Abstract:
The invention is a process for forming a preform for use in a structure having at least one curved portion of a specific length, including the steps of: 1) providing a preform having cable of expanding in length threads in rows parallel to the direction of curvature over a length equal to the length requiring curvature, such that the gaps between each thread row are spaced from the gaps in the adjacent tread rows; and 2) stretching the portions of the preform requiring curvature in a sine-wave pattern.

Description:
This is a continuation in part of application Ser. No. 10/779,901, filed Feb. 17, 2004. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to the field of composite structure fabrication techniques and, in particular, to a process for forming curves in woven composite preforms. 
     2. Description of Related Art 
     Typically T shaped composite structures are fabricated by joining the vertical member to the horizontal member by bonding a multi-number of reinforcing sheets across the joint (extending from the horizontal member up along the side of the vertical member). Such a joint is disclosed in WIPO Publication WO 01/64387 A1 Production, Forming, Bonding Joining And Repair Systems For Composite And Metal Components by N. Graham. Two honeycomb sheets are joined by layers of composite cloth to the horizontal member on each side of the vertical member that extend up each side of the vertical member. The disadvantage is that the joint&#39;s strength is dependent on the layers of composite cloth. 
     Recently, three-dimensional weaving has allowed very complex shapes to be woven. For example, U.S. Pat. No. 6,007,319 Continuous Forming Of Complex Molded Shapes by T. L. Jacobson, et al. discloses a method of weaving complex preform shapes. More recently a process for making woven 3D PI cross-section shapes in U.S. Pat. No. 6,446,675 Minimum Distortion 3D Woven Preforms by J. Goering. Such preforms can be impregnated with a resin and partially cured (called B stage) and stored for relatively long periods at low temperature until use is required. However, attempts to use such a preform in a curved structure has resulted in severe distortion. It is also difficult to form 2 dimensional woven composite preforms into curved shapes. 
     Thus, it is a primary object of the invention to provide a process for making curved preforms from woven composite materials. 
     It is another primary object of the invention to provide a process for making curved 3D woven PI preforms structures. 
     It is a further object of the invention to provide a process for making such 3D woven PI preforms in curved structures that does not significantly reduce the strength of the preform. 
     SUMMARY OF THE INVENTION 
     The invention is a process for forming woven materials, and in, particular a 3D woven PI shaped cross-section preform having a first and second upstanding leg portions and first and second foot portions for use in a structure having at least one curved portion of a specific length. The process includes the steps of:
     1. Cutting the threads parallel to the direction of curvature over a length equal to the length of the curve, such that the cuts in each thread are spaced from the cuts in the adjacent treads. Preferably, the first and second upstanding leg portions are folded over the first and second bottom foot portions, prior to the step of cutting.   2. Stretching the portions of the preform requiring curvature. If the preform must be curved in the plane of the bottom foot portions with the first bottom portion requires greater stretching than the second bottom foot portion, both upstanding legs are bent over on to one of the bottom foot portions in a “cactus” configuration. The preform is then placed between matched tapered sign-wave dies, with a small amplitude end and a large amplitude end, with the first portion positioned in the small amplitude end and the second end in the large amplitude end. Stretching is accomplished by closing the die halves. If the completed preform requires concave curvature of the bottom foot portions, the bottom foot and upstanding leg portions of the preform are bent toward each other in an “H” configuration, but only the upstanding legs are placed in a small to large amplitude tapered sign wave die for stretching while the foot portion is stretched at a constant amount in the large amplitude section of the die. If on the other hand the bottom foot portions require a convex shape, the bottom foot and upstanding leg portions are again folded together in an “H” configuration. However the die shape is tapered over the length of the upstanding legs from small to large amplitude and the foot bottom portions are outside of the stretching sinewave die.   3. Forming the curvature in the preform. After the step of stretching, the preform is expanded about a die surface having the final desired shape of the preform.   

