Abstract:
The present invention relates to a blade made of composite material ( 100 ) comprising an airfoil ( 100 A) formed of filaments or fibers, optionally woven, impregnated with a heat-curable resin, with an intrados wall and an extrados wall between the leading edge and the trailing edge. 
     It is characterized in that a device for damping the vibrations is incorporated in one or other of the intrados and extrados walls, and is formed of at least one layer made of viscoelastic material ( 111 ) and a layer made of rigid material ( 110 ), these layers being superposed. According to one embodiment, the layer made of rigid material comprises a first zone ( 110   1 ) and a second zone ( 110   2 ), the layer made of viscoelastic material ( 111 ) being interposed between the airfoil and said first zone ( 110   1 ) of the rigid layer ( 110 ), and said second zone ( 110   2 ) of the rigid layer being attached to the wall of the airfoil ( 100 A) without interposition of viscoelastic material. 
     The invention relates in particular to a turbomachine blade, such as a fan blade of a turbojet engine or a propeller airfoil of a turboprop engine.

Description:
[0001]    The present invention relates to the field of turbomachines. It relates to the damping of a blade made of a composite material and its object is more particularly the damping of a fan blade in a turbojet engine or else a propeller airfoil in a turboprop engine. 
       BACKGROUND OF THE INVENTION 
       [0002]    The blades, notably of a fan, but also of a low-pressure compressor, made of composite material with carbon fibers are made in different ways. According to one manufacturing method, a stack of one-way plies or preimpregnated wovens is produced that is placed in a mold while orienting the successive plies differently before compacting and polymerization in an autoclave. According to another method, woven preforms of dry fibers are prepared that are assembled by sewing or else a single preform made of woven fibers in three dimensions, that is impregnated with resin by injection in a closed mold. The blade is formed in a single piece comprising the root with the airfoil. It comprises various protections in order to reinforce the thermomechanical resistance thereof. A metallic protection is therefore attached to the leading edge or to the whole contour of the airfoil comprising the leading edge, the blade tip and the trailing edge, for example in the form of a titanium part bonded over the whole surface of the leading edge and over a front portion of the outer surfaces of the extrados wall by mounting a protective film that can be made of a synthetic material, polyurethane for example, and directly bonded to the intermediate part. 
         [0003]    The object of the invention is this type of blade with at least one protection along the leading edge. An example of manufacture is described in patent EP1777063 in the name of the present applicant. 
         [0004]    The object of the invention is more generally any type of blade made of composite material, the airfoil of which is formed of fibers, threads or filaments, optionally woven, impregnated with a heat-curable resin. 
         [0005]    Flutter is a phenomenon of coupling between the aerodynamic and elastic characteristics of the blade creating unstable situations. A flutter manifests itself asynchronously. Subsonic flutter is distinct from supersonic flutter. The fan blade is mainly concerned by subsonic flutter. 
         [0006]    Flutter is a phenomenon that is difficult to predict, because of the complexity of the coupling between the aerodynamic and mechanical responses. Moreover, the mechanical damping of the blade is usually not very well known. Finally, in the current design of increasingly loaded blading, flutter is a phenomenon that must be particularly taken into account. 
         [0007]    During the design of the fan blade, a margin of flutter is estimated, which measures, at a given flow rate, the difference between the line of flutter and the line of operation. This value is usually established based on a known reference (the closest) by adding thereto the differences calculated between this reference configuration and the new configuration. The criteria used nowadays for subsonic flutter on  1 F and  1 T modes and on zero diameter coupled mode are:
       Twist Bend Coupling (TBC), representing the ratio between the movements in twisting mode and in bending mode. The higher the TBC parameter, the greater the risk of flutter also.   The reduced speed or Strouhal criterion given by the following formula:
 
VR=W/C*f*pi, where W is the relative speed, C the chord of the blade at a given height and f the frequency of the blade mode in question. This criterion represents the coherence between the vibrational frequency of the blading and the frequency of the unsteadiness of the flow along this blading.
       
 
         [0010]    Other factors may influence the flutter margin and may occasionally be used when the phenomenon is encountered during tests: reduction of the specific flow rate, reduction of the number of blades or increase in the chord, lubrication of the blade root, detuning. 
