Abstract:
A fiber blank woven as a single piece by three-dimensional weaving to make a closed box-structure platform out of composite material for a turbine engine fan. In each plane of the fiber blank, a set of warp yarns interlinks layers of weft yarns in first, second, and third portions of the fiber blank, while leaving a closed non-interlinked zone separating the first and second portions over a fraction of the dimension of the fiber blank in the warp direction between an upstream non-interlinking limit and a downstream non-interlinking limit, and while leaving at least one open non-interlinked zone separating the second and third portions over a fraction of the dimension of the fiber blank in the warp direction from a non-interlinking limit to a downstream edge of the fiber blank. A method of fabricating a preform for the closed box-structure platform can use such a fiber blank.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to the general field of making a fiber blank by three-dimensional (3D) weaving for fabricating a blade platform out of composite material for a fan of an aviation turbine engine. 
         [0002]    Fan blade platforms for turbine engines, and in particular for turbojets, are arranged between the blades of the fan so as to extend an inlet cone of the fan. They serve in particular to define the inside of the annular air inlet passage into the fan, which passage is defined on the outside by a casing. 
         [0003]    It is known to have recourse to composite materials for making various parts of an aviation turbine engine. Thus, a composite material part may be obtained by making a fiber preform and densifying the preform with a matrix. In the intended application, the preform may be made of glass, carbon, or ceramic fibers, and the matrix may be made out of an organic material (polymer), out of carbon, or out of ceramic. 
         [0004]    For parts presenting a relatively complex geometrical shape, it is also known to make a fiber blank or structure as a single piece by 3D or multilayer weaving and to shape the fiber structure so as to obtain a fiber preform having a shape that is close to the shape of the part that is to be fabricated. 
         [0005]    Proposals have thus already been made to use 3D weaving to make a fiber preform of π-shaped section for a platform. Such platforms with a π-shaped section comprising a base and two legs form stiffeners that extend from a face of the base and that serve to stiffen the platform so as to avoid any movement of the platform under the centrifugal force generated by the speed of rotation of the fan. 
         [0006]    With such platforms, it has been found that the mere presence of stiffeners does not always give sufficient strength against centrifugal force. It has thus been found necessary to add a wall between the free ends of the stiffeners in order to form a closed box structure under the base of the platform so as to reinforce its strength. Unfortunately, that implies providing a covering on the stiffeners of the platform preform, which operation is difficult to perform because of the size of the fibers involved. Consequently, this operation can easily lead to a part being rejected. 
       OBJECT AND SUMMARY OF THE INVENTION 
       [0007]    An object of the invention is thus to provide a fiber blank woven as a single piece by three-dimensional weaving for making a platform of closed box structure out of composite material for a turbine engine fan. 
         [0008]    In a first aspect of the invention, this object is achieved by a fiber blank woven as a single piece by three-dimensional weaving, the fiber blank having opposite surfaces and presenting: 
         [0009]    a first portion, a second portion, and a third portion, each comprising a plurality of layers of weft yarns and each forming a portion of the thickness of the fiber blank between its opposite surfaces, the weft yarns of the fiber blank being arranged in columns, each column having weft yarns in all three portions; 
         [0010]    in each plane of the fiber blank, a set of warp yarns interlinking the layers of weft yarns of the first portion, of the second portion, and of the third portion, while forming:
       a closed non-interlinked zone separating the first portion from the second portion over a fraction of the dimension of the fiber blank in the warp direction between an upstream non-interlinking limit and a downstream non-interlinking limit; and   at least one open non-interlinked zone separating the second portion from the third portion over a fraction of the dimension of the fiber blank in the warp direction from a non-interlinking limit to a downstream edge of the fiber blank;       
 
         [0013]    one or more first warp yarns interlinking layers of weft yarns in the first portion of the fiber blank adjacent to the closed non-interlinked zone, and layers of weft yarns in the second portion of the fiber blank before and after the closed non-interlinked zone; 
         [0014]    one or more second warp yarns interlinking layers of weft yarns in the second portion of the fiber blank adjacent to the open non-interlinked zone, and layers of weft yarns in the first portion of the fiber blank before and after the closed non-interlinked zone; 
         [0015]    one or more third warp yarns interlinking layers of weft yarns in the second portion of the fiber blank adjacent to the open non-interlinked zone, and layers of weft yarns in the third portion of the fiber blank before the open non-interlinked zone; and 
         [0016]    one or more fourth warp yarns interlinking layers of weft yarns in the third portion of the fiber blank adjacent to the open non-interlinked zone, and layers of weft yarns in the second portion of the fiber blank before the open non-interlinked zone. 
