1) Field of the Invention
Embodiments of the present invention relate to composite structures and, more particularly, to the intersection of composite reinforcements for integration with various structures.
2) Description of Related Art
Composite structures provide weight, fatigue strength, and corrosion resistance advantages over metallic structures. Unitization and new manufacturing processes have reduced the cost of composite structures and yielded additional weight savings. Applying composites to complex, highly loaded structures, however, has required multiple manufacturing operations and costly assembly processes. With respect to the aircraft industry, large, unitized, grid-stiffened structures have been developed by integrating axial and longitudinal stiffeners with the aircraft skin. But, there has been no efficient mechanism to transfer loads though the intersections of the stiffeners, which has required additional weight and complexity at the intersections.
In particular, utilization of a longitudinal stiffener and circumferential frame approach consists of large composite or metallic frames that are mechanically fastened at locations along the length of the fuselage and fastened circumferentially to the fuselage's composite skin. This design also includes longitudinal stiffeners that are co-cured, co-bonded, or mechanically fastened to the fuselage's composite skin. This design is not the optimum approach since the frames cannot transverse through the stiffeners. Therefore, the longitudinal stiffeners terminate before and after each frame, which reduces the ability of the structure to control buckling. As such, the load that each longitudinal stiffener carries is directed through the skins. To carry the extra loads the skin thickness must be increased along with the stiffener and frames which increases the weight and complexity of the structure. Furthermore, since the frames are mechanically fastened to the fuselage the fabrication cost is very high.
Composite structures lend themselves to be fabricated as a monolithic or unitized structure. In other words, it is generally less complex and costly to produce a composite fuselage that is co-cured with the skins, longitudinal stiffeners, and frames than it is to build up each component in smaller sections. There are generally three ways to reduce the cost of fabricating composite structures: minimize or eliminate tooling, mechanization of the lay-up process, and unitization of the structure. To minimize tooling costs multifunctional tools were developed. For example, tools have been developed that allow a user to lay-up, cure, and trim on a single tool eliminating the need for two additional tools. The mechanization of composite parts is accomplished through various processes such as: fiber placement, filament winding, braiding, and tape placement. The development of large unitized structures, such as grid-stiffened structural skins, reduces the cost of composite structures due to the elimination of lay-up and cure processes.
One limitation of these large, unitized grid-stiffened structures is that there is no efficient mechanism to transfer loads in both directions through the stiffeners. Current state of the art technology uses primarily braiding or woven intersections for composite intersection reinforcement. However, when braided and woven intersections are unfolded after fabrication, the intersections may have line length differences which typically lead to significant tow waviness through the intersection of the composite intersection reinforcement. Moreover, it is difficult to control the fiber distortion during curing of these composite fiber materials.
U.S. Pat. No. 4,584,226 to Vitale et al. discloses an alternative technique for transferring loads through a structure that includes laminated sheets and fiber strands formed into webs that intersect at a common junction and carry loads through the intersection. The '226 patent also discloses that a single fiber strand may be utilized in an interweaving tool to direct the strand in a repetitive path, such as a clover leaf or figure eight pattern, to obtain a cruciform load transfer structure. The tool generally includes a primary set of mandrels and shuttle carrier rings, where the rings are configured to rotate in a fixed orbit while the mandrels are shifted relative to the rings. Laminated sheets of carbon fiber cloth and tape are positioned to underlie or enclose the cruciform shaped strand. The structure is then cured in a heating chamber or autoclave in order to harden the structure.
Despite these advantages in developing unitized grid stiffened structures, there is a need for more efficiently manufacturing composite intersection reinforcements. In addition, there is a need for a composite intersection reinforcement that effectively transfers loads through the intersection of a plurality of structures.
It would therefore be advantageous to provide an apparatus and method for efficiently and effectively manufacturing composite reinforcements for integration with various structures. In addition, it would be advantageous to provide composite reinforcements that effectively transfer loads through the intersection of a plurality of structures.