A TURBINE ENGINE BLADE MADE OF COMPOSITE MATERIAL WITH A
BULB-SHAPED ROOT+
Background of the invention
5 The present invention relates to the general field
of turbine engine blades made of composite material and
having a root in the form of a bulb for mounting on a
rotor disk via a dovetail type connection.
The intended field is that of gas turbine blades for
10 aeroengines or for industrial turbines.
Proposals have already been made to make turbine
engine blades out of composite material. By way of
example, reference may be made to International patent
application number WO2010/061140 filed jointly in the
15 names of Snecma and Snecma Propulsion Solide, which
describes fabricating a turbine engine blade by making a
fiber preform by three-dimensional weaving and by
densifying the preform with a matrix.
Furthermore, for mounting such a blade on a rotor
20 disk, it is known to give the blade root the shape of a
bulb. The blade root with its bulb shape co-operates
with a slot of complementary shape formed in the
periphery of the rotor disk in order to retain the blade
radially on the disk by a dovetail type connection.
25 With a blade made of composite material, the bulb
shape of the blade root is generally made during weaving
of the fiber blank that is that constitute the blade by
forming extra thickness in the blade root, this extra
thickness subseguently being machined to the final shape
30 of the bulb. In practice, the extra thickness is usually
obtained by adding an insert during the weaving of the
fiber blank.
Nevertheless, such a method of fabricating a
composite material blade with a bulb-shaped root presents
35 numerous drawbacks. Specifically, making the insert and
t
2
putting it into position during weaving of the fiber
blank for the blade constitute operations that are very
difficult. Also, that method requires the extra
thickness of the blade root to be machined to its final
5 shape, thereby having the consequence of spoiling the
intrinsic properties of the composite material, in
particular by cutting fibers in the bearing surfaces of
the blade root. This causes degradation of the
attachment of the blade in terms of its mechanical
10 strength.
Also known, from document FR 2 941 487, is a
solution for mounting a blade made of composite material
on a rotor disk, in which the blade root is clamped
between metal plates that are fastened by means of a
15 welded peg. With that solution, the main force retaining
the blade on the rotor disk is taken up by shear in the
peg and by compression against the hole in the composite
material. Nevertheless, the difference in expansion
between the metal of the plates and the composite
20 material of the root either gives rise to thermal shear
stresses if the fastening is rigid, or else to
uncertainty about the positioning of the bearing surfaces
if the fastening is made with slack.
25 Obj ect and summary of the invention
A main object of the present invention is thus to
propose a blade made of composite material that is
attached to a rotor disk by a dovetail type connection
without presenting the above-described drawbacks.
30 This object is .achieved by a turbine engine blade
made of composite material comprising fiber reinforcement
obtained by three dimensionally weaving yarns and
densified with a matrix, the blade comprising an airfoil
and a blade root forming a single part, the blade root
35 having two opposite lateral flanks that are substantially
plane, and in which, in accordance with the invention the
blade root is clamped between two independent pads made
3
of composite material, which pads are fastened against
the lateral flanks of the blade root so as to form a
blade root that is bulb-shaped.
The blade root of the invention thus possesses three
5 portions are made of composite material, namely the
plate-shaped root (with its plane lateral flanks), and
the two pads that reconstitute the profile of a bulb.
Such a blade presents numerous advantages.
In terms of manufacture, the blade of the invention
10 is of simplified design since the plate-shaped portion of
the blade root is easier to obtain than is a bulb shape.
This makes it possible to improve the quality of the
blade root and to reduce the cost of fabricating it.
Furthermore, cohesion between the various portions of the
15 blade root is provided via interfaces that are plane, and
that are easy to produce correctly {in terms of
fabrication and of inspection).
In terms of the mechanical strength of the
attachment of such a blade, the pads of the blade root
20 may be fabricated out of composite material that is well
adapted to the very high level of mechanical stress to
which the root is subjected. In particular, it is
preferable to use a material having the highest possible
warp-to-weft ratio, or indeed a material that is
25 unidirectional {i.e. 100% warp, with the warp direction
extending in the longitudinal direction of the blade).
The level of stress that can be accepted by such a
material is considerably higher than that which can be
accepted by a material obtained from a conventionally
30 woven blade root. Furthermore, the pads may be made in
such a manner as to obtain a face of "net" shape on the
outside, i.e. a face without any cut fibers in the zone
that is subjected to the greatest mechanical stress.
