Turbomachine blade made of composite material

A turbomachine blade, particularly a fan blade for an aircraft jet engine, is made of a composite material comprising reinforcing fibres embedded in a thermosetting resin matrix, wherein the reinforcing fibres form a multilayer woven fabric in which the warp fibres of each layer are interwoven not only with the weft fibres of the layer but also with the weft fibres of at least the layers immediately above and below it so that the layers are all interconnected. Also, each layer only partly covers the layer below it so that the ends of the warp and weft fibres of a layer extend to near the surface of the blade, and the thickness of the blade at any point is determined by the number of layers present at that point.

BACKGROUND TO THE INVENTION 
1. Field of the Invention 
This invention relates to turbomachine blades made of a composite material 
having an organic matrix, and is applicable particularly, but not 
exclusively, to the fan blades of aircraft jet engines. 
2. Summary of the Prior Art 
Turbomachine blades made of a composite material comprising reinforcing 
fibres impregnated by an organic matrix are used in aircraft jet engines 
and are valued for their lightness compared to metal blades, and also for 
their strength. Such blades are conventionally made using glass or carbon 
fibre or Kevlar or the like, together with a high-strength thermosetting 
resin matrix. Materials of this kind have satisfactory strength in the 
direction of the fibres but are less strong perpendicularly thereto. 
Similar considerations apply to rigidity when using fibres having a high 
modulus of elasticity, such as carbon. The fibres are disposed in bundles 
and/or superposed sheets of fabric arranged in shell-fashion or draped 
around a core. 
The superposed fabric sheets provide satisfactory strength in the sheet 
plane, particularly in the directions of their constituent weft and warp 
fibres, but their strength in directions perpendicular to the sheets is 
poor. The unsticking of the fabric sheets of the composite material from 
one another is called "delamination". In order to improve the ability of 
the blade to withstand impacts by foreign bodies the fabric sheets are 
conventionally disposed without interruptions along the blade surface, the 
arrival of the end of a fabric sheet in the surface of the blade leading 
to a tendency to delamination at this position. 
This technology is unsatisfactory for the fan blades of aircraft jet 
engines, particularly in the case of wide-chord blades--i.e. blades in 
which there is a considerable distance between the leading edge and the 
trailing edge. Blades of this kind may be up to 1200 mm long and have a 
distance of 500 mm between the leading edge and the trailing edge, yet 
must still be thin and light. Also, they are particularly exposed to 
impacts from foreign bodies, such as birds, sucked in by the jet engine. 
Of the various stresses experienced by these blades, two require 
conflicting technical solutions. 
1) The blade vibrates in various modes, notably in bending and torsion. To 
counter this, blade rigidity must be increased, and substantial densities 
of fibres made of a material having a high modulus of elasticity must be 
disposed in the body of the blade. 
2) The blade experiences impacts from foreign bodies which can cause 
rupture of the matrix between the fabric sheets so that they disengage 
from one another. This rupture or delamination starts at the point of 
impact, then spreads between the fabric sheets concerned. The problem is 
that delamination is boosted by the rigidity which the blade must have in 
order to prevent absorption of the impact shocks. 
Fabrics having a number of layers which are directly woven together by 
supplementary fibres, the latter extending through the layers and being 
woven with warp and weft fibres of each layer, are known. These fabrics 
are called 3D, 4D, 5D and so on, D denoting dimension. The supplementary 
fibres provide the fabric with substantial resistance to delamination, but 
increase the weight thereof without improving the strength of the material 
in the plane of the layers. 
Also known from French Patent 2 610 951 is a multilayer fabric whose warp 
fibres each extend through a number of layers and which can provide thin 
structures, notably for heat protection elements for space craft. For a 
given weight these fabrics are stronger than the 3D fabrics previously 
mentioned, but they do not solve the problem of delamination between the 
fabric sheets which are subsequently assembled in successive layers to 
form a blade. 