         [0017]    Such 3D weaving makes it possible to make a fiber blank as a single piece for fabricating a platform that has a closed box structure under the base of the platform that serves to define the inside of the annular air inlet passage into the fan. 
         [0018]    In an embodiment, the fiber blank further comprises one or more fifth warp yarns interlinking layers of weft yarns in the first portion of the fiber blank before the closed non-interlinked zone and adjacent thereto, and layers of weft yarns in the second portion of the fiber blank after the closed non-interlinked zone; and one or more sixth warp yarns interlinking layers of weft yarns in the second portion of the fiber blank before the closed non-interlinked zone and adjacent thereto, and layers of weft yarns in the first portion of the fiber blank after the closed non-interlinked zone. 
         [0019]    The paths of the fifth warp yarn(s) and of the sixth warp yarn(s) advantageously cross in at least one transition zone extending in the fiber blank from the downstream limit of the closed non-interlinked zone, the transition zone extending in the warp direction over a distance longer than one pitch step between adjacent columns of weft yarns. Such crossing reinforces the downstream limit of the closed non-interlinked zone and may give rise to less stress on the yarns while unfolding a portion of the fiber blank adjacent to the closed non-interlink zone. 
         [0020]    The non-interlinking limit of the open non-interlinked zone may be situated in the warp direction between the upstream and downstream limits of the closed non-interlinked zone. 
         [0021]    In another embodiment, the fiber blank further includes a second open non-interlinked zone separating the second portion from the third portion over a fraction of the dimension of the fiber blank in the warp direction from an upstream edge of the fiber blank opposite from the downstream edge, up to a non-interlinking limit. 
         [0022]    The third warp yarn(s) may interlink layers of weft yarns in the second portion of the fiber blank adjacent to the open non-interlinked zones, and layers of weft yarns in the third portion of the fiber blank between the open non-interlinked zones, and the fourth warp yarn(s) may interlink layers of weft yarns in the third portion of the fiber blank adjacent to the open non-interlinked zones, and layers of weft yarns in the second portion of the fiber blank between the open non-interlinked zones. 
         [0023]    The non-interlinking limits of the open non-interlinked zones may be situated in the warp direction between the upstream and downstream limits of the closed non-interlinked zone. 
         [0024]    In yet another embodiment, the fiber blank further includes two closed non-interlinked zones separating the second portion from the third portion over a fraction of the dimension of the fiber blank in the warp direction between the upstream and downstream limits of the closed non-interlinked zone, said two closed non-interlinked zones being for forming a platform box structure with a honeycomb arrangement. Such a honeycomb arrangement makes it possible to reinforce the buckling strength of the box structure of the platform while keeping control over thickness. 
         [0025]    Under such circumstances, one or more sixth warp yarns may interlink layers of weft yarns in the second portion of the fiber blank before and after the two closed non-interlinked zones, and one or more seventh warp yarns may interlink layers of weft yarns in the third portion of the fiber blank before and after the two closed non-interlinked zones, the paths of the sixth warp yarns and of the seventh warp yarns crossing on three occasions in order to create the two non-interlinked zones. 
         [0026]    Likewise, one or more eighth warp yarns may interlink layers of weft yarns in the second portion of the fiber blank before the two closed non-interlinked zones, and layers of weft yarns in the third portion of the fiber blank after the two closed non-interlinked zones, and one or more ninth warp yarns may interlink layers of weft yarns in the third portion of the fiber blank before the two closed non-interlinked zones, and layers of weft yarns in the second portion of the fiber blank after the two closed non-interlinked zones, the paths of the eighth warp yarn(s) and of the ninth warp yarn(s) crossing in a middle region of the two closed non-interlinked zones. 