Finally, since the various portions of the blade root are
35 made of composite material, there is no differential
expansion between the parts and thus no thermal shear
stress at the interface between these portions.
4
Preferably, each pad comprises a substantially plane
lateral flank for coming into contact with a lateral
flank of the blade root, and an opposite lateral flank
that presents a varying profile reproducing a blade root
5 bearing surface. Under such circumstances, the pads may
be obtained by molding a fiber preform and densifying the
molded preform.
The pads may be fastened against the lateral flanks
of the blade root by brazing, by co-densification, or by
10 matrix deposition.
The blade root and the pads may be made of ceramic
matrix composite material (CMC). Preferably, the pads
are made from fiber reinforcement based on SiC fibers.
The invention also provides a turbine engine
15 including a plurality of blades as defined above.
Brief description of the drawing
Other characteristics and advantages of the presentinvention
appear from the following description made with
20 reference to the accompanying drawings, which shov; an
implementation having no limiting character. In the
figures:
• Figure 1 is a view showing how a turbine engine
blade in accordance with the invention is assembled;
25 • Figure 2 is a perspective view of the Figure 1
blade once assembled;
• Figure 3 is a profile view of the Figure 2 blade;
and
• Figure 4 is a section view of blade roots in
30 accordance with the invention when mounted on a rotor
disk.
Detailed description of the invention
The invention is applicable to various turbine
35 engine blades made of composite material, and in
particular to compressor blades and to turbine blades of
various spools of a gas turbine engine, for example low5
pressure turbine blades such as those shown in Figures 1
to 4.
In known manner, the blade 10 as shown in these
figures comprises an airfoil 12, a root 14 extended by a
5 tang 16, and a platform 18 situated between the tang 16
and the airfoil 12. The blade could also have an outer
platform {not shown) in the vicinity of its free end 20
{or tip).
The airfoil 12 of the blade presents a curved
10 aerodynamic profile that extends (in a longitudinal
direction) from the platform 18 to its tip 20. This
profile is of varying thickness and is made up of a
pressure side surface 12a and a suction side surface 12b
that are connected together transversely by a leading
15 edge 12c and by a trailing edge 12d.
The root 14 of the blade in this example is bulbshaped
and is for mounting in a slot formed in the
periphery of a rotor disk by means of a dovetail type
connection.
20 The blade 10 is made of composite material,
preferably of ceramic matrix composite (CMC) material.
By way of example, reference may be made to International
patent application number WO 2010/061140, which describes
an example of fabricating a turbine engine blade by
25 making a fiber preform by three-dimensional weaving and
by densifying the preform with a matrix.
More particularly, that method provides the making
of a fiber blank as a single piece by three-dimensional
weaving, shaping the fiber blank to obtain a fiber
30 preform as a single piece having a first portion forming
a preform for the airfoil and the root of the blade and
at least one second portion forming a preform for an
inner or outer platform of a blade, and then densifying
the preform with a matrix. The method thus makes it
35 possible to obtain a blade made of composite material
having fiber reinforcement constituted by the preform and
6
densified by the matrix, and forming a single piece with
an incorporated {inner and/or outer) platform.
By virtue of its particular fabrication method, the
blade root 14 presents the shape of a plate (i.e. of a
5 rectangular parallelepiped) with two opposite lateral
flanks 22 that are substantially plane and that are
formed extending the pressure side and suction side
surfaces 12a and 12b respectively of the airfoil 12.
According to the invention, the root 14 of the blade
10 10 is clamped between two independent pads 24 made of
composite material, which pads are fastened against the
lateral flanks 22 of the blade root so as to form a blade
root that is bulb-shaped.
Each of the composite material pads 24 has a lateral
15 flank 26 that is substantially plane (referred to below
as the "plane lateral flank") for coming into plane
contact against a lateral flank 22 of the blade root 14,
and an opposite lateral flank 28 that presents a varying
profile reproducing a blade root bearing surface
20 (referred to below as the "profiled lateral flank").
The pads 24 are preferably obtained by threedimensionally
weaving a fiber blank, followed by molding
the fiber blank in order to obtain a fiber preform for a
place having the desired geometrical shapes, and then
25 densifying the fiber preform with a matrix. In
particular, the lateral face of the fiber blank
corresponding to the profiled lateral flank of the pad is
molded so as to give it the profile of a blade root
bearing surface.
