Abstract:
The invention provides a method of fabricating a lug on a structural element of composite material made at least locally out of a stack of primary plies of composite fibers defining at least one extension for forming the lug. The method includes the step of separating the primary plies at least in the extension and of inserting intermediate plies between the primary plies.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is entitled to the benefit of French Patent Application No. 0511846 filed on Nov. 23, 2005. 
     FIELD OF THE INVENTION 
     The invention relates to a method of fabricating a lug on a structural element of composite material, in particular a connecting rod. 
     BACKGROUND OF THE INVENTION 
     Connecting rods are known that comprise a hollow body of composite material, e.g. obtained by winding filaments around a mandrel, or indeed by winding a ply of woven fibers. 
     The thickness of the hollow body is obtained by winding an appropriate number of turns. 
     Composite material connecting rods are also known in which the solid body is made by stacking plies. 
     It is also known to provide extensions, either from the wall of the hollow body or from the solid body that serve to become the lugs of coupling forks. After the body has been polymerized, it suffices to pierce holes in the extensions and possibly to cut them to shape in order to obtain the lugs. 
     Nevertheless, the thickness of the lugs obtained in that way is the same as the thickness of the wall of the hollow body or the thickness of the solid body. Unfortunately that thickness is not necessarily sufficient. The state of the art is illustrated by the following patent documents: FR 2 060 049, DE 37 26 340, FR 2 705 610, U.S. Pat. No. 5,279,892, U.S. Pat. No. 6,036,904. 
     OBJECT OF THE INVENTION 
     An object of the invention is to propose a novel method of producing one or more lugs on a structural element of composite material. 
     BRIEF DESCRIPTION OF THE INVENTION 
     To achieve this object, the invention provides a method of fabricating a lug on a structural element of composite material made at least locally out of a stack of primary plies of composite fibers defining at least one extension for forming the lug, the method including the step of separating the primary plies at least in the extension and of inserting intermediate plies between the primary plies. 
     Thus, the thickness of the extension is no longer tied to the thickness of the structural element. In particular the extension can be made thicker in order to obtain a lug of suitable thickness. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be better understood in the light of the following description given with reference to the figures in the accompanying drawings, in which: 
         FIG. 1  is a perspective view of a connecting rod obtained by the method of the invention; 
         FIG. 2  is a face view of a cut-out pattern for fabricating a connecting rod of the invention; 
         FIG. 3  is a section on line III-III through the body of the  FIG. 1  connecting rod; 
         FIG. 4  is a fragmentary view of the  FIG. 1  pattern seen edge-on; 
         FIG. 5  is a section view on line V-V of  FIG. 1 ; and 
         FIG. 6  is a diagrammatic view of a fabric comprising a plurality of bonded-together plies suitable for use in implementing the method of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 1 , the method of the invention serves to obtain a completely composite connecting rod  100  comprising a tubular body  102  with two forks  103 , each comprising two facing lugs  104 . 
     According to a particular aspect of the invention shown in  FIG. 2 , an initial step lies in cutting out a pattern  110  from a fiber fabric, a carbon fiber fabric in this example, which pattern  110  has a central portion  111  with two opposite edges  112  and has four extensions  113  projecting therefrom, comprising two extensions at each end of the central portion  111 , on either side of an axis of symmetry  114  of the pattern. 
     The fiber fabric is preferably obtained from a so-called “2.5 D” weave, comprising a plurality of primary plies having weft fibers interconnected by warp fibers that extend from one primary ply to another in order to bond the primary plies together. Such bonding between the primary plies enables them to be secured to one another, while allowing for relative sliding between the primary plies while the pattern is being shaped. 