         [0027]    Regardless of the embodiment, the outer layers of weft yarns adjacent to the opposite surfaces of the fiber blank are advantageously woven with the same warp yarns extending continuously over the entire dimension of the fiber blank in the warp direction, thus preserving yarn continuity at the surface. 
         [0028]    In a second aspect of the invention, the intended object is achieved with a fiber blank as defined above but with warp and weft interchanged. 
         [0029]    In a third aspect of the invention, the invention provides a method of fabricating a preform for a closed box-structure platform out of composite material for a turbine engine fan, the method comprising making a fiber preform by shaping a fiber blank as defined in the first aspect of the invention, the shaping comprising unfolding fractions of the first portion and of the second portion of the fiber blank that are adjacent to the closed non-interlinked zone and to the open non-interlinked zone, cutting off the fractions of the first and second portions of the fiber blank after the closed non-interlinked zone, and densifying the preform with a matrix. 
         [0030]    In a fourth aspect of the invention, the invention provides a method of fabricating a preform for a closed box-structure platform out of composite material for a turbine engine fan, the method comprising making a fiber preform by shaping a fiber blank as defined in the second aspect of the invention, the shaping comprising unfolding fractions of the first portion and of the second portion of the fiber blank that are adjacent to the closed non-interlinked zone and to the first and second open non-interlinked zones, cutting off the fractions of the first and second portions of the fiber blank before and after the closed non-interlinked zone, and densifying the preform with a matrix. 
         [0031]    The shaping may further comprise shaping fractions of the second and third portions of the fiber blank that are situated between the upstream and downstream limits of the closed non-interlinked zone to form undulations in the box structure of the platform. A box structure provided with undulations presents improved buckling strength while keeping thickness under control. 
         [0032]    In a fifth aspect of the invention, the invention provides a closed box-structure platform made out of composite material for a turbine engine fan, the platform being obtained by the method of the third or fourth aspect of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    Other characteristics and advantages of the present invention appear from the following description given with reference to the accompanying drawings, which show an embodiment having no limiting character. In the figures: 
           [0034]      FIG. 1  is a diagrammatic view of a closed box-structure platform made of composite material for a turbine engine fan; 
           [0035]      FIG. 2  is a diagrammatic view of a plane of a 3D woven fiber blank in an embodiment of the invention; 
           [0036]      FIGS. 3A ,  3 B, and  3 C are enlargements of  FIG. 2 ; 
           [0037]      FIGS. 4 to 6  are very diagrammatic section views showing how the fiber blank of  FIG. 2  is shaped to obtain a preform for a box-structure platform; 
           [0038]      FIGS. 7A ,  7 B and  7 C are views of the plane of the preform obtained by shaping the fiber blank corresponding to the views of  FIGS. 3A ,  3 B, and  3 C respectively; 
           [0039]      FIG. 8  is a diagrammatic view of a plane of a 3D woven fiber blank in another embodiment of the invention; 
           [0040]      FIG. 9  is an enlargement of  FIG. 8 ; 
           [0041]      FIGS. 10 and 11  are very diagrammatic section views showing how the fiber blank of  FIG. 8  is shaped to obtain a preform for a box-structure platform; 
           [0042]      FIG. 12  is a fragmentary view of the plane of the preform obtained by shaping the fiber blank of  FIG. 8 ; 
           [0043]      FIGS. 13 and 14  are cross-section views of preforms for closed box-structure platforms in variant embodiments of the invention; and 
           [0044]      FIG. 15  is a fragmentary view of a plane of a fiber blank for obtaining the  FIG. 14  platform preform. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0045]      FIG. 1  is a very diagrammatic view of a closed box-structure platform  10  made of composite material for a turbine engine fan. 
         [0046]    The platform  10  comprises a base  12  having a top face  12   a  and a bottom face  12   b,  together with two legs  14 ,  16  forming stiffeners and extending from the bottom face  12   b.  At their bottom ends, the two legs  14  and  16  are connected together by a stiffener wall  18  so as to form a closed box structure  20  under the base  12 , as represented by shading in  FIG. 1 . 
         [0047]    The platform  10  is for mounting in a gap between two fan blades, in the vicinity of their roots, so as to define the inside of an annular air inlet passage into the fan, the passage being defined on the outside by a fan casing. 
