Patent Publication Number: US-2022228490-A1

Title: Inter-blade platform with a sacrificial box section

Description:
FIELD OF THE INVENTION 
     The invention relates to the general field of inter-blade platforms in fans of aeronautical turbines, particularly when these platforms are made of a composite material comprising a fibrous reinforcement densified by a matrix. 
     TECHNOLOGICAL BACKGROUND 
     Inter-blade platforms of turbomachine, particularly turbojet, fans are arranged between the fan blades in an extension of its inlet cone. They allow in particular delimiting, on the inside, the annular air intake path in the fan, this path being delimited, on the outside, by a casing. These platforms generally comprise a base, configured to delimit the path, and a box section extending radially inward from the base in order to allows support of the platform on the fan disk. The box section is further configured to stiffen the platform in order to ensure continuity of the aerodynamic flow in the fan. 
     It is known to create inter-blade platforms of fans of composite material. The composite material generally comprises a fibrous reinforcement densified by a matrix. Depending on the application contemplated, the preform can be of glass, carbon or ceramic fibers and the matrix can be of an organic material (polymer), of carbon or of ceramic. For parts with a relatively complex geometric shape, it is also known to create a fibrous structure or blank in a single piece by 3D or multilayer weaving and to form the fibrous structure to obtain a fibrous preform having a shape near that of the part to be manufactured. The creation by 3D weaving of a fibrous preform with a π-(Pi) shaped cross section for the platform has thus already been proposed in document WO 2013/088040. These platforms have a π-shaped cross section with a base and two flanks forming stiffeners which extend from one face of the base and serve to stiffen the platform so as to avoid any displacement of it under the centrifugal load due to the speed of rotation of the fan. 
     In order to reinforce the resistance of the platform to centrifugal loads, it has been proposed to add a wall between the free ends of the flanks to form a closed box section under the base of the platform. Document no. FR 2 989 977, in the name of the Applicant, may in particular be referred to, which describes an example of a fibrous blank woven in a single piece by three-dimensional weaving for creating a platform with a closed box section. A blank of this type does indeed allow obtaining a platform of composite material and a closed box section limiting the risk of rejection. The box section thus allows providing stiffness to the platform in the face of the various mechanical stresses encountered (centrifugal force in particular and various ingestions) and the retention in position of the platform upon stopping the turbomachine. 
     However, the numerous operations necessary for the manufacture of a platform of this type (in particular the manufacture of the preform, the debonding, the injection and the pocketing) remain considerable and complex and are not necessarily justified for all types of engines, particularly when the fan does not undergo too great a centrifugal force during operation. In addition, in the event of impact with an object, the box section of such platforms has a tendency to allow propagation of the shock to adjacent blades, which risks penalizing the fan severely. 
     Document FR 3 029 563 describes an inter-blade platform comprising a base, two flanks extending radially from the base and a U-shaped structure forming a box section which extends over a portion of the length of the base. The portion of the platform which is bereft of a box section does not come into contact with the disk. 
     Documents FR 3 018 473 and EP 1 503 044 describe an inter-blade platform comprising a base and two flanks. 
     Document FR 3 033 180 describes an inter-blade platform comprising a closed box section. 
     SUMMARY OF THE INVENTION 
     One objective of the invention is therefore to optimize the costs linked to the manufacture of fan inter-blade platforms for turbomachines, particularly when they are made of a composite material such as a fibrous reinforcement densified by a polymer matrix, as well as their mass. 
     To this end, the invention proposes an inter-blade platform of a turbomachine fan comprising:
         a base comprising a first surface configured to delimit a flow path in the fan and a second surface opposite to the first surface and   two flanks, extending radially next to the second surface, each of the flanks having a sacrificial free end configured to bear against a fan disk.       

     What is meant here by sacrificial is that the free end of the flanks wears before the rest of the flanks and before damaging the radial face (or if applicable a protective strip applied to the fan disk), so as to limit the propagation of shocks to the blades. 
     Certain preferred but non-limiting features of the platform described above are the following, taken individually or in combination:
         the free end of the flanks has a thickness that is smaller than an average thickness of the rest of the flanks;   the base and the two flanks are made of a composite material comprising a fibrous reinforcement densified by a polymer matrix;   a thickness of the fibrous reinforcement at the free end of the flanks is smaller than an average thickness of the fibrous reinforcement in the rest of the flanks;   the thickness of the sacrificial free end of the flanks is at most equal to 75% of the average thickness of the rest of the flanks, preferably at most equal to 50% of the average thickness of the rest of the flanks;   the free end of the flanks is machined;   the base and the flanks are formed integrally and in a single piece;   each flank comprises a wall formed integrally and in a single piece with the base and a plate, applied and attached to the wall, and in which either the plate extends from the base without covering the sacrificial free end, or the plate extending from the base and beyond the free end of the wall; and/or   the base comprises a first edge and a second edge, said edges extending on either side of the flanks, the first edge and the second edge being sacrificial.       

