Abstract:
A method of joining two components utilizes a fibrous preform at the joint. The preform has two lateral portions and a longitudinally extending flow path. The preform is sandwiched between the two components and enclosed within a vacuum bag. Resin is introduced at an inlet of the flowpath while a vacuum pump creates a suction at an outlet end and along the lateral side edges. The flow path has greater permeability than the remaining portions of the preform, causing the resin to flow to the outlet end. When the resin reaches the outlet end, the suction at the outlet end is reduced, but the suction at the lateral edges remains without reduction to cause the resin to flow laterally into the preform.

Description:
FIELD OF THE INVENTION 
     This invention relates in general to joining fiber and resin composite structures, and in particular to a method using a fibrous preform between the components that is infused with resin after the components are placed against the preform. 
     BACKGROUND OF THE INVENTION 
     Composite structures formed of resin and fibers such as glass, aramid or carbon are used in many industries because of light weight and strength. A number of methods exist for forming composite structures. In one method, composite components are formed and pre-cured, then secured together. One method of securing the components is by the use of adhesive, which saves cost and weight as compared to drilling holes and mechanically attaching the components. Components may also be co-cured and bonded to each other at the same time, but co-curing requires tooling that holds the uncured components in position. 
     Process sensitivity and quality assurance issues have limited the use of adhesive bonding of pre-cured composites in some applications, such as in aerospace. These issues include sensitivity to surface preparation, bond line thickness, pressure distribution, moisture, and time-temperature curing profiles. In particular, the application of adhesive bonding on large composite structures has proven to be difficult for a number of reasons, as follows: 
     1. insuring uniform pressure application over large bonded areas; 
     2. insuring that no voids or bridged areas exist along the bond line; 
     3. demonstrating that variations in bond line thickness, which are characteristic of the interface between large structures, are acceptable; and 
     4. properly applying the adhesive on the large areas to be bonded in a timely and consistent manner to insure producibility from part to part. 
     One known technique uses a preform between the two components to be joined. A preform is a fabric member that may be woven or nonwoven. Normally, the preform is impregnated initially with a resin, but it will be in an uncured state. An adhesive film may be placed between the preform and each of the pre-cured components. Pressure and heat are applied to cure the preform and bond the components together. The pressure may be applied by vacuum bagging techniques. The preform has a thickness that causes it to compress under the pressure. 
     SUMMARY OF THE INVENTION 
     In this invention, a preform of composite material is utilized. The preform is a fabric member that locates between two components to be joined before the preform is infused with resin. The preform has a thickness such that it can conform to variable thicknesses and adjust to local manufacturing tolerances. For example, the preform may be non-woven fibrous material or felt. The preform has a flow path extending through it with an inlet and an outlet. The flow path has a greater permeability than the remaining portions of the preform. 
     After the preform is placed between the first and second components, the preform and at least portions of the components are placed within a vacuum bag. A resin source connected to the inlet supplies resin to the flow path. A vacuum source at the outlet of the flow path creates suction to cause the resin to flow along the flow path. Preferably, lateral outlets are located along the sides of the preform to induce resin flow from the flow path laterally outward into the preform. After the preform has been properly infused with resin, the resin is cured, thereby bonding the first and second components to each other. 
     In one embodiment, the preform has flaps along each lateral side. The first component is placed in contact with the preform between the flaps. The flaps are then folded over onto portions of the first component prior to the vacuum bag step. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view a preform with a first embodiment of a flow path and shown in one stage of a method in accordance with this invention. 
         FIG. 2  is a sectional view illustrating a second embodiment of the flow path of the preform of  FIG. 1 . 
         FIG. 3  is a schematic sectional view illustrating another step of the method of this invention, and with a preform having a third embodiment of a flow path. 
         FIG. 4  is a sectional view of a preform having a fourth embodiment of a flow path. 
         FIG. 5  is a perspective view illustrating another step of the method of this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , a fiber preform  11  is shown. Preform  11  is a fabric, preferably of a non-woven or fibrous material, such as felt, but woven material are also feasible. Preferably, preform  11  is free of resin at the point in the method shown in  FIG. 1 . Preform  11  has a base  13  that has two lateral portions  13   a ,  13   b . Each lateral portion  13   a ,  13   b  has a side edge  15 . 
