  17  are held in place by retention means, such as snap-in clips, springs, or detent devices. 
     Slip-in ribs  17  may have a wide variety of shapes and configurations. Four possible configurations are shown in  FIG. 2 . Slip-in rib  19  is a solid plug, slip-in rib  21  has a C-channel configuration with a solid web portion  27 . Slip-in rib  23  has an open C-channel configuration with a central aperture or truss structure  29 , and slip-in rib  25  has an open I-beam configuration having a central aperture or truss structure  31 . It will be appreciated that these and other configurations may be used depending various factors, such as the location of the rib, the need to pass other components through the rib, and the load requirements. In an alternate version of this embodiment, tail boom  11  is formed by a flat skin with longitudinal stiffeners wrapped into a conical shape to form a tailboom structure. 
     Referring now to  FIGS. 3 and 4  in the drawings, a structural box beam  111  according to the present invention is illustrated. Box beam  111  is a one-piece, thin-walled hollow structure having walls  113 ,  115 ,  117 , and  119 . Walls  113  and  115  form upper and lower surfaces of box beam  111 , and walls  117  and  119  form side walls of box beam  111 . As is shown, walls  113 ,  115 ,  117 , and  119  may include additional material at corners  114 ,  116 ,  118 , and  120  to provide longitudinal stiffness to box beam  111 , if desired. 
     Box beam  111  is internally supported by at least one slip-in rib  121 . Slip-in rib  121  has an internal web portion  123  and a peripheral flange portion  125 . Although web portion  123  has been shown as a solid plate member, it should be understood that web portion  123  may include apertures and/or a truss arrangement (not shown) that would allow cables, wiring, and other component parts to pass longitudinally through box beam  111 . Flange portion  125  includes a recessed channel  127  that extends along the periphery of slip-in rib  121 . Web portion  123  and flange portion  125  cooperate to allow slip-in rib  121  to function as an I-beam. This configuration allows slip-in rib  121  to be very strong in compression, which the primary functional load transfer path of the present invention. 
     Walls  113 ,  115 ,  117 , and  119  are typically integrally formed by filament winding, braiding, fiber placement or hand lay-up with either prepreg or resin infusion methods such as resin transfer molding, or variations such as, pultrusion, or extrusion, or roll forming from metallic sheets or panels of fiber composite laminates, and slip-in ribs  121  are preferably fabricated from either a metallic or non-metallic fiber filled machining-tolerant plate material. Compression or injection molding or reaction injection molding may also be used to produce net molded ribs. However, it will be appreciated that walls  113 ,  115 ,  117 , and  119  and slip-in ribs  121  may be formed from other suitable materials, depending upon load, assembly, and application requirements. 
     In conventional box beam structures, walls  113 ,  115 ,  117 , and  119  cannot be integrated or pre-assembled without attaching the internal ribs and other support structures. One major benefit of the present invention is that walls  113 ,  115 ,  117 , and  119  can be integrally produced or assembled prior to installation of the internal support network. This is because slip-in ribs  121  are configured to facilitate installation into an integral or one piece box beam  111 . 
     The location of slip-in rib  121  after installation into box beam  111  is shown in dashed lines in  FIG. 4 . As is shown, a small clearance, or gap  131 , exists between the exterior edges of flange  125  and the interior surfaces of walls  113 ,  115 ,  117 , and  119 . This gap allows slip-in ribs  121  to be quickly and easily inserted into the interior of box beam  111 . As will be explained in more detail below, gap  131  allows slip-in ribs  121  to accommodate certain manufacturing tolerances, as well. Although only one slip-in rib  121  is shown, it will be appreciated that as many slip-in ribs  121  as are necessary to provide the desired support for box beam  111  may be installed into box beam  111 . Slip-in ribs  121  may be installed from either end of box beam  111 . 
     Referring now to  FIG. 5  in the drawings, box beam  111  with slip-in rib  121  installed is shown in a cross-sectional view taken at V-V of  FIG. 4 . As is shown, it is preferred that the peripheral shape and contour of slip-in rib  121  closely match the internal shape and contour of box beam  111 . This ensures that gap  131  between the exterior edges of flange  125  and the interior surfaces of walls  113 ,  115 ,  117 , and  119  is generally uniform, even though manufacturing tolerances may dictate that there be a finite gap of varying size therebetween. 