French Patent 2 664 941 discloses a composite blade in which resistance to 
delamination caused by impact has been improved by interleaving resilient 
connecting agents between different layers. Unfortunately, this solution 
to the problem reduces the rigidity of the blade and therefore lowers its 
natural resonance modes. 
To increase the resistance of the blade to delamination the said French 
Patent 2 664 941 also proposes that the various fabric sheets be stitched 
together by supplementary fibres, but the stitching lines and points 
remain, of necessity, at a distance from one another and commencement of 
delamination may still occur in the resulting gaps. Increasing the stitch 
point density does not solve the problem since the stitch points would 
have to be very close together to counter delamination commencement 
effectively. This amount of stitching is out of the question for large 
blades. It would increase blade weight and cause distortion in the network 
formed by the fibres of the fabrics. 
U.S. Pat. No. 5,279,892 discloses a blade comprising a multilayer fabric 
central part sandwiched between two cambered shells each formed by a stack 
of fabric layers, the whole being maintained, for example, by stitches as 
in the techniques previously mentioned. In each shell each fabric layer 
projects beyond a more inward fabric layer and covers it completely to 
ensure that the edges of the fabric layers are not flush with the blade 
surface except for the outermost layers. This blade also fails to solve 
the problem of delamination between the fabric layers except for the 
central multilayer fabric part, but this is unsatisfactory because the 
central part is the least stressed part and is protected by the shells 
from impacts by foreign bodies, and because the leading edge must in any 
case be covered by a harder covering since it is the part most exposed to 
impacts by foreign bodies. 
We are unaware of any commercial wide-chord composite fan blade providing 
dynamic strength and shock resistance performance meeting the 
specifications of the Federal Aviation Administration (F.A.A.), which is 
the United States body which sets the standards recognised by most 
countries. 
SUMMARY OF THE INVENTION 
With this in mind it is an object of the invention to provide a composite 
blade comprising reinforcing fibres embedded in an injectable hardenable 
matrix, the blade having an impact strength compatible with its operating 
conditions yet being light and very rigid. 
Accordingly, there is provided a turbomachine blade made of a composite 
material comprising reinforcing fibres embedded in a matrix of injectable 
and hardenable material, wherein said reinforcing fibres form a multilayer 
fabric consisting of a plurality of parallel layers disposed one on top of 
another such that each layer partly covers the layer below it, the number 
of layers present at any position determining the thickness of said blade 
at that position, and wherein each of said layers is formed by weft fibres 
and warp fibres interwoven with said weft fibres, said weft fibres of each 
layer partly covered by another layer being connected by warp fibres 
thereof to the weft fibres of at least one layer thereabove over the 
extent of the surface covered by said at least one layer thereabove, and 
said weft fibres of each layer partly covering another layer being 
connected by warp fibres thereof to the weft fibres of at least one layer 
therebelow. 
The multilayer fabric used is therefore integral and preferably extends 
without interruption from the tip of the blade to the base of the root 
thereof. It is also preferably flush with the surface of the blade over at 
least half, and preferably two-thirds, of the length of the aerofoil 
portion and is continuous between the intrados and the extrados faces of 
the aerofoil portion. 
The invention leads to a feature contrary to conventional wisdom for 
high-strength blades, since the layers of the fabric open on to the blade 
surface at their periphery with a variable angle of inclination, the 
periphery not necessarily being covered by the layer above. In the case of 
a conventional blade this feature would be very disadvantageous near the 
leading edge, particularly on the extrados side, since foreign bodies 
would strike the layers at their ends and would therefore tend to pull 
them out. This disadvantage does not occur with the present invention 
since the layers are directly connected to the layers below by the warp 
fibres--i.e., with connection points which are very close together and 
which provide great and well-distributed strength. 
Advantageously, however, the weft fibres in the portion of each layer not 
covered by another layer are also connected to the weft fibres of at least 
one layer therebelow by the warp fibres. 
To produce the blade a fibre preform is prepared in the manner hereinbefore 
described, placed in a mould, and impregnated by the injection of a matrix 
material which is subsequently subjected to a polymerisation treatment. 
