Title of the invention A fiber structure woven as a single piece by 3 D weaving, and an application thereof to fabricating a composite material part Background of the invention The invention relates to making a fiber structure , woven as a single piece by three-dimensional ( 3 0 ) weaving, in particular for fabricating a composite material part. One particular,,but non-exclusive, field of application of the invention lies in making fiber structures for preforms of composite material parts for aircraft ok aeroengines, in particular for airplane turbink engines. In well-known manner, a composite material part may be obtained by making a fiber preform and by densifying the preform with a matrix. Depending on the intended application, the preform may be made of glass, .carbon, or ceramic fibers, and the matrix'may be made of an organic material (a polymer), of carbon, or of ceramic. For parts that are relatively. complex in shape, it is known to make a fiber structure or blank as a single piece by 3~ or multiple-layer weaving, and to shape the blank in order to obtain a fiber pre,form that presents a shape that is close ~ -t--o- the- s hape of --t he,part ~~ -~ th--a t ~ is to - ~- -- be fabricated. + In order. to facilitate such shaplng, and in order to' ~'~~ avoid making incisions that result in yarns- being cut and. that lead to a reduction in mechanical strength, it is known to leave'one &.more non-interlinked zones within the fiber .structure while it is being woven. Such'noninterlinked zones may be obtained by locally omitting any interlinking of the layers of adjacent yarns, thereby making it possible to fold out portions of the fiber structure adjacent to the non-interlinked zones. The making of composite material parts that are complex in shape from woven structures with noninterlinked zones is described in particular in documents WO 2010/061139 and WO 2010/103213. Nevertheless, the shaping of a fiber structure by folding out portions that are separated by a non- 5 interlhked zone can give rise to weakness at the end of the non-interlinked zone and to excessive levels of stress on the ygrns that are subjected to stress while performing such shaping. 10 2 An object of the invention is to remedy such drawbacks. . In a first aspect of the invention, th:s object is achieved with a fiber structure woven as a single piece 15 by three-dimensional weaving, the fiber stxucture having opposite surfaces and presenting: a first portion having a plurality of layers of warp yarns and forming a first portion of the thickness of the fiber structure between its opposite surfaces; 2 0 a second portion having a plurality of layers of warp yarns and forming a second portion of thickness of the fiber structure, the warp yarns being arranged in columns each of which includes warp yarns ok the first portion and of the secqnd portion; and t 25 a se-t o f ~ weft yar-n~s interlinking the layers o-f ~ ._ -~ -- - 1 ~- warp yarns of the first portion and of the second portion , while leaving at least one non-interlinked zone . . separating the first and second portions over a portion of the dimension of the fiber structure in the weft 30 direction from an edge of the fiber structure to an knd of non-interlinked zone, . . in which fiber structure, in each plane: one or more same first weft' yarns interlink layers of warp yarns of the first portion of the fiber structure 35 adjacent to the non-interlinked zone and layers of warp yarns of the second portion of the fiber structure beyond the non-interlinked zone; and one or more same second warp yarns interlink layers of weft yarns of the second portion of the fiber structure adjacent to >he non-interlinked zone and layers of warp yarns of the first portion of the fiber structure beyond the non-interlinked zone; such that the paths of the first weft yarn(s) and of the second weft yarn(s) cross over in at least'one transition zone extending in the fiber structure from the end of the non-interlinked zone; and the transition zone extends in the weft direction over a distance that is greater than the pitch between adjacent warp columns. The crossing-over of weft yarns in a transition zone adjacent to the end of the non-inforlinked zone reinforces said end and can give rise to reduced