IE62818B1

IE62818B1 - High speed composite turbine wheel

Info

Publication number: IE62818B1
Application number: IE309989A
Authority: IE
Inventors: Jean-Pierre Labrouche; Bernard Broquere; Jean-Pierre Parenteau; Jacques Etienne; Alain Lacombe
Original assignee: Europ Propulsion
Priority date: 1988-09-30
Filing date: 1989-09-27
Publication date: 1995-03-08
Also published as: NO893887D0; EP0362074B1; NO893887L; JP2950554B2; CA1315696C; DE68906498T2; FR2637319B1; FR2637319A1; IE893099L; ES2041023T3; PT91839B; PT91839A; ATE89364T1; EP0362074A1; JPH02176102A; DE68906498D1

Abstract

The wheel, comprising a central part or rim equipped with blades on its periphery, is produced in one piece from a composite material consisting of a fibrous reinforcement densified by a matrix. The fibrous reinforcement is formed by means of a helical fabric (20) with radially oriented fibres and circumferentially oriented fibres, the count of fibres in the radial direction and the count of fibres in the circumferential direction varying along a radius of the wheel as a function of the variation of the radial stresses and circumferential stresses, respectively, exerted on the wheel during its use, and at least some of the circumferentially directed fibres in the rim are made of a material having a high mechanical resistance whilst at least some of the circumferentially directed fibres in the blades and of the radially directed fibres, are made of a material having a high resistance to elevated temperatures and to chemical attacks. <IMAGE>

Description

High speed turbine wheel in composite material.

The present invention relates to a composite material turbine wheel designed for high-speed operation, in particular for use in an aeronautical engine. Here, the term high-speed is understood to mean linear peripheral speeds in excess of 500 m/s.

It has already been proposed, e.g. in document FR-A-2,476,766, to produce a one-piece turbine wheel in composite material comprising a central portion, or rim, fitted with peripheral blades. The composite material consists of a fibrous reinforcement densified by a matrix, with the orientation of the reinforcing fibers determined as a function of the stresses exerted on the wheel when in operation.

It has also been proposed. in document EP-A-0 176 386, to use a fibrous reinforcement constituted by two woven helical textures, imbricated in each other, the external diameter of one of the textures being smaller than that of the other texture to form a part which is thicker at the level of the hub. Each helical texture has circumferentially oriented yarns and radially-oriented yarns. The density of the circumferential yarns can vary along a radius of the wheel, for example it can be greater on the small diameter side.

The object of the present invention is to provide a turbine wheel of the above type but having significantly improved performances, both mechanically and as regards resistance to high temperatures and chemical attacks.

This object is achieved by the fact that the fibrous reinforcement is formed by means of a helical fabric with radially oriented fibers and circumferentially oriented fibers, in which according to the invention: - the ratio (r/c) of the density (r) of radially oriented fibers and the rate (c) of circumferentially oriented fibers varies along a radius of the wheel from a value which is smaller than i at the level of the inner diameter of the rim up to a value which is greater than 1 at the level of the outer diameter of the rims in the anchoring zone of the blades, thereafter decreasing as far as the end orf the blades such that the distribution of the · densities of radially oriented fibers and circumferentially oriented fibers along a radius of the wheel varies as a function of the distribution of radial stresses and of circumferential stresses, respectively, exerted on the wheel when said wheel is in operation, and in that at least part of said circumferentially oriented fibers in the rim are made of material having substantial mechanical resistance, while at least part of said circumferentially oriented fibers in the blades and of said radially oriented fibers are made of a material resistant to high temperature and chemical attacks, such that the radially oriented fibers and the circumferentially oriented fibers are distributed along a radius of the wheel as a function of the distribution of the radial and circumferential stresses, respectively which are exerted on the wheel when said wheel is in operation, and - at least part of the circumferentially oriented fibers in the rim are made of a material resistant to mechanical stresses, such as carbon, while at least part of the circumferentially oriented fibers in the blades and of the radially oriented fibers are made of material resistant to high temperatures and to chemical attacks, such as silicon carbide.

The combination of a fiber density that is graded in both a radial direction and a circumferential direction as a function of the stresses exerted in the wheel, with a selection of materials having specific properties adapted to different parts of the wheel, possible to produce a turbine wheel that can high stresses while having good longevity, a chemically and thermally aggressive makes it withstand even in environment.