     Referring now to  FIG. 6  in the drawings, box beam  111  with slip-in rib  121  installed is shown in a cross-sectional view taken at VI-VI of  FIG. 5 . As is shown, a filler material  141  is disposed in channel  127 . Filler material  141  may partially fill channel  127 , fully fill channel  127 , or overfill channel  127 . In the preferred embodiment, the filler material  141  is an expandable foam-type adhesive that is installed in channel  127  prior to the insertion of slip-in rib  121  into the interior of box beam  111 , and then activated so as to make the adhesive expand and fill channel  127  and gap  131 . The process for activating expandable adhesive  141  is typically an increased temperature and or increased pressure curing process in which the temperatures and duration of curing depend upon the particular expandable adhesive used and the load and response characteristics desired. This process allows a relatively large clearance to be maintained for ease of insertion and assembly of slip-in ribs  121 . Although the entire structure can be heated in a large oven to activate the expandable adhesive, local means of heat can also be used including by placing resistive heating elements or strips in proximity of the adhesive or inside or adjacent to the adhesive and passing a current through the resistive material through contacts. Resistive heating elements can be embedded into the skin lay-up adjacent to the rib and a network of contacts or wires can be routed to a point of external access. Induction heating can be employed by using magnetic material in proximity or in the adhesive and introducing a magnetic field. Induction heating can also be used with a magnetic layer embedded into the skin lay-up in proximity of the adhesive. 
     In  FIG. 6 , the upper portion of channel  127  shows expandable filler material  141  prior to activation, and the lower portion of channel  127  shows expandable filler material after activation and expansion. As is shown, after expandable filler material  141  expands, channel  127  and gap  131  are filled with expandable filler material  141 . 
     Referring now to  FIGS. 7A and 7B  in the drawings, two alternate embodiments of the present invention are illustrated. In these embodiments, the interface between flange  125  and the internal surface of walls  113 ,  115 ,  117 , and  119  is modified to improve strength for loads perpendicular to the plane of slip-in rib  121 . In these embodiments, filler material  141  is an expandable filler material that may not bond with slip-in rib  121  or walls  113 ,  115 ,  117 , and  119 , but still provides an acceptable mechanical lock to react lateral pressure loads on slip-in rib  121 . 
     In the embodiment of  FIG. 7A , a recessed groove or slot  118  is added to the interior surface of walls  113 ,  115 ,  117 , and  119  opposite flange  125 . Channel  127  and slot  118  are filled with filler material  141 . After curing, filler material  141  forms a mechanical lock which provides longitudinal stability and helps prevent slip-in rib  121  from moving longitudinally relative to walls  113 ,  115 ,  117 , and  119 . In the embodiment of  FIG. 7B , circumferential grooves  151  are added at the bond interface to trap filler material  141  as filler material  141  expands. After curing filler material  141 , grooves  151  provide longitudinal stability and help prevent slip-in rib  121  from moving longitudinally relative to walls  113 ,  115 ,  117 , and  119  by means of mechanical lock. 
     In the embodiment of  FIGS. 7A and 7B , the attachment between slip-in rib  121  and the interior of walls  113 ,  115 ,  117 , and  119  is not critically loaded. Slip-in rib  121  can provide buckling support and shape retention through compressive load transfer only. Thus, the structural bond formed by filler material  141  is only secondary. 
     The foregoing discussion and the embodiments of  FIGS. 1-7B  are particularly well suited for secondary structural ribs, i.e., ribs that only react compressive loads. However, primary structural ribs require a structural adhesive bond or mechanical fastening between the rib and the box beam skins. Therefore, it is necessary to provide a means for structurally securing the rib within the box beam. This can be accomplished using either a structural adhesive or mechanical retainers. 
     Referring now to  FIGS. 8-10  in the drawings, two embodiments of slip-in ribs according to the present invention that are suitable for use as both secondary and primary structural ribs are illustrated. For primary structural ribs, the rib must be bonded using a structural adhesive and/or mechanically fastened to the skin of the structural box beam in the local area of load introduction. Other regions of the primary rib can still rely on compression only contact. Although mechanical fasteners may be used, in the preferred embodiment of the present invention, structural adhesives are used to form the bond between the slip-in rib and the skin of the box beam and fasteners are installed after this adhesive material cures as necessary. 
     As is shown, a structural box beam  211  includes a skin  213  and a slip-in rib  221 . Slip-in rib  221  includes an internal web portion  223  and a peripheral flange portion  225 . As with slip-in rib  121 , although web portion  223  has been shown as a solid plate member, it should be understood that web portion  223  may include void spaces and/or a truss arrangement (not shown) that would allow cables, wiring, and other component parts to pass longitudinally through box beam  211 . 
     Flange portion  225  includes a recessed channel  227  that extends along the periphery of slip-in rib  221  to receive a structural adhesive material  241 . As is shown, a small clearance, or gap  231 , exists between the exterior edges of flange  225  and the interior surfaces of skin  213 . In these embodiments, small peripheral grooves  235  may be included in both sides of flange  225  for receiving optional seal members, such as an O-rings  237 . O-rings  237  help to initially seat slip-in rib  221  and help to contain adhesive  241  if required. 