stress on the yarns while folding out a fraction of the'fiber structure adjacent to. the non-interlinked zone In an embodiment, a plurality,of first weft yarns, as well as a plurality of second weft yarns, follow similar paths between the ends in the weft direction of the transition zone. . In another embodiment, a plurality of first weft yarns, as well as a plurality of second weft yarns, follow similar paths that are mutually dffset in the weft direction in the transiti.o~ n ~ zone (s) . ~p~ ~ -- ~dvantageously, the outer layers of warp yarns adjacent to the opposite .surfaces-of the fiber structure ~~~~~ are woven with the same weft yarns extending continuously over the entire dimension of'the fiber structure in the weft direction, thus makYng it possibl& to preserve continuity of surface yarns. Also advantageously, in at least one of the first and second portions of the fiber structure, the'warp yarns of the outer layers of warp yarns adjacent to a surface of the fiber structure are woven with the same weft yarns having paths that cross over at a location corresponding substantially to that of the transition zone, thus making it possible, while folding out a fraction of the fiber structure adjacent to the noninterlinked zone, to limit the curvature that is imposed on the weft yarns adjacent to the surface. 5 In an embodiment, the fiber structure presents at least two non-interlinked zones separating the first and second portions over a portion of the dimension of the fiber structure in the weft direction from opposite edges of.the fiber structure as far as respective ends of non- 10 interlinked zones, thus making it possible, after . shaping, to.obtain a fiber preform having a section that is x-shaped or I-shaped. +In another aspect of the ihvention, the intended object is achieved with a fyber structure as defined 15 above in which the terms "weft" and "warp" are intkrchanged. In yet another aspect of the invention, the invention provides a method of fabricating a composite material part comprising making a fiber preform by ' 20 shaping a fiber structure as defined above, the shaping. including at least folding out a fraction of the first or the second portion of the fiber structure adjacent to a non-interlinked zone, and densifying the preform with a matrix. -~ ~-25 p~~ ~ - A.~c cording .t o . yet another aspect of the ~ invention, ~ ~~ -~~ - *- - I the invention provides a method of fabricating a ~ ~~ 'composite materia.1 part having a substantially .x-shaped . .section, the method comprising making a fiber preform by shaping a fiber' structure as defined above with twonon- 30 interlinked zones, the shaping incl~dinq~foldinogu t . ~~ fractions of the first or the second portion of the fiber structure adjacent to the two non-interlinked zones, and dehsifying the preform with a matrix. By way of example, such a part having a section that ' 35 is substantially n-shaped may be a fan blade platfbrm for a turbine engine' According to another aspect of the invention, the invention provides a method of fabricating a composite material part of substantially I-shaped section, the method comprising making a fiber preform by shaping a 5 fiber structure as defined above with two non-interlinked zones, the shaping including folding out fractions of the first and second portions of the fibdr structure adjacent fo the two non-interlinked zones, and densifying the preform with a matrix. 10 By way of example, such a part of section that is substantially I-shaped may be an outlet guide vane of a turbine engine. According to yet other aspects, tzhe invention provides a hollow propeller blade for an aeroengine 15 obtained by a method as defined above. Brief description of the drawings The invention can.be better understood on reading the following description given by way of non-limiting 20 indication and with reference to the accompanying drawings,.in which: Figure 1 is a highly diagrammatic section view of a 3D woven fiber structure in an embodiment of the invention; , 25 -- ~- ~ ~- ~ i ~.