Patent Abstract:
A composite structure is provided. In another aspect of the present invention, a composite structure has relative layer-to-layer fiber orientations of between approximately 5° and 15°, inclusive. A further aspect of the present invention employs relative fiber offset angles less than 30° on a curved section. Yet another aspect of the present invention provides a three-dimensionally woven configuration where the first sheet is interwoven or mechanically linked with both the adjacent second layer and the opposite third or deeper layer.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Ser. No. 60/846,429, filed Sep. 22, 2006, which is incorporated by reference herein. 
    
    
     GOVERNMENT FUNDING 
     The present invention was funded under U.S. Army TARDEC Contract No. DAAE07-00-C-L075. The U.S. government may have certain rights to this invention. 
    
    
     BACKGROUND 
     This invention relates generally to composites and more particularly to a composite structure having a specific fiber or shape configuration. 
     It is known to employ prepreg composites with stacked material layers. Each layer typically has resin and fibers, with the fibers being oriented at 45°/0°/−45°/90°, 30°/90°/0°/90° or 30°/60°/90°/0° for relative adjacent layers. For example, traditional constructions are disclosed in Ambur et al., “Effect of Curvature on the Impact Damage Characteristics and Residual Strength of Composite Plates,” American Institute of Aeronautics and Astronautics, AIAA 98-1881 (Apr. 20-23, 1998); Z. Cui et al., “Buckling and Large Deformation Behaviour of Composite Domes Compressed Between Rigid Platens,” Composite Structures 66 (2004), pp. 591-599 (Elsevier); and S. Spottswood et al., “Progressive Failure Analysis of a Composite Shell,” Composite Structures 53 (2001), pp. 117-131 (Elsevier). Such conventional fiber patterns, however, are prone to severe delamination, cracking and fiber breakage upon projectile impact. 
     SUMMARY 
     In accordance with the present invention, a composite structure is provided. In another aspect of the present invention, a composite structure has relative layer-to-layer fiber orientations of between approximately 5° and 15°, inclusive. A further aspect of the present invention employs relative fiber offset angles less than 30° on a curved section. Yet another aspect of the present invention provides a three-dimensionally woven configuration where the first sheet is interwoven or mechanically linked with both the adjacent second layer and the opposite third or deeper layer. A method of making a composite structure is also provided. 
     The composite structure of the present invention is advantageous over traditional constructions in that the present invention allows for fiber bridging spanning the curved shape, generally without significant delamination, upon projectile impact. This allows for up to 90% energy absorption of the impact without significant structural composite failure. The present invention is thereby ideally suited for use in armor plating without the conventional weight of metallic materials and with the ease of forming curved yet thin shapes. Additional advantages and features of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side elevational view showing the composite structure of the present invention employed on an airplane; 
         FIG. 2  is a side elevational view showing the composite structure of the present invention employed on a military tank; 
         FIG. 3  is a cross-sectional view, taken along line  3 - 3  of  FIG. 1 , showing a first preferred embodiment of the composite structure; 
         FIG. 4  is a fragmentary and perspective view, taken from  FIG. 2 , showing a second preferred embodiment of the composite structure; 
         FIG. 5  is a cross-sectional view, taken along lines  5 - 5  of  FIG. 4 , showing the second preferred embodiment composite structure; 
         FIG. 6  is a perspective view showing a third preferred embodiment of the composite structure; 
         FIG. 7  is an exploded and diagrammatic, perspective view showing the preferred embodiments of the composite structure prior to shaping; 
         FIG. 8  is a diagrammatic side view showing the preferred embodiments of the composite structure prior to shaping; 
         FIG. 9  is an exaggerated and diagrammatic top view showing the preferred embodiments of the composite structure, employing a two-dimensional fabric weave, prior to shaping; 
         FIG. 10  is a diagrammatic side view showing a weaving pattern of the preferred embodiments of the composite structure, employing a two-dimensional fabric weave, prior to shaping; 
         FIG. 11  is a perspective view showing the second preferred embodiment composite structure after a projectile impact; 
         FIG. 12  is a fragmentary perspective view showing a first alternate embodiment of the composite structure; 
         FIG. 13  is a fragmentary perspective view showing a second alternate embodiment of the composite structure; 
         FIG. 14  is a fragmentary perspective view showing a third alternate embodiment of the composite structure; 
         FIG. 15  is a perspective view showing a fourth alternate embodiment of the composite structure; 
         FIG. 16  is a cross-sectional view, taken along line  16 - 16  of  FIG. 15 , showing the fourth alternate embodiment composite structure; 
         FIG. 17  is a diagrammatic perspective view showing a first alternate embodiment of a weaving pattern employed with the composite structure; and 
         FIG. 18  is a diagrammatic perspective view showing a second alternate embodiment of a weaving pattern employed with the composite structure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first preferred embodiment topologically controlled composite structure  21  of the present invention is shown in  FIGS. 1 and 3 . Composite structure  21  includes multiple curved composite layers  23 , outer skin composite layers  25  and inner skin composite layers  27 , all permanently joined together. Curved composite layers  23  have a repeating corrugated shape. Skin layers  25  and  27  span between and bridge the corrugations such that the skins only contact tangents, in other words the peaks and valleys, of curved composite layers  23 . Outer skin layers  25  act as an aerodynamic surface on an aerospace craft or vehicle such as a leading edge of an airplane wing  29 , the underside of a helicopter, or an outer shield for a satellite; alternately outer skin layers  25  can be used as an outer surface of a ship hull, other marine vehicle, or the like. 
