Method for manufacturing a mechanical part made of composite material, having a bore formed from two fibrous preforms

Abstract

The invention relates to a method for manufacturing a mechanical part (10) made of composite material which comprises an elongate body and an articulation bore arranged at each end of the body, the method comprising the following steps: - producing a first preform forming a central portion (17) of the mechanical part (10) and comprising two opposing semicircular ends; - producing a second preform forming a belt (18) surrounding the central portion (17) and comprising, at two opposing ends, a semicircular edge; and - assembling the second preform around the first preform so that each semicircular end substantially forms, with the corresponding semicircular edge of the second preform, the articulation bore. According to the invention, the ends of the first preform have a thickened region, and the method comprises a step of machining the thickened region after the first and the second preform have been densified by a matrix.

Description

Description TITLE: METHOD FOR MANUFACTURING A MECHANICAL PART MADE OF COMPOSITE MATERIAL HAVING A BORE FORMED FROM TWO FIBROUS PREFORMS Technical field of the invention

[0001] The present invention relates to the aeronautical field and in particular to a mechanical part made of composite material intended to be articulated with other members at its ends and a method of manufacturing such a part. Technological background

[0002] The prior art includes patent documents FR-A1 -2543054 and CH-A1 - 652176.

[0003] In the aeronautical field, aircraft and / or turbomachines may be equipped with one or more mechanical parts made of composite material articulated at their ends. An example of an articulated mechanical part made of composite material is a connecting rod which is installed in an aircraft landing gear and which is subjected in operation to significant mechanical forces, in particular compression and traction.

[0004] Figure 1 illustrates a connecting rod 1 made of composite material also known as a brake bar. The connecting rod 1 comprises an elongated body 2, along an elongation axis 3, and two articulation bores 4 arranged respectively at each end 5 of the elongated body 2. In particular, the connecting rod 1 is produced using two fiber preforms, a first preform forming a central part 6 of the connecting rod and a second preform forming a belt 7 surrounding the central part. The articulation bores 4 are formed in the first preform. The two preforms are assembled in a mold into which a matrix is ​​injected. The assembly then undergoes densification to obtain the final connecting rod with the bores in the central part.

[0005] However, the interface between the central part 6 and the belt 7 may be damaged due to the different temperature variations that the part undergoes during its manufacturing process. An experimental study has revealed that circumferential cracks are likely to appear and propagate in a circular zone 8 (surrounded by dotted lines) close to the bores 4 at the ends 5 of the connecting rod. The cracks can be due to the deformation of the different parts, for example during cooling following polymerization, because each part has its own coefficient of thermal expansion, which generates significant residual stresses in this area.

[0006] There is a need to address some or all of the above drawbacks. Summary of the invention

[0007] The objective of the present invention is to provide a simple, economical and robust solution, which makes it possible to reduce the risks of cracking of a composite material part during its manufacturing process.

[0008] We achieve this objective in accordance with the invention by means of a method for manufacturing a mechanical part made of composite material, the mechanical part comprising an elongated body between two ends and an articulation bore arranged at each end, the method comprising the following steps of: - production of a first preform intended to form a central part of the mechanical part and comprising two opposite ends each having a semi-circular shape, - production of a second preform intended to form a belt surrounding the central part and comprising at two opposite ends a semi-circular edge, and - assembling the second preform around the first preform so that each semi-circular end forms, substantially, with the corresponding semi-circular edge of the second preform, the articulation bore, the ends having an excess thickness and the method comprising a step of machining the excess thickness occurring after a step of densifying the first and second preforms by a die.

[0009] Thus, this solution makes it possible to achieve the aforementioned objective. In particular, this predetermined excess thickness at the rounded end of the central part or core makes it possible to reduce residual stresses at the mold outlet and to avoid cracking of the matrix between the first preform and the second densified preform. The interface between the central part and the belt of the part is offset at the bore to eliminate the interface that was likely to crack. Furthermore, such a manufacturing process is inexpensive and allows for economic gains through material savings. The properties of the mechanical part are reinforced and its service life extended.

