Abstract:
The present invention generally relates to a woven preform for reinforced composite materials and a method of making thereof. Specifically, the present invention is a method of machine weaving fiber preforms for polymer matrix composites that consist of closed perimeters with multiple intersecting members in their interiors. More specifically, the invention is a woven preform and a method of forming thereof with closed cells at the outer edges with continuous hoop reinforcement in each cell of the preform. The woven preform is woven flat in both the warp and weft directions, and then unfolded to achieve the final shape of the structure, and can be processed into composite structural components using known methods such as resin transfer molding or chemical vapor infiltration. Thus, complicated shapes of all sizes can be woven on a conventional loom using the instant method.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to woven preforms for reinforced composite materials and, in particular, to a method for machine weaving fiber preforms that consist of closed perimeters with multiple intersecting members in their interiors. 
     2. Incorporation by Reference 
     All patents, patent applications, documents, references, manufacturer&#39;s instructions, descriptions, product specifications, and product sheets for any products mentioned herein are incorporated herein by reference, and may be employed in the practice of the invention. 
     3. Description of the Prior Art 
     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, 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, 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, nonwoven 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 interstitial 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 mass 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 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 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”. 
     The current state of the art for these types of structures is to lay individual layers of tackified fabric or prepreg to form the final shape. The resulting laminated preform is then resin transfer molded (for the case where tackified fabric is used) or vacuum bagged and cured (for the case when prepreg is used). In related art, U.S. Pat. No. 5,451,448 relates to a composite multilayered flexible blanket insulation including a top woven fabric layer having multiple layers of continuous woven fabric, a bottom woven fabric layer, high temperature insulation layer, and optional reflection shield layers and spaces, all secured using a woven ceramic fabric. The top fabric and bottom fabric layers are secured to each other by a rib structure of woven ceramic fabric at an angle from the surface of either the top fabric layer or bottom fabric layer, thus creating triangular prism or trapezoidal prism shaped spaces between the top fabric layer and bottom fabric layer and the rib structure. 
     U.S. Pat. No. 6,418,973 is a woven preform for a ceramic composite having a plurality of layers of woven yarns of fibrous material, and structural members extending between the layers. The structural members may be walls that, along with the layers, define channels. The method disclosed therein requires weaving preforms with the desired distances between the individual sheets or layers, such that the sheets are physically spaced apart at a predetermined distance at the time of weaving. This not only limits the size and shape of the structures that can be produced, but also fails to provide the ease of being produced on a conventional loom. The preform formed therein, additionally, does not have closed cells on its outer edges, and the paths of the weft yarns cannot be selected such that they result in continuous hoop reinforcement in each cell, resulting in open cells on the outer edges of the structure and cells that are much weaker with respect to internal pressure loads. Therefore, there is a need in the art to provide woven preforms and a method of forming thereof with closed cells at the outer edges with continuous hoop reinforcement in each cell of the preform. 
     The present invention overcomes the drawbacks of the prior art and provides further advantages such as requiring less work to produce the woven preform by adopting a unique flat machine weaving technique, forming a preform that is woven flat at first and then subsequently folded open to attain its final shape. 
     SUMMARY OF THE INVENTION 
     The present invention is a woven preform for reinforced composite materials and a method of forming thereof. Specifically, the present invention is a method of machine weaving fiber preforms for polymer matrix composites that consist of closed perimeters with multiple intersecting members in their interiors. 
     Accordingly, one embodiment of the present invention is directed to a fiber reinforced composite including a woven preform having a plurality of horizontal layers and a plurality of vertical layers of woven material integrally woven with the plurality of horizontal layers. The plurality of vertical layers extend as structural members between the horizontal layers, and the plurality of horizontal layers together with the plurality of vertical layers define a plurality of through cells within the preform. The woven preform may be impregnated with a polymer matrix material to form the final structure. The preform has a closed perimeter with closed cells at outer edges defined by circumferentially woven weft fibers. These cells may extend in a warp direction (where outer edges are defined by circumferentially woven weft fibers) or weft direction and may be square or rectangular in shape. The preform has continuous hoop reinforcement in each cell defined by circumferentially woven weft fibers. 
     One embodiment of the invention is a method of making a fiber reinforced composite including the steps of forming a preform by integrally weaving a plurality of horizontal layers with a plurality of vertical layers, and opening the preform after being woven so that the plurality of vertical layers extend as structural members between the horizontal layers, and the plurality of horizontal layers together with the plurality of vertical layers define a plurality of through cells within the preform. The woven preform may be impregnated in a polymer matrix material by transfer molding or chemical vapor infiltration. The preform may be woven to have a closed perimeter with closed cells on outer edges by circumferentially weaving weft fibers. The cells extend in a warp direction (where outer edges are defined by circumferentially woven weft fibers) or weft direction and may be square or rectangular in shape. The preform is formed to have continuous hoop reinforcement in each cell defined by circumferentially woven weft fibers. 