~- u - r2 ei s a highly diagrammatic section view of - ~ ~ -- - .. * a preform obtained by shaping the Figure 1 fiber structure; ' ~-~ Figure 3 is. a diagrammatic. plan view of a 3D woven 3 fiber structure in an embodiment of the invention; ' 30 Figur6 4 is a diagrammatic plan view of a n-shaped . . preform obtained by shaping the Figure 3 fiber structure; Figure 5 is a highly diagrammatic section view of a 3D woven fiber structure in an embodiment of the invention; 3 5 Figure 6 is a highly diagrammatic section view of a preform obtained by shaping the Figure 5 fiber structure; Figure 7 is a diagrammatic plan view of a n-section preform in an embodiment of the invention; Figure 8 is a highly diagrammatic section view of a 3D woven fiber structure in an embodiment of the 5 invention; Figure 9 is a highly diagrammatic section view of a preform obtained by shaping the Figure 8 fiber structure; Figure 10 is a diagrammatic plan view of a 10 n-section preform in an embodiment of the invention; Figure 11 is a highly diagrammatic section view of a 3D woven fiber structure in an embodiment of the. invention; Figure 12 is a highly diagrammatic section view, of 15 a preform obtained by shaping the Figure 11 fiber structure; Figure 13 is a diagrammatic perspective view of a fan blade.platform obtained by densifying a preform of . substantially n-shaped section; 2 0 Figure 14 is a highly diagrammatic section view of a 3D woven structure in an embodiment of the invention; Figure 15 is a highly diagrammatic section view of 'a preform of I-shaped profile obtained by shaping the . Figure 14 fiber structure; - 2 5 -~ ~ .o Figure 16 is a diagramm-a~ tic perspedti-v ~e- ~ view p~~ of an ~~pp ~~ outlet guide vane obtained bydensifying a preform of I-shaped profile; ~~~ ~~ ~ Figure 17 is a diagrammatic section view of a.3D woven structure in an embodiment of the invention; 3 0 Figure 18 is a highl~.diagrammatic section view of a preform of V-shap.ed profile obtained by shaping the Figure 17 fiber structure; and ~igure 19 is a diagrammatic view of a propeller obtained by densifying a preform of V-shaped profile. 3 5 Detailed description of embodiments In order to avoid overcrowding the drawings, in Figures 1, 2, 3, 4, 5, 7, 8, 10 and 11, the paths of the weft yarns are drawn as straight lines while the warp yarns, shown in section, are represented by dots. Since 3D weaving is involved, it will be understood that the 5 weft yarns follow sinuous paths so as to interlink warp yarns belonging to different layers of warp yarns, with the exceptions of non-interlinked zones, it being observed that 3D weaving, and in particular using an interlock weave, may include 2.0 weaving at the surface. 10 By way of example, various 3D weaves may be used, such as interlock, multiple-satin, or multiple-plain weaves, as described in particular in document WO 2006/136755. Figure 1 shows highly diagrammatically a weft p'lane in a 3D yoven fiber structure 10 constituting a 'single 15 piece having opposite faces 10a and lob. The terms "weft plane" is used herein td mean a section plane + perpendicular to the warp direction and showing one column of.ra~eft yarns. The fiber structure .lo comprises two portions 12 and 14 respectively forming first and 20. second portions of the thickness of the fiber structure . 10. Each portion 12, 14 comprises a plurality of superposed layers of warp yarns, four of them in the example shown, the number of layers of warp yarns ' poteneially being any desired number not less than two, 2'5 depen- d-- ing on the --d esired ~ thickness. ~~ ~-~ Furthermore, the ' . . number of layers of warp yarns in the portions 12 and 14 'could .be different from each other. It is also possible to have.a number of Layers of warp barns that is not constant along the entire weft direction. The warp yarns 30 &re arranged in columns, each comprising both warp yarns. of the portion 12 and warp yarns of the.portion 14 qf the fiber structure 10. Over a portion of the dimension of the fiber ptructure 10 in the weft direction (t), the two portions 35 12 and 14 of the fiber structure are totally separated from each other by a non-interlinked zone 16 that extends from an edge 10c of the fiber structure 10 to an end 16a of the non-interlinked zone. The term non-interlinked zone is used herein to mean a zone that is not crossed by weft yarns interlinking the warp yarns in the layers belonging respectively to the portions 12 and 14 of the 5 fiber structure 10. Except in the non-interlinked zone, the layers of warp yarns are interlinked by weft yarns belonging to a * plurality of layers of weft yarns. In the example shown, in each plane of the fiber 10 structure 10, first weft yarns tll to t,, interlink the warp yarns of the layers of warp yarns in the fraction 12a of the portion 12 adjacent to the non-interlinked zone 1'6, and also warp yarns of the warp yarn layers of t the portion 16 beyond the non-interlinked.zone 16. 