         [0011]    The object of the invention is to improve the harmonic response of the blade to synchronous aerodynamic excitations such as:
       inlet duct distortions generated by flying conditions in angle of incidence—climbs, descents, crosswinds,   the harmonic excitations generated by a residual imbalance,   backpressure fluctuations, induced by a fixed impeller of the stator type on a fan impeller,   wake or backpressure fluctuations induced by a moving fan impeller on its neighbor in the case of an architecture with two contrarotating rotors.       
 
       DESCRIPTION OF THE PRIOR ART 
       [0016]    A damping technology has been studied for several years by the present applicant with a first evaluation on one-piece bladed disks. The latter terms designate a disk-and-blade assembly manufactured in a single piece. The operating principle of the damping system is based on the dissipation of energy through the shearing of a suitably placed viscoelastic material. The correct behavior of the damping system is dependent on the dimensions of the material and on good adhesion between the material and the engine part. 
         [0017]    Also known is U.S. Pat. No. 6,471,484 which describes a system for damping vibrations in a gas turbine engine rotor comprising a single-piece bladed disk. The airfoils of the blades are provided with a cavity hollowed in an intrados face or extrados face and containing a layer of damping material with a stress layer. A covering sheet covers the cavity. In operation, the damping of the vibrations is promoted by the shearing stresses induced in the damping material between the airfoil and the stress layer on the one hand and in the damping material situated between the stress layer and the covering sheet on the other hand. Patent application EP926312 describes a similar damping device applied to a fan blade of a turbojet engine. 
       SUMMARY OF THE INVENTION 
       [0018]    The object of the present invention is an improvement to this technique in an application to a blade made of composite material with, in particular, optimization of the location of the damping treatment. 
         [0019]    According to the invention, the turbomachine blade made of composite material, such as a fan blade of a turbojet engine or a propeller airfoil of a turboprop engine, comprising an airfoil formed of filaments or fibers, optionally woven, impregnated with a heat-curable resin, with an intrados wall and an extrados wall between the leading edge and the trailing edge, is characterized in that a device for damping the vibrations is incorporated in at least one of the intrados and extrados walls, and is formed of at least one layer made of viscoelastic material and a layer made of rigid material, these layers being superposed, the layer made of rigid material comprises a first zone and a second zone, the layer made of viscoelastic material being interposed between the airfoil and said first zone of the rigid layer, and said second zone of the rigid layer being attached to the wall of the airfoil without interposition of viscoelastic material and the rigid layer comprises said first zone and said second zone, and is arranged so that the first zone of the rigid layer comprises at least two lobes, the two lobes being attached to the second zone. 
         [0020]    According to one embodiment, the layer made of viscoelastic material is attached, by the vulcanization of a film of viscoelastic material, by bonding by means of an adhesive material or by a mechanical connection such as by bolting or screwing, to the rigid layer on one side and to the airfoil on the other side. 
         [0021]    The thickness of the viscoelastic layer is advantageously situated between 0.1 mm and 1 mm. The layer of rigid material, for its part, preferably has a thickness of between 0.5 mm and 1 mm. 
         [0022]    For this other embodiment, the second zone, by which the rigid layer is secured to the airfoil, is preferably situated radially on the side of the root of the blade relative to the first zone. More particularly, the second zone of the rigid layer is bonded to the airfoil. Optionally the rigid layer may be held and immobilized along this second zone by mechanical means. 
         [0023]    The functions of the viscoelastic layer are to introduce a mechanical damping for the vibratory responses of the blade, in particular in the case of a fan blade for the bending mode  1 F and twisting mode  1 T, and also to play a damping role during phenomena such as during a bird strike, by absorbing a fraction of the impact energy and thus limiting the damage to the blade. The damping device is applied in the zones that are subjected to high deformation energy levels of the vibratory mode to be damped. The zones in which the levels are at a maximum are particularly sought. 
         [0024]    According to another objective of the invention, there has been an attempt to optimize the shape of the device in order to obtain the best possible damping. It has therefore been determined that the shearing stresses were at a maximum in the peripheral zone of the rigid layer. The invention therefore proposes to form the rigid layer so that its contour is as long as possible taking account of the surface covered by the device. 
         [0025]    The rigid layer comprises at least two lobes. The shape of the lobes may be varied. The lobes may advantageously have an elongated shape and extend in a radial direction relative to the root of the blade. Therefore the shape may be comb-like, star-like or another shape. According to another particular embodiment, an overall contour of increased length is created by arranging cuts along closed, for example circular, lines in the rigid layer. 