     A second preferred embodiment composite structure  41  is employed as armor on a land vehicle such as a military tank  43 , personnel carrier or automobile. This is shown in  FIGS. 2 ,  4  and  5 . Each composite structure  41  has a curved middle segment  45  bordered by flanges  47  which are attached to an underlying skin  49 . Skin  49  can be made from a composite, steel or other material. Middle segment  45  has a generally semi-cylindrically curved shape. Referring to  FIG. 5 , exemplary dimensions for composite structure  41  are as follows: d 1  is approximately 3.0 inches, d 2  is approximately 1.0 inches and d 3  is approximately 0.3-0.8 inches. It should be appreciated, however, that these dimensions may vary depending upon the actual aerospace, land vehicle or watercraft application. 
     A third preferred embodiment composite structure  61  of the present invention can be observed in  FIG. 6 . A curved segment  63  has a generally semi-spherical curved shape or dome shape projecting from a generally planar base segment  65 . It should be appreciated that inner and outer skins such as those shown in  FIG. 3  may also be employed with any of these preferred embodiments disclosed herein depending upon whether aerodynamic or aesthetic covering of the curved shape is desired. 
     All of the presently disclosed preferred embodiment composite structures are made by: (1) stacking and overlapping sheets or layers made of fibers and resin as shown in  FIGS. 7 and 8 ; or (2) a “two-dimensional” weave between two adjacent fiber bundles, with each woven layer or ply then being stacked upon each other in an overlapping manner with resin applied before or after the weaving to join the fabric layers, such as is shown in  FIG. 10 . With either approach, the adjacent layer-to-layer fiber orientation is approximately 0° and the immediately adjacent fiber orientation is between 5° and 29°, inclusive, and more preferably about 0° and between 5°-15°, inclusive. It should be appreciated that the desired fiber orientation is the primary or average fiber orientation as the fibers may not be perfectly straight and may have a slight tolerance variation during manufacturing. Furthermore, the 0° orientation is simply a base reference angle from which the adjacent layer orientation is measured. This fiber orientation is shown in an exaggerated fashion in  FIGS. 7 and 9  wherein a first layer  81  has a reference orientation of 0° and the immediately adjacent layer  83  has a relative reference fiber orientation of 5°. Similarly, in the woven scenario of  FIG. 10 , a warp fiber  85  has a reference orientation of 0° while a woven and interlinking weft fiber  87  has a reference orientation of about 5°. 
       FIG. 11  illustrates composite structure  41  after the curved segment is impacted by a bullet-type projectile P. In this scenario, the fibers tear out of the resin matrix and bridge across the back, concave side of the curved segment without significant layer-to-layer delamination. Improved results are expected for larger radii or a greater curved height dimension d 3 . The combination of small fiber angle offsets between immediately adjacent layers and the curved shape provides significant energy absorption without complete composite structure failure. 
     The laminated version of the present invention can be made from a prepreg tape or fabric, or a fiber preform. Glass fibers, fabrics and braided preforms are preferred for land vehicles and marine structures due to their high specific strength properties. Polymeric fibers, fabrics and braided preforms, such as Kevlar® aramid, Spectra® polyethylene or Dyneema® polyethylene, are preferred for military applications due to their high specific energy absorption characteristics. Furthermore, carbon fibers, fabrics and braided preforms are preferred for aerospace vehicles due to their high specific stiffness properties. Toughened epoxy resins are desirable in the specific curing temperatures is dependent on the type of epoxy resin used. For example, if manufacturing convenience is of primary concern, then a low temperature and low viscosity epoxy resin should be employed. If the structure is to be used in high temperature environments, however, a high temperature epoxy would be desirable. 