30 The fiber blank is preferably woven so as to present
a warp-to-weft ratio that is as high as possible, or
indeed solely with yarns in the warp direction (the warp
direction corresponding to the longitudinal direction of
the blade that is to be fabricated). As a result, the
35 stress that such a material can accept is considerably
greater than the stress of the material used for
fabricating the blade root.
7
Likewise, because of the molding, the pads may be
made in such a manner as to obtain a profiled lateral
flank 28 of "net" shape, i.e. a face without any cut
fibers. Since this is the zone of the root that is
5 subjected to the highest levels of mechanical stress,
such a net shape method greatly improves the mechanical
strength of the blade root against the rotor disk.
For the composite material, it is preferable to
select a ceramic matrix composite material (as for making
10 the blade). Advantageously, the fiber reinforcement is
based on silicon carbide fiber sold under the name "Hi-
Nicalon® of type S" by the supplier Nippon Carbon Co.,
Ltd. Such fibers present the advantage of locally
imparting excellent mechanical strength to the blade
15 root.
Furthermore, it is possible to apply a specific
surface coating to the profiled lateral flanks 28 of the
pads 24 for the purpose of improving the friction
behavior of the pads. This coating may be different from
20 the coating optionally applied to the airfoil of the
blade. For example, it is possible to apply coatings
having a lubricating function such as graphite,
CoCrAlYSi, or MoS2.
The pads 24 are fastened to the lateral flanks 22 of
25 the blade root 14 by any method known for fastening parts
made of composite material. Thus, it is possible to have
recourse to brazing, to co-densification, to matrix
deposition, or to any other equivalent method. This
fastening takes place between two surfaces that are
30 substantially plane, thereby making it easier to perform.
It should be observed that it is possible to fasten
additional elements to the blade root in the same manner
as for fastening the pads that give the root its bulb
shape. These elements can make it possible to provide
35 the blade with sealing and/or anti-tilting functions,
thereby simplifying the provision of the blade platform.
Alternatively, these elements could be directly
incorporated in the pads.
Figure 4 shows blades 10 as described above that are
mounted in slots 30 (or sockets) formed in the periphery
5 of a rotor disk 32. Typically, such slots extend axially
between the two lateral faces of the rotor disk, and each
is of a shape complementary to the bulb shape of the
blade roots.
In this figure, there can be seen the warp and the
0 weft yarns of the weaving of the fiber blanks for the
blades and for the pads. In particular, compared with
weaving the blade root 14, it can be seen that the fiber
blanks of the pads 24 are advantageously woven solely
with yarns in the warp direction, thereby significantly
5 increasing the stress that these pads can accept.

CLAIMS
1. A turbine engine blade (10) made of composite material
comprising fiber reinforcement obtained by three
dimensionally weaving yarns and densified with a matrix,
5 the blade comprising an airfoil (12) and a blade root
(14) forming a single part, the blade root having two
opposite lateral flanks (22) that are substantially
plane, the blade being characterized in that the blade
root is clamped between two independent pads (24) made of
10 composite material, which pads are fastened against the
lateral flanks of the blade root so as to form a blade
root that is bulb-shaped.
2. A blade according to claim 1, wherein each pad (24)
15 comprises a substantially plane lateral flank (26) for
coming into contact with a lateral flank of the blade
root, and an opposite lateral flank (28) that presents a
varying profile reproducing a blade root bearing surface.
20 3. A blade according to claim 2, wherein the pads (24)
are obtained by molding a fiber preform and densifying
the molded preform.
4. A blade according to any one of claims 1 to 3, wherein
25 the pads (24) are fastened against the lateral flanks of
the blade root by brazing, by co-densification, or by
matrix deposition.
5. A blade according to any one of claims 1 to 4, further
30 including a platform (18) formed integrally with the
airfoil and with the blade root.
6. A blade according to any one of claims 1 to 5, wherein
the blade root (14) and the pads (24) are made of ceramic
35 matrix composite material.
10
7. A blade according to claim 6, wherein the pads (24) .
are made from fiber reinforcement based on SiC fibers.
8. A turbine engine including a plurality of blades (10)
5 according to any one of claims 1 to 7.