     In this respect, the preferred fabric is the fabric described in patent document FR 2 759 096, and described below with reference to  FIG. 6 . The fabric comprises a basic weave that is constituted:
         firstly by at least twenty-eight weft fibers  1  to  28  organized in at least eight columns C 1  to C 8  each extending in the thickness direction E of the fabric, and disposed in a staggered configuration with alternation between columns C 2 , C 4 , C 6 , C 8  having at least three superposed weft fibers spaced apart at a predetermined pitch P, and columns C 1 , C 3 , C 5 , C 7  having at least four superposed weft fibers spaced apart by the same pitch P, the weft fibers  1  to  28  extending to define at least seven primary plies N 1  to N 7 ; and   secondly, by at least twelve warp fibers  29  to  40  disposed in at least four parallel planes P 1 , P 2 , P 3 , P 4  that are offset in the weft fiber direction, each plane containing three superposed parallel warp fibers arranged in each of these planes as follows:
           a first warp fiber (respectively numbered  29 ,  32 ,  35 ,  38 ) connects the topmost warp fiber ( 1 ,  8 ,  15 ,  22 ) of a four-weft fiber column (C 1 , C 3 , C 5 , C 7 ) to an upper intermediate weft fiber ( 16 ,  23 ,  2 ,  9 ) of a four-weft fiber column (C 5 , C 7 , C 1 , C 3 ) that is spaced apart from the preceding column by at least two pitch steps P, the first warp fiber returning over a top end weft fiber ( 1 ,  8 ,  15 ,  22 ) of a four-weft fiber column (C 1 , C 3 , C 5 , C 7 ) that is spaced apart from the first column by at least four pitch steps P;   a second warp fiber (respectively numbered  30 ,  33 ,  36 ,  39 ) connecting a top intermediate weft fiber ( 2 ,  9 ,  16 ,  23 ) of a four-weft fiber column (C 1 , C 3 , C 7 ) to a lower intermediate weft fiber ( 17 ,  24 ,  3 ,  10 ) of a four-weft fiber column (C 5 , C 7 , C 1 , C 3 ) that is spaced apart from the preceding column by at least two pitch steps P, the second warp fiber returning over an upper intermediate weft fiber ( 2 ,  9 ,  16 ,  23 ) of a four-weft fiber column (C 1 , C 3 , C 5 , C 7 ) that is spaced apart from the first column by at least four pitch steps P; and   a third warp fiber (respectively numbered  31 ,  34 ,  37 ,  40 ) connecting a lower intermediate weft fiber ( 3 ,  10 ,  17 ,  24 ) of a four-weft fiber column (C 1 , C 3 , C 5 , C 7 ) to the bottommost weft fiber ( 18 ,  25 ,  4 ,  11 ) of a four-weft fiber column (C 5 , C 7 , C 1 , C 3 ) spaced apart from the preceding column by at least two pitch steps P, the third warp fiber returning over a lower intermediate weft fiber ( 3 ,  10 ,  17 ,  24 ) of a four-weft fiber column (C 1 , C 3 , C 5 , C 7 ) that is spaced apart from the first column by at least four pitch steps P.   
               

     The positions of the parallel warp fibers ( 29 ,  30 ,  31 ;  32 ,  33 ,  34 ;  35 ,  36 ,  37 ;  38 ,  39 ,  40 ) are offset longitudinally by one pitch step P from one plane to another. Continuous lines represent the warp fibers  29 ,  30 ,  31  of plane P 1 , short dashed lines represent the warp fibers  23 ,  33 ,  34  of plane P 2 , chain-dotted lines represent the warp fibers  35 ,  36 ,  37  of plane P 3 , and finally long dashed lines represent the warp fibers  38 ,  39 ,  40  of the plane P 4 . The offset can be seen particularly clearly. 
     Returning to  FIG. 2 , the pattern  110  is cut out from said fabric in such a manner that the weft fibers extend along the axis of symmetry  114  of the pattern  110 . 
     According to a particular aspect of the invention, the pattern  110  is then rolled up to form a tube by bringing its edges  112  close together. As shown diagrammatically in  FIG. 3 , the plies of the fabric slide relative to one another, with sliding being zero on the axis of symmetry  114  and at its maximum in the vicinity at the edges  112 , such that the edges take on a chamfered shape. 
     The edges  112  are then placed against one another. Preferably, the end face of one of the edges  112  bears against the inside face of the pattern  110  so that the thickness of the resulting tube is substantially constant in the join zone. 
