Abstract:
The piece made of composite material comprises a plate and a plurality of stiffeners integrally formed with the plate. A metal wall is fixed to the edges of the stiffeners and is provided with passages for circulating the fluid. The plate exposed to the heat flux radiates towards the metal wall through the gaps between the stiffeners, and it also performs the structural function. The invention can be used with structures that are exposed to very high heat flux: the walls of jet combustion chambers, the walls of space plane vehicles, the walls of nuclear fusion reactors.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a structural part including a portion that is made of thermostructural material cooled by fluid circulation. 
     The invention finds applications in making structures that are exposed in operation to very high heat flux or to aggressive environmental conditions at very high temperatures. It relates in particular to wall structures that are exposed to hot gas flows, e.g. wall structures for the combustion chambers of jet or rocket engines, or wall structures exposed to intense heating, e.g. wall structures of spaceplane vehicles, particularly during atmosphere reentry stages, or indeed wall structures that form heat screens, e.g. in nuclear fusion reactors. 
     Because of their mechanical properties which make them suitable for constituting structural elements, and because of their capacity to retain these properties up to high temperatures, thermostructural composite materials are commonly used in such applications. Specifically, the materials are carbon-carbon (C/C) composite materials comprising carbon fiber reinforcement densified with a carbon matrix, and ceramic matrix composites (CMCs) comprising reinforcement of refractory fibers (e.g. carbon or ceramic) densified by a ceramic matrix. 
     When thermostructural composite materials are used under very severe environmental conditions (intense heat flux or an atmosphere that is aggressive, in particular that is oxidizing), it is necessary to limit the surface temperature thereof in order to obtain satisfactory lifetime. 
     A conventional method of cooling consists in circulating a cooling or “heat-conveying” fluid. 
     Proposals have been made to form fluid circulation passages within a composite material or in its rear face. Because thermostructural composite materials inevitably present residual porosity because of the way in which they are made, it is necessary to guarantee fluid tightness for the walls of the fluid circulation passages. Reference can be made for example to U.S. Pat. No. 5,583,895. 
     Proposals have also been made to connect a piece of composite material to a substrate made of metal, e.g. copper, which is cooled by a circulating cooling fluid. A problem which is difficult to solve is that of bonding together two parts that have coefficients of thermal expansion that are significantly different from each other. Reference can be made in particular to document WO 98/03297. 
     Those parts of known structure therefore present difficult problems of preparation. In addition, they are relatively heavy, which at least in certain applications constitutes a severe drawback. 
     OBJECT AND BRIEF SUMMARY OF THE INVENTION 
     An object of the invention is to provide a structural part having a piece of thermostructural composite material cooled by fluid circulation and which can be prepared without raising the same difficulties as in the prior art. 
     Another object of the invention is to propose such a part which is substantially lighter than in the prior art. 
     These objects are achieved with a structural part comprising a piece of thermostructural composite material connected to a metal wall provided with fluid circulation passages, in which part the composite material piece comprises a plate and a plurality of stiffeners formed integrally with the plate and situated on one side thereof, being spaced apart from one another, and the metal wall is fixed to the end edges of the stiffeners. 
     Thus, the piece of thermostructural composite material withstands heat flux and mechanical loads, and radiates from its rear face towards the metal piece through the gaps between the stiffeners. In contrast with the prior art, this decoupling between thermal and mechanical effects simplifies manufacture of the structural part. 
     In particular, the following advantages are obtained: 
     the plate of thermostructural composite material stiffened by the stiffeners can itself be thin, so as to constitute merely a skin, imparting lightness to the assembly while still performing its structural function; 
     since the plate of composite material is exposed to the heat flux via its front face opposite from its face provided with the stiffeners, heat transfer takes place by conduction through the plate, by radiation in the gaps situated between the stiffeners from the back face of the plate to the metal wall, and by forced convection of the fluid travelling in the passages of the metal wall; 
     the metal wall and the piece of thermostructural composite material are assembled together solely via the end edges of the stiffeners, i.e. in discontinuous manner and at the locations that are furthest from the plate exposed to the heat flux; as a result, thermomechanical stresses are minimized and assembly is simplified; 
     since the structural function is performed by the piece made of thermostructural composite material, it is possible to use a thin metal wall having fluid circulation pipes fitted thereto, thus minimizing the overall mass of the part; and 
     the piece made of thermostructural composite material and the metal wall can be assembled together during final assembly of the part, e.g. by brazing or mechanically, thereby making it possible to manufacture structures of complex shape, e.g. having dual curvature. 
     The invention also seeks to provide a method of manufacturing a part having the structure as defined above. 
     In the method, a fiber preform for the composite material piece is made by assembling together preform elements corresponding to the plate and to the stiffeners, the preform is densified so as to obtain the composite material piece, and the metal wall is fixed to the end edges of the stiffeners. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following description of embodiments of the invention given by way of non-limiting indication, reference is made to the accompanying drawings, in which: 
     FIG. 1 is a cross-section view through an embodiment of a structural part in accordance with the invention; 
     FIG. 2 is a cross-section view showing a variant embodiment of the FIG. 1 part; 
     FIGS. 3A to  3 C are diagrams showing successive steps in a first implementation of a method for preparing the piece made of thermostructural composite material of a structural part such as that shown in FIG. 1; 
     FIGS. 4A and 4B are diagrams showing the successive steps in a second implementation of a method for preparing the piece made of thermostructural composite material for a structural part such as that of FIG. 1; and 
     FIGS. 5A and 5B are diagrams showing successive steps in a third implementation of a method of preparing the piece made of thermostructural composite material for a structural part. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is a cross-section through a structural part comprising a thin plate or skin  10  of composite material, stiffeners  12  that are integral with the skin  10 , that are situated on the rear face  10   a  thereof, and that are spaced apart from one another, and a metal wall  14  provided with passages  16  for circulating a cooling fluid, the metal wall being fixed on the end edges of the stiffeners. 