     According to a second aspect, the invention also proposes a fan comprising a fan disk, at least one blade and at least one inter-blade platform as described above. A fan with a plurality of blades will be described as an exemplary embodiment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features, aims and advantages of the present invention will appear more clearly upon reading the detailed description which follows, and with reference to the appended drawings given by way of non-limiting examples and in which: 
         FIG. 1  illustrates an exemplary embodiment of a fan conforming to one embodiment of the invention. 
         FIG. 2 a    is a transverse section view of an exemplary embodiment of an inter-blade platform conforming to a first embodiment of the invention. 
         FIG. 2 b    is a transverse section view of an exemplary embodiment of an inter-blade platform conforming to a second embodiment of the invention. 
         FIG. 3  is a partial perspective view of a section of a fan, seen from an upstream face of said fan. 
         FIG. 4  is a schematic view of an example of a fibrous three-dimensionally woven blank according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF ONE EMBODIMENT 
     In the present application, upstream and downstream are defined relative to the normal direction of flow of the gas in the fan  1  through the turbomachine. Moreover, what is called the axis of revolution of the fan  1  of the turbomachine, the radial axis X of symmetry of the fan  1 . The axial direction corresponds to the direction of the axis X of the fan  1 , and a radial direction is a direction perpendicular to this axis and passing through it. Absent a contrary declaration, inner and outer will be used, respectively, with reference to a radial direction so that the inner portion or face (i.e. radially inner) of an element is closer to the axis X than the outer portion or face (i.e. radially outer) of the same element. 
     A turbomachine fan  1  comprises a fan  1  disk  10  bearing a plurality of fan blades  2 , associated with inter-blade platforms  20 . 
     The blades  2  are engaged in axial grooves  12  formed in a radial face  14  of the fan  1  disk  10 . Optionally the fan disk  10  can comprise a sacrificial protective strip  16 , or foil. In a manner known per se, the protective strip  16  has as its function, during use, to protect the radial face  14  of the disk  10  by deteriorating before the disk  10 . 
     Each blade  2  has a root, engaged in one of the grooves, a head (or tip), a leading edge  3  and a trailing edge. The leading edge  3  is configured to extend facing the flow of gas entering into the turbomachine. It corresponds to the anterior portion of an aerodynamic profile which faces the flow of air, and which divides the flow of air into a pressure side flow and into a suction side flow. The trailing edge, for its part, corresponds to the posterior portion of the aerodynamic profile, where the pressure side and suction side flows are rejoined. 
     The blades  2  are associated at their radially inner end with inter-blade platforms  20 , which are arranged in the extension of an inlet cone. 
     Each platform  20  comprises:
         a base  21  comprising a first surface  22  configured to delimit a flow path in the fan  1  and a second surface  23  opposite to the first surface  22 ,   two flanks  25 , extending radially next to the second surface  23 , each of the flanks  25  having a free end  26  configured to be supported against a fan  1  disk  10 . The free end  26  of the flanks  25  extends radially to the inside relative to the base  21  and is configured to bear against the radial face  14  of the disk  10 , or if applicable against the outer face of the protective strip  16 .       

     The free end  26  of each flank  25  is sacrificial. What is meant here by sacrificial is that the free end  26  of the flanks  25  wears before the rest of the flanks and before damaging the radial face  14 , or if applicable the protective strip  16 , so as to limit the propagation of shocks to the blades  2 . 
     The inter-blade platform  20  is therefore simple to produce, but also lighter, being bereft of a wall linking the radially inner end of the flanks  25 , while limiting the risk of propagation of shocks to the blades  2  adjacent to the platform  20  in the event of impact. In fact, in the event of an impact, the sacrificial portion of the flanks  25  tends to be damaged, thus reducing the stiffness of the platform  20  and consequently avoiding damage to the adjacent blades  2 , in particular to their root. 
     In one embodiment, a thickness e 1  of each sacrificial free end  26  is less than an average thickness e 2  of the rest of the flanks  25 . What is meant here by thickness e 1 , e 2 , is the dimension along an axis normal to the flanks  25 . 
