FR2655976A1 - Process for the production of a composite structure, strip of staples of composite material enabling the process to be implemented and process for the manufacture of the strip of staples - Google Patents

Abstract

A composite structure consisting of a fibrous reinforcing texture densified with a matrix is obtained by a process comprising the stages which consist in separately producing the constituent elements of the fibrous texture for reinforcing the structure, assembling the texture elements, and densifying the assembled texture, using the constituent material of the matrix, in which process the assembly of the texture elements is produced at least partially by stapling by means of staples made of composite material. The staples are available in the form of a strip of staples (10) of U-shaped cross-section, consisting of a fibrous reinforcing texture densified with a matrix and comprising weakened or precut parts (14, 15, 16, 18) which define breakage regions making it easier to separate the strip of staples into individual staples.

Description

 The present invention relates to the production of composite structures, that is to say structures made up of a fibrous reinforcement texture densified by a matrix.

For the production of composite structures, in particular
Binding of a structure of large dimension or of complex shape, it is known to use a process comprising the steps which consist in carrying out separately the elements constituting the fibrous texture of reinforcement of the structure, assembling the elements of texture, and densifying The texture assembled, by the material constituting the matrix
Such a process is, for example, used for the production of composite structures made up of a section and stiffeners.

 The texture elements are for example produced by stacking layers. These can be bidirectional (fabrics), unidirectional (sheets of threads parallel to each other), or multilayer (such as an assembly of layers of fabric and pre-sharpened felt). The stacked strata can be linked between them by sharpening or setting up threads in one or more transverse directions relative to the strata.

The texture elements can be assembled in the dry state or prepreg. In the latter case, the impregnation is carried out, for example, by means of a resin making it possible to shape and to maintain the shape of the texture elements. The resin can be a fugitive resin, that is to say which disappears without leaving a residue under the effect of the rise in temperature produced during densification by the matrix. It is also possible to use a resin Leaving a residue compatible with
The material of the matrix.

 Densification is generally carried out by chemical infiltration in the vapor phase, the dry or prepreg texture elements being kept assembled by means of a tool or conformation preferably in graphite until consolidation of your texture. Several infiltration cycles are generally necessary, including after removal of the tool, when the texture is consolidated to reach the desired densification rate, that is to say limit the residual porosity to the desired value.

 It has been noted that the composite structure obtained after removal from the tool does not offer a total guarantee of non-cracking at the interface between the texture elements, whether during final densification or, subsequently, during of the use of the texture.

 Also, according to a first aspect, the invention aims to propose a method allowing the production of a composite structure by co-densification of assembled fibrous texture elements, the assembly of the texture elements being carried out so to strengthen the connections between them and to avoid further cracking.

 This object is achieved, in accordance with the invention, because the assembly of the texture elements is carried out at least in part by stapling by means of staples made of composite material.

 According to another aspect, the invention proposes a strip of staples with a U-shaped transverse section characterized in that it is constituted by a fibrous reinforcement texture densified by a matrix and comprises weakening parts or precuts which define zones. breaking facilitating the separation of the strip of staples into individual staples.

 Advantageously, the aids of the strip of staples are shaped so as to give the stems individual staples intended to penetrate elements to assemble a relief capable of promoting their anchoring in textures.

 According to yet another aspect, the invention provides a process for manufacturing a strip of staples made of composite material, a process characterized in that it comprises the steps which consist in producing a fibrous reinforcement texture in the form of a flat strip, shaping the reinforcing texture to produce a preform of strip of staples with a U-shaped cross section, densifying the fibrous texture with a material forming a matrix, and forming by machining weakening parts or precuts intended to constitute zones of break facilitating the separation of the strip of staples into individual staples.