[0010] The method also comprises one or more of the following features and / or steps, taken alone or in combination: - the excess thickness is between 5 mm and 10 mm. - the method comprises a step of impregnating the first and second preforms with a matrix after the step of assembling the second preform and the first preform having the excess thickness at each end. - the first preform and the second preform are obtained by three-dimensional weaving. - in the step of producing the second preform, the latter is obtained by winding several wires or at least one sheet around a tool so as to produce several layers and compacting the stacked layers. - a stack of four to eight layers is obtained after winding. - the second preform is in the form of a single continuous and closed strip. - the layers of the second preform are identical or at least one layer is different from the other layers. Brief description of the figures

[0011] The invention will be better understood, and other aims, details, characteristics and advantages thereof will appear more clearly on reading the detailed explanatory description which follows, of embodiments of the invention given as purely illustrative and non-limiting examples, with reference to the appended schematic drawings in which: - Figure 1 is an axial sectional view of an example of a mechanical part according to the prior art; - Figure 2 is an axial sectional view of an example of a mechanical part made of composite material according to the invention; - Figure 3 is an axial sectional view of another example of embodiment of a mechanical part made of composite material according to the invention; - Figure 4 is a perspective view of an example of a fiber preform intended to form a central part of a mechanical part according to the invention; - Figure 5 is a perspective view of an example of a fiber preform intended to form a belt of a mechanical part according to the invention; - Figure 6 illustrates a flowchart of an example of a method for manufacturing a mechanical part made of composite material according to the invention; - Figure 7 is a perspective and schematic view of an example of tooling for implementing the manufacturing method according to Figure 6; and - Figure 8 is a detailed view of an interface between two parts of a composite material part according to the invention. Detailed description of the invention

[0012] Figure 1 represents a mechanical part which has already been described.

[0013] Figure 2 represents a mechanical part 10 made of composite material obtained by a manufacturing method 100 according to the invention.

[0014] The mechanical part 10 made of composite material is, for example, a landing gear connecting rod, but not exclusively. A landing gear connecting rod made of composite material makes it possible to reduce the mass and optimize its mechanical performance. The mechanical part 10 made of composite material can be used in other types of applications in an aircraft or in an aircraft turbomachine.

[0015] The mechanical part 10 comprises an elongated body 11 extending between a first end 12 and a second end 13 along an elongation axis 14. The mechanical part 10 comprises two articulation bores 15 which are arranged at the first end 12 and the second end 13. The two articulation bores 15 each pass through the elongated body 11 on either side along an axis 16. The axes 16 are parallel to each other and are parallel to a plane perpendicular to the elongation axis 14. The mechanical part 10 is articulated between two members of the turbomachine or of the aircraft around the axes 16.

[0016] In the present embodiment, the articulation bores 15 are cylindrical and have an identical diameter. Alternatively and as shown in FIG. 3, the diameters of the articulation bores 15 are different.

[0017] The mechanical part 10 comprises a central part 17 and a belt 18 which surrounds the central part 17.

[0018] The central part 17 is made from a first fibrous preform 20 (or fibrous reinforcement) which is shown in Figure 4. The first preform 20 is intended to resist compression forces during operation of the mechanical part 10.

[0019] The belt 18 is made from a second fiber preform 21 which is shown in Figure 5. The second preform 21 is intended to withstand tensile forces during operation. The first and second fiber preforms are embedded in a matrix or resin.

[0020] According to a remarkable characteristic, each articulation bore 15 is formed from a first semi-circular portion 17a of the central part 17 and a second semi-circular portion 18a of the belt 18. The first portion 17a and the second portion 18b are circular or semi-circular.

[0021] We will now describe the manufacturing process 100 of the mechanical part. The different stages of the process are illustrated in Figure 6.

[0022] The manufacturing method 100 comprises a step 101 of producing the first fibrous preform 20. The first fibrous preform 20 is advantageously obtained from a three-dimensional weaving (3D weaving).

[0023] In the present invention, we understand by "three-dimensional weaving" a weaving in which certain weft threads and warp threads are linked on several weft layers. Three-dimensional weaving promotes good bonding between the layers and good mechanical strength of the preform and subsequently of the composite material part.

[0024] The fibers used for weaving the first preform are chosen from carbon, aramid, glass or other fibers.