     The following preferred manufacturing steps are employed with the laminated versions of the preferred embodiment composite structures. First, the fiber preform is prepared by selecting laminated fabrics or sheets with small relative offset angles between adjacent layers or two-dimensional fabrics with small angle differences between warp and weft yarns, or three-dimensional fabrics with small angular differences between linked layers. Second, a clean mold with the designated curved geometry is made. Third, the mold is waxed, and fourth, a sealant tape is applied around a perimeter of the mold. Fifth, the operator cuts a peel ply and places it on the mold surface. Sixth, the operator places the fiber preform over the peel ply and marks its outline on the peel ply before removing it from the mold. Seventh, the user prepares the epoxy matrix by mixing a hardener and the resin into the designated ratio. Furthermore, eighth, the user applies the mixed epoxy to the peel ply on the marked area for the fiber preform. Ninth, the user places the fiber preform on the epoxy and tenth, adds a second layer of peel ply over the fiber preform. Eleventh, the user adds a bleeder/breather fabric on the top of the second peel ply. Twelfth, a vacuum bag is applied to the sealant tape on the mold and thirteenth, a vacuum gauge is inserted at one end of the mold. Fourteenth, the user sets up the vacuum pump and piping, and fifteenth, turns on the pump to impregnate the fiber preform with the epoxy matrix and cures the epoxy. Sixteenth, the composite structure is trimmed and seventeenth, the user inspects the composite structure for design compliance. Eighteenth, the composite component is applied to a secondary assembly with adhesive bonding and nineteenth, the operator conducts a final assembly inspection. Finally, the composite structure assembly is packaged and shipped. 
       FIG. 12  illustrates an alternate embodiment composite structure  91 . In this embodiment, a corrugated curved shape segment  93 , including multiple joined fiber and resin sheets, is attached between outer composite skin layers  95  and middle composite skin layers  97 . Additionally, a second corrugated curved composite segment  99 , having peaks and valleys offset from the first corrugated segment  93 , is affixed between middle skin layers  97  and inner composite skin layers  101 . The skin layers bridge and span between the peaks and valleys of each corrugation segment, essentially only contacting the corrugated segments at the tangents of their respective curves. 
       FIG. 13  discloses a series of elongated, tube-like curved composite segments  111  affixed between spanning composite outer and inner skins  113  and  115 , respectively. Tubular segments  111  each have a generally cylindrical shape. Each of the composite segments and skins includes multiple sheets of fiber and resin layers. The tubular segments  111  only contact each other along outer tangents and only contact the bridging skins at their corresponding tangents thereby leaving somewhat triangular gaps defined by adjacent pairs of tubular segments  111  and the adjacent skin  113  or  115 . The inside of each tubular segment  111  is also open or hollow. The tubular segments  111  are further generally parallel to each other. Referring to  FIG. 14 , another alternate composite structure  121  is similar to that shown in  FIG. 13 , however, a smaller elongated and tubular curved segment  123  is located in each of the gaps between the much larger diameter tubular segments  111 ′ and the adjacent skins  113 ′ or  115 ′. The smaller diameter tubular segments  123  are also of a multi-layer fiber and resin composite structure. This exemplary embodiment creates a hollow and multi-cellular curved configuration. 
     Moreover, referring to  FIGS. 15 and 16 , another alternate embodiment composite structure  131  has a central curved segment  133  and a pair of outboard flanges  135 . Three or more fiber and resin layers  137 ,  139  and  141  are joined together in an overlapping and contacting manner at flanges  135 , however, these layers are spaced away from each other and have air gaps  143  and  145  therebetween at the curved segment  133 . It should also be appreciated that each layer  137 ,  139  and  141  may include multiple laminated or woven sheets therein. 
       FIGS. 17 and 18  illustrate alternate embodiments of “three-dimensionally” woven composite structures  161  and  171 . With the embodiment of  FIG. 17 , a first warp fiber  163  is woven around first and second weft fibers  165  and  167  in a repeating manner. Thus, a first ply is woven with a third ply, a third ply is woven with a fifth ply, a fifth ply is woven with a seventh ply, a seventh ply is woven with a ninth ply, a ninth ply is woven with an eleventh ply, and the eleventh ply is woven with a twelfth ply. With the embodiment of  FIG. 18 , a first ply is woven with a second ply, a second ply is woven with a fourth ply, a fourth ply is woven with a sixth ply, a sixth ply is woven with an eighth ply, an eighth ply is woven with a tenth ply and a tenth ply is woven with a twelfth ply. Accordingly, there is no need for a separate transverse stitch to sew together multiple ply layers as the present invention integrally links multiple depth plies together during the initial weaving process. These composite structures  161  and  171  also contain an epoxy or other polymeric resin. These three-dimensionally woven composite structures  161  and  171  are preferably employed with small angular offsets between adjacent ply layers and within a curved segment after shaping and curing, however, they do not necessarily need to have small fiber angle offsets and curved final shapes if they are employed in other non-impact final use applications. 
     While various aspects of the present invention have been disclosed, it should be appreciated that other variations may fall within the scope of the present invention. For example, a single very large dome-shaped composite structure can be employed on the side of an armored land vehicle with multiple underlying smaller dome, corrugated or tubular curved composite structures thereunder. It should also be appreciated that various numbers of composite layers have been shown by way of example, but a greater or lesser number of composite layers may actually be employed depending upon the end use applications and specific materials chosen. It is intended by the following claims to cover these and any other departures from the disclosed embodiment which falls within the true spirit of this invention.

Technology Classification (CPC): 8