 The invention will be better understood on reading the description given below, by way of indication but not limitation, with reference to the appended drawings in which: - Figure 1 schematically illustrates a strip of staples according to the invention, - Figure 2 is a detail view on an enlarged scale of a section of the strip of staples of Figure 1, - Figure 3 shows schematically, in section, two elements assembled by means of a staple from the strip d staples of Figure 1, - Figures 4A to 4E illustrate the successive stages of manufacturing a strip of staples of composite material such as that of Figure 1, - Figures 5 to 8 illustrate different possibilities of machining ends of the stems of the staples aimed at promoting the deformation of the stems during their penetration into textures to be assembled, - Figures 9 and 10 illustrate various non-limiting possibilities and by way of example of the formation of cutouts d in the wings of the staple strip to facilitate the separation into individual staples and give the stems thereof a relief favoring their anchoring in textures to be assembled, - Figures 11 and 12 illustrate examples of assembly of elements of fibrous reinforcement texture by means of staples, in accordance with the invention.

 FIG. 1 illustrates an example of a strip of staples 10 made of composite material according to the invention. Conventionally, this strip of staples has a U-shaped cross section, the wings 12 of the U corresponding to the rods 22 of the staples 20. The dimensions of the strip 10 can be similar to those of the standard metal staple strips. Likewise, the strip 10 contains a number of staples which is similar to that contained in conventional metal staple strips. Thus, the staple strip 10 can be used with conventional mechanical or electrical staplers.

 As described below in detail, the strip 10 is constituted by a composite structure in one piece in which are made weakenings and / or precuts which define break zones facilitating the separation into individual staples.

 Thus, as shown in FIG. 2, notches 14 are formed at regular intervals in the longitudinal edges of the wings 12 in order to separate the ends 24 of the rods 22 from each other, the shape of the notches 14 being determined to give the ends 24 the desired shape. In addition, still at the rupture zones between staples 20, are formed holes 15 which pass through the wings 12 and external grooves 16. These can be located in the bent parts connecting the wings 12 to the rest of the strip 10 , or in the wings 12 or in the central part of the strip 10. The grooves 16 are formed on only part of the thickness of the strip 10. It is however possible to produce deeper grooves, which pass through the strip 10 throughout its thickness over at least part of their length. Finally, corrugations 18 are formed by longitudinal grooves on the outer side of the wings 12.

 FIG. 3 shows a clip 20 after it has been placed in elements to be assembled 30, 32. These are for example in the form of overlapping strips or plates. Of course, the elements 30, 32 are made of a material with a hardness substantially lower than that of the composite material constituting the strip of staples 10.

According to a preferential application of the invention, the elements 30, 32 are elements of dry or prepreg fibrous texture intended to constitute a reinforcement of a part made of composite material.

 The separation of a clip 20 from the rest of the strip 10 and its positioning in the elements to be assembled 30, 32 are conventionally carried out by exerting a force P on the base of the clip (see FIG. 3).

 The ends 24 of the rods 22 are for example tapered and bevelled, their length being greater on the inside of the wings 12 than on the outside (see FIG. 2). In this way, when the staple is put in place, the rods 22 bend towards one another under the effect of the reaction of the elements 30, 32 to the force P. This bending promotes the anchoring of the rods 22 in the elements 30, 32, all the more since it is combined with the presence of the reliefs 28 formed by the corrugations 18. In fact, the deflection of the rods 22 being produced within the limit of elastic deformation, the rods exert a force tending to penetrate the reliefs 28 in the materials constituting the elements 30,32, causing a blockage when the staple is torn off.

 Notches 25 located laterally on the rods 22, and coming from the formation of the holes 15, also contribute to the anchoring of the rods 22, giving them a harpoon shape.

In the example illustrated in FIG. 2, the number of holes 15 in each wing 12, for each rupture zone, is equal to one.

Of course, this number can be greater than one, in which case it can be envisaged to do without the grooves 16.

 In certain cases, if the blocking by the reliefs 28 is sufficient, and if the notches 14 and grooves 16 are sufficient to allow the staples to be easily separated, it is not necessary to provide holes 15. Conversely, if the blocking by the notches 25 is sufficient, it is not necessary to provide the reliefs 28. In this way, strips of staples can be produced which do not include holes 15, or no corrugations 18.