     The preform can thereafter be used in the making of curved composite structures, primarily as a transition member between sheet type structural members. Note that the preform can be pre-impregnated with a resin prior to any forming steps. 
     It should be understood, that the above-described PI shaped preform required that the threads in the direction of curvature be cut. However, it is possible to make curved preforms that do not require this step. For example, the preform could be manufactured with threads made from tows made up of short discontinuous fibers. The “bundled” fibers act as a continuous thread allowing their weaving into a preform, but also will allow stretching during sine wave die forming. U.S. Pat. No. 6,477,740 “Stretch Breaking Of Fibers” by N. W. Hansen discloses a method of stretch braking of fibers of the thread discontinuous threads in the direction of curvature. Either method will work, and the process would only comprise the steps of stretching and forming. 
     The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description in connection with the accompanying drawings in which the presently preferred embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for purposes of illustration and description only and are not intended as a definition of the limits of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the PI shaped preform made of woven filamentary material. 
         FIG. 2  is a top view of the PI shaped preform shown in  FIG. 1  illustrating the effect of attempting to curve the preform in the “as is” condition. 
         FIG. 3  is a perspective view of the desired curved shape for the preform shown in  FIG. 1 . 
         FIG. 4  is an end view of the preform with the legs folded over on the bottom portions installed in a darting die assembly. 
         FIG. 5  is a side view of the bottom portion of the die illustrating the placement of the cutters used for darting the preform. 
         FIG. 6  is an enlarged view of a portion of the preform, after darting illustrating the darting pattern. 
         FIG. 7  is a perspective view of a sine wave forming die assembly used for selectively stretching the preform. 
         FIG. 8  is a cross-sectional view of the die assembly shown in  FIG. 7  illustrating a first method of stretching in order to form the preform shown in  FIG. 3 . 
         FIG. 9  is a top view of  FIG. 3 . 
         FIG. 10  is a cross-sectional view of a completed structure using the preform shown in  FIG. 3 . 
         FIG. 11  is a perspective view of the preform having a convex curvature. 
         FIG. 12  is a cross-sectional view of the die assembly shown in  FIG. 7  illustrating a second method of stretching the preform to obtain the curvature shown in  FIG. 10 . 
         FIG. 13  is a perspective view of the preform having a concave curvature. 
         FIG. 14  is a cross-sectional view of the die assembly shown in  FIG. 7  illustrating a third method of stretching the preform to obtain the curvature shown in  FIG. 12 . 
         FIG. 15  is a perspective view of a second L shaped preform. 
         FIG. 16  is a perspective view of the preform shown in  FIG. 14  formed into a curved shape. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The typical PI woven preform is illustrated in  FIGS. 1 and 2 , indicated by numeral  10 . The preform  10  includes upstanding legs  12  and  14  and bottom foot portions  16  and  17 , with center or root  18 . The warp fibers  20  run parallel to the legs  12  and  14 , while the threads  21  run perpendicular to the upstanding legs. If one tries to form a curve, indicated by arrows  22 A and  22 B, the legs  12  and  14  tend to bend over indicated by arrow  24 . Attempts to “bend” the preform  10  into other shapes also cause one or more portions to warp. The subject process will eliminate this problem. 
     A completed preform  10 A is shown in  FIG. 3 , having a curved length  26 , with a radius  28 . To accomplish this, the upstanding legs  12  and  14  are pushed over onto the one of the bottom portions  16  or  17  as shown in  FIG. 14 . The folded preform is then placed in the die  30  shown in  FIG. 4  which includes a cutter head  31  and an receiver pad  32 . The cutter head  31  incorporates staggered blades  33  having a width  34  as shown in  FIG. 4  slightly greater than the width of the warp threads  20 , allowing for some mismatch in warp thread location. This allows the warp threads  20  to be cut (darted) periodically into segments such that the cuts in each tread are spaced from the cuts in the adjacent treads as shown in  FIG. 6 . The spacing  35  of the cuts should be as large a distance as possible, but still allowing the curved length  26  to be formed. Thus some experimentation may be required to obtain the optimum spacing. 