         [0026]    According to another embodiment, the layer of viscoelastic material is contained at least partially in a cavity arranged in the composite material of the airfoil. 
         [0027]    The viscoelastic material is chosen from materials such as rubber, silicone, polymer elastomer or epoxy resin. It may have a single layer or many layers and the layers may if necessary be formed of different materials, depending on the environment, the materials used and the damping characteristics sought. For example, the damping characteristics may differ to cover more extensive temperature ranges. 
         [0028]    The rigid layer is metallic or else made of the same material, based on filaments, as the airfoil. “Filaments” also means threads and fibers. To the extent that the rigid layer has sufficient rigidity in the direction of the deformations of the airfoil, the layer may be thin and not disrupt its aerodynamism. 
         [0029]    According to another embodiment, at least one additional layer made of rigid material is interposed at least partly between the rigid layer and the airfoil, with two layers of viscoelastic material placed on either side of the additional layer, the viscoelastic layers being formed of different or identical materials. 
         [0030]    Preferably the layer of viscoelastic material is secured to the rigid layer and/or to the airfoil by vulcanization of a film of viscoelastic material or else by bonding by means of an adhesive material. The layers may also be secured to one another by a mechanical connection means, such as bolting or screwing. 
         [0031]    According to a particular application, the leading edge of the airfoil comprises a protective coating formed of a metal strip bonded to the airfoil on at least a portion of its surface. 
         [0032]    In the latter case, advantageously a layer of viscoelastic material is interposed at least partly between the metal strip and the airfoil so as to form a second vibration-damping device. 
         [0033]    According to a variant, said metal strip is rigidly connected to the rigid layer of the damping device applied to the intrados or extrados wall. 
         [0034]    Finally, the invention relates to a method for producing a damping device on a blade according to which the shape of the lobes of the first zone of the rigid layer is determined so as to have a contour of the rigid layer that is as high as possible taking account of its surface area. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]    A nonlimiting embodiment of the invention will now be described in greater detail with reference to the appended drawings in which: 
           [0036]      FIG. 1  represents schematically a turbojet engine with a front fan. 
           [0037]      FIG. 2  shows a fan blade made of composite material with a protective element protecting the leading edge having zones comprising a viscoelastic damper. 
           [0038]      FIG. 3  shows, in a top view of the blade of  FIG. 2 , the region of the leading edge of the blade. 
           [0039]      FIG. 4  shows a schematic view in perspective of a blade comprising a vibration-damping device according to the invention. 
           [0040]      FIG. 5  shows a schematic view in perspective of a blade comprising a vibration-damping device according to a variant embodiment of the invention. 
           [0041]      FIG. 6  shows an arrangement of the damping device of the invention. 
           [0042]      FIG. 7  shows another arrangement of the damping device of the invention. 
           [0043]      FIG. 8  shows another arrangement of the damping device of the invention. 
           [0044]      FIG. 9  shows a schematic view in perspective of a blade comprising a variant of a vibration-damping device according to the invention. 
           [0045]      FIG. 10  is a graph showing the damping properties of a device comprising a combination of viscoelastic materials, over a range of temperatures. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0046]    With reference to  FIG. 1 , shown schematically is an example of a turbo-machine in the form of a twin-spool bypass turbojet engine  1 . A fan  2  at the front supplies the engine with air. The air compressed by the fan is divided into two concentric flows F 1  and F 2 . The secondary flow F 2  is discharged directly into the atmosphere and provides an essential portion of the motive thrust. The primary flow F 1  is guided through several compression stages  3  to the combustion chamber  4  where it is mixed with the fuel and burned. The hot gases supply the various turbine stages  5  which drive the fan  2  and the compression rotors  3 . The gases are then discharged into the atmosphere. 
         [0047]      FIGS. 2 and 3  show a fan blade  10  capable of being used on this type of engine. It is a blade made of composite material. In general, the airfoil  10 A, made of composite material, of the blade consists of fibers or filaments linked together by a heat-curable resin. The filaments or fibers are made of carbon or another material such as glass, silica, silicon carbide, alumina, aramid or an aromatic polyamide. The filaments are, according to a known method of assembly, in the form of woven elements. The leading edge is in this instance coated with a metallic protective means  10 B. It is, for example, a titanium foil bonded by the layer  30  to the composite material extending along the leading edge, with a strip forming a wing on each side: one wing  10 Bi on the intrados wall downstream of the leading edge and one wing  10 Be on the extrados wall downstream of the leading edge. The two wings are connected along the leading edge by a thicker portion  10 B 2 . 
