Abstract:
A woven preform for a reinforced composite material which may be woven flat and folded into shape with the fibers at the fold being so woven so as to compensate for folding.

Description:
FIELD OF THE INVENTION 
     This invention relates to woven preform for a reinforced composite material which can be woven flat and folded into its final shape without producing undesirable distortion in the fibers. 
     BACKGROUND OF THE INVENTION 
     The use of reinforced composite materials to produce structural components is now widespread, particularly in applications where their desirable characteristics are sought of being light in weight, strong, tough, thermally resistant, self-supporting and adaptable to being formed and shaped. Such components are used, for example, in aeronautical, aerospace, satellite, battery, recreational (as in racing boats and autos), and other applications. 
     Typically such components consist of reinforcement materials embedded in matrix materials. The reinforcement component may be made from materials such as glass, carbon, ceramic, aramid (e.g., “KEVLAR”), polyethylene, and/or other materials which exhibit desired physical, thermal, chemical and/or other properties, chief among which is great strength against stress failure. Through the use of such reinforcement materials, which ultimately become a constituent element of the completed component, the desired characteristics of the reinforcement materials, such as very high strength, are imparted to the completed composite component. The constituent reinforcement materials typically, may be woven, knitted or otherwise oriented into desired configurations and shapes for reinforcement preforms. Usually particular attention is paid to ensure the optimum utilization of the properties for which the constituent reinforcing materials have been selected. Usually such reinforcement preforms are combined with matrix material to form desired finished components or to produce working stock for the ultimate production of finished components. 
     After the desired reinforcement preform has been constructed, matrix material may be introduced to and into the preform, so that typically the reinforcement preform becomes encased in the matrix material and matrix material fills the intersticial areas between the constituent elements of the reinforcement preform. The matrix material may be any of a wide variety of materials, such as epoxy, polyester, vinyl-ester, ceramic, carbon and/or other materials, which also exhibit desired physical, thermal, chemical and/or other properties. The materials chosen for use as the matrix may or may not be the same as that of the reinforcement preform and may or may not have comparable physical, chemical thermal or other properties. Typically, however, they it will not be of the same materials or have comparable physical, chemical thermal or other properties, since a usual objective sought in using composites in the first place is to achieve a combination of characteristics in the finished product that is not attainable through the use of one constituent material alone. So combined, the reinforcement preform and the matrix material may then be cured and stabilized in the same operation by thermosetting or other known methods, and then subjected to other operations toward producing the desired component. It is significant to note at this point that after being so cured, the then solidified masses of the matrix material normally are very strongly adhered to the reinforcing material (e.g., the reinforcement preform). As a result, stress on the finished component, particularly via its matrix material acting as an adhesive between fibers, may be effectively transferred to and borne by the constituent material of the reinforcing reinforcement preform. 
     Frequently, it is desired to produce components in configurations that are other than such simple geometric shapes as (per se) plates, sheets, rectangular or square solids, etc. A way to do this is to combine such basic geometric shapes into the desired more complex forms. One such typical combination is made by joining reinforcement preforms made as described above at an angle (typically a right-angle) with respect to each, other. Usual purposes for such angular arrangements of joined reinforcement preforms are to create a desired shape to form a reinforcement preform that includes one or more end walls or “T” intersections for example, or to strengthen the resulting combination of reinforcement preforms and the composite structure which it produces against deflection or failure upon it being exposed to exterior forces, such as pressure or tension. In any case, a related consideration is to make each juncture between the constituent components as strong as possible. Given the desired very high strength of the reinforcement preform constituents per se, weakness of the juncture becomes, effectively, a “weak link” in a structural “chain”. 
     An example of an intersecting configuration is set forth in U.S. Pat. No. 6,103,337, the disclosure of which is incorporated herein by reference. This reference sets forth an effective means of joining together two reinforcing plates into a T form. 
     Various other proposals have been made in the past for making such junctures. It has been proposed to form and cure a panel element and an angled stiffening element separate from each other, with the latter having a single panel contact surface or being bifurcated at one end to form two divergent, co-planar panel contact surfaces. The two components are then joined by adhesively bonding the panel contact surface(s) of the stiffening element to a contact surface of the other component using thermosetting adhesive or other adhesive material. However, when tension is applied to the cured panel or the skin of the composite structure, loads at unacceptably low values resulted in “peel” forces which separate the stiffening element from the panel at their interface since the effective strength of the join is that of the matrix material and not of the adhesive. 