     Since the edges  112  are not parallel in this example, a tubular portion is obtained that is conical in shape. However it would be possible to obtain a cylindrical tubular portion in the same manner by cutting the pattern  110  to have edges  112  that are parallel. 
     According to a particular aspect of the invention, as shown in  FIG. 4 , the warp fibers are removed from the ends of the extensions  113  in order to separate the primary plies formed by the weft fibers. This produces primary plies N 1  to N 7  (seen edge-on and represented by thick lines) that can be spaced apart from one another. Intermediate plies  116  (represented by fine lines with only one intermediate ply being given a reference) are inserted between adjacent primary plies so that the fibers constituting the intermediate plies  116  extend obliquely, preferably at 45° relative to the weft fibers making up the primary plies N 1  to N 7 . 
     The intermediate plies  116  are preferably disposed in such a manner as give the extensions  113  thickness that varies progressively so as to reach an end thickness that is constant and substantially twice that of the fabric. To do this, intermediate plies  116  are inserted of lengths that increase with increasing distance from the center of the extension  113 . 
     Transverse fibers  117  are then introduced across the primary plies N 1  to N 7  and the intermediate plies  116  in order to reinforce the ends of the extensions  113  (the transverse fibers are represented by dashed lines, with only one of them carrying a reference in the figure. This gives a three-dimensional structure to said end that is particularly strong and that prevents the plies from sliding one on another. The transverse fibers are preferably inserted by stitching. 
     The pattern fitted with its intermediate plies is shaped on a mandrel (not shown). Thereafter, using the conventional resin transfer molding (RTM) technique, resin is diffused between the fibers of the pattern and of the intermediate plies. 
     The overlapping edges  112  are thus bonded together by the resin. The overlapping chamfers provide a larger bonding area between the two edges  112  such that the join (visible in  FIG. 1 ) is very strong and makes the connecting rod suitable for withstanding stresses both in tension and in compression. 
     This produces a strong tubular body with two arms of increased thickness at each end formed by the extensions, said arms extending facing each other in pairs. It then remains to cut the arms to shape and to pierce them in order to transform them into the lugs  104 . This produces the connecting rod shown in  FIG. 1  that is made entirely out of composite material. 
     Preferably, and as shown in  FIG. 5 , the lugs are each provided with a pair of rings  120 , each pair comprising a first ring  121  having a cylindrical portion  122  extending in the hole in one of the lugs  104 , together with a collar  123  extending against one of the flanks of the lugs  104 , and a second ring  125  having a cylindrical portion  126  extending tightly inside the cylindrical portion  122  of the first ring  121 , together with a collar  127  that bears against the end of said cylindrical portion  122 . The length of said cylindrical portion  122  is preferably very slightly shorter than the width of the lug  104  so that the lug is lightly clamped between the collars  123  and  127 . 
     Such a connecting rod is advantageously used for constituting folding braces or stays for landing gear. Such braces comprise two connecting rod elements that are hinged together and that work essentially in traction and compression, such that the connecting rod of the invention can advantageously be used in such an application. In addition, it is known that such braces or stays can also be subjected to impacts, e.g. from stones thrown up by the tires. The “2.5 D” fabric used is specifically well-known for its high resistance to impacts and to delamination. 
     Dimensioning has shown that the saving in weight compared with metal braces or stays is significant. Furthermore, manufacturing time is considerably shortened. 
     The invention is not limited to the description above, but on the contrary covers any variant coming within the ambit defined by the claims. 
     In particular, although the use of a particular fabric is described with reference to  FIG. 6 , it is possible to use a similar fabric having a larger number of primary plies, or indeed to use other fabrics that allow primary plies to slide relative to one another. Such a fabric can be obtained by superposing primary plies and stitching them together loosely. 
     In order to reinforce the edge-to-edge join, it is possible to stitch the two edges together before polymerization. 
     Although the method of the invention is associated with a connecting rod, the method of the invention can be applied equally well to any other structural element made of composite material.