         [0048]      FIG. 2  is a diagrammatic view of a warp plane of a 3D-woven fiber blank  100  from which a platform fiber preform can be shaped, prior to injecting resin or densifying with a matrix, and possible machining, in order to obtain a fan platform made of composite material, such as platform shown in  FIG. 1 . 
         [0049]    In 3D weaving, it should be understood that the warp yarns follow sinuous paths so as to interlink weft yarns belonging to different layers of weft yarns, with the exception of non-interlinked zones, it being understood that 3D weaving, in particular when using an interlock weave, may include 2D weaving at the surface. Various 3D weaves can be used, such as interlock, multi-satin, or multi-plain weaves, for example, as described in particular in Document WO 2006/136755. 
         [0050]    In  FIG. 2 , the fiber blank  100  presents opposite surfaces  100   a  and  100   b,  and it comprises a first portion  102 , a second portion  104 , and a third portion  106 , the second portion  104  being positioned between the other two portions  102  and  106 . These three portions form respectively first, second, and third portions of the thickness of the fiber blank between its opposite surfaces  100   a  and  100   b.    
         [0051]    Each portion  102  to  106  of the fiber blank has a plurality of superposed layers of weft yarns, four in the example shown, it being possible for the number of weft yarns to be any desired number not less than two, depending on the desired thickness. In addition, the number of layers of weft yarns in the portions  102  to  106  may differ from one another. The weft yarns are arranged in columns, each comprising weft yarns of the first, second, and third portions of the fiber blank. 
         [0052]    Over a fraction of the dimension of the fiber blank  100  in the warp direction (c), the first portion  102  and the second portion  104  of the fiber blank are completely separated from each other by a closed non-interlinked zone  108  that extends between an upstream limit  108   a  and a downstream limit  108   b  for the non-interlinked zone. The term “closed” non-interlinked zone is used herein to mean a zone that is closed at both of its ends and that does not have any warp yarns passing therethrough to interlink weft yarns in layers belonging respectively to the first portion  102  and to the second portion  104  of the fiber blank  100 . 
         [0053]    Over another fraction of the dimension of the fiber blank  100  in the warp direction, the second portion  104  and the third portion  106  of the fiber blank are completely separated from each other by an open non-interlinked zone  110  that extends from a non-interlinking limit  110   a  to a downstream edge  100   c  of the fiber blank. The term “open” non-interlinked zone is used herein to mean a zone that is closed at one end and open at an opposite end and that does not have any warp yarns passing therethrough for interlinking the weft yarns of layers belonging respectively to the second portion  104  and to the third portion  106  of the fiber blank  100 . 
         [0054]    In this embodiment, the non-interlinking limit  110   a  of the open non-interlinked zone  110  is situated in the warp direction between the upstream and downstream limits  108   a  and  108   b  of the closed non-interlinked zone  108 . 
         [0055]    Except in the closed non-interlinked zone  108  and in the open non-interlinked zone  110 , the layers of weft yarns are interlinked by warp yarns over a plurality of warp yarn layers c 11  to c 22 . 
         [0056]    In the example shown more particularly in  FIGS. 3A to 3C , a common first warp yarn c 15  interlinks the layers of weft yarns in the first portion  102  of the fiber blank adjacent to the closed non-interlinked zone  108  and layers of weft yarns in the second portion  104  of the fiber blank before and after the closed non-interlinked zone, i.e. before the upstream limit  108   a  and after the downstream limit  108   b  of the closed non-interlinked zone. Naturally, this interlinking could be performed by a plurality of first warp yarns. 
         [0057]    Conversely, a common second warp yarn c 14  interlinks layers of weft yarns in the second portion  104  of the fiber blank adjacent to the open non-interlinked zone  110  and layers of weft yarns in the first portion  102  of the fiber blank before and after the closed non-interlinked zone. Naturally, this interlinking could be performed by a plurality of second warp yarns. 
         [0058]    Thus, the path of the warp yarn c 15  and the path of the warp yarn c 14  cross both at the upstream limit  108   a  of the closed non-interlinked zone  108  and at the downstream limit  108   b  of this closed non-interlinked zone. 