[0025] As illustrated in Figure 4, the first preform 20 is elongated and extends between two opposite ends 20a, 20b. It has a substantially parallelepiped shape. These ends 20a, 20b each have an arcuate or semi-circular shape. The semi-circular shape of the ends 20a, 20b is intended to form a portion of the articulation bore 15 of the part mechanical 10 and in particular the first semi-circular portion 17a of the central part 17. Advantageously, the first preform 20 is woven so as to obtain a predetermined excess thickness SE which is arranged at each end 20a, 20b of semi-circular shape. Advantageously, but not limitingly, the predetermined excess thickness SE extends the end of the part and follows the semi-circular shape while being close to the expected final shape. The predetermined excess thickness SE is between 5 and 10 mm. In the present example, the predetermined excess thickness SE is of the order of 6 mm. A predetermined excess thickness of less than 5 mm could induce complexity in the manufacturing (because continuity of the fibers during weaving is necessary) and a lack of repeatability in the manufacturing process. The mechanical properties would vary from one mechanical part to another, which is not desirable.Conversely, a predetermined excess thickness greater than 10 mm could increase the rigidity and residual stresses in this area, i.e. a risk of cracking. Of course, this predetermined excess thickness depends on several parameters. Examples of parameters are the dimensions and configuration of the mechanical part 10, its stiffness, the orientation of the fibers, the coefficient of expansion of the first and second preforms, the diameter of the articulation bores of the mechanical part to estimate the residual stresses in the interfaces, constraints linked to the manufacture of the part to ensure the continuity of the warp threads, the ease of subsequent machining, etc. The predetermined excess thickness is neither too high nor too low.

[0026] The predetermined excess thickness SE is advantageously obtained using a numerical model.

[0027] The manufacturing method 100 comprises a step 102 of producing the second fiber preform 21. The second fiber preform 21 is also obtained from 3D weaving. In the context of this 3D weaving, the weft and warp threads are wound around a first tool 50 having a substantially oblong shape. Preferably, several layers of a 3D woven sheet are wound around the first tool 50 (along the peripheral surface of the tool). The layers can be produced using several sheets. An example of a first tool 50 is shown schematically in FIG. 7 where a weft thread 51 is wound around a peripheral surface 52 of this one. The shape of the tooling 50 guarantees the shape of the second fiber preform 21, here the shape of the belt 18. In the case of a mechanical part 10 with bores of different sizes, the belt 18 will have a shape corresponding to this difference in dimensions as well as the tooling.

[0028] Advantageously, but not limited to, the weft and warp threads are wound so as to obtain a stack of several layers. The stack may comprise eight layers. Of course, the number of layers may be lower or higher depending on the dimensions of the desired belt 18. The different layers are then compacted to obtain the second preform 21. For this, the stack of the different layers of wound threads is transferred to a second tool (not shown) so as to carry out the compacting. After winding and compacting, the second preform 21 is in the form of a single continuous and closed strip. Such a production step 102 makes it possible to control the thickness of the second preform.

[0029] With reference to Figure 5, the second preform 21 has a generally circular, or even oblong, shape. The second preform 21 comprises two circular edges 21a, 21b which are located at the opposite ends thereof.

[0030] Advantageously, but not limited to, the layers constituting the preform are identical. In particular, the percentage of weft threads and warp threads are identical in each layer. For example, in each layer, there may be a percentage of 70% of warp threads and a percentage of 30% of weft threads. The warp threads provide mechanical properties in the loading direction, which implies that their percentage is greater than that of the weft threads.

[0031] Alternatively, the layers are different. The layers may be different from each other or at least one layer may be different from the others. That is to say, the second preform 21 may comprise a succession of two layers with a percentage of 70% warp threads and 30% weft threads, two layers with a percentage of 50% weft threads and 50% warp threads, and two layers with a percentage of 80% warp threads and 20% weft threads. The compositions of the layers will of course depend on the mechanical characteristics expected in the belt 18. We obtain flexibility in the choice of layers with such a production step 102.

[0032] The manufacturing method 100 comprises a step 103 of assembling the first preform 20 and the second preform 21. The preforms are assembled so that each semi-circular end 20a, 20b of the first preform 20 forms, substantially, with the corresponding circular edge of the second preform 21, the articulation bore 15. Advantageously, the first preform 20 and the second preform 21 are assembled in a mold which has the shape of the mechanical part (before machining).