To simplify the weaving of the preform the latter may have a simple shape 
such as a rectangular parallelepiped, and after the injection and setting 
in the mould the blade is then machined to the required shape. However, 
this procedure has two disadvantages. Firstly, the weft fibres and warp 
fibres flush with the surface of the finished blade are partly or 
completely cut undesirably during the machining of the blade, so that the 
surface strength thereof, especially regarding impacts, is reduced. 
Secondly, the procedure is expensive since the machining causes 
substantial wastage of costly material which has to meet aeronautical 
standards of performance and quality. 
Advantageously, weaving of the preform stops when its shape is that of the 
finished blade, and the projecting weft fibres and warp fibres are 
preferably severed at a distance from the respective warp and weft which 
is from 100% to 150% of the basic warp or weft spacing. The resulting 
preform is placed in a mould having the shape of the finished blade and 
the matrix material is injected and cured. The ends of the weft and warp 
fibres therefore arrive woven over their complete cross-section in the 
vicinity of the blade surface, so that the surface strength of the blade 
is improved. 
Also, the multilayer fabric with its interlayer connections provided 
directly by the warp fibres in the dry state--i.e., when not impregnated 
with resin--is readily deformable by bending of the layers. The preform 
can therefore be woven flat and will readily take up the camber of the 
blade when placed in the mould. 
The weft fibres may be disposed in the direction of the blade length to 
improve resistance to centrifugal force. However, in the case of fan 
blades it is preferred to have the weft fibres oriented in the direction 
of the "chord" of the aerofoil portion --i.e., from the end of the leading 
edge to the end of the trailing edge. This arrangement improves the 
torsional rigidity of the aerofoil portion and is particularly recommended 
for "large-chord" fan blades in which the width of the aerofoil portion 
may be substantially equal to the length thereof. 
Another important aspect of the invention is the actual texture of the 
fabric, it having been found that the warp fibres interconnecting the 
various fabric layers provide an impact delamination resistance which 
remains adequate even when the angle of inclination of the warp fibres 
relative to the fabric layers is low. Since the angle of inclination is 
low, the equivalent strength and elasticity modulus in the plane of the 
fabric are improved, thus enabling the production of blades reconciling 
lightness with satisfactory rigidity and satisfactory impact strength, in 
contrast to the heavier 3D fabrics. In practice, a warp fibre inclination 
angle of from 5.degree. to 15.degree. is used, the warp fibre changing 
layers only after having passed at least one, and preferably two or three, 
weft fibres. 
This angle of inclination is noteworthy since the strength of the fabric is 
virtually independent of the number of layers--2, 3, 4 or more--passed 
through by each warp fibre. 
In order not to leave excessive delamination-promoting gaps between the 
fibres and not to multiply excessively the number of warp fibres required 
to be wound separately for weaving, it is preferred to use fibres having 
from 12000 to 48000 elementary strands and a diameter of the order of 5 
mm, at least 60% of the fibre volume being allotted to the warp fibres and 
at most 40% the volume being allotted to the weft fibres. 
Preferably, the weft fibres of each layer are offset relative to the weft 
fibres of the adjacent layers by a distance equal to half the weft spacing 
P--i.e., the weft fibres of successive layers are in a staggered 
arrangement--this arrangement helping to reduce the gaps left between the 
fibres and thus to improve the resistance to delamination. 
Alternatively, the weft fibres may be in line with one another in 
successive layers. In this case supplementary warp fibres are added to the 
uncovered portions of the layers in order to reduce the gaps between the 
fibres, the supplementary warp fibres each being restricted to a single 
layer. 
The blade is very thin over most of its aerofoil portion and fairly thick 
at its root, the root thickness decreasing in the transition zone between 
the root and the remainder of the aerofoil portion. 