15 Conversely, second weft yarns t,, to t,, interlink the warp yarns of the warp yarn layers in the fraction 14a of the ' portion 14 adjacent to the non-interlinked zone 16 and . also warp yarns of the layers of warp yarns, in the portion 12 beyond the non-interlinked zone 16. 20 Naturally, the portions 12 and.14 of the fiber structure 10 beyond the non-interlinked zone 16 are themselves interlinked. By way of example, it is possible to adopt a satin weave on the surface for the weft yarns t,, and t,, in the ~ -~. . 25 fractions 12a and 12b that ~~ ~~~ a--r--e sep-a~ra ted ~ by ~ the non- ~ interlinked zone 16, with weaving continuingtwith an .interlink,weave beyond the non-interlinked zone 16. Thus, the paths of the yarns t,, to .t,, and the paths of the yarns t,, to t,, cross in a transition zone 18 that 30 extends from the end 16a of the nbn-interlinked zone 16. In the..weft direction, this transition zone 18.extends over a distance of more than one pitch p between adjacent columns of warp yarns, and preferably of not less than ' 2p. In the example shown, this distance is equal to 4p. 35 In the transition zone 18; the yarns t,, to t,,, like the yarns t15 to t,,, follow similar parallel paths between the ends of the transition zone 18 in the weft direction. A fiber preform 19 of substantially T-shaped profile (Figure 2) is obtained by folding out the fractions 12a and 14a on either side of the non-interlinked zone 16. Because the weft yarns pass through the layers of warp 5 yarns in the transition zone 18 in a progressive manner, the weft yarns are less exposed to any risk of damage in comparison dith crossing more suddenly through a gap between two columns of warp yarns. Furthermore, the fact of having a- transition zone that extends in the weft 10 direction over a length that is relatively long imparts better capacity for deformation. Figure 3 is a plan view of a fiber structure 20 that . has a base portion with an outside face'20a and an inside face 20b. In its thickness, the fiber structure includes 15 two fractions 22 and 24 that are mutually separated over a portion of the dimension of the fiber structure in the weft direction by non-interlinked zones 26 and 26'. The non-interlinked zones 26 and 26' extend from opposite edges 20c and 20d of the fiber structure 20 to respective 20 ends 26a and 26'a of the non-interlinked zones, with the central fraction of the fiber structure 20 not including any non-interlinked zone. Each portion 22'and 24 of the fiber structure has a plurality 'of layers. of warp yarns, the numbers of layers ~~ 25 of warp w~a~~~r~ p~ y- a-~r ns ~ ~~ in the - portions ~ . e~~ - 22 and 24 being di-f f-e rent ~ ~ ~ ~ ~~~ + in this example. - ~ In kach' plane of the-fiber Structure 20, the same first weft yarns t21, ti$, t23, t24 interlink the warp yarns in the portion 24 beyond the non-interlinked zone 26' and 30 aiso interlink the warp yarns in the fraction 22&of the portion 22 beside the non-interlinked zane. Conversely, .the same second weft yarns t25, t26, t27, t28 interlink the warp yarns'in the fraction 22'a of the portion 22 beside the non-interlinked zone 26' and also interlink the warp 35 - yarns in the portion 24 before the non-interlinked zone. Thus, the paths of Lhe yarns tzl, t22, t23, t24 cross the paths of the yarns tzs, tz6, t27, tz8 in a transition zone 28 situated in the central portion of the fiber structure 20 between the ends 26a and 26'a of the noninterlinked zones 26 and 26'. As in the embodiment of Figure 1, the paths of the weft yarns t~l, t22, t~3, tz4 and also the paths of the weft yarns tzs; t26, t27, t28 between the ends of the transition zone 28 are similar, with the transition zone 28 extending over a distance kn the weft direction'that is greater than -p, here equal to 4p. It should be observed that from