         [0059]    In the same manner, still in the example in  FIGS. 3A to 3C , a common third warp yarn c 19  interlinks the layers of weft yarns in the second portion  104  of the fiber blank adjacent to the open non-interlinked zone  110 , and layers of weft yarns in the third portion  106  of the fiber blank before the open non-interlinked zone, i.e. before the non-interlinking limit  110   a  of this open non-interlinked zone. Naturally, this interlinking could be performed by a plurality of third warp yarns. 
         [0060]    Conversely, a common fourth warp yarn c 18  interlinks layers of weft yarns in the third portion  106  of the fiber blank adjacent to the open non-interlinked zone  110 , and layers of weft yarns in the second portion  104  of the fiber blank before the open non-interlinked zone. Naturally, this interlinking could be performed by a plurality of fourth warp yarns. 
         [0061]    Thus, the path of the warp yarn c 19  and the path of the warp yarn c 18  cross at the non-interlinking limit  110   a  of the open non-interlinked zone  110 . 
         [0062]    Furthermore, fifth warp yarns c 12 , c 13  interlink layers of weft yarns in the first portion  102  of the fiber blank before the closed non-interlinked zone  108  and adjacent thereto, and layers of weft yarns in the second portion  104  of the fiber blank after the closed non-interlinked zone. 
         [0063]    Likewise, sixth warp yarns c 16 , c 17  interlink layers of weft yarns in the second portion  104  of the fiber blank before the closed non-interlinked portion  108  and adjacent thereto, and layers of weft yarns in the first portion  102  of the fiber blank after the closed non-interlinked zone. 
         [0064]    As shown in  FIG. 3C , the paths of the fifth warp yarns c 12 , c 13  and the paths of the sixth warp yarns c 16 , c 17 , cross in a transition zone  112  extending in the fiber blank from the downstream limit  108   b  of the closed non-interlinked zone  108 . This transition zone  112  extends in the warp direction over a distance that is longer than one pitch step p between adjacent columns of weft yarns, e.g. over a distance equal to 2p. 
         [0065]    Such crossing between the fifth warp yarns c 12 , c 13  and the sixth warp yarns c 16 , c 17  reinforces the downstream limit  108   b  of the closed non-interlinked zone  108  and may give rise to less stress on the yarns while unfolding a portion of the fiber blank adjacent to the closed non-interlinked zone. 
         [0066]    The outer layers of weft yarns adjacent to the opposite surfaces  100   a  and  100   b  of the fiber blank  100  are woven using the same warp yarns, respectively c 11  and c 22 , that extend continuously over the entire dimension of the fiber blank in the warp direction. By way of example, it is possible to use a surface satin weave for the warp yarns c 11  and c 22 . Likewise, it is also possible to use a surface satin weave for the warp yarns c 14  and c 15  in those fractions of the first and second portions of the fiber blank that are separated by the closed non-interlinked zone  108 , and also for the warp yarns c 18  and c 19  in those fractions of the second and third portions of the fiber blank that are separated by the open non-interlinked zone  110 . 
         [0067]    A fiber preform for a closed box-structure platform (such as the platform shown in  FIG. 1 ) may be obtained from such a fiber blank, in the manner described below. 
         [0068]    After weaving, the fiber blank  100  presents a shape as shown very diagrammatically in  FIG. 4 . The fractions of the first and second portions  102  and  104  in the fiber blank that are adjacent to the closed non-interlinked zone  108  and to the open non-interlinked zone  110  are unfolded as shown in  FIG. 5 , while the third portion  106  of the fiber blank is not manipulated. The final preform as obtained at the end of such unfolding is as shown in  FIG. 6 . 
         [0069]    The unfolding of these fiber blank fractions is shown more particularly in  FIGS. 7A to 7C . In particular,  FIG. 7A  corresponds to the enlargement of  FIG. 3A  and shows how the fraction of the first portion  102  of the fiber blank that is adjacent to the closed non-interlinked zone is unfolded. This unfolding takes place perpendicularly to the fraction of the second portion of the fiber blank that is adjacent to the closed non-interlinked zone. 