 An embodiment of a strip of staples such as that illustrated in FIGS. 1 and 2 will now be described with reference to FIGS. 4A to 4E.

 The strip of staples is made of a composite material made up of a fibrous reinforcement texture densified by a matrix.

 The reinforcing texture 40 is for example formed by filament winding around a mandrel 42 of rectangular section (FIG. 4A). This has a side whose length corresponds to the width of the central part of a strip of staples and an adjacent side whose length corresponds to twice the width of a wing of a strip of staples, so that the preforms of two strips of staples opposite each other are formed simultaneously. The winding around the mandrel is made so that the direction of the wires corresponds to that of the staples, that is to say perpendicular to the longitudinal direction of the strip of staples.

 The reinforcing texture 40 is placed in an enclosure in order to densify the texture with the material constituting the matrix. This densification is carried out for example by chemical vapor infiltration, which is continued until the desired degree of densification is obtained.

 After infiltration, the densified texture 44 is separated from the mandrel 42 (FIG. 4B) then by cutting it into two equal parts, each constituting a strip 46 with a rough surface state of infiltration (FIG. 4C). In order to facilitate the separation between the densified texture and the mandrel, the winding on the latter may advantageously have been carried out with the interposition of a layer, such as a layer of fabric, of a material having a different coefficient of thermal expansion those of the mandrel and the reinforcement texture.

The use of such an interposition layer is described in French patent application No. 89 15929 in the name of the applicant.

 The strip 46 is then machined to the desired final shape.

 Machining consists in making finishing passes to give the corrugations 18 the desired shape, for example that illustrated in FIG. 4D or, as a variant, that illustrated in FIG. 4E.

 The machining also consists in making the notches 14, the holes 15 and the grooves 16 to result in a strip as illustrated in FIG. 2. For this purpose, it is advantageous to use the technique of cutting by water jet at very high pressure, or the laser cutting technique.

The composite material constituting the staple strip is chosen according to the intended use. As part of the application to the assembly of texture elements intended for the manufacture of thermostructural composite parts, generally carbon-carbon, reinforcing texture in carbon fibers and matrix essentially in carbon, or in carbon-ceramic, or ceramic-ceramic, the staple strip is preferably also carbon-carbon, carbon-ceramic or ceramic-ceramic, respectively, in order to avoid problems caused by differential expansions. The fibers constituting the texture 40 are then carbon fibers, or ceramic fibers, for example silicon carbide. The densification of the texture 40 is then essentially carried out with pyrocarbon or a ceramic such as silicon carbide. The techniques of infiltration in the vapor phase of carbon or other refractory materials are well known. By way of example, reference may be made to documents FR 2 189 207,
FR 2 401 888 and EP 0 172 082.

 As already indicated, the machining of the notches 14 is carried out so as to give the ends 24 of the rods 22 the desired shape, in particular so that their penetration into the elements to be assembled is accompanied by bending of the rods 22.

 Figures 5 to 8 illustrate alternative embodiments of the ends 24 other than that shown in Figure 2 In these figures, the notches 25 and / or reliefs 28 are not shown, for the sake of simplification.

 In the case of FIG. 5, the end 24 is not tapered, but is bevelled with a longer length of the rod 22 on the inner side, which causes, when the staple is put in place, a bending of the rod inward, toward the other rod. When the bevel is made on the inner side, with a longer type length on the outer side (Figure 6), the bending of the rod occurs in the opposite direction, outward.

 In the case of FIGS. 7 and 8, the end 24 of the rod, not tapered or tapered, is beveled laterally, so that the deformation of the rod occurs by twisting in a direction perpendicular to the base of the staple. The bevels on the ends of the two stems of a staple are then formed on one and the other side of the staple, respectively, so that the twists of the stems occur in opposite directions to each other .

