Abstract:
The invention relates to a method of fabricating a structural part comprising a tubular portion from which there projects a structural element made of composite material, the method comprising the steps of:
       shaping a plane preform comprising a stack of a plurality of superposed fiber plies such that over at least a fraction of the preform that is to form a structural portion, the plies are secured to one another, and over at least another fraction of the preform that is to form a portion for connecting to the tube, the plies are separable so as to form two facing sheets;   inserting a tube between the sheets of the connection portion so that the sheets surround the tube at least in part; and   bonding the tube to the preform such that the tube forms the tubular portion and the preform forms the structural element of the structural part.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/FR2009/001453 filed Dec. 18, 2009, claiming priority based on French Patent Application No. 08 07448, filed Dec. 24, 2008, the contents of all of which are incorporated herein by reference in their entirety. 
     The present invention relates to a method of fabricating a structural part comprising a structural element that is made of composite material and that is connected to a tube, in particular for use in aviation. 
     BACKGROUND OF THE INVENTION 
     Aircraft include landing gear in which a main structural part, referred to as a “strut”, is generally pivotally mounted to the aircraft to enable the landing gear to be retracted into a housing within the aircraft after takeoff. The strut comprises a tube having a rod slidably mounted therein, which rod carries the wheel(s) at its bottom end. The strut also includes structural elements such as arms or webs for Connecting the tube to the aircraft. In particular, certain struts include side arms that extend on either side of the tube and that are terminated by spools for receiving pins for hinging the strut to the aircraft. 
     A strut of that type is generally made of metal, e.g. of aluminum or of steel, in particular as a casting or as a forging. It is then easy to provide connection shapes between the tube and the arms in order to ensure that the connections between these elements are mechanically strong. 
     It is also known to make tubes out of composite material by filamentary winding of fibers onto a mandrel. That method of fabrication is easily industrializable and makes it possible to control the orientation and the distribution of the fibers so as to make a tube that is light in weight while also being mechanically strong. 
     The invention seeks to provide a method of fitting said tube with structural elements, e.g. lateral arms or beams for transmitting forces. 
     Proposals have been made to use reinforcing parts that are adhesively bonded, being fitted astride the structural element and the tube. However that type of connection is not satisfactory. The fibers of the structural element terminate in register with the tube and no continuity is provided. In order to make that connection mechanically strong, it is necessary to use reinforcing parts that are thick and of large area. The assembly then becomes heavier and more difficult to make, and the connection becomes bulkier. 
     OBJECT OF THE INVENTION 
     A particular object of the invention is to avoid the drawbacks of the prior art, and to propose a method for use with composite material elements that is effective and inexpensive for bonding structural elements to a tube. The method makes it possible in particular to make struts for aircraft landing gear. 
     BRIEF DESCRIPTION OF THE INVENTION 
     To this end, the invention provides a method of fabricating a structural part comprising a tubular portion from which there projects a structural element made of composite material, the method comprising the steps of:
         shaping a plane preform comprising a stack of a plurality of superposed fiber plies such that over at least a fraction of the preform that is to form a structural portion, the plies are secured to one another, and over at least another fraction of the preform that is to form a portion for connecting to the tube, the plies are separable so as to form two facing sheets;   inserting a tube between the sheets of the connection portion so that the sheets surround the tube at least in part; and   bonding the tube to the preform such that the tube forms the tubular portion and the preform forms the structural element of the structural part.       

     The fibers of the preform present continuity between the structural portion proper, e.g. forming an arm, and the portion for connection to the tube. The fibers of the preform thus fit around the tube over a large connection area, thus enabling stresses to be spread over large areas and enabling forces to be transferred from the structural element of the tube with excellent continuity. 