In a first embodiment, the multilayer fabric preform extends continuously 
between the intrados and the extrados faces over the entire area of the 
aerofoil portion. To form the extra thickness of the root the two adjacent 
fabric layers disposed at the centre of the thickness of the preform are 
not connected by the warp fibres in the root zone. Consequently, the 
fabric can be divided at the root into two portions which are kept apart 
by an insert during moulding. However, this structure has the disadvantage 
of complicating the weaving because of the large number of fibres and 
because of the interruption of weaving between the two adjacent central 
layers at the root. 
In a second embodiment which simplifies weaving, the preform is assembled 
and possibly stitched together in three parts comprising an integral woven 
multilayer preform having the shape of the aerofoil portion and extended 
into the transition zone and the root, and two conventional preforms 
making up the remaining volume of the transition zone and the root and 
possibly stitched to the flanks of the preform defining the aerofoil 
portion. The root thickness may be further increased by inserts placed 
locally between the layers. 
Technically, the present blade cannot be confused with the blade described 
in French Patent 2 664 941 since the performance of the stitched 
interlayer connections of the latter is very inferior to the directly 
woven interlayer connections of the invention, which have a large number 
of close-together connection points and therefore provide a substantial 
and well-distributed interlayer strength. Also, the matrix gaps left 
unconnected between two layers are very much reduced, which inhibits the 
commencement of any delamination. 
In other words, the effect of using a multilayer woven fabric extending 
unbroken between the intrados and extrados faces is to inhibit the start 
and spreading of delamination, whereas the use of a stitched assembly of a 
plurality of fabric layers inhibits the spreading of delamination but not 
the start thereof. These two different arrangements cannot therefore be 
regarded as equivalents. 
Nor can the invention be confused with that disclosed by U.S. Pat. No. 
5,279,892, because all the layers are parallel to one another and are 
interconnected by the warp fibres to form a single multilayer fabric 
extending continuously between the intrados face and the extrados face at 
least in the top half, and preferably the top two-thirds, of the blade. 
The resulting blade is particularly resistant to delamination, especially 
in its upper and thinnest part. 
The multilayer fabric in accordance with the invention, in which the layers 
are interconnected directly by the warp fibres, also cannot be compared in 
its present use to the conventional 3D, 4D etc. fabrics. With the 
parameters specified the multilayer fabric of the invention provides a 
weight-stiffness compromise with sufficient impact strength to enable a 
wide-chord fan blade to be made to the F.A.A. specifications. It also 
enables the preform to be woven flat directly in the shape of the blade, 
or at least the aerofoil portion, with final cambering and twisting being 
provided by the mould. Furthermore, no supplementary draping of fibres on 
the blade surface is necessary. 
French Patent 2 610 951 describing this kind of fabric suggests its use in 
a thin structure such as elements for protecting space craft at re-entry 
into the atmosphere, but it does not suggest a use for profiled articles 
with weaving of the preform to the shape of the article. 
With a carbon fibre having a tensile modulus E&gt;270 
GPa(270.multidot.10.sup.9 pascals), a tough epoxy resin and a fibre 
content of 60% by volume, we have obtained tensile elasticity moduli in 
the directions of the weft fibres, warp fibres and perpendicularly to the 
plane of the top layers of 40 GPa, 10 GPa and 10 GPa respectively, a 
torsion modulus in the plane of the layers of more than 5.5 GPa, and a 
propagation energy of delamination cracks between the layers of more than 
2000 J/m.sup.2. 
The invention will now be described in more detail with reference to the 
preferred embodiments, given by way of non-limitative example, and with 
reference to the accompanying diagrammatic drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring first to FIG. 1, the blade 1 comprises an aerofoil portion 2 
integrally joined to a root 3 through a transition zone 4. The blade 1 is 
made of a composite material having a carbon fibre base embedded in a 
tough epoxy resin matrix, and has, in known manner, a leading edge 5 of 
TA6V titanium alloy fitted and bonded to the rest of the blade, the 
leading edge 5 extending over the whole length of the aerofoil portion 2. 