     Accordingly, an object of the invention is to lock all fibers together at the joints thus eliminating the resin layers between the individual plies that would be present in a conventional laminate composite, which eliminates the potential for delamination and improves damage tolerance. 
     Another object of the invention is to provide all of the fiber in a horizontal wall of the preform to be continuous through the vertical walls and vice versa, thereby improving the strength of the structure at the intersections. 
     Yet another object of the invention is to weave the preform in such a manner that the weft fibers in the interior wall weave into the exterior wall, thereby improving strength and damage tolerance. 
     Yet another object of the invention is to eliminate practically all of the hand labor required to assemble these structures in conventional lamination techniques. The preform of the invention is woven to shape, and placed into the mold as a single piece, thereby reducing manufacturing time and cost. 
     Yet another object of the invention is to avoid the additional steps associated with joining multiple structural components, by producing an integrally woven preform with improved performance and structural efficiency, at lower production costs. 
     Yet another object of the invention is to avoid trimming at the edges, such that there is no extra material along the edge that needs to be cut, thereby reducing time and cost; and also avoiding the need for any tackifier, thus eliminating potential incompatibility problems with the primary resin. 
     Yet another object of the invention is to provide integrally woven preforms for use in composite structures that do not need holes to be drilled for bolts or rivets, which are traditionally used to join metal components to form complex metal structures. 
     Yet another object of the invention is to provide integrally woven preforms that are very stable over a wide range of temperatures, such that they do not experience changes in size and shape as is the case with traditional metal structures. 
     Yet another object of the invention is to provide an integrally woven preform that is woven flat in both the warp and weft directions. Thus, complicated shapes of all sizes can be woven on a conventional loom. The woven preform of the present invention is woven flat, and then unfolded to achieve the final shape of the structure, resulting in increased flexibility in the types of structures that can be produced using the instant method. 
     The present invention can be used in a myriad of applications such as but not limited to heat exchangers where fluid is run through the open channels, structural panels with raceways that permit electrical wiring and other elements such as hydraulic lines to be run unimpeded through the structure, and as chaff dispenser systems in aircrafts where the chaff cartridges are loaded into the open channels. 
     These and other objects and advantages will be apparent from the present invention. The fiber reinforcement is one that may be woven on conventional weaving machinery and then folded into its final shape prior to impregnation of resin without producing an undesirable distortion in the fibers. 
     For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying descriptive matter in which preferred, but non-limiting, embodiments of the invention are illustrated. 
     The terms “fibers” and “yarns” are used interchangeably throughout and have an equivalent meaning. Terms “comprising” and “comprises” in this disclosure can mean “including” and “includes” or can have the meaning commonly given to the term “comprising” or “comprises” in US Patent Law. Terms “consisting essentially of” or “consists essentially of” if used in the claims have the meaning ascribed to them in US Patent Law. Other aspects of the invention are described in or are obvious from (and within the ambit of the invention) the following disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention, are incorporated in and constitute a part of this specification. The drawings presented herein illustrate different embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings: 
         FIGS. 1(   a )-( c ) show steps in the formation of a cruciform joint in a woven preform according to one aspect of the invention; 
         FIGS. 2(   a )-( b ) show steps in the formation of a woven preform according to one aspect of the invention; 
         FIGS. 3(   a )-( c ) are profile views of a preform, illustrating steps involved in the formation of the preform according to one aspect of the invention; 
         FIG. 4(   a ) shows a typical weft path or pick sequence followed by a weft yarn in the semi-helix approach according to one aspect of the invention; 
         FIGS. 4(   b )-( c ) are cross-sectional views of a woven preform in a weft direction according to one aspect of the invention; 
         FIG. 5  shows a profile view of a woven preform having woven side walls, closed cells on both edges and continuous hoop reinforcement in each cell according to one aspect of the invention; 
         FIGS. 6(   a )-( b ) show profile views of woven preforms having woven side walls, closed cells on both edges and continuous hoop reinforcement in each cell according to one aspect of the invention; 
         FIGS. 7(   a )-( b ) show a woven preform having woven side walls, closed cells on both edges and continuous hoop reinforcement in each cell according to one aspect of the invention; and 
         FIG. 8  shows a typical pick sequence or weft path for weaving a preform with three horizontal cells and three vertical cells according to one aspect of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The instant invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein. Rather, these illustrated embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
     In the following description, like reference characters designate like or corresponding parts throughout the figures. Additionally, in the following description, it is understood that such terms as “vertical,” “horizontal,” “outer” and “inner” and the like are words of convenience and are not to be construed as limiting terms. 