The invention will be more clearly understood from the following description, given as a non-limiting example, with reference to the attached which : Irawxngs in - Figure i is a diagrammatical view of a turbine wheel, - Figure 2 is a diagrammatical view of a length of helical fabric used for forming the fibrous reinforcement of the composite material constituting the turbine wheel according to the invention, - Figure 3 shows the variations in circumferential and radial stresses as a function of the wheel diameter, - Figure 4 shows a typical stress versus strain curve for a composite material comprising a refractory fiber reinforcement and a ceramic matrix, and - Figure 5 shows the evolution of fiber density in the helical fabric along circumferential and radial directions, taking into account the stresses, whose variation is illustrated in figure 3.

Figure 1 shows a turbine wheel 10 comprising, in conventional manner, a rim 12 in the form of an annular disk whose central portion forms a hub 14, and blades 16 distributed around the periphery of the rim.

In accordance with the invention, the wheel is made from a single piece of composite material whose fibrous reinforcement is obtained by means of a helical fabric, as shown in figure 2. fa Helical fabrics and their manufacturing processes are well known in the art. In the illustrated example, the fabric 20 is made of warp threads 22 oriented in the circumferential direction and weft threads 24 oriented in the radial direction. The density of warp threads from one edge of the fabric to the other can be decreased or increased by spreading or bunching the threads. The density of weft threads from one edge of the fabric to the other, i.e. along a radius, can also be varied by inserting the weft threads over all or part of the width of the warp, not necessarily starting from the edge of the latter.

A preform of the wheel is obtained by tightening together the spirals of the helical fabric, whose number and fiber density correspond to the thickness required for the preform, as shown in figure 2. The outer diameter of the preform is deliberately made greater than that of the finished wheel, inclusive of the blades, to account for the reduction in dimensions resulting from the final machining operation.

According to the invention, the densities of circumferentially oriented fibers and radially oriented fibers vary with the radius so as to be adapted to the stresses exerted on the wheel during its operation.

Figure 3 shows the variations in circumferential and radial stresses in a turbine wheel made of linearly elastic isotropic material, such as that shown in figure 1, having an internal diameter of 33 mm, an external diameter of 220 mm (including blades) and an external rim diameter of 155 mm. /° •a As shown by curve C in figure 3, the circumferential stresses decrease along the radius, starting from the inner radius of the rim with a stronger decrease in the section forming the hub.

In contrast, curve R in figure 3 shows that radial stresses increase from the inner radius of the rim, in the section forming the hub, and thereafter decrease up to the external radius of the rim. A large and abrupt increase in radial stresses is observed at the blade roots, beyond which radial stresses steadily decrease when going to the external radius of the wheel.

The use of a composite material consisting of a refractory ceramic fiber (such as carbon, silicon carbide, alumina, alumina-silica, etc...) and a ceramic or refractory matrix such as silicon carbide, makes it possible to considerably reduce the calculated maximum stresses in the wheel.

Indeed, as is shown in figure 4, the tensile strength curve for such a material reveals a '’plastic88 phase, beyond the elastic phase (zone A), which is generally attributed to a micro-cracking of the matrix. This type of ceramic composite material therefore accommodates local over-stresses without fragile breakage or subsequent propagation of cracks to the rest of the wheel. Such materials make it possible to reduce circumferential stresses at the level of the bore by about 20 to 25%.

Adapting of the fiber density to the stress values in the circumferential and radial directions is achieved by acting on the relative proportions of warp threads (circumferential threads) and weft threads (radial threads) between the inner and outer edges of the helical fabric (i.e. along a radius). In other words, the proportion of warp threads is greater than that of weft threads in areas where circumferential stresses exceed radial stresses, and vice-versa.

Figure 5, which shows a section of helical fabric, indicates how the ratio r/c evolves along a radius, where r is the relative proportion of radial threads and £ is the relative proportion of circumferential threads. In a first zone corresponding to the hub portion of the rim, the ratio r/c is on average equal to 30/70, the stresses at that level being essentially in a circumferential direction. In a second zone, corresponding to the rest of the rim except for its peripheral portions where the blades are rooted, the ratio r/c is on average equal to 50/50. In a third zone, corresponding to part where the blades are rooted in the rim (the base of the blades), the ratio r/c is on average equal to 70/30, the stresses being essentially exerted in a radial direction. Finally, in a fourth zone corresponding to the blades outside the rim, the ratio r/c progresses from 70/30 to 33/6G, this being naturally achieved with the use of radial threads that extend throughout the length of the blades, without returning in between (whence a gradual decrease in the radial thread density) and with regularly spaced circumferential threads (hence a substantially constant circumferential thread density). It will be understood that there is no sudden discontinuity in the ratio r/c when going from one zone to the other, the changes in this ratio being progressive.