The blade 1 is reinforced by a carbon fibre preform (not shown in FIG. 1) 
extending from the tip 6 of the aerofoil portion 2 to the base 7 of the 
root 3, and from the leading edge 5 to the trailing edge 8. The aerofoil 
portion 2 reduces in thickness towards the leading edge 5, the tip 6 and 
the trailing edge 8, and is thicker at its centre and towards the root 3, 
this thickness variation being represented by isoclinal curves 9. 
Referring now to FIG. 2, the aerofoil portion 2 is reinforced by a 
multilayer woven carbon fibre fabric 15. the reference 16 denotes any 
layer, the references 16a denote any layer above the layer 16, and the 
references 16b denote any layer below the layer 16. At any point on the 
aerofoil portion 2 the number of superposed layers 16 present determines 
the thickness of the portion 2 at that position so as to form the shape of 
the portion 2. Except for the top layer 16c each layer 16 is completely or 
partly overlapped by at least one layer 16a above it. Conversely, each 
layer 16, including the top layer 16c, completely or partly overlaps at 
least one lower layer 16b. Lines 17, 17a respectively define the outer 
limits of layers 16, 16a, and it will be seen that each layer 16 has an 
uncovered area 18 between the lines 17 and 17a, the remainder of its 
surface 19 being covered by the layer 16a immediately above it. The fabric 
15 is embedded in the matrix bounded by the surface 20 of the aerofoil 
portion. The fitted leading edge 5 has an outer surface 21 which is an 
extension of the surface 20, and an inner surface 22 which is 
complementary to the surface 23 at the corresponding edge of the aerofoil 
portion 2, the surfaces 22 and 23 being glued together by tehcniques 
familiar to the skilled addressee to fix the leading edge 5 to the 
aerofoil portion 2. 
The various layers 16 each comprise weft fibres arranged in the direction 
of the chord of the aerofoil portion 2--i.e., in directions parallel to 
the direction from the front 25 of the leading edge 5 to the rear 8 of the 
trailing edge. The warp fibres interconnecting the weft fibres of each 
layer 16 also each connect either the weft fibres of the adjacent layer 
16a above it or the weft fibres of the adjacent layer 16b below it, and 
thus directly provide woven cohesion between the layers 16 of the entire 
fabric 15. The top layer 16c is of course connected only to the layer 
immediately below it. Also, the uncovered area 18 of each layer 16 is 
connected by weaving to the layer 16b immediately below. 
Clearly, all the layers 16 are progressively interconnected by the warp 
fibres to form a single multilayer woven fabric which is thus continuous 
from the intrados face 20a to the extrados face 20b of the aerofoil 
portion 2. The layers 16 are parallel to one another and to an imaginary 
surface disposed midway between the intrados face 20a and the extrados 
face 20b. 
Referring now to FIG. 3, the layers 16, 16a, 16b comprise weft fibres 30 
interconnected by warp fibres 31, the weft fibres 30 being perpendicular 
to the plane of FIG. 3 and being shown end-on. Some warp fibres 31 connect 
the weft fibres 30 of layer 16 to the weft fibres 30 of the layer 16a 
immediately above, while other warp fibres 31 connect the same weft fibres 
30 of the layer 16 to the weft fibres 30 of the layer 16b immediately 
below. This arrangement is repeated from layer to layer to form an 
integral fabric. The weft fibres 30 of the layer 16 are aligned with the 
immediately opposite weft fibres 30 of the adjacent layers 16a, 16b. In 
other words, seen end-on, the weft fibres 30 form a network of which the 
basic mesh is a rectangle. 
The warp fibres 31 form a maximum angle .alpha. of from 5.degree. to 
15.degree. with respect to the layer 16. The weft fibres 30 and the warp 
fibres 31 comprise from 12000 to 48000 elementary strands twisted not too 
tightly so that after weaving the fibres have a flattened cross-section 
verging on an ellipse. The warp fibres 31 make up at least 60% of the 
fibre volume, and the fibre volume makes up about 60% of the total volume. 
