Patent Publication Number: US-2013248093-A1

Title: Composite structure forming on coefficient of thermal expansion mismatched tooling

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention relate to systems and methods for forming composite structures. More particularly, embodiments of the present invention relate to systems and methods for forming composite structures with a forming tool that has a large coefficient of thermal expansion. 
     2. Description of the Related Art 
     Composite structures may be formed by winding composite material, such as carbon fiber, around a mold, or mandrel, in the shape of the final structure. The combination of the mandrel and the composite material is often subjected to high temperature in order to cure the composite material. Wrapping the material around the mandrel may form a closed loop. During heating, both the mandrel and the material will expand. If the coefficient of thermal expansion (CTE) of the mandrel is greater than the CTE of the material, then the mandrel will experience greater expansion than the material. If the material is wound tightly against the mandrel, then the expansion of the mandrel will exert a force on the material that creates tension in the fibers. A large enough tension may break a sufficient number of fibers to weaken the final structure and render it unusable. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention solve the above-mentioned problems and provide a distinct advance in the art of forming composite structures. More particularly, embodiments of the invention provide a method for forming composite structures with a mandrel that has a greater coefficient of thermal expansion than that of the composite structure material. 
     A mandrel is provided in the shape of a composite part in its final form. The mandrel may include an outer surface on which composite material is placed, and a groove in which splice material is filled. The groove may be positioned on the outer surface along the length of the mandrel. The groove may also provide a path along which first and second ends of the composite material are aligned such that the composite material contacts the splice material and bonds to the splice material when the composite material and the mandrel are cured. A method for forming the composite part includes the steps of providing a mandrel that includes a groove aligned with its longitudinal axis, filling the groove with splice material, cutting the composite material to match the shape and the size of the mandrel, placing the composite material around the outer surface of the mandrel, and curing the composite material and the mandrel. 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. 
     Other aspects and advantages of the present invention will be apparent from the following detailed description of the embodiments and the accompanying drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       Embodiments of the present invention are described in detail below with reference to the attached drawing figures, wherein: 
         FIG. 1  is a perspective view of a mandrel including a longitudinal groove used to form a composite structure in accordance with various embodiments of the present invention; 
         FIG. 2  is an enlarged front view of the groove; 
         FIG. 3  is a perspective view of the mandrel depicting the groove filled with splice material; 
         FIG. 4  is an enlarged front view of the groove filled with splice material; 
         FIG. 5  is a perspective view of the mandrel depicting a cut in the composite material over the groove; 
         FIG. 6  is an enlarged front view of the groove depicting the cut in the composite material over the groove; 
         FIG. 7  is a perspective view of a finished composite structure; and 
         FIG. 8  is a flow diagram depicting a least a portion of the steps of a method of forming composite parts. 
     
    
    