     Turning now to the figures,  FIGS. 1(   a )-( c ) show the details of formation of a cruciform joint  60  in a woven fabric or preform  50  according to one embodiment of the invention, where weft yarns travel from one internal wall to another internal wall within the preform.  FIG. 1(   a ) particularly shows two layers of a fabric  50  formed by interweaving two layers of warp fibers/yarns  30 ,  40  with two layers of weft fibers/yarns  10 ,  20 . In that, warp yarns  30  weave only with weft yarns  10  in a desired pattern to form the upper layer, and warp yarns  40  weave with weft yarns  20  in a desired pattern to form the lower layer of the fabric  50 , until a desired height “h” of the internal vertical wall and desired length “l” of the internal horizontal wall of the preform is woven. The desired height “h” of the internal vertical wall and desired length “l” of the internal horizontal wall are based on the final dimensions of the preform to be produced. After the desired height “h” and desired length “l” are reached, warp yarns  30  interchange positions with warp yarns  40 , thereby resulting in warp yarns  40  being on the upper layer and warp yarns  30  being on the lower layer of the fabric. However, weft yarns  10 ,  20  maintain their positions such that now weft yarns  10  interweave with warp yarns  40  and weft yarns  20  interweave with warp yarns  30 . It is to be noted that the two woven layers of the fabric  50  pass through one another without any breaks in the warp and weft fiber/yarn, and this consistency can be observed throughout the entire structure. 
     After the fabric  50  is woven, the preform comes off the loom as a flat sheet. When the preform is pulled open in the directions as shown in  FIG. 1(   b ), there is continuous fiber across the intersection with no breaks in warp and weft yarns, making a very strong cruciform joint  60 . 
       FIGS. 2(   a )-( b ) show formation of a woven fabric or preform  150  according to one embodiment of the invention.  FIG. 2(   a ) particularly shows the cross section in a warp direction of a preform  150  formed according to the method described above, using five layers of weft yarns and fiver layers of warp yarns  51 - 55 . It can be observed that these warp yarn layers  51 - 55  interchange positions at equal lengths according to a desired dimension of the preform as shown in the  FIG. 2(   a ).  FIG. 2(   b ) is a front view of the woven preform  150  in its final shape when the flat preform  150  is pulled open after being removed from off the loom. It can be noticed that the preform  150  takes the shape of an ‘egg crate’ structure when pulled open, which is nothing but a collection of cruciform joints discussed in the previous embodiment. 
       FIGS. 3(   a )-( c ) are profile views of the preform  150 , illustrating steps involved in the formation of the final shape of preform  150 .  FIG. 3(   a ) particularly shows flat preform  150  as woven and taken off the loom.  FIG. 3(   b ) shows a profile view of the preform  150  when it is partially opened and  FIG. 3(   c ) shows a profile view of the preform  150  in its final shape. Although shuttle looms are found to be most appropriate for producing the above discussed shapes of preforms, other weaving techniques such as Jacquard weaving and Rapier looms may be employed in producing the woven preform of the invention. Weave patterns such as plain, satin, twill, etc. can be selected based on the required thickness and density of the preform. 
     Referring to  FIGS. 4(   a )-( c ), the invention according to one embodiment is a method of forming a woven preform  250  with a single weft yarn  200  that traverses throughout the entire structure of the preform. The weft yarn adopts a semi-helix approach to interweave with two or more layers of warp yarns to form closed warp channels or cells  220  in the preform  250 . A typical weft path or pick sequence followed by weft yarn  200  in this semi-helix approach is depicted in steps  1 - 8  of  FIG. 4(   a ). The preform herein is woven in a manner such that the weft fiber/yarn  200  is continuous in a hoop direction, and the resulting structure has closed edges on all sides of the preform  250  when pulled opened, as shown in the cross-sectional view in  FIG. 4(   c ). The length “l 1 ” of the preform  250  is only limited by the width of the loom, however, the height of the walls “h 1 ” can be practically any length, since the preform  250  comes off the loom as a continuous piece in the warp direction, such as shown in  FIG. 4(   b ).  FIG. 5  shows a profile view of the preform  250  having woven side walls, closed cells  220  on both edges and continuous hoop reinforcement in each cell. 