         [0048]    Such a blade is manufactured for example according to the technique described in patent EP 1,777,063 in the name of the present applicant. 
         [0049]    According to the latter technique, a preform is constructed by weaving filaments in three dimensions. The one-piece woven preform is then trimmed to shape by cutting the contour according to a three-dimensional graphic. The part is placed in a forming mold. Then, after appropriate deformation, the part is placed in a compacting mold which stiffens the deformed preform. The leading edge is overcompacted so as to allow the placement of the protective element along the leading edge. This is an element in the form of a longitudinal half-sleeve with two wings designed to cover a portion of the extrados wall and intrados wall downstream of the leading edge. As explained in the patent cited above, the protective element is placed in a mounting device capable of parting the wings. The protective element is placed, via its leading edge pre-coated with adhesive, between the two wings and then the latter are released. 
         [0050]    The assembly is placed in an injection mold into which a binder comprising a heat-curable resin is injected so as to impregnate the whole preform. Finally the mold is heated to the curing temperature of the resin. It is then sufficient to remove the part from the mold. 
         [0051]    In patent application FR 0706430 dated Sep. 13, 2007 in the name of the present applicant, a description is given of the production of a vibration-damping means by incorporating, between one of the intrados or extrados walls of the airfoil and the protective element  10 B, at least one layer of a viscoelastic material, located for example in one of the zones  11 ,  12  or  13 . The metal protective element  10 B forms a rigid backing layer for the vibration-damping system that it forms with the layer of viscoelastic material. 
         [0052]    According to the invention, a damping device is placed on the intrados wall and/or the extrados wall. In  FIG. 4 , which represents schematically a blade provided with a damping device, the damping is achieved on the extrados wall of the airfoil  100 A. In this instance, this airfoil comprises a protective element  100 B fitted to the leading edge. The airfoil is obtained as in the airfoil of application FR 0706430 reported above based on filaments or fibers impregnated with a heat-curable resin. 
         [0053]    The damping device comprises a viscoelastic layer  111  interposed between the wall  100 A, intrados or extrados, of the airfoil and a rigid layer  110 . 
         [0054]    Viscoelasticity is a property of a solid or a liquid which, when it is deformed, exhibits a behavior that is both viscous and elastic by simultaneous dissipation and storage of mechanical energy. 
         [0055]    A rigid material in the vibration-damping system is more rigid than the viscoelastic material of the layer. In other words, the isotropic or anisotropic characteristics of elasticity of the material of the backing layer are greater than the isotropic or anisotropic characteristics of the viscoelastic material in the desired thermal and frequency operating range. The material of the viscoelastic layer is of the rubber, silicone, polymer elastomer, epoxy resin or thermoplastic material type. 
         [0056]    The embodiment shown in  FIG. 4  illustrates a device covering approximately ⅓ of the intrados surface area and/or extrados surface area of the blade. This ratio of one third of the intrados or extrados surface area to which the device is applied preferably corresponds more generally to the extent of the device of the invention. 
         [0057]    The arrangement of the damping device can be seen in  FIG. 6  which shows a partial section of the blade in the direction A-A in  FIG. 4 . The rigid layer  110  is attached by means of a rigid attachment means  112  directly to the wall  100 A of the airfoil. The attachment means is advantageously a layer of adhesive material. However, a mechanical means is equally suitable. 
         [0058]    In this embodiment, the rigid layer comprises two zones: a first zone  110   1  and a second zone  110   2 . The attachment means  112  extends along the second zone  110   2 . A viscoelastic material  111  is interposed between the wall  100 A of the airfoil and the first zone  110   1  to which this material is attached. 
         [0059]    Preferably the second zone  110   2  by which the rigid layer is attached to the blade is on the side of the blade root relative to the first zone. Depending on the bending or twisting deformation, the rigid layer moves relative to the blade wall and the viscoelastic layer opposes by resisting the movement. The result of this is an action of damping against the vibrations sustained by the blade. The device is preferably placed on the portions of the blade sustaining maximum deformation for the mode in question. 