one weft plane to another, the location of the transition zone may be offset in the weft direction in order to avoid having.any portion with a greater number of yarn crossovers than some other portion between the non-interlinked. zones 26 and 26'. The shaping of the fiber structure 20 in order to obtain a fiber preform 29 of substantially %'shaped structure comprises folding out the fractions.of the portion 24 of the fiber structure beside the noninterlinked zones 26 and 26', as shown in Figure 4, so as to form in section.the legs 24a and 24'a of the n-shape, which legs extend.from the inside face 20b. In the portion of the fiber structure 20 and of the fiber preform 29 adjacent of the outside face 20a, it should be observed that weaving is performed with a satin weave (yarn ~ t29) at ~ t-h e surface ~ so as ~ -t~o ~-p~ provide ~ ~ surface ~~-~p - ~- ~ ~ -- . . continuity without passing through the layers of warp yarns and-without crdssing.any other weft'yarn. In the example shown;it should also.be observed that the fractions of the'portion 24 of the fiber structure 20 that are' to form the lkgs 24a and 24'a extend beyond the edges of the portion 22 by adding columns of warp yarns, so as to impart a desired length to the legs 24a and 24'a. Figure 5 shows, .highly diagrammatically, a singlepiece 3D woven fiber structure 40 in a second embodiment of the invention. Elements that are common between the fiber structure 40 of Figure 4 and the fiber structure 10 of Figure 1 are given the same references and they are not described again. The fiber structure 40 differs from the fiber , structure 10 in the paths followed through the layers of warp yarns by the weft yarns that cross in the transition zone 18. Thus, each weft yarn tIl, t,,, tI3, tI4 passes therethrough over a distance in the weft direction that is equal to the pitch -p between the columns of warp yarns, however the paths of the weft yarns tl, tot,, are mutually offset in the weft direction, with the offset in the example shown being equal to the pitch p. The same A applies to the weft yarns t;,, tI6, tu, and t,,. There is thus a transition zone 18 that, as in the above-described embodiment, extends in the weft direction over a distance that is greater than the pitch - p, specifically over a .distance of 4p. Compared with the embodiment of . Figure 1, greater stress is exerted on the weft yarns as they pass through the transition zone, but the dimension thereof imparts good capacity for deformation. Figure 6 shows a fiber preform 49 of substantially T-shaped section obtained after folding out the fractions 12a and i4a on either side'of the non-interlinked zone 16 of the.portions 12 and 14 of the fiber structure 40. Figure 7 is a plan view of a fiber structure 50 , ' . . ,~ ~ - suitable for obtaining a preform of substantially i -x-shaped section. Elements copon between 'the fiber ' structure 50 of Figure 7 and the fiber structure 20 of Figure 3 are given the same references and they are not ' described again. The fiber .structure 50.. differs from the fiber structure 20 by the paths followed through the layers of warp yarns by the weft yarns. Thus, each of the weft yarns t21, t22, tz3r t24 passes through the layers over a distance in the weft',direction that is equal to the pitch -p between the columns of warp yarns, with the same applying to the paths followed by each of the weft yarnstzs, t26, t2~, t28. Nevertheless, the locations where the yarns t,, to t,, and likewise the locations where the yarns t,, to t,, cross one another on passing through are offset relative to one another in the 5 weft direction. In the.example of Figure 7, the transition zone 28 extends over a relatively long distance between the ends 26a and 26'a of the non- ' interlinked zones 26 and 26', being formed over a plurality of individual transition zones 28,, 28,, 28,, 10 and 28,, with the crossovers thus being distributed in the weft direction over the fraction of the f.iber structure that extends between the non-interlinked zones 26 and 26'. A fiber prefprm of section that is substantially 15 n-shaped is obtained by folding out the fractions of the portion.24 of the fiber structure that are adjacent to the non-interlinked zones 26 and 26', as in Figure 4. Figure 8 is a,highly diagrammatic view of a singlepiece 3D woven fiber structure 60 in a third embodiment 20 of the invention. Elements that are common to the fiber structure 60 of Figure 8 and the structures 10 and 40 of Figures 1 and 5 are given the same references and they are not described again. The fiber. structure 60 diifers froc the fiber 25 structure 10 in that, in each plane, only some of the ~ - ~~. -~ ~ - -- ~ ~ ~ -' .