         [0070]      FIG. 7B  likewise shows the folding that takes place in a region of the fiber blank that corresponds to the enlargement of  FIG. 3B . In this example, the fraction of the second portion  104  of the fiber blank that is adjacent to the open non-interlinked zone is unfolded perpendicularly to the fraction of the third portion  106  of the fiber blank that is adjacent to the open non-interlinked zone. 
         [0071]    Finally,  FIG. 7C  shows the unfolding that is performed in the region of the fiber blank that corresponds to the enlargement of  FIG. 3C , i.e. at the downstream limit  108   b  of the closed non-interlinked zone  108 . In this region, the fraction of the second portion  104  of the fiber blank that is adjacent to the closed non-interlinked zone is unfolded perpendicularly to the fraction of the first portion  102  of the fiber blank that is adjacent to the closed non-interlinked zone. After this unfolding, the fractions of the first and second portions of the fiber blank that are situated beyond the closed non-interlinked zone, i.e. beyond the downstream end  108   b  of the non-interlinked zone  108 , are cut off along a cutting plane D (see also  FIG. 6 ). 
         [0072]    The shaping of the fiber blank  100  thus makes it possible to obtain a preform for a closed box-structure platform as described above with reference to  FIG. 1 . 
         [0073]      FIG. 8  is a diagram showing a warp plane in a 3D woven fiber blank  100 ′ for obtaining a platform of closed box structure in another embodiment of the invention. Elements in common between the fiber blank  100 ′ of  FIG. 8  and the fiber blank  100  of  FIG. 2  are given the same references and are not described again. 
         [0074]    The fiber blank  100 ′ differs from the fiber blank of  FIG. 2  by the presence of a second open non-interlinked zone  110 ′ between the second portion  104  and the third portion  106  over a fraction of the fiber blank in the warp direction (c), this second open non-interlinked zone  110 ′ extending from an upstream edge  100   d  of the fiber blank opposite from its downstream edge  100   c  and up to a non-interlinking limit  110 ′ a.    
         [0075]    For this purpose, and as shown more particularly in  FIG. 9 , a common third warp yarn c 19  interlinks layers of weft yarns in the second portion  104  of the fiber blank  100 ′ that are adjacent to both of the open non-interlinked zones  110 ,  110 ′, and layers of weft yarns in the third portion  106  of the fiber blank between these open non-interlinked zones, i.e. between the respective non-interlinking limits  110 ′ a  and  110   a  of these open non-interlinked zones. 
         [0076]    Likewise, a common fourth warp yarn c 18  interlinks layers of weft yarns in the third portion  106  of the fiber blank  100 ′ adjacent to both of the open non-interlinked zones  110 ,  110 ′, and layers of weft yarns in the second portion  104  of the fiber blank between these open non-interlinked zones. 
         [0077]    Naturally, this interlinking by the third and fourth warp yarns could be performed by pluralities of third and fourth warp yarns. It is also possible to use a surface satin weave for the warp yarns c 18  and c 19  in the fractions of the second and third portions of the fiber blank that are separated by the two open non-interlinked zones  110 ,  110 ′. 
         [0078]    The second open non-interlinked zone  110 ′ between the second and third portions  104  and  106  of the fiber blank  100 ′ is identical to the closed non-interlinked zone  110  described with reference to the embodiment of  FIG. 2 . The same applies to the closed non-interlinked zone  108  between the first and second portions  102  and  104  of the fiber blank. 
         [0079]    In addition, the non-interlinking limits  110   a,    110 ′ a  of the two open non-interlinked zones  110 ,  110 ′ of the fiber blank  100 ′ are situated in the warp direction between the upstream and downstream limits  108   a  and  108   b  of the closed non-interlinked zone  108 . 
         [0080]    A fiber preform for a closed box-structure platform (such as the platform shown in  FIG. 1 ) can be obtained from such a fiber blank in the manner described below. 
         [0081]    After weaving, the fiber blank  100 ′ presents a shape as shown very diagrammatically in  FIG. 10 . The fractions of the first portion  102  and of the second portion  104  of the fiber blank that are adjacent to the closed non-interlinked zone  108  and to the two open non-interlinked zones  110 ,  110 ′ are unfolded as shown in  FIG. 11 , while the third portion  106  of the fiber blank is not manipulated. 