 FIGS. 9 and 10 illustrate non-limiting variants and by way of example of embodiment of holes in the wings of the strip of staples, other than that shown in FIG. 2.

 In the case of FIG. 9, several holes 15, 15 ′ are formed in each wing at the level of each rupture zone, at least one of the holes (15 ′) having a direction not perpendicular to the wing of the clip.

 In the case of FIG. 10, a single bore 15 is formed in each wing, at the level of each rupture zone, this bore having an oblong shape, elongated in the direction of the rupture zone.

 The staple strip according to the invention is more particularly intended for the assembly of dry or prepreg fibrous texture elements which have been manufactured separately, this in order to constitute a reinforcement texture for obtaining a part made of composite material, in particular a large part and / or of complex shape, by densification of the reinforcing texture.

 FIG. 11 shows the stapling of fibrous texture elements 52 pre-impregnated in the form of L-section profiles on a plane texture element 50 of relatively small thickness, with a view to producing a part constituted by a web provided with stiffeners . In this case, the use of composite staples does not completely dispense with a holding tool during the densification of the assembled texture, but aims to prevent cracking at the interfaces between assembled texture elements.

 FIG. 12 shows a graphite mandrel 60 on which is wound, stretched, a strip of fabric 62 whose free end is stapled to the wound texture. This arrangement makes it possible to obtain a tube of composite material after densification and after removal of the mandrel.

 The composite staples provide a reinforcement of the connections between the texture elements which prevents the appearance of cracks at the interfaces between these elements during their codensification by the material constituting the matrix of the composite material of the part to be produced, or during the use of it.

 The composite staples can be used as the only means of connection between the texture elements, or in addition to other means of connection (for example implantation of wires).

Claims (10)

 1. Strip of staples with U-shaped cross section, characterized in that it is constituted by a fibrous reinforcement texture densified by a matrix and comprises weakening parts or precuts which define rupture zones facilitating the separation of the strip d staples into individual staples.  2. strip of staples according to claim 1, characterized in that the wings of the strip of staples are shaped so as to give the rods individual staples intended to penetrate elements to assemble a relief suitable for promoting their anchoring in textures.  3. strip of staples according to any one of claims 1 and 2, characterized in that the edges of the wings of the strip of staples have regularly distributed notches, located at the rupture zones and whose shape defines that the ends of the staples of the staples.  4. Strip of staples according to claim 3, characterized in that the ends of the stems of the staples are bevelled so as to induce a bending deformation of the stems of the staples during penetration of the latter into the elements to be assembled.  5. Strip of staples according to any one of claims 1 to 4, characterized in that the wings have longitudinal undulations on their outer face.  6. Strip of staples according to any one of claims 1 to 5, characterized in that the wings have holes at the rupture zones.  7. strip of staples according to any one of claims 1 to 6, characterized in that the fibrous reinforcing texture is made of a material chosen from carbon and ceramics and the matrix is essentially made of a material chosen from carbon and ceramics.  8. A method of manufacturing a strip of staples made of composite material, according to any one of claims 1 to 7, characterized in that it comprises the steps which consist in producing a fibrous reinforcement texture making it possible to obtain a staple strip preform, densify the fibrous texture with a material forming a matrix, and obtain the staple strip by machining in particular to form weakening parts or precuts intended to constitute rupture zones facilitating the separation of the strip from staples to individual staples.  9. Method according to claim 8, characterized in that the fibrous reinforcement texture is produced by filament winding on a mandrel of rectangular section in order to obtain preforms of two strips of staples opposite to each other.  10. Method for producing a composite structure made up of a fibrous reinforcement texture densified by a matrix, method comprising the steps which consist in producing separately the constituent elements of the fibrous reinforcement texture of the structure, assembling the texture elements, and densifying the assembled texture, by the material constituting the matrix, characterized in that the assembly of the texture elements is carried out at least in part by stapling by means of staples of composite material coming from a strip of staples according to any one of claims 1 to 7.