The absolute values for fiber densities t in a r radial direction and fiber densities t in a c circumferential direction are chosen to provide the finished product with the mechanical resistance required to withstand the stresses exerted thereon. For instance, the density of circumferential fibers at the level of the inner diameter will be chosen to make the latter withstand the circumferential forces at that level. For the rest of the wheel, the density values i and t are chosen so that they satisfy the predetermined evolution of the ratio r/c. It should be ensured that there is a sufficient density of radial or circumferential fibers in the most exposed zones; to this end, there should at least be a minimum density of radial fibers at the blade root base to ensure good anchoring of the blades. What is meant here by fiber density for a zone, is the percentage of that zone occupied by the fibers.

According to another characteristic of the invention, the fibers are chosen so as to present properties adapted to the operating conditions of the wheel.

When the wheel is active, especially in a turbojet, it is exposed, particularly at its peripheral portion, to high temperatures and to chemical attacks. Consequently, the radial fibers, which can extend up to the external radius, and the circumferential fibers in the part corresponding to the blades, are chosen to be at least partially made of material that is above all capable of withstanding high temperatures and chemical attacks, such as for example silicon carbide, even though its mechanical characteristics are inferior to those of carbon threads. On the other hand, in the rim, where circumferential stresses are high but where thermal and chemical attacks are less severe, the circumferential fibers are selected to be at least partially made of a material that is above all capable of withstanding high mechanical stresses, such as carbon, even though it does not resist so well to high temperatures and to chemical attacks as silicon carbide. Therefore, the helical fabric is woven so that the warp fibers are made for example of carbon for the part corresponding to the rim, and of silicon carbide for the part corresponding to the blades, while the weft threads are made of silicon carbide.

Once the wheel preform has been produced by tightening of the spirals of the helical fabric, as 4) explained above, it is kept in shape by impregnation with a fugitive resin so as to be machinable in view of obtaining a wheel blank. The latter is then into a tool for densification by the introduced material matrix . constituting Densification infiltration the composite material is preferably achieved by chemical vapor of the matrix material, for example with silicon carbide. Chemical vapor infiltration of silicon carbide is a well known process, in particular described in document FR-A-2 401 888. The fugitive resin is in this case eliminated during the temperature rise preceding the infiltration, with the blank being held by the tool. When the densification is completed, the wheel is machined to its final dimensions.

Claims

1. High-speed turbine wheel comprising a central portion, or rim, provided with blades at its peripheral portion and made of a single piece of composite material formed by a fibrous reinforcement densified by a matrix, said fibrous reinforcement being formed by means of a helical fabric (20) with radially oriented fibers and circumferentiallv oriented fibers, characterized in that the ratio (r/c) of the density (r) of radially oriented fibers to the density (c) of circumferentially oriented fibers varies along a radius of the wheel from a value which is smaller than i at the level of the inner diameter of the rim up to a value which is greater than 1 at the level of the outer diameter of the rim, in the anchoring zone of the blades, thereafter decreasing as far as the end of the blades such that the distribution of the densities of radially oriented fibers and circumferentially oriented fibers along a radius of the wheel varies as a function of the distribution of radial stresses and of circumferential stresses, respectively, which are exerted on the wheel when said wheel is in operation, and in that at least part of said circumferentially oriented fibers in the rim are made of a material which is resistant to mechanical stresses, while at least part of said circumferentially oriented fibers in the blades and of said radially oriented fibers are made of a material having substantial resistance to high temperatures and to chemical attacks.

2. Wheel according to claim 1, characterized in that at least part of said circumferentially oriented fibers in the rim are made of carbon.

3. Wheel according to any one of claims 1 and 2, characterized in that at least part of said 4. ¾ circumferentially oriented fibers in the blades and of said radially oriented fibers are made of silicon carbide.

4. iaShee.i according to any one of claims 1 to 3. characterized in that the matrix is made of silicon carbide. \

5. , A high speed turbine wheel according to claim 1„ substantially as. hereinbefore described with particular i reference to and/' as illustrated in the accompanying drawings