         [0082]    More precisely, unfolding the fractions of the first and second portions  102  and  104  of the fiber blank  100 ′ that are adjacent to the closed non-interlinked zone  108  causes this closed non-interlinked zone to be opened through 180° at its non-interlinking limit (see  FIG. 11 ). 
         [0083]    Thus,  FIG. 12  shows such unfolding in a region of the fiber blank corresponding to the upstream non-interlinking limit  108   a  of the closed non-interlinked zone. In this region, the fractions of the first and second portions  102  and  104  of the fiber blank that are adjacent to the closed non-interlinked zone are unfolded so that they form between them an angle of about 180°. 
         [0084]    After the operation of unfolding the fiber blank, the fractions of the first and second portions of the fiber blank  100 ′ that are situated before and after the closed non-interlinked zone  108 , i.e. before the upstream non-interlinking limit  108   a  and after the downstream non-interlinking limit  108   b  thereof, are cut off on cutting planes D′ (see also  FIG. 11 ). 
         [0085]    In the embodiments described, it should be understood that the weft and warp directions could be interchanged. 
         [0086]    Furthermore, in the embodiments described, the fiber blank  100 ,  100 ′ is formed by 3D weaving with yarns of nature that is selected as a function of the intended application, e.g. yarns made of glass, carbon, or ceramic fibers. 
         [0087]    The matrix is deposited in the fiber preform (as obtained by shaping the fiber blank) in order to form a closed box-structure platform made of composite material by holding the preform in a mold until the preform has been stiffened (or consolidated). Prior to putting the preform in the mold, a core is arranged inside the closed non-interlinked zone of the preform. 
         [0088]    The nature of the matrix is selected as a function of the intended application, for example an organic matrix obtained in particular from a resin that is a precursor for a polymer matrix such as an epoxy, bismaleimide, or polyimide matrix, or that is a precursor for a carbon matrix or for a ceramic matrix. For an organic matrix, the fiber preform is impregnated by a composition containing the matrix precursor resin, prior to being shaped in tooling, or after shaping, with impregnation then being performed by infusion or by a process of the resin transfer molding (RTM) type, for example. For a carbon matrix or a ceramic matrix, densification may be performed by chemical vapor infiltration (CVI) or by impregnating with a liquid composition containing a precursor resin for carbon or for ceramic and by performing pyrolysis heat treatment or ceramization of the precursor, which methods are themselves well known. The platform is machined to its final dimensions after the fiber preform has been injected/densified. 
         [0089]      FIGS. 13 and 14  show variant embodiments of preforms for making platforms of closed box structure. 
         [0090]    In these variant embodiments, the stiffener wall  18  of the closed box structure under the base  12  of the platform presents increased buckling strength without any need to increase the thickness of the stiffener wall. Thus, in the variant embodiment of  FIG. 13 , the stiffener wall  18  presents undulations  18   a.  Likewise, in the variant embodiment of  FIG. 14 , the stiffener wall  18  presents a honeycomb arrangement  18   b.    
         [0091]    These particular structures  18   a,    18   b  serve to reinforce the ability of the box structure of the platform to withstand the compression forces to which the platform is subjected. 
         [0092]    The structure with an undulation  18   a  as shown in  FIG. 13  is obtained during the step of shaping the fiber blank, that is itself obtained as described above. In particular, the fiber blank may be made using the embodiment described with reference to  FIGS. 2 and 3A  to  3 C or the embodiment described with reference to  FIGS. 8 and 9 . 
         [0093]    During the step of shaping the fiber blank, the fractions of the second and third portions of the fiber blank that are to form the preform for the stiffener wall (i.e. that are situated between the upstream and downstream limits of the closed non-interlinked zone of the fiber blank) are themselves deformed with the help of special tooling for forming undulations of the kind shown in  FIG. 13 . The number and the amplitude of the undulations that are created may vary depending on mechanical requirements. 
         [0094]    The honeycomb arrangement  18   b  of  FIG. 14  is obtained by a variation to the weaving of the fiber blank as obtained in either of the two embodiments described above. 