     For example, the thickness e 1  of the sacrificial free end  26  of the flanks  25  is equal at most to 75% of the average thickness e 2  of the rest of the flanks  25 , preferably equal at most to 50% of the thickness e 2  of the rest of the flanks  25 . 
     For each sacrificial free end  26 , a height h of the sacrificial free end  26  can be comprised between three times and six times an average thickness e 2  of the rest of the flanks  25 . What is meant here by height h is a dimension along an axis substantially normal to the second surface  23  of the base  21 . 
     Thus, in one exemplary embodiment, each platform  20  has flanks  25  of which the average thickness e 2  (aside from the free end  26 ) is equal to 4 mm. The thickness e 1  of the free end  26  can then be comprised between 2 mm and 3 mm while its height h can be comprised between 12 mm and 24 mm. 
     The base  21  and the flanks  25  of each platform  20  are formed integrally and in a single piece. 
     In one embodiment, the base  21  and the flanks  25  can be made of a composite material comprising a fibrous reinforcement densified by a polymer matrix. 
     The fibrous reinforcement can be formed starting with a fibrous preform obtained by three-dimensional weaving with evolving thickness. It can in particular comprise carbon, glass, aramid and/or ceramic fibers. The matrix, for its part, is typically a polymer matrix, for example epoxy, bismaleimide or polyimide. The blade  1  is then formed by molding by means of a vacuum resin injection method of the RTM (for “Resin Transfer Molding”), or even VARRTM (for Vacuum Resin Transfer Molding). 
     As a variant, the base  21  and the flanks  25  can be made of metal. 
     In a first embodiment, each sacrificial free end  26  is obtained by local thinning of the flanks  25 . For each flank  25 , the thinning can be accomplished on each face  27 ,  28  of the flank  25 . As a variant, the thinning can be accomplished on the facing faces  27  of the flanks  25 . According to yet another variant, the thinning can be accomplished on the opposite faces  28  of the flanks  26 . 
     For example, when the platform  20  is made of metal or of composite material, particularly including a fibrous reinforcement densified by a polymer matrix, the sacrificial free end  26  can be machined. For example, the flanks  25  of the platform  20  can be machined after molding. 
     As a variant, when the platform  20  is made of a composite material, the sacrificial free end  26  can be obtained by creating an open debonding in the fibrous reinforcement.  FIG. 4  in particular can be referred to, which shows schematically a chain plan of a three-dimensionally woven fibrous blank  100  from which a fibrous platform  20  preform can be formed, prior to injection of resin or densification by a matrix and possible machining, in order to obtain a fan  1  platform  20  of composite material like that illustrated in  FIGS. 1 to 4 . What is meant by three-dimensional weaving is that the warp yarns C 1 -C 8  follow winding trajectories in order to link together the weft yarns T belonging to different layers of warp thread, with the exception of debonding sites  106 , it being noted that a three-dimensional weave particularly with an interlock weave pattern, can include 2D weaving on the surface. Different three-dimensional weave patterns can be used, such as interlock, multi-satin or multi-ply weave patterns, for example, as described in particular in document WO 2006/136755. In  FIG. 4 , the fibrous blank  100  has two opposite surfaces  100   a ,  100   b  and comprises a first portion  102  and a second portion  104 . These two portions  102 ,  104  form respectively a first and a second portion of the thickness e of the fibrous blank  100  between its opposite surfaces  100   a ,  100   b.    
     Each portion  102 ,  104  of the fibrous blank comprises a plurality of superimposed layers of weft yarns T, four in the example illustrated, the number of weft yarns T being able to be any desired number, at least equal to two, depending on the desired thickness e. In addition, the number of layers of weft yarns in the portions  102  and  104  can be different from one another. The weft yarns T are arranged in columns each comprising weft yarns T from the first and from the second portion  102 ,  104  of the fibrous blank. On one portion of the dimension of the fibrous blank  100  in the warp direction C, the first portion  102  and the second portion  104  of the fibrous blank  100  are totally separated from one another by an open debonding site.  106  which extends from an upstream limit  106   a  to a downstream edge  100   c  of the fibrous blank  100 . What is meant here by an open debonding site is a zone closed at one end and open at an opposite end which does not have the warp yarns C 1 -C 8 , linking together the weft yarns T of layers belonging respectively to two of the layers passing through it, in the example here the second portion  104  and the second portion  104  of the fibrous blank  100 . 