There are well-known empirical formulae availably to the skilled addressee 
which show the relationship between these parameters and the thickness of 
the layers. In this example the angle .alpha., which is low, is obtained 
by the warp fibres 31 of the layer 16 changing direction every two weft 
fibre intervals, each warp fibre 31 returning to the initial layer every 
four weft fibre intervals. The warp fibres 31 in planes parallel to the 
plane of FIG. 3 are offset by one weft interval at each change of plane, 
and therefore return to an identical position every four planes. 
Advantageously, in order to increase the fibre density in the uncovered 
areas 18 of the layers 16, supplementary warp fibres 32 are provided which 
are woven only with the weft fibres 30 in the uncovered area 18, the 
supplementary warp fibres 32 passing alternately above and below 
successive weft fibres 30. 
Referring now to FIG. 4, in this embodiment the extra thickness of the root 
3 is obtained by interrupting the weaving between the two adjacent layers 
16d disposed at the centre of the fabric thickness, but only in the zone 
of the root 3, and by providing an insert 37 between the two layers 16d to 
keep them apart. The assembly is then placed in the mould, impregnated 
with resin and polymerised, the space 38 at the apex of the insert 37 
being filled by the resin, Such a structure is very strong and light. 
Referring back to FIG. 1, in an alternative embodiment the fabric is 
integral over the whole of the blade 1 up to a thickness corresponding to 
the isocline 9a, i.e. 21 mm in this example. Beyond this, the additional 
fabric thickness is obtained by the application of supplementary fitted 
and stitched single-layer fabric sheets 35. Stitched fabric sheets can be 
used in this zone--i.e. at the centre and at the bottom of the aerofoil 
portion 2 towards the root 3--since there is very little exposure to 
impacts from foreign bodies in this region. 
The apex 9b of the isocline 9a--i.e., the furthest point of the isocline 9a 
from the bottom 7 of the blade--is separated from the bottom 7 of the 
blade by a distance d1 corresponding at most to half the length d2 of the 
blade 1 taken between the bottom 7 and the tip 6, in order to limit the 
area of fitted fabric 35 which is less resistant to shocks caused by 
foreign bodies. Preferably, however, the relative fitted fabric height 
d1/d2 is of the order of 1/3, which is still sufficient to support the 
blade 1 satisfactorily while leaving the fitted fabric 35 in the least 
exposed lower zones. The warp and weft fibres of the fabric sheets 35 are 
preferably inclined at 45.degree. to the chord of the blade in order to 
increase the torsional resistance thereof. 
Referring now to FIG. 5, the single multilayer fabric 15 is clamped on 
opposite sides by the fitted and stitched fabric sheets 35, the number of 
supplementary sheets increasing with the thickness of the blade towards 
the root 3. The flared shape of the root 3 is produced by placing inserts 
36 between the sheets 35 by a technique which is familiar to the skilled 
addressee. 
The flat dry preform thus formed is then placed in a mould having the shape 
of the blade, particularly as regards the twist and camber of the aerofoil 
portion, and the injection of the thermosetting resin and the curing heat 
cycle for the resin are performed by techniques familiar to the skilled 
addressee. 
Clearly, the main parameters of the structure of the fabric in accordance 
with the invention are the angle of inclination .alpha. of the warp fibres 
31, the number of strands making up the warp fibres 31 and the weft fibres 
30, the volume distribution of the warp and weft fibres and the total 
volume percentages of fibre. The other elements depend mainly upon the 
weaver's skill, for example, the number of layers passed through by each 
warp fibre, the number of basic weft fibre intervals passed through by the 
warp fibre at each change of layer, and the aligned or staggered 
arrangement of the weft fibres. 
Clearly, too, the maximum thickness of the multilayer fabric depends upon 
the cross-section of the fibres and upon the number of fibre bobbins which 
the weaver can use to produce the fabric. 
Another advantage of the invention is that the blade 1, at least so far as 
the aerofoil portion 2 is concerned, can be produced directly by moulding 
to its final shape without additional machining. Similar considerations 
apply to the surface 20 of the aerofoil portion, except for the fitted 
leading edge 5, and to the surface 23 to which the leading edge 5 is 
fixed.