         [0060]    The solution of the invention makes it possible to produce a thin device. The rigid layer, the thickness of which is between 0.5 and 1 mm, is formed so as not to affect the air flow along the airfoil and the aerodynamic properties of the latter. If necessary, the airfoil is slightly hollowed in order to contain the damping device so that its outer profile is in the continuity of the blade wall. 
         [0061]    The invention is not limited to this embodiment, the layer made of rigid material is, according to one embodiment, attached to the airfoil by means of the viscoelastic layer which adheres or is bonded both to the layer of rigid material and to the airfoil. 
         [0062]    According to a variant illustrated by  FIG. 7 , the damping device comprises an additional rigid layer  117  between the rigid covering layer  110  and the wall  100 A. The additional rigid layer  117  is held between two layers of viscoelastic material: one layer  111  between the two rigid layers  117  and  110 , one layer  115  between the wall  110 A and the additional rigid layer  117 . The rigid layers may be formed of the same material or of different materials. The same applies to the two viscoelastic layers  111  and  115  which may be made of different materials depending on the environment and the damping sought. 
         [0063]    The additional layer  117  is not attached to the wall; it is preferably free to move to the extent that the deformation of the viscoelastic material allows. The viscoelastic layer is secured to the layers and walls that are in contact with it by vulcanization, to the extent of the possibilities offered by the material. It can also be bonded as can be seen in  FIG. 8 . The layer  111  is bonded by means of a film of an appropriate adhesive substance,  118  and  119  respectively, to the rigid layer  110  and to the wall  100 A. 
         [0064]    The embodiments of  FIGS. 7 and 8  show the attachment  112  of the rigid layer to the wall. 
         [0065]    However, the invention also includes the embodiment in which the connection of the rigid layer to the airfoil is obtained only by means of the viscoelastic and rigid intermediate layers. 
         [0066]      FIG. 5  shows a variant damping device on a blade  200  combining the damping achieved by the rigid layer  210  on the wall, in this instance the intrados, of the airfoil and the damping achieved by the protective layer  200 B of the leading edge of the airfoil associated with a viscoelastic layer. According to this combination, the two layers  200 B and  210  are secured in a coupling zone  210 B at the tangency between the two plates forming the layers. 
         [0067]    The coupling zone  210 B between the rigid layer  210  and the leading edge may have a greater or lesser extent. Nevertheless, preferably, the aim is to minimize it in order to prevent a phenomenon of mechanical locking of shearing on the damper of the airfoil wall. Specifically, the larger the coupling zone, the less the vibration energy is dissipated. 
         [0068]    It should be noted that the coupling zone  210   b , although formed of rigid material, allows relative movement between the rigid layer  210  and the protective layer  200 B. 
         [0069]    In this case, the two layers  210  and  200 B are cut from one and the same material. 
         [0070]    In the embodiment with two zones of the rigid layer, the viscoelastic layer is placed in a first zone between the two rigid layers and the airfoil. A second zone of this assembly can be rigidly attached to the airfoil. 
         [0071]      FIG. 9  shows a variant of the invention. On the basis of an observation that the damping is more effective at the border of the rigid layer, the rigid layer is formed so as to maximize the extent of this border. According to the nonlimiting example illustrated by  FIG. 9 , the first zone beneath which the viscoelastic layer is interposed is in the form of lobes or in this instance of fingers  310   1 ; these lobes preferably extend radially relative to the root  300 C of the blade, substantially parallel to the plate  300 B protecting the leading edge. The lobes are connected to the second zone  310   2  of the rigid layer  310  which is rigidly attached to the wall  300 A of the blade. To the extent that the gradients of movement between the rigid layer and the application surface are at a maximum on the edges of the device, a structure with lobes allows an effective damping by shearing of the viscoelastic layer. Shearing is zero in the middle of the rigid layer because of the symmetry of the bending movement and is at the maximum on the edges where the gradient of movement between the rigid backing layer and the supporting structure is at the maximum. 
         [0072]    In  FIG. 10 , the curves show the value there would be in combining several viscoelastic materials when it is a matter of being operational over a relatively extensive temperature range. Each material exhibits a peak for a given temperature. Here there are three materials, the effectiveness ranges of which cover distinct temperature ranges. By combining the three materials, the effectiveness of the damping device is extended over a temperature range covering the three temperatures T 1 , T 2  and T 3 . 
         [0073]    The materials are placed in layers side by side on the airfoil and/or superposed.