     The drawing figures do not limit the present invention to the specific embodiments disclosed and described herein. The drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the invention. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following detailed description of the invention references the accompanying drawings that illustrate specific embodiments in which the invention can be practiced. The embodiments are intended to describe aspects of the invention in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments can be utilized and changes can be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense. The scope of the present invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled. 
     In this description, references to “one embodiment”, “an embodiment”, or “embodiments” mean that the feature or features being referred to are included in at least one embodiment of the technology. Separate references to “one embodiment”, “an embodiment”, or “embodiments” in this description do not necessarily refer to the same embodiment and are also not mutually exclusive unless so stated and/or except as will be readily apparent to those skilled in the art from the description. For example, a feature, structure, act, etc. described in one embodiment may also be included in other embodiments, but is not necessarily included. Thus, the present technology can include a variety of combinations and/or integrations of the embodiments described herein. 
     A mandrel  10  for forming composite structures using composite material, constructed in accordance with various embodiments of the current invention, is shown in  FIGS. 1-6 . The mandrel  10  may present the shape of the composite structure to be formed. For example, when forming a composite structure used to manufacture an aircraft fuselage, the mandrel  10  may present a generally elongated cylindrical shape with dimensions that are similar to those of the fuselage. In various embodiments, the mandrel  10  may be solid. In other embodiments, the mandrel  10  may be hollow or have a generally tubular shape. The mandrel  10  is typically constructed from a metal, such as aluminum, that can withstand compressive forces when the composite structure is being formed. Generally, the mandrel  10  includes an outer surface  12  along its circumference, a first end  14 , and an opposing second end  16 . The mandrel  10  may have an exemplary diameter of approximately 6 feet to approximately 30 feet and an exemplary length of approximately 10 feet to approximately 100 feet. 
     The mandrel  10  further includes a groove  18  that is generally positioned on the mandrel  10  in the area or areas where composite material fibers will be wound. Furthermore, the groove  18  is typically oriented such that at any point along the groove  18 , the groove  18  is positioned transverse or at an approximate right angle to the direction of the winding of the fibers. In various embodiments, the groove  18  may have a shape as required by the final part. In some embodiments, the groove  18  may have a curvature along its length. In exemplary embodiments, the groove  18  may be positioned along the outer surface  12  and in alignment with the longitudinal axis of the mandrel  10  such that the groove  18  extends from the first end  14  to the second end  16 . The groove  18  may include a first sidewall  20  and an opposing second sidewall  22  that extend inward from the outer surface  12  of the mandrel  10  at an angle. A generally curved bottom wall  24  (that matches the curvature of the mandrel  10 ) may connect from the first sidewall  20  to the second sidewall  22 . In various embodiments, the first sidewall  20 , the second sidewall  22 , and the bottom wall  24  may have different shapes, dimensions, or relative angles, such that the groove  18  may have a variety of cross-sectional profiles. An exemplary groove  18  may have dimensions of approximately 6 inches to approximately 24 inches in width. In general, the groove  18  is shaped to retain material that is placed within the groove  18  to form a co-cured splice. 
     The composite material  28 , as shown in  FIGS. 5-6 , generally includes at least two constituent components—a reinforcement material and a matrix material. The reinforcement material generally provides mechanical strengthening properties, such as high tensile strength, to the composite material, while the matrix material acts as a binder to hold the reinforcement material together. The reinforcement material and the matrix material may possess additional properties not discussed herein. Furthermore, the composite material may include additional components not discussed herein. 
     Examples of the reinforcement material that may be used with the current invention include, but are not limited to, fiber materials such as carbon fiber, boron fiber, fiberglass, aramid fiber, ceramic fiber, and the like. In the case of fiber-based reinforcement materials, the fiber may exist in one of at least two forms—either preimpregnated (prepreg), in which the fiber may be coated with a matrix material that is uncured, such as uncured resin, or as dry fiber, with no matrix material incorporated prior to part manufacture. The matrix material may typically be in the form of polymer resins, such as epoxies, bismaleimides, vinyl esters, and the like, among others. 
     The composite material  28  may exist in a fiber or bundle of fibers form, or it may exist as a sheet or a weave of fibers. 
     At least a portion of the steps of a method  100  of forming composite structures using the mandrel  10  in accordance with various embodiments of the present invention is listed in  FIG. 8 . The steps may be performed in the order as shown in  FIG. 8 , or they may be performed in a different order. Furthermore, some steps may be performed concurrently as opposed to sequentially. In addition, some steps may be omitted. 
     Referring to step  101 , a mandrel  10  is provided that includes a groove  18  aligned with its longitudinal axis, as seen in  FIGS. 1-2 . In some embodiments, more than one mandrel  10  may be provided. The mandrel  10  may present other shapes as necessary to match the shape of the final part to be formed. The mandrel  10  may include an outer surface  12  in which the groove  18  is placed, such that the groove  18  extends from a first end  14  of the mandrel  10  to an opposing second end  16 . The groove  18  may further include a first sidewall  20 , a second sidewall  22 , and a bottom wall  24 , and may be formed using techniques such as machining. 
     Referring to step  102 , the groove  18  is filled with composite material, as seen in  FIGS. 3-4 , referred to as splice material  26 , which may be cured or uncured. In some embodiments, the splice material  26  may also be a metal such as titanium. 
     Referring to step  103 , the composite material  28  is cut to match the size and shape of the mandrel  10 . In the case of fiber or fiber bundle composite material  28 , the composite material  28  is cut to form strands with a length that is approximately equal to the circumference of the mandrel  10 . If the composite material  28  is a sheet or a weave, then the composite material  28  is cut to have a width approximately equal to the circumference of the mandrel  10 . Cutting the composite material  28  generally creates a first material end  30  and a second material end  32  on opposing sides of the composite material  28 . 
     Referring to step  104 , the composite material  28  is placed on the mandrel  10 , such that the first material end  30  and the second material end  32  abut each other over the groove  18  and the splice material  26 . The composite material  28  may be placed using automated techniques or manual techniques. In various embodiments, fiber tow placement may be used to place the composite material  28  on the mandrel  10 . Generally, after placement, the first material end  30  and the second material end  32  should touch or abut one another over the center of the groove. In addition, it is desirable for the first material end  30  and the second material end  32  to touch one another, although a gap therebetween of up to 0.05 inches may be acceptable. 
     Referring to step  105 , the mandrel  10 , the composite material  28 , and the splice material  26  are cured. The curing may be performed in an autoclave or an oven. During the curing, the mandrel  10  may expand as a result of the high temperature. The circumference of the mandrel  10  may increase thereby creating tension on the composite material  28  and increasing the separation distance between the first material end  30  and the second material end  32 . The portions of the first material end  30  and the second material end  32  that contact the splice material  26  may bond to the splice material  26  and cure as a unit such that the composite material  28  (including the first material end  30  and the second material end  32 ) and the splice material  26  form a single monolithic finished composite part  34 . 
     Referring to step  106 , the part  34 , shown in  FIG. 7 , is removed from the mandrel  10 . The part  34  is generally ready for use. 
     The method of various embodiments of the current invention allows for lighter weight, lower cost metals with a relatively higher coefficient of thermal expansion, such as aluminum, to be used for the mandrel  10  while still providing a monolithic finished composite part. Furthermore, the method results in virtually no broken fibers and a reduced number of marcelled fibers. Thus, finished parts are of a higher quality for a lower cost. 
     Although the invention has been described with reference to the embodiments illustrated in the attached drawing figures, it is noted that equivalents may be employed and substitutions made herein without departing from the scope of the invention as recited in the claims.