     Preform  11  has a flow path  17 , which in  FIG. 1  comprises an open gap between lateral portions  13   a ,  13   b . In this embodiment, flow path  17  extends in a straight line parallel to lateral side edges  15 . Flow path  17  extends from one end of base  13  to the other. 
     As shown in  FIG. 2 , flow path  17  ( FIG. 1 ) could alternately comprise a conduit  19  or tube of a variety of materials. Tube  19  is shown with perforations  21  along the side edges to enable lateral outward flow of resin from the hollow interior of conduit  19 . Rather than perforations  21 , conduit  19  could be made of porous material that freely allows the flow of resin through its sidewalls. Tube  19  is preferably secured to lateral portions  13   a ,  13   b , or it may be imbedded within base  13  or located in a channel (not shown) in base  13 . 
     The flow path could also be made of a fabric, such as flow path  23  in  FIG. 3 . Flow path  23  is formed of fibrous material that has a greater permeability than base  13 . In  FIG. 3 , the thickness of flow path  23  is the same as the thickness of base  13 , but the fibers contained therein are fewer in number or smaller in diameter to provide less resistance to resin flow than base portions  13   a ,  13   b . Flow path  23  is located in the same plane as base portions  13   a ,  13   b.  Preferably flow path  23  is joined to base lateral portions  13   a ,  13   b  by stitching, weaving or adhesives. 
     In  FIG. 4 , flow path  25  is also preferably a fabric similar to flow path  23  ( FIG. 3 ). However, base portions  13   a ,  13   b  join each other, and flow path  25  is shown on top of base  13 . Alternately, flow path  25  could be within a groove or channel (not shown) formed in base  13  or embedded within base  13 . Whether the flow path is gap  17  ( FIG. 1 ), tube  19  ( FIG. 2 ), in-plane fabric strip  23  ( FIG. 3 ), or out-of-plane fabric strip  25 , the resistance to flow of resin is less than in base  13 . 
     Referring again to  FIG. 1 , base  13  preferably has a pair of wings or flaps  27  that are laterally spaced apart from each and on opposite side edges of flow path  17 . Each flap  27  is located on one of the lateral portions  13   a ,  13   b . Each flap  27  is a rectangular strip of fabric that may be the same type and thickness as base  13  or different. Each flap  27  has a stationary portion  29  that overlies one of the base portions  13   a  or  13   b  and is preferably stitched to base  13  by stitching  31 . Each flap  27  is flexible relative to its stationary portion  29  so that it can be folded generally upright or 90 degrees relative to stationary portion  29 . Flaps  27  are also preferably free of resin in the step shown in  FIG. 1 . 
     In  FIG. 1 , a first component  33  is shown resting on the upper surface of base  13 . In this example, first component  33  comprises a spar such as used for an aircraft wing. Spar  33  has a flange  35  on its lower side. Flange  35  is flat and has lateral side edges  37  in this embodiment. Lateral side edges  37  are spaced inward from flaps  27  in the step shown in  FIG. 1 . Spar  33  also has a web  39  that extends at 90 degrees relative to flange  35 . An upper flange  40  ( FIG. 5 ) may be located on the upper end of web  39  parallel to flange  35 . Web  39  is preferably centered over flow path  17 . Preferably first component  33  is of a composite resin and fiber material that has been cured prior to placing it on preform  11 , but it alternately could be a metal. 
     The opposite or lower side of base  13  is in contact with a second component  41 . Second component  41  is also preferably a pre-cured composite structure, but it could be of another material such as metal. Second component  41  may comprise a skin of a wing, for example. Flange  35  and skin  41  define upper and lower sides for flow path  17 . 