         [0095]    In particular, the fiber blank  100 ,  100 ′ as shown in part in  FIG. 15  (with weft yarns omitted for reasons of clarity) further includes a pair of closed non-interlinked zones  114  that separate the second portion  104  from the third portion  106  over a fraction  116  of the dimension of the fiber blank in the warp direction. This fraction  116  also extends between the upstream and downstream limits  108   a  and  108   b  of the closed non-interlinked zone  108  shown in  FIGS. 2 and 8 . 
         [0096]    More precisely, the fraction  116  of the fiber blank in which the pair of closed non-interlinked zones  114  is made is situated, for a fiber blank  100  of the embodiment shown in  FIG. 2 , between the upstream limit  108   a  of the closed non-interlinked zone and the non-interlinking limit  110   a  of the open non-interlinked zone  110 , and for a fiber blank  100 ′ of the embodiment shown in  FIG. 8 , between the non-interlinking limit  110 ′ a  of the second open non-interlinked zone  110 ′ and the non-interlinking limit  110   a  of the open non-interlinked zone  110 . 
         [0097]    Furthermore, the pair of closed non-interlinked zones  114  between the second and third portions  104  and  106  of the fiber blank consists of two closed non-interlinked zones that are adjacent to each other and given respective references  114   a  and  114   b.    
         [0098]    More precisely, one or more sixth warp yarns c i1  and C i4  interlink layers of weft yarns in the second portion  104  of the fiber blank before and after the pair of closed non-interlinked zones  114 , and one or more seventh warp yarns c j1  and c j4  interlinked layers of weft yarns in the third portion  106  of the fiber blank before and after the pair of closed non-interlinked zones. The paths of the sixth warp yarns c i1, c   i4  and of the seventh warp yarns c j1 , c j4  cross at three locations in order to create the two closed non-interlinked zones. 
         [0099]    Furthermore, one or more eighth warp yarns c i2  and c i3  interlink layers of weft yarns in the second portion  104  of the fiber blank before the pair of closed non-interlinked zones  114 , and layers of weft yarns in the third portion  106  of the fiber blank after the pair of closed non-interlinked zones. 
         [0100]    Likewise, one or more ninth warp yarns c j2  and c j3  interlink layers of weft yarns in the third portion  106  of the fiber blank before the pair of closed non-interlinked zones  114 , and layers of weft yarns in the second portion  104  of the fiber blank after the pair of closed non-interlinked zones. 
         [0101]    The paths of the eighth warp yarn(s) c i2 , c i3  and of the ninth warp yarn(s) c j2 , c j3  cross in a middle region of the pair of closed non-interlinked zones  114 , i.e. level with the junction between the two closed non-interlinked zones  114   a  and  114   b.    
         [0102]    It should be observed that with a fiber blank obtained in the embodiment of  FIGS. 2 and 3A  to  3 C, the sixth above-mentioned warp yarns c i1 , c i4  correspond respectively to the warp yarns c 14  and c 18  mentioned in that embodiment, with the seventh warp yarns c j1 , c j4  corresponding respectively to the warp yarns c 19  and c 22 , the eighth warp yarns c i2 , c i3  corresponding respectively to the warp yarns c 16  and c 17 , and the ninth warp yarns c j2 , c j3  corresponding respectively to the warp yarns c 20  and c 21 . 
         [0103]    Likewise, in a fiber blank obtained in the embodiment of  FIGS. 8 and 9 , the sixth above-mentioned warp yarns c i1 , c i4  correspond respectively to the warp yarns c 14  and c 19  mentioned in that embodiment, with the seventh warp yarns c j1 , c j4  corresponding respectively to the warp yarns c 18  and c 22 , with the eighth warp yarns c i2 , c i3  corresponding respectively to the warp yarns c 16  and c 17 , and with the ninth warp yarns c j2 , c j3  corresponding respectively to the warp yarns c 20  and c 21 . 
         [0104]    It should also be observed that the weft and warp yarns could be interchanged. 
         [0105]    The preform for the closed box-structure platform that is provided with a honeycomb arrangement  18   b  as shown in  FIG. 14  is obtained by shaping the fiber blank woven in this way. In particular, the closed non-interlinked zones  114   a  and  114   b  making up the two closed non-interlinked zones  114  are unfolded so as to form two adjacent cells. The number and the dimensions of the cells may be varied depending on mechanical requirements.