     The preform may be obtained in various ways:
         it is possible to start from a three-dimensional (3D) or a two-and-a-half-dimensional (2.5D) fabric made up of a plurality of superposed plies, and then in the connection portion to unlink the plies so as to enable them to be separated in two sheets;   it is also possible to start from two blanks cut out from a fabric made up of a plurality of superposed plies, the two blanks being superposed and stitched together in the structural portions, and being left free in the connection portion, with each of the blanks then forming a respective one of the sheets; and   it is also possible to use a 2.5D fabric woven in such a manner that the plies are intimately linked together in the structural portions while being left free in the connection portion, so that the plies can be separated to form two sheets.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be better understood on reading the following description given by way of example with reference to the accompanying drawings, in which: 
         FIG. 1  is a perspective view of a prior art aircraft undercarriage with a metal strut; 
         FIG. 2  is a perspective view of a composite material tube during fabrication; 
         FIGS. 3 to 5  are views of a structural element preform and of its assembly with a tube in a first implementation of the invention, the structural element being seen edge-on; 
         FIGS. 6 to 8  are views showing an assembly in a second implementation of the invention; 
         FIG. 9  shows a variant of  FIG. 8 ; 
         FIG. 10  is a view showing an assembly in a third implementation of the invention; 
         FIG. 11  is a view of a strut of composite material made using the method of the invention as shown in  FIGS. 6 to 8 ; 
         FIG. 12  is a view showing an assembly in a fourth implementation of the invention; and 
         FIG. 13  is a diagrammatic section view of a 2.5D fabric adapted to implementing the method of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is described below in application to the field of aviation, and in particular the field of landing gear. Naturally, the invention is not limited to this type of application. 
       FIG. 1  shows an aircraft nosewheel undercarriage comprising a strut  1  made of aluminum alloy that comprises a cylindrical portion  2  with two side arms  4 ,  6  terminating at the top in spools  8 ,  9  that receive hinge pins for connecting the strut to the aircraft. The strut  1  also includes a fastener member  12 , here in the form of two lugs forming a fork for receiving a strut (not shown) for stabilizing the undercarriage in the extended position. 
     A rod  14  is mounted to slide vertically in the cylindrical portion  2  of the strut  1 . The bottom portion of the rod  14  carries axles  16  that receive wheels. The rod  14  is connected by a torque linkage  18  to a steering member  20  fastened to the strut  1 , here comprising hydraulic actuators for turning the rod, thereby enabling the wheels to be swiveled in order to steer the airplane while taxiing. 
     The invention seeks in particular to enable a strut to be made of composite material, the strut comprising a tubular element and a structural element such as an arm or a beam. 
     A tube of composite material is made initially. In this respect,  FIG. 2  shows a tube  30  made by filamentary winding of fibers on a mandrel, e.g. for laying successive layers of fibers  32  along a longitudinal axis followed by layers of crossed fibers  34 ,  36 , the method enabling the distribution of the various fibers to be adjusted. During winding, the fibers are impregnated with a resin that is subsequently polymerized. By controlling the composition and the orientation of the fibers, this method makes it possible to obtain a tube that is light in weight, while presenting strong mechanical characteristics. This method is well known. In a variant, the tube may be obtained by other methods, e.g. by braiding. 
     Thereafter, the structural element is made and subsequently assembled to the tube  30 . For this purpose, a preform is prepared initially. 
     In a first implementation of the invention, as shown in  FIGS. 3 to 5 , the preform  49  is made using two blanks cut from a fiber pad made by superposing plies that are connected together, e.g. by weaving or by stitching, or as to obtain good cohesion between the plies and allow the pads to be handled. By way of example, the fibers may be fibers of glass, aramid, or carbon. Nevertheless, the bonding between the superposed plies is preferably sufficiently loose to allow the plies to slide relative to one another while the blanks are being shaped. 
     Fabrication of the preform  49  begins by cutting blanks  40  out from a fiber pad. As can be seen in  FIG. 3 , the blanks  40  are subsequently superposed and bonded together, by stitching  42  in this example, while taking care to leave the ends of the blanks  40  free. A preform  49  is thus defined that has a structural portion  41  corresponding to the stitched fractions, which structural portion is to form the arm of the structural element, and a connection portion for connecting to the tube, corresponding to the free end fractions of the blanks  40 , which form two facing sheets  44  that are separable. 