     The use of metal bolts or rivets at the interface of such components is unacceptable because such additions at least partially destroy and weaken the integrity of composite structures themselves, add weight, and introduce differences in the coefficient of thermal expansion as between such elements and the surrounding material. 
     Other approaches to solving this problem have been based on the concept of introducing high strength fibers across the joint area through the use of such methods as stitching one of the components to the other and relying upon the stitching thread to introduce such strengthening fibers into and across the juncture site. One such approach is shown in U.S. Pat. No. 4,331,495 and its method divisional counterpart, U.S. Pat. No. 4,256,790. These patents disclose junctures having been made between a first and second composite panels made from adhesively bonded fiber plies. The first panel is bifurcated at one end to form two divergent, co-planar panel contact surfaces in the prior art manner, that have been joined to the second panel by stitches of uncured flexible composite thread through both panels. The panels and thread have then been “co-cured”: i.e., cured simultaneously. 
     Another method to improve upon junction strength is set forth in U.S. Pat. No. 5,429,853. 
     While the prior art has sought to improve upon the structural integrity of the reinforced composite and has achieved success, particularly in the case of U.S. Pat. No. 6,103,337, there exists a desire to improve thereon or address the problem through an approach different from the use of adhesives or mechanical coupling. In this regard, one approach might be by creating a woven three dimensional (“3D”) structure by specialized machines. However, the expense involved is considerable and rarely is it desirable to have a weaving machine directed to creating a simple structure. 
     Another approach would be to weave a two dimensional (“2D”) structure and fold it into shape. However, this typically results in parts that distort when the preform is folded. The distortion occurs because the lengths of fiber as-woven are different than what they should be when the preform is folded. This causes dimples and ripples in areas where the as-woven fiber lengths are too short, and buckles in the areas where fiber lengths are too long. These distortions cause undesirable surface anomalies and reduce the strength and stiffness of the component. While this may be relieved by cutting and darting, such procedure is undesirable, since it is labor intensive or otherwise may compromise the integrity of the preform. 
     As aforesaid, 3D preforms which can be processed into fiber reinforced composite components are desirable because they provide increased strength relative to 2D laminated composites. These preforms are particularly useful in applications that require the composite to carry out-of-plane loads. 
     Accordingly, there is a need for a 3D preform which provides for an alternative approach and/or an improved method of creating 3D preforms and/or reinforced composite structures. 
     SUMMARY OF THE INVENTION 
     It is therefore a principal object of the invention to provide for a 3D preform which is of a design which is an alternative to and/or an improvement on existing preforms and/or reinforced composite structures heretofore available. 
     It is a further object of the invention to provide for such a 3D preform which may be folded into shape without distortion of the fibers comprising the preform. 
     A yet further object of the invention is to provide for a 3D preform which is particularly useful in forming T-shaped reinforced composites. 
     These and other objects and advantages are achieved by providing for a 3D woven preform that can be woven flat and then folded into its final shape prior to impregnation of resin without producing an undesirable distortion in the fibers. This is accomplished by adjusting the length of the fibers during weaving such that so that some are too short in some areas and too long in others. The fibers are then equalized when the preform is folded into shape providing for a smooth transition at the fold. This technique, while particularly suited for form T-shaped woven preforms, may be utilized with various shapes. Also, while reference is made to woven preforms, its applicability to non-wovens, such as braided or stitched bonded, for example, will be apparent to one skilled in the art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Thus by the present invention its objects and advantages will be realized, the description of which should be taken and in conjunction with the drawings wherein: 
     FIGS. 1A and 1B are respective side sectional views of a woven fabric as-woven and as-folded for use as a preform for a reinforced composite. 
     FIGS. 2A and 2B are respective side sectional views of a woven fabric as-woven and as-folded for use as a preform for a reinforced composite, incorporating the teaching of the present invention. 
     FIGS. 3A and 3B are respective side sectional views of a woven fabric as-folded illustrating a plurality of weft fibers at varying lengths, incorporating the teachings of the present invention. 
     FIGS. 4A and 4B are respective side sectional views of a woven fabric and as-folded having a different configuration, incorporating the teachings of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning now to the drawings in more detail, where like parts will be similarly numbered, in FIG. 1A there is shown a woven fabric  10  comprising fill or weft fibers  12 A, B and C (for illustration purposes) and warp fibers  14 . As aforesaid, the fabrics may be made from a variety of materials with, however, carbon or carbon based fiber typically utilized in aeronautical applications. 