     In these embodiments, it is necessary to form a structural bond between slip-in rib  221  and the interior surface of skin  213 . Although mechanical fasteners may be used, it is preferred that structural adhesive be used to form the structural bond. One method of forming this bond is to use a self-contained displacement mechanism to force structural adhesive  241  outward into contact with the interior surface of skin  213 . This method is illustrated in  FIG. 8 . Another method of forming this bond is to inject structural adhesive  241  through apertures  243  in recessed channel  227 . This method is illustrated in  FIG. 9 . It will be appreciated that there may be many other ways to form this bond that fall within the scope of the present invention.  FIG. 10  is a perspective view of slip-in rib  221 . 
     In the method of  FIG. 8 , channel  227  is lined with a self-contained displacement mechanism, such as an evacuated or collapsed inflatable tubular bladder  245 . Structural adhesive  241  is then located on top of the displacement mechanism, inflatable bladder  245  in this example. Channel  227  is sized and shaped to accommodate the reduced volume of bladder  245  and additional volume to accommodate enough structural adhesive  241  to fill the interface area between slip-in rib  221  and skin  213 . Bladder  245  is preferably made of a material, such as nylon, rubber, or other elastomeric material, such that when inflated or otherwise activated, structural adhesive  241  is displaced outward into contact with the interior surface of skin  213 , thereby forming the structural bond between slip-in rib  221  and skin  213 . Bladder  245  may then be sealed so as to remain inflated, become a sacrificial “fly-away” tool, or be extracted from channel  227  after structural adhesive  241  cures. In this embodiment, apertures  243  may be used to allow access to one or more valve stems  247  for inflating bladder  245 . The use of a self-contained displacement mechanism, such as bladder  245 , allows a measured or metered volume of a fluid adhesive to be dispensed. This eliminates the requirement to control adhesive volume using conventional methods, such as calendared or film adhesive. 
     In the method of  FIG. 9 , slip-in rib  221  is inserted dry into place within skin  213 . Then, structural adhesive  241  is injected through apertures  243 , so as to flow around and fill channel  227 . Channel  227  is sized and shaped to allow adhesive  241  to flow around the perimeter of slip-in rib  221  without squeezing out of flange  227 . As is shown in  FIG. 10 , apertures  243 , which may be located at varying locations around flange  225 , and which may be simple inspection ports, may be used to ensure that adhesive  241  has adequately filled channel  227 . Structural adhesive  241  is forced into channel  227  with sufficient pressure to force structural adhesive into contact with the interior surface of skin  213 , thereby forming the structural bond between slip-in rib  221  and skin  213 . 
     It should be understood that although the embodiments of  FIG. 8-10  have been shown and described with continuous adhesion, it is not necessary that structural adhesive  241  form a continuous bond with skin  213 . According to the present invention, only local adhesion is required. For example, portions of the interface between slip-in rib  221  and skin  213  may be treated so as to release structural adhesive  241  at selected locations and maintain the bond at other locations. 
     As forth above, mechanical retainers may be used instead of structural adhesive to secure the slip-in ribs in place within the box beam. Some mechanical retainers include: shims with snap-in receivers, spring biased buttons, and leaf springs, to name a few. 
     The slip-in ribs of the present invention reduce the complexity of the assembly of box beam structures, particularly in applications where the number of parts has been reduced. This is because the slip-in ribs of the present invention are not directly dependent upon the number of joints where manufacturing tolerances are accommodated. Also, the installation of fasteners into box beam structure presents difficulties such as access to small interior spaces and sealing requirements. The process by which the slip-in ribs of the present invention are installed accommodates tolerance buildup, resulting in reduced part count. This reduces manufacturing labor because fastener installation is eliminated. 
     By using the slip-in ribs according to the present invention, the costs associated with manufacturing a closed-box structure can be reduced due to relaxed tolerances and the ability to reduce part count without incurring additional assembly cost. Because the rib/box bond is secondary, failure of the rib/box bond is not a significant structural concern as long as the rib position in the box is retained. 
     Using slip-in ribs that can be bonded in place with a process that accommodates tolerance build-up results in reduced part count, as installing one-piece ribs in one-piece structural box beams is possible; and manufacturing labor is reduced, as fastener installation is eliminated. 
     Although the present invention has been described with respect to a slip-in rib having basically the same shape and contour as the geometrical cross-section of the structural box beam, it should be understood that the concept of the present invention, i.e., the ability to internally support the box beam using compression-only load paths, may be achieved by other means, as well, including the use of rigid, elongated rods placed between ribs or spars, the rods not being fixed to the internal surface of the box beam. These rods resist buckling of the skins by maintaining the desired spacing between internal stiffening members. 
     It is apparent that an invention with significant advantages has been described and illustrated. Although the present invention is shown in a limited number of forms, it is not limited to just these forms, but is amenable to various changes and modifications without departing from the spirit thereof.