     The piece comprising the plate  10  and the stiffeners  12  is made of a thermostructural composite material, in particular a C/C or a CMC composite material, e.g. a C/SiC composite material comprising carbon fiber reinforcement densified by a matrix of silicon carbide or essentially of silicon carbide. The presence of the stiffeners  12  makes it possible to impart a structural character to the structural part while nevertheless having a plate  10  and a metal wall  14  that are relatively thin, thereby ensuring that the final assembly thereof is of very low mass. 
     In the example shown, the stiffeners  12  are in the form of mutually parallel rectilinear ribs leaving gaps  20  between one another. 
     By way of example, the metal plate  14  is a thin plate of copper with pipes  18  forming the passages  16  being fixed to the rear face  14   a  thereof by welding. 
     In the example of FIG. 1, the metal wall  14  is fixed to the end edges of the stiffeners  12  by brazing. To this end, a layer of brazing alloy  22  is deposited on the end edges of the stiffeners, the metal wall  14  together with the pipes  18  is brought into contact with the stiffeners having the brazing alloy thereon, and the assembly is heated. 
     The discontinuity of the bonding to the metal wall and the distance between the bonding zone and the skin  10  which is exposed to the heat flux limit the thermo-mechanical stresses that result from the difference between the coefficients of thermal expansion of the thermostructural composite material constituting the stiffeners  12  and the metal constituting the wall  14 , thereby simplifying the brazing process. In particular, it need not be necessary to metallize the edges of the stiffeners in order to improve adhesion of the brazing alloy. 
     Amongst the brazing alloys that can be used, mention can be made, for example, of the alloys sold under the name “Cu-ABA” or TiCuSil by the US company Wesgo, Inc. 
     In the example of FIG. 2, the metal wall  14  is fixed to the end edges of the stiffeners  12  by screws  24 . By way of example, the screws can be made of a refractory metal, e.g. molybdenum or a molybdenum alloy, stainless steel, an alloy of nickel and cobalt, or a ceramic material. In order to be able to receive the screws  24 , the end portions  12   a  of the stiffeners are preferably enlarged. 
     In use, the structural part is exposed to heat flux via the front face  10   a  of the skin  10 . Heat transfer takes place by conduction through the skin  10  and then by radiation in the gaps  20  between the stiffeners  12  and towards the wall  14 , with the heat then being taken up by the cooling fluid circulating in the passages  16 . 
     There is practically no conduction along the stiffeners  12 . In addition, because the skin  10  is thin, the material constituting the skin  10  and the stiffeners can be selected from materials that do not necessarily have a high coefficient of thermal conductivity. Depending on the application, and in particular depending on environmental conditions, it is thus possible to select C/C composite materials or CMCs, with CMCs being particularly suitable for use in an oxidizing atmosphere. Suitable CMCs include composite materials of C/SiC types. 
     FIGS. 3A to  3 C show a first implementation of a method of making an assembly comprising the skin  10  and the stiffeners  12 . 
     Fiber preforms  10 ′ and  12 ′ (FIGS. 3A and 3B) are made separately for the skin  10  and for the stiffeners  12 . 
     By way of example, the preforms  10 ′ are obtained by superposing plies of fiber fabric, such as plies of cloth or felt, or unidirectional sheets of threads or cables superposed in different directions. The plies can be bonded together, e.g. by needling performed transversely relative to the plies. 
     The preforms  12 ′ have a T-shaped cross-section. They can also be made by superposing plies of fiber fabric, with the plies being bent though a right angle. The preforms  12 ′ can be kept in shape by consolidation. To this end, the plies are impregnated with a binder and they are maintained in the desired shape by means of tooling until the binder has consolidated. By way of example the binder can be a resin. The quantity of binder used is selected to be just sufficient to give the preforms enough mechanical strength to enable them to be handled. 
     The preforms  12 ′ are assembled to the preform  10 ′ (which could also be consolidated), e.g. by sewing or stitching (FIG.  3 C). 
     Thereafter, the preforms are densified with the matrix of the thermostructural composite material. Densification techniques making use of gas (chemical vapor infiltration) or of liquid (impregnation with a precursor for the matrix followed by heat treatment) are well known for making C/C and CMC composites. The binder used for consolidating the preforms  12 ′ and possibly also the preform  10 ′ is either eliminated by the heat used during densification, or else it is transformed into a carbon residue, e.g. when the binder is a resin having a non-zero coke content. 
     FIGS. 4A and 4B show another implementation of the assembly comprising the skin  10  and the stiffeners  12 . 
     A fiber preform  13 ′ having both preform portions for the skin  10  and for the stiffeners  12  is made directly by stacking plies of fiber fabric to which the necessary shapes and dimensions have been imparted (FIG.  4 A). 
     The preform  13 ′ can be consolidated mechanically by sewing and stitching (FIG. 4B) prior to being densified by the matrix of the composite material, or it can be densified directly. At least at the beginning of the densification process, the preform  13 ′ is held in shape by means of tooling. 
     Finally, FIGS. 5A and 5B show another implementation of the assembly comprising the skin  10  and the stiffeners  12 . 
     A blank  15  in the form of a thick plate of thermo-structural composite material is made (FIG. 5A) by densifying plies of a fiber preform  16  made up of fiber fabric that is optionally bonded together by needling. In the example shown, the plies  16  are superposed flat. 
     The stiffeners  12  are formed by machining the gaps  20  (FIG.  5 B). 
     This implementation presents the advantage of making the preform for the thermostructural composite material easier to manufacture, but has the drawback of giving rise to a large loss of material.