     What is shown in FIGS. 1A and 1B might be considered a conventional approach to create a woven preform which may be folded into shape. In this regard, a space  16  is provided in the woven fabric where the lower four rows of warp fibers  14  are absent from the weave. This is to allow the adjacent portions or legs  18  and  20  of the fabric  10  to be folded perpendicular to the fabric  10  as shown in FIG.  1 B. 
     The problem that results, however, is that, since the lengths of the weft fibers  12 A, B and C are the same when woven flat and when folded, upon folding, this causes dimples or ripples in the areas  22  and  24  where the fibers  12 B are too short and buckles in areas  26  and  28  where the weft fibers  12 C are too long. The dimples or ripples in areas  22  and  24  occur since the weft fibers  12 B which are too short upon folding cause the warp fibers to pull downward. In areas  26  and  28 , since the weft fibers  12 C are too long upon folding, they buckle outward. The foregoing results in undesirable surface anomalies and reduce the strength and stiffness of the component. 
     Turning now to FIGS. 2A and 2B, like parts are similarly numbered. As can be seen in FIG. 2A, weft fiber  12 B′, is shown merely for illustration purposes. Due to the differences in the length of the fiber in the as-woven (shown in Figu re  2 A) versus the as-folded (shown in FIG. 2B) the fiber lengths are woven too short in area  30  and too long in area  32 . These differences in length are equalized when the preform is folded into shape so that the desired lengths are obtained. FIG. 2B shows the typical fiber path for a preform which, when folded, has a “pi” cross section. Fiber  12 B′ is intentionally woven in a path that renders it too short in area  30  and too long in area  32 . This causes a tensioning in the short portion which, when folded into the “pi” shape, causes a drawing in of the long portion so that the fiber  12 B′ length on both sides is equal and correct in the folded portion and avoids the distortions and buckles as heretofore discussed. 
     Of course FIGS. 2A and 2B just illustrate the basic concept showing the use of a single fiber  12 B. There would be many fibers along the length of the preform which would have the same fiber path. Also, there would be additional weft fibers woven in a similar manner to provide for the different lengths necessary depending upon their position in the weave, as will be discussed. 
     Turning now to FIGS. 3A and 3B, there is shown, respectively, fabric  10  as-woven and as-folded for a pi-shaped preform. The preform shape is for illustration purposes only, since the present invention can be implemented with regard to many shapes as will be apparent to one skilled in the art. 
     As shown, the top portion  34  of the fabric  10  of the pi preform comprises layers of warp fibers  14  land weft or fill fibers generally designated  12 ′. The leg portions  36  and  38  of the pi preform similarly comprise layers of warp fibers  14  and weft fibers  12 . The fabric  10 , as would be in the prior examples, is woven in a desired weave pattern (e.g. satin, etc.) suitable for the purpose. Also, as in the immediate prior example, the invention is directed towards eliminating distortions and buckles which would normally occur when going from a flat woven to a folded situation. 
     As can be seen in FIG. 3A, depending upon their position in the weave, this will determine the fiber path. For example, weft fibers  40  are of equal length on both sides of space  16 , Fibers  42  are too long on the left hand side of FIG.  3 A and too short on the right hand side. Similarly, fibers  44  are too short on the left hand side and too long on the right hand side. 
     Because of this, when the leg portions  36  and  38  are folded down as shown in FIG. 3B, the fiber lengths equalize in the same manner as discussed with regard to FIGS. 2A and 2B. Note that, the space between the leg portions  36  and  38  and the top portion  34  is exaggerated for illustration purposes. 
     Turning now to FIGS. 4A and 4B, there is shown a layout for making a T shaped preform  50 . 
     FIG. 4A is the as-woven fabric with FIG. 4B being the as-folded fabric. 
     The top portion of the T is made up from two leg portions  52  and  54 . The bottom of the T comprises portion  56 . As in the earlier examples, the preform is woven in a desired pattern suitable for the purpose with, however, the modification of the weft fibers  58  so as to implement the advantages of the present invention. In this regard, fiber  58  is too long on the top and too short on the bottom. Fiber  60  is too short on the top and too long on the bottom. Fibers  62  are of equal length both top and bottom. 
     As shown in FIG. 4B, fibers  58  and  60  act as previously described due to the too long and too short fiber paths. They serve to interconnect portions  52  and  54 . Fibers  62  serve to interconnect portions  52  and  54  with portions  56 . 
     As in the case of all of the preforms of the present invention, once folded into shape, they can then be impregnated or coated with a resin to create the desired shaped composite structure. 
     Thus by the present invention its objects and advantages are realized and although preferred embodiments have been disclosed and described in detail herein, its scope should not be limited thereby rather its scope should be determined by that of the appended claims.