- 4 weft yarns.are concerned by the process of passing through and cr.ossing over, these weft yarns being those that interlink.the warp yarns.of the layers of warp yarns in the fractions of the fiber structure 60 adjacent to . 30 the non-interlinked zone 16, while the warp yarns situated in the fractions of the fiber structure adjacent to its faces 10a and.lOb extend continuously along these surfaces without passing through warp layers or crossing other weft yarns. In this way, it is possible to 35 reinforce the fiber structure at the end of the noninterlinked zone, while preserving surface continuity that encourages a good surface state for a composite material part as finally obtained. In the example shown, the weft yarns t,,, tlZ, t,,, and t,, extend continuously between the edges 10c and 10d 5 of the fiber structure 60 without crossing other weft yarns. In contrast, the weft yarns t,, and t,, in the fraction 12a of the portion 12 of the fiber structure 60 adjacent to the non-interlinke'd zone 16 pass through layers of warp yarns immediately beyond the end 16a of 10 the non-interlinked zone 16 so as to. enter into the portion 14 of the fiber structure 60. Conversely, the weft yarns tl, and t,, in the fraction 14a of the portion 14 of the fibe; structure 60 adjacent to the non- . interlinked zone 16 pass through layers of warp yarns 15 immediately beyond the end 16a of the non-interlinked zone 16, crossing the weft yarns t,, and t,, in order to enter into the portion 12 of the fiber,structure 10. The paths.through the warp yarns and the crossovers with the weft yarns take place in a transition zone 18 that 20 presents a dimension in the weft direction that is greater than the pitch -p between columns of warp yarns, this dimension being equal to 2p in the present example. The configuration with weft yarns extending continuously L close to the faces 10a and lob, and weft yarns involved ~ ~ 25 . . in the process of passing ~ -- t--h r. ough and'crossjng ~ ~ o~ver - - inside the fiber structure 60 is to be found in each. plane of the fiber structure. - ~ ~ ~ Naturally, the number of weft yarns situated in the fractions 12a and 14a adjacent to the non-int6rlinked 30 zone and c0ncerned.b~ the .process of passjing through and crossing over rpay be other than two, and it must be not less than one. Similarly, the number of weft yarns adjacent to the faces 10a and lob and extending contPnuously without crossovers between the edges 10c and 35 lob may be other than two, being at least equal to one. Figure 9 shows a fiber preform 69 of substantially T-shaped section obtained after folding out the fractions 12a and 14a on either side of the non-interlinked zone 16 of the portions 12 and 14 of the fiber structure 60. The weft yarns t,,, t,,, t,,, tle that are not concerned by the process of passing through and crossing over follow a smooth path. through the curved zones. Figure 10 shows a plan of a fiber structure 70 suitable for obtainiAg a fiber preform of substantially z-shaped section. Elements common between the fiber structure 70 of Figure 11 and the fiber structures 2.0 and 50 of Figures 3 and 7 are given the same references and they are not described again. The fiber structure 70 differs from the fiber structures 20 and 50 in particular by the preseLce of a weft.yarn tIzg that extends continuously alodg the inside face 20b and along the faces of the fractions of the portion 24 beside the non-interlinked zones 26 and 26', thereby providing continuity for the surface of the preform on the inside. In addition, the crossovers between the weft yarns take place in two transition zones 28' and 28" that are situated in the immediate proximity of the ends 26a and 26'a of the non-interlinked zones 26 and 26'. Each transition zone extends in the weft direction over a distance that is greater than the pitch