     One embodiment of the invention is a method of machine weaving a fiber preform  100  that consists of closed perimeters with multiple intersecting members in its interior. The preform  100  is woven flat as shown in  FIG. 7(   a ) and then folded open to form a cellular structure, as shown in  FIG. 7(   b ). Weaving is done on a shuttle loom so that the weft fiber/yarn  110  is continuous through the intersection of interior walls, resulting in closed edges an all sides of the preform  100 , as shown in  FIG. 7(   b ). A typical pick sequence or weft path for weaving a preform  450  with three horizontal cells  445  and three vertical cells  445  as shown in  FIG. 6(   a ) is shown in  FIG. 8 . It should be noted as shown in  FIG. 6(   a ) that weft fiber/yarn weaves continuously from interior walls  400  into exterior walls  410  to form a t-joint  370 , and weaves continuously from interior walls  400  into interior walls  400  to form a cruciform joint  360  respectively. It should also be noted that in  FIG. 8 , a full repeat of the pattern is broken into five subsets, two that weave the horizontal interior walls  400  into the external walls  410 , two that weave the vertical interior walls  400  into the vertical external walls  410 , and one that provides continuous fiber through all external walls. Additional repeats of the fifth subset can be included to increase the thickness of exterior walls, relative to the thickness of the internal walls as desired. 
     The five pick subsets can also be woven in any order. The pattern in the warp direction can use any design that will lock all the weft fiber together (i.e. plain weave, satin, twill, etc.). The pattern shown in  FIG. 8  will result in a preform that has a single warp layer in each wall. The same approach can be used to weave a preform that has multiple warp layers in each wall. In this case, the pick paths shown have to be repeated once for each warp layer, and the warp fibers have to weave a pattern that will lock multiple layers together (i.e. through thickness angle interlock, through thickness orthogonal, ply-to-ply interlock, etc.). For example, in a through thickness angle interlock architecture, warp fibers pass through the full thickness of the preform at an angle that is defined by the pick or weft spacing. In a through thickness orthogonal architecture, warp fibers pass through the full thickness of the preform between adjacent pick or weft columns, so the through thickness component is more or less orthogonal to the faces of the fabric. This type of architecture usually includes warp ‘stuffers’ that simply pass between two layers of picks without weaving. In a ply to ply interlock architecture, warp fibers only pass part way through the full thickness of the preform, locking two or more layers together. The warps usually have an interlock angle similar to angle interlock architectures, but can be orthogonal also. For instance, layer  1  in the preform can be locked to layer  2 , layer  2  to layer  3 , and so on. The invention according to one embodiment is a method of forming a 3D integrally woven preform  350  comprising two woven external walls  320  and one or more woven internal horizontal wall  330 , as shown in  FIG. 6(   b ). The woven external and internal horizontal walls  320 ,  330  are joined together with multiple woven internal vertical walls  340 . The multiple woven internal vertical walls  340  may or may not be included on the vertical outer edges of the woven preform  350 . The resulting structure is a multi-cell woven preform  350  with integral joints  360 ,  370  as shown in  FIG. 6(   b ). A variant of this embodiment is shown in  FIG. 6(   a ) wherein a woven preform  450  comprises at least four spaced layers  410 - 440  with vertical walls  400  extending between the layers, where the layers and the walls define channels  445  extending in a warp direction of the structure. 
     The fiber preform formed according to the invention can be processed into a composite structural component using methods such as resin transfer molding or chemical vapor infiltration. Thus, the invention according to one embodiment is a fiber reinforced composite. 
     Typical uses for the resulting structure include applications that require stiffening in two directions (i.e. substructures for stiffened panels) and applications that require multiple connected compartments. The preforms of the invention can be used in a myriad of applications such as but not limited to heat exchangers where fluid is run through the open channels, structural panels with raceways that permit electrical wiring and other elements such as hydraulic lines to be run unimpeded through the structure, and as chaff dispenser systems in aircraft where the chaff cartridges are loaded into the open channels. 
     Some of the advantages of the invention over prior art are:
         All fibers are locked together at the joints, eliminating the resin layers between individual plies that would be present in a conventional laminated composite, thereby eliminating the potential for delamination and improving damage tolerance.   At internal walls, the weft fibers in a horizontal wall are continuous through the vertical walls and vice versa, thereby improving strength. The joint at the intersection of two internal walls is referred to as a cruciform joint  360 , such as shown in  FIGS. 6(   a )-( b ).   At external walls, the weft fibers in the interior wall weave into the exterior wall, thereby improving strength and damage tolerance. The joint at the intersection of an internal wall and an external wall is referred to as a T joint  370 , such as shown in  FIGS. 6(   a )-( b ).   Practically all of the hand labor required to assemble these structures using conventional lamination techniques is eliminated using the method of the instant invention. The preform is woven to shape, and placed into the mold as a single piece, thereby reducing manufacturing time and cost.   The preform thus formed has closed edges on either sides, thus eliminating the trimming step, also reducing time and cost.   There is no need for any tackifier, saving time and cost, and eliminating potential incompatibility problems with the primary resin.       

     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 and objects should not be limited thereby; rather its scope should be determined by that of the appended claims.