     Referring to Figures, if the part is to be simply curved shape as shown in  FIG. 3 , the darted preform  10  is folded as shown in  FIG. 8  Cactus with the legs  12  and  14  bent over on to leg  17 . The preform  10  is placed in a sine-wave shaped die assembly  40  having matched die halves  41  and  42  with mating sign-wave shaped forming surfaces  43  and  44  respectively. The sign-wave pattern is on forming surface  43  is tapered from ends  45  and  46  on die half  41  and the forming surface  44  is tapered from ends  47  and  48  on forming surface  44 . What the sine wave forming accomplishes is a stretching that is zero at the end of bottom portion  17  and a maximum at the end of bottom portion  16 . 
     The now stretched preform  10  can be placed in a die assembly (not shown) and formed into its final shape. Alternately the stretched preform can be shaped by hand. Referring to  FIG. 9 , it can thereafter be resin infused by any of several existing resin infusion processes and be used to join to structural elements together. For example, structural elements  52 A and  52 B, by the process set forth in WIPO Publication WO 01/64387 A1 Production, Forming, Bonding Joining And Repair Systems For Composite And Metal Components by N. Graham. Of course, the preform could be resin infused prior to darting and stretching. 
     If the completed preform requires curvature in a convex shape as illustrated in  FIG. 11  and designated by numeral  10 B, the preform  10  is folded the shape as illustrated in  FIG. 12  with the legs  12  and  14  folded together and portions  16  and  17  folded together. As illustrated the die halves  41 A and  42 A have forming  43 A and  44 A. Stretching would only from the center outward toward the end of the legs  12  and  14  where stretching would be at a maximum. 
     If on the other hand, the preform final shape shown in  FIG. 13 , and designated by numeral  10 C, is desired, then, as illustrated in  FIG. 12 , the legs  12  and  14 , and portions  16  and  17  are brought together as in the previous example, and placed in the die assembly  40 B having die halves  41 B and  42 B with forming surfaces  43 B and  44 B. However, stretching is accomplished by placing the folded preform  10  in the sine-wave dies such that stretching of the legs  12  and  14  is a minimum at there ends and becomes a maximum at the center. Thereafter, stretching of the bottom portions  16  and  17  is held constant. 
     Thus it can be seen that the process will allow the PI shaped preform to molded into numerous curved shapes, many more than have been described herein. While there is a weakening of the preform due to the cutting of the warp fiber, the loss of strength has proven acceptable in most applications, particularly where the primary loads and distributed along the fill fiber. 
     It should be understood, that the above-described PI shaped preform required that the threads in the direction of curvature be cut. However, it is possible to make curved preforms that do not require this step. For example, the preform could be manufactured with threads made from tows made up of short discontinuous fibers. The “bundled” fibers act as a continuous thread allowing their weaving into a preform, but also will allow stretching during sine wave die forming. U.S. Pat. No. 6,477,740 “Stretch Breaking Of Fibers” by N. W. Hansen discloses a method of stretch braking of fibers of the thread. The thread thereafter can be used to weave preforms. Illustrated in  FIG. 15  is a right angle preform, generally indicated by numeral  60 , having horizontal leg  62  and vertical leg  64 , while illustrated in  FIG. 16  is preform, now indicated by numeral  60 A formed into a curved shape. Such a preform  60  could be formed by folding the vertical leg  64  over and on to the horizontal leg  62  and forming in a die assembly similar to one shown in  FIG. 8 . 
     While the invention has been described with reference to a particular embodiment, it should be understood that the embodiment is merely illustrative, as there are numerous variations and modifications, which may be made by those skilled in the art. Thus, the invention is to be construed as being limited only by the spirit and scope of the appended claims. 
     INDUSTRIAL APPLICABILITY 
     The invention has applicability to industries manufacturing composite structures, particularly, the aircraft industry.