p between columns of- t- h~e~ p~~ warp - yarns, ~ ~ specifically over a distance equal to ~ - . ' ~~~ ~~ 2P. , Figure 11 is a diagram of a single-p.iece 3D woven ftber structure 80 in a fourth kmbodiment of the Invention. Elements that are chon between the iiber structure i)0 of Figure 11 and-the structures 10, 40; and " + 60 of Figures 1, 5, and 8 are given-the same rezerences . ~ and they. are not described again. The fiber strscture 80 differs from the fiber structure 60 in that, in each plane, the weft yarns that weave the warp yarns of the layers of warp yarns closest to the faces 10a and lob, specifically the weft yarns t,, and t,, and also the weft yarns t17 and tle, cross over on their paths between the opposite edges 10c and IOd without crossing any other weft yarns. -These crossovers are situated substantially .at the end of the noninterlinked zone 16, i.e. at the connections 12'a and 5 14'a between the fractions 12a and 14a and the remainder of the fiber structure 80, when it is shaped, as shown in Figure 12. The effect of this crossover configuration in the connection zones 12'a and 14'a is for the yarns t,,, t,,, 10 t,,, and tla to present smaller amounts of curvature, i.e. to follow greater radii of curvature, in comparison with the embodiment of Figures 8 and 9. The yarns t,,, t,,, t,,, and t,, are thus less stressed during shaping, in . ' particular when the angle at which the, fraction 12a or 15 L4a is folded out is relatively large. In the various embodiments described, the fiber structure is formed by 3D.weaving with yarns of a nature that is selected as a function of the intended application, e.g. yarns made of glass, of carbon, or of 20 ceramic. After the fiber structure has been shaped, the fiber preform is densified by forming a matrix that is likewise o£ a nature that is selected as a function of the intended application, e. g. an organ'ic matrix obtained in .. I 25 particular from ~~ a ~ r esin that- ~is- , - a p-r ec~-u-r~s or of a polymer ~~ -~ . - . matrix, such as .an epoxy, bismaleimide, or polyimide resin, or a carbon matrix, or'a ceramic matrix. Fbr a -- carbon matrix or a ceramic matrix, densification may be performed by chemical vapor infiltration (CVI) or by 30 impregnatin& with a liquid coriposition that 'contains a carbon or ceramic precursor resin and by applying heat treatment to pyrolize or to ceramize the precursor, which methods are themselves well known. Figure 13 is a highly diagrammatic view of a fan 35 platform 30 for an aviation turbine engine,~the platform being made of composite material of the kind that can be obtained by densifying a fiber preform having a substantially n-shaped section, as shown in Figure 4 or as obtained from the fiber structures of Figures 7 and 10. The fibers are preferably carbon fibers and the matrix is preferably a polymer matrix. 5 . The platform 30 comprises a base 32 having a top face 32a and a bottom face 32b, and two legs 34 and 36 that serbe in particular to form stiffening webs and that extend from the bottom face 32b of the platform 30,' which thus has a n-shaped section, as shown in dashed lines. 10 The platform 30 is designed to be mounted in the gap between two fan blades, in the vicinity of their roots, so as to define the inside of an annular air inlet passage through the fan, which passAge is defined on the outside by a fan casing. The platform 30 is machined to 15 its final dimensions after the fiber preform has been densified. Fiber preforms obtained from fiber structures presenting one or more nonsinterlinked zones and in accordance with the invention may be used for fabricating 20 other composite material parts of aeroengines. Thus, Figure 14 is a highly diagrammatic view of a weft plane of a 3D woven fiber structure 90 that differs from the fiber structure 10 of Figure 1 in particular in that the portio.ns 14 and 16 are separated from each other , ~~ 25 a-l-on g two ~~ ~ non-interl. i.n ked zones 16 and~- ~ 16' -t- hat extend ~ -~~ ~ - I ~ from opposite edges. 10c and 10d of the fiber stricture 90 - ~ . ~ ~ to respective non-interlinked zone ends 16 and 16'a: ,The paths of 18. A hollow prop+ller blade for an aeroengine and made '30. of composite inaterial, the blade being obtained by the . . method of claim 13.