     Aside from the open debonding site  108 , the layers of weft yarns T are linked together by warp yarns of a plurality of warp yarns C 1  to C 8 . In the example illustrated more precisely in  FIG. 5 , the same first warp yarn C 4  links together layers of weft yarns T of the first portion  102  of the fibrous blank adjacent to the debonding  106  and the to the layers of weft yarns T of the second portion  102  of the fibrous blank beyond the debonding  106 , i.e. before the upstream limit  106   a . Naturally, this connection could be accomplished by several first warp yarns. 
     Conversely, the same second warp yarn C 5  links together layers of weft yarns T of the second portion  104  of the fibrous blank adjacent to the open debonding  106  and to the layers of weft yarns of the first portion  102  of the fibrous blank beyond the closed debonding site. Of course, this connection could be accomplished by several second warp yarns. Thus, the trajectory of the warp yarn C 5  and that of the warp yarn C 6  cross at the upstream limit  106   a  of the open debonding site  106 . 
     The fibrous preform  10  therefore comprises, in the direction of the warp yarns C, a first portion  24  in which the first portion  102  and the second portion  104  are attached securely so as to form, after injection of the matrix, the flank of the platform  20 , and a second portion  25  extending between the upstream limit  106   a  of the debonding  106  and the downstream edge  100   c  of the preform, intended to form the sacrificial free end  26 . To this end it is sufficient, after weaving, to separate the two portions  102  and  104  and to cut one of them, then place the preform after cutting in a suitable mold in order to inject the matrix into it under vacuum, in conformity with the methods habitually used (for example by an RTM or VARRTM method). 
     In this first embodiment, the base  21  and the flanks  25  can be formed integrally and in a single piece (monolithically). In the case where the inter-blade platform  20  is made of composite material, the flanks  25  can then be obtained by creating an open debonding site at the two opposite edges of the fibrous reinforcement, before injection of the matrix under vacuum. 
     In a second embodiment, each of the flanks  25  can comprise a wall  29 , formed integrally and in a single piece with the base  21 , and a plate  30 , applied and attached to the wall  29 . 
     In a first variant embodiment, the plate  30  extends from the base  21  without covering the sacrificial free end  26 . The sacrificial end  26  of the flanks  25  is therefore formed by the uncovered portion of the walls  29 . The thickness of the plate  30  and of the wall  29  being constant, the thickness of the flank  25  is therefore reduced at its free end  26 . 
     In a second variant embodiment, the plate  30  extends from the base  21  and beyond the free end  26  of the wall  29 . The sacrificial free end  26  of the flanks  25  then corresponds to the portion of the plates  30  which extends beyond the walls  29 . The thickness e of the plate  30  and of the wall  29  being constant, the thickness e of the flank  25  is therefore reduced at its free end  26 . 
     The plates  30  can be attached to the facing faces  27  of the walls  29  of the flanks  25 , so as to extend one facing the other, under the base  21 , or as a variant on the opposite faces  28  of the walls  29  of the flanks  25  so as to extend on either side of said walls  29  ( FIG. 2 b   ). 
     The base  21  has two lateral edges  24 , extending substantially parallel to the flanks  25 , on either side of said flanks  25 . 
     Optionally, in order to further reduce the risks of damaging the fan  1  blades  2  in the event of an impact, the lateral edges  24  are sacrificial. The technology of creating the sacrificial lateral edges  24  of the base  21  can be substantially identical to that of the sacrificial free ends  26  of the flanks  25 , and comprise in particular machining, the creation of a debonding or the attachment of a plate  30  to the radial face of the base  21 , said plate  30  then forming the first surface  22  of the base  21 . 
     Moreover, a thickness e 1  of each sacrificial lateral edge  24  is less than an average thickness of the rest of the base  21 . What is meant here by thickness is the dimension along an axis normal to the base. 
     For example, the thickness of the sacrificial lateral walls  24  is at most equal to 75% of the average thickness of the rest of the base  21 , preferably equal at most to 50% of the thickness of the rest of the base  21 . 
     For each sacrificial lateral edge  24 , a length of the sacrificial lateral edge  24  can be comprised between three times and six times an average thickness of the rest of the base  21 . What is meant here by length is a dimension along a circumferential axis extending between the lateral edges  24  of the base  21 , which is normal to the flanks  25 . 
     In this second embodiment, the base  21  and the walls  29  of the flanks  25  can be formed integrally and in a single piece (monolithically). In the case where the inter-blade platform is made of composite material, the walls  29  of the flanks  25  can then be obtained by accomplishing an open debonding at the two opposite edges of the fibrous reinforcement, before the injection of the matrix under vacuum.