[0033] Optionally, the method comprises a step of drying the second preform 21 before the assembly step. Beforehand, the first and second preforms 20, 21 are preferably wetted with water and then compacted. The wetting and compacting make it possible to guarantee the volume ratio of the fibers after impregnation with the resin at a predefined level.

[0034] The manufacturing method 100 comprises a step 104 of impregnating the fibers of the first and second preforms 20, 21 with a matrix. Advantageously, this step 104 occurs after the assembly step 103. The matrix is ​​advantageously, but not limited to, an organic matrix. An example of an organic matrix is ​​a polymer matrix such as an epoxy-based resin or a thermoplastic polymer matrix (in addition to a thermosetting polymer such as epoxy bases). Optionally, the impregnation is an injection of the matrix which takes place in a mold (not shown) in which the first and second preforms 20, 21 are previously arranged. The injection can be carried out according to an RTM type molding process (meaning in English “Resin Transfer Molding”). The molding process can be carried out under vacuum.

[0035] The manufacturing method 100 comprises a densification step 105 so as to obtain a rigid mechanical part 10. The densification can be obtained in a device making it possible to raise the temperature which causes the polymerization (or curing) of the matrix. The temperature can reach a value of the order of 200°C concomitantly with a pressure of 2 to 10 bars. With an epoxy-based thermosetting polymer matrix, the polymerization temperature is 180°C and the injection pressure is between 2 and 30 bars during the cycle. This step can also take place in the injection mold of the die.

[0036] The densified mechanical part 10 includes this excess thickness SE which projects inside each bore and along the rounded surface of the bore portion 17a as can be seen in Figure 8. The risk of stress in this area with the excess thickness is almost halved.

[0037] The manufacturing method 100 further comprises a step 106 of machining the excess thickness SE. The machining makes it possible to obtain the final outline corresponding to the second portion of the bore. In particular, the semi-circular end with the excess thickness SE is close to the diameter of the final articulation bore, which implies, on the one hand, a reduction in rigidity in this area and, on the other hand, a minimum of material to be machined.

[0038] In this way, such a mechanical part 10 obtained with the method of the invention can resist shear or cleavage fractures and have a longer service life thanks to a reduction in residual stresses at the interface between the preforms at the bore.

Claims

Claims [1] Method for manufacturing (100) a mechanical part (10) made of composite material, the mechanical part (10) comprising an elongated body (11) between two ends (12, 13) and an articulation bore (15) arranged at each end (12, 13), the method comprising the following steps of: - production (101) of a first preform (20) intended to form a central part (17) of the mechanical part (10) and comprising two opposite ends (20a, 20b) each having a semi-circular shape, - production (102) of a second preform (21) intended to form a belt (18) surrounding the central part (17) and comprising at two opposite ends an edge (21a, 21b) of semi-circular shape, and - assembly (103) of the second preform (21) around the first preform (20) so that each end (20a, 20b) of semi-circular shape forms, substantially, with the corresponding semi-circular edge of the second preform (21), the articulation bore (15), characterized in that the ends (20a, 20b) of the first preform (20) have an excess thickness (SE) and in that the method comprises a step of machining (106) the excess thickness (SE) occurring after a step of densification (105) of the first and second preforms (20, 21) by a die. [2] Method (100) according to the preceding claim, characterized in that the excess thickness (SE) is between 5 mm and 10 mm. [3] Method (100) according to one of the preceding claims, characterized in that the method comprises a step of impregnation (104) of the first and second preforms (20; 21) by a matrix after the step of assembly (103) of the second preform (21) and the first preform (20) having the excess thickness (SE) at each end (20a, 20b). [4] Method (100) according to one of the preceding claims, characterized in that the first preform (20) and the second preform (21) are obtained by three-dimensional weaving. [5] Method (100) according to one of the preceding claims, characterized in that in the production step (102), the second preform (21) is obtained by winding several wires or at least one sheet around a tool (50) so as to produce several layers and compacting the stacked layers. [6] Method (100) according to the preceding claim, characterized in that a stack of six layers is obtained after winding. [7] Method (100) according to one of claims 5 to 6, characterized in that the second preform (21) is in the form of a single continuous and closed strip. [8] Method (100) according to one of claims 5 to 7, characterized in that the layers of the second preform (21) are identical or at least one layer is different from the other layers.

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