Process for making a curved preform made from woven composite materials

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.

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'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.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The typical PI woven preform is illustrated inFIGS. 1 and 2, indicated by numeral10. The preform10includes upstanding legs12and14and bottom foot portions16and17, with center or root18. The warp fibers20run parallel to the legs12and14, while the threads21run perpendicular to the upstanding legs. If one tries to form a curve, indicated by arrows22A and22B, the legs12and14tend to bend over indicated by arrow24. Attempts to “bend” the preform10into other shapes also cause one or more portions to warp. The subject process will eliminate this problem.

A completed preform10A is shown inFIG. 3, having a curved length26, with a radius28. To accomplish this, the upstanding legs12and14are pushed over onto the one of the bottom portions16or17as shown inFIG. 14. The folded preform is then placed in the die30shown inFIG. 4which includes a cutter head31and an receiver pad32. The cutter head31incorporates staggered blades33having a width34as shown inFIG. 4slightly greater than the width of the warp threads20, allowing for some mismatch in warp thread location. This allows the warp threads20to be cut (darted) periodically into segments such that the cuts in each tread are spaced from the cuts in the adjacent treads as shown inFIG. 6. The spacing35of the cuts should be as large a distance as possible, but still allowing the curved length26to 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 inFIG. 3, the darted preform10is folded as shown inFIG. 8Cactus with the legs12and14bent over on to leg17. The preform10is placed in a sine-wave shaped die assembly40having matched die halves41and42with mating sign-wave shaped forming surfaces43and44respectively. The sign-wave pattern is on forming surface43is tapered from ends45and46on die half41and the forming surface44is tapered from ends47and48on forming surface44. What the sine wave forming accomplishes is a stretching that is zero at the end of bottom portion17and a maximum at the end of bottom portion16.

The now stretched preform10can be placed in a die assembly (not shown) and formed into its final shape. Alternately the stretched preform can be shaped by hand. Referring toFIG. 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 elements52A and52B, 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 inFIG. 11and designated by numeral10B, the preform10is folded the shape as illustrated inFIG. 12with the legs12and14folded together and portions16and17folded together. As illustrated the die halves41A and42A have forming43A and44A. Stretching would only from the center outward toward the end of the legs12and14where stretching would be at a maximum.

If on the other hand, the preform final shape shown inFIG. 13, and designated by numeral10C, is desired, then, as illustrated inFIG. 12, the legs12and14, and portions16and17are brought together as in the previous example, and placed in the die assembly40B having die halves41B and42B with forming surfaces43B and44B. However, stretching is accomplished by placing the folded preform10in the sine-wave dies such that stretching of the legs12and14is a minimum at there ends and becomes a maximum at the center. Thereafter, stretching of the bottom portions16and17is 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 inFIG. 15is a right angle preform, generally indicated by numeral60, having horizontal leg62and vertical leg64, while illustrated inFIG. 16is preform, now indicated by numeral60A formed into a curved shape. Such a preform60could be formed by folding the vertical leg64over and on to the horizontal leg62and forming in a die assembly similar to one shown inFIG. 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.