     As can be seen in  FIG. 4 , the sheets  44  are moved apart. Thereafter, in a following step shown in  FIG. 5 , the tube  30  is inserted between the sheets  44  so that the sheets  44  surround the tube and join together at a point  43  diametrically opposite from the beam  41 . In a variant, the two ends  44  may be shorter and not joined together at the point  43 , or they may be longer and overlie each other, depending on the strength that is to be given to the connection. The assembly as made in this way is then impregnated with resin, e.g. by a known transfer method such as injecting resin under pressure or infiltration under a vacuum. The resin is subsequently hardened by polymerization. 
     This forms a connection between the structural element and the tube  30  by providing fiber continuity from the structural element of the structural portion (the stitched fraction that is cantilevered out from the tube) to the connection portion (the non-stitched free fraction that surrounds the tube). The connection portion provides a large contact area, thereby making it possible for the connection between the structural element and the tube to be mechanically strong. Furthermore, implementation is simple. The size occupied by the connection is small, and both the weight and the cost of the assembly are therefore reduced. 
     Preferably, an insert  47  of substantially triangular shape, e.g. made of pultruded material, is inserted between the tube  30  and the structural element and serves to fill the space between the sheets in register with the stitching  48  that defines the boundary between the connection portion and the structural portion of the structural element  41 . 
       FIGS. 6 to 8  show a second implementation of the invention that comprises making a structural element having two arms  51 ,  53  that are to extend on either side of the tube  30 . Fabrication begins by preparing the preform  59 . Two blanks  50  are initially cut out from a fiber pad. The blanks  50  are superposed and bonded together in end zones thereof by stitching  52 , taking care to leave a central zone free in which the blanks  50  are not stitched together. 
     A structural element is thus defined that has structural portions  51 ,  53  that form the arms, corresponding to the stitched zones, and a connection portion that corresponds to the non-stitched zone in which the central fractions  54  form separable sheets. A slot or space  56  is thus defined between the facing sheets. 
     As shown in  FIG. 7 , the sheets  54  are then spaced apart to form a substantially circular central passage. In a following step shown in  FIG. 8 , the tube  30  is inserted between the sheets  54  such that the sheets surround the tube  30  completely. Inserts  57  are interposed between the sheets and the tube where the sheets start. The assembly as made in this way is then impregnated with resin that is hardened by polymerization. 
     This provides a connection between the tube  30  and the two arms  51 ,  53  that are cantilevered out from the tube, with the same advantages as in the above-described embodiments. 
     In a variant, in plan view, the two beams may form between them an angle as shown in  FIG. 9 . Under such circumstances, a preform  69  is prepared by cutting out two blanks  60 ,  66  having two non-stitched central fractions  64  and  68  forming sheets that are of different lengths, the sheet  64  formed by the blank  60  that is situated within the angle being shorter than the sheet  68  formed by the other blank  66 . Inserts  67  are placed between the tube and the sheets, where the sheets start. The assembly is then polymerized. 
     When assembling the two blanks  60 ,  66  together, the ends of these blanks are superposed and stitched together so as to define two arms  61 ,  63 , with the central fractions not being stitched together so as to be left free and form two sheets  64 ,  68 . Thereafter, and in the same manner as described above, the sheets  64 ,  68  are spaced apart and the tube  30  is inserted between them. The assembly is impregnated with resin that is hardened by polymerization. 
       FIG. 10  shows a third implementation of the invention. The fabrication of the preform  79  begins by cutting out a single blank  70  of elongate shape from a fiber pad. The blank  70  is then folded in half so that its ends  72  are superposed and stitched together to form the structural portion  71 . The central fraction  74  of the blank is left free to form two facing sheets that join together continuously. The tube  30  is then inserted in the central fraction  74  that completely surrounds the tube. The assembly as made in this way is impregnated with resin that is hardened by polymerization. 
     With the method of the invention, it is possible to give various shapes to the beams  41 ,  51 ,  53 ,  61 ,  71  depending on how the blanks constituting these beams are cut out and assembled together. 