     In  FIG. 3 , a technician has folded flaps  27  downward so that each flap  27  overlies a portion of the upper side of spar flange  35 . Also, the technician has installed a vacuum bag assembly  43 . Vacuum bag assembly  43  comprises one or more sheets of flexible plastic film that are arranged to form an airtight enclosure around preform  11 . In this embodiment, two edges of vacuum bag assembly  43  are secured by sealant tape  45  to web  39 . Two other edges are secured by sealant tape  47  to skin  41  outward from preform lateral edges  15 . Vacuum bag assembly  43  also has end portions that extend around each longitudinal end of spar  33 . Because spar  33  and skin  41  are pre-cured, they are impermeable and substantially airtight, thus there is no need for enclosing them entirely within vacuum bag assembly  43 . If the two components to be joined were small enough, the entire assembly could be enclosed within a vacuum bag. 
     Referring to  FIG. 5 , a technician provides an inlet port  49  at one longitudinal end of vacuum bag assembly  43 . Inlet port  49  communicates with one end of flow path  17  (or flow paths  19 ,  23  or  25  if one of those is utilized). A resin source  51 , which comprises a container containing a liquid resin, connects to inlet port  49 . The technician also connects one or more lateral outlet ports  53  to vacuum bag assembly  43  along the lateral edges of spar  33 . In this example, three outlet ports  53  are located along each lateral side of spar  33 . Also, an end outlet port  55  with a valve  61  locates on an end of spar  33  opposite from inlet port  49 . End outlet port  55  communicates with the opposite end of flow path  17 . A series of tubes  57  extend from a vacuum pump  59  to each of the outlet ports  53  and to valve  61 . Inlet and outlet ports  49 ,  53  and  55  communicate with the interior of vacuum bag  43 , but need not be physically joined to any portion of preform  11 . 
     The operator turns on vacuum pump  59  and opens valve  61 , causing air to be withdrawn from vacuum bag  43  as well as from flow path  17  ( FIG. 1 ). The suction created at valve  61  causes resin to flow from resin source  51  through inlet port  49  and along flow path  17  ( FIG. 1 ). Because the permeability of flow path  17  is greater than the permeability of base portions  13   a,    13   b  ( FIG. 1 ), the resin will flow more readily toward end outlet port  55  than laterally outward into preform lateral portions  13   a ,  13   b  ( FIG. 1 .) When the resin nears end outlet port  55  of flow path  17  ( FIG. 1 ), the technician closes valve  61  or at least substantially reduces the air flow through valve  61 . The suction created by vacuum pump  59  continues at lateral outlet ports  53 , inducing resin flow from flow path  17  laterally outward through base lateral portions  13   a ,  13   b  ( FIG. 1 ). The resin also flows into and infuses flaps  27  ( FIG. 3 ). 
     Because spar  33  and skin  41  are pre-cured, resin does not flow into these components. Vacuum bag assembly  43  collapses on the components and applies pressure that causes spar  39  to move more closely toward skin  41 , compressing the thickness of base  11 . If flow path  25  of  FIG. 4  is utilized, rather than flow paths  17  ( FIG. 1 ),  19  ( FIG. 2 ) or  23  ( FIG. 3 ), the vacuum pressure will cause flow path  25  to compress and to compress portions of base  13  so that after evacuation, flow path  25  will be substantially flush with the upper surface of preform base  11 . 
     After preform  11  is entirely infused with resin, the resin flow is stopped by stopping vacuum pump  59  or by closing a valve (not shown) at inlet port  49 . The resin within preform  11  is then allowed to cure, preferably while still under a vacuum, at an appropriate temperature to consolidate and strengthen the assembly. Once cured, vacuum bag  43  can be removed. Preferably, heat is also applied during the curing process or during the resin infusion step. 
     The invention has significant advantages. Since the preform is not pre-impregnated with resin initially, there is no issue related to whether the preform is still within its shelf life. Because of its thickness, the preform of this invention will conform to fabrication and assembly tolerances associated with the components to be joined. The resin infusion process is easy to implement and clearly establishes that all joint surfaces have been infiltrated with resin. Being of fibrous material, the preform forms a composite once the resin has cured, providing a stronger joint than joints that are bonded with only adhesive. Tooling and processing requirements are less intensive than those required for co-curing or co-bonding. The infusion joining process has a potential for a higher degree of repeatability or producibility as compared to other joining methods. 
     While the invention has been shown in only a few of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes without departing from the scope of the invention. For example, if desired, suction at the lateral outlets could be delayed until the resin reaches the end outlet.