       FIG. 11  shows an example of a strut made by the method of the invention. The structural portions in this example form webs. As described above, fabrication of the preform  99  begins by cutting out the blanks  80 ,  82  from a fiber pad. The blanks  80 ,  82  are then superposed and stitched together by stitching  90  so as to form the two arms  81 ,  83  while leaving a central connection zone free in which the blanks form separable sheets  94  between which the tube  30  is inserted (only one of the sheets is visible in the figure). 
     It should be observed that the stitched fractions of the blanks are not stitched together over their entire height. The edges are left free so as to enable them to be folded along the lines of stitching  92 . The edges as folded out in this way form flanges  86 ,  88  that serve to stiffen the arms  81 ,  83 , and in particular the webs constituted by the stitched fractions of the arms. 
     Reinforcement  95  in the form of a braided sleeve may be added, the reinforcement having a cylindrical portion  96  that fits over the tube  30  and a collar  97  that is pressed against the ends of the flanges  86  where they are close to the tube  30 . Said ends are stitched to the collar  97 . 
     The assembly as made in this way is impregnated with resin that is hardened by polmerization in order to form two stiffened webs  81  and  83  that are strongly connected to the tube  30 . 
     Finally, another implementation of the invention is shown in  FIG. 12 . 
     The preform  109  is obtained from two blanks  100  and  106 , the blank  106  being considerably longer than the blank  100 . The ends of the blanks  100  and  106  are superposed and stitched together to form arms  101  and  103 . The central fractions are left free to so as to form sheets  104  and  105  between which the tube  30  is inserted. The sheet  105  is considerably longer than the sheet  104 . The arms  101  and  103  extend in parallel. They may for example serve as lugs for receiving the pin at the end of a stay. Inserts  107  are inserted between the tube and the sheets. 
     Thus, in all of the implementations shown, one or more blanks are cut out and two fractions are superposed, either belonging to the same blank or else to two distinct blanks, which fractions are then stitched together in part so as to define at least one structural portion in which the facing fractions are stitched together, and at least one connection portion in which the facing fractions are free. Thereafter, the tube is inserted between the fractions in the connection zone where the fractions have been left free so as to surround the tube, thus making it possible firstly to have fiber continuity between the structural portions that are cantilevered out from the tube and the connection portion that extends around the tube. 
     In the particular implementation shown in  FIG. 10 , only one blank is used, with the superposed fractions being joined together in the connection portion so that in said portion the facing fractions that have been left free form a ring. 
     In general, the stitching may be performed in various ways, in particular by using a tufting stitch, with the number of stitches being variable to achieve particular reinforcement of the connections. 
     The orientation of the reinforcing fibers constituting the blanks may be adapted to favor one particular direction as a function of the orientation of the stresses to which the beam is subjected, so as to improve mechanical strength in a method that is well known. 
     In order to improve the strength of the assembly, the structural element may advantageously be stitched to the tube prior to polymerization. Such stitching is shown in  FIG. 11 . It extends parallel to the axis of the tube and it connects the sheet to the tube. 
     In all of the examples shown, the structural element is obtained from a preform that is prepared using one or two blanks that are superposed and stitched together in a structural portion that is to form an arm, a lug, or a web that projects from the tube, while another portion is left free to form the sheets. 
     The preform could be made differently. In  FIG. 13 , there is shown in highly diagrammatic manner, a fabric comprising four superposed plies A, B, C, and D of fibers that are connected together by weft yarns that, in fractions E and F link the plies A, B, C, and D intimately together to form the structural portions of the preform, whereas in a fraction G they leave the plies free so that they can be separated into two sheets. The potential separation is represented by a dashed line between the two sheets. 
     It is also possible to start from a 3D or 2.5D fabric comprising a plurality of mutually superposed plies that are linked together by weft yarns, and to locally unlink the plies in order to make them mutually separable in the connection portion, so as to form the sheets. 
     The invention is not limited to the above description, but covers any variant coming within the ambit defined by the claims. In particular, although it is stated above that the tube is itself made of composite material, so as to enable it to be impregnated simultaneously with the structural element, it is possible to implement the invention using a tube that is not made of composite material, e.g. a metal tube. The resin then needs to be selected to favor strong adhesion between the structural element and the tube.