Patent Publication Number: US-2022228496-A1

Title: Airfoil with wishbone fiber structure

Description:
BACKGROUND 
     A gas turbine engine typically includes a fan section, a compressor section, a combustor section and a turbine section. Air entering the compressor section is compressed and delivered into the combustion section where it is mixed with fuel and ignited to generate a high pressure and temperature gas flow. The high pressure and temperature gas flow expands through the turbine section to drive the compressor and the fan section. The compressor section may include low and high pressure compressors, and the turbine section may also include low and high pressure turbines. 
     Airfoils in the turbine section are typically formed of a superalloy and may include thermal barrier coatings to extend temperature capability and lifetime. Ceramic matrix composite (“CMC”) materials are also being considered for airfoils. Among other attractive properties, CMCs have high temperature resistance. Despite this attribute, however, there are unique challenges to implementing CMCs in airfoils. 
     SUMMARY 
     An airfoil according to an example of the present disclosure includes an airfoil wall that defines a leading end, a trailing end, and pressure and suction sides that join the leading end and the trailing end. The airfoil wall has a wishbone-shaped fiber layer structure. The wishbone-shaped fiber layer structure includes a pair of arms that merge into a single leg. The pair of arms are formed by first and second S-shaped fiber layers each of which is comprised of a network of fiber tows. The first and second S-shaped fiber layers merge to form the single leg. The single leg includes fiber tows from each of the first and second S-shaped fiber layers that are interwoven. The single leg forms at least a portion of the trailing end of the airfoil wall. 
     In a further embodiment of any of the foregoing embodiments, the wishbone-shaped fiber layer structure is situated between an exterior pressure side face skin fiber layer and an exterior suction side face skin fiber layer. 
     In a further embodiment of any of the foregoing embodiments, there is filler material between the single leg of the wishbone-shaped fiber layer structure and the exterior pressure side face skin fiber layer and between the single leg and the exterior suction side face skin fiber layer. 
     In a further embodiment of any of the foregoing embodiments, the filler material, the exterior pressure side face skin fiber layer, and the exterior suction side face skin fiber layer are of equivalent compositions. 
     In a further embodiment of any of the foregoing embodiments, the first and second S-shaped fiber layers are also of the equivalent composition. 
     In a further embodiment of any of the foregoing embodiments, the filler fiber material is selected from a monolithic ceramic, a 3-D woven fabric, fully or partially densified fiber material, or combinations thereof. 
     In a further embodiment of any of the foregoing embodiments, the pair of arms define an airfoil cavity there between. 
     In a further embodiment of any of the foregoing embodiments, the first and second S-shaped fiber layers are ceramic matrix composite. 
     In a further embodiment of any of the foregoing embodiments, the first and second S-shaped fiber layers define, respectively, thicknesses t 1  and t 2 , the single leg defines a thickness t 3 , and t 3  is less than or equal to the sum of t 1  and t 2 . 
     In a further embodiment of any of the foregoing embodiments, each of the first and second S-shaped fiber layers defines two inflection points and a region between the two inflection points over which the respective first or second S-shaped fiber layer is straight. 
     An airfoil according to an example of the present disclosure includes an airfoil wall defining a leading end, a trailing end, and pressure and suction sides that join the leading end and the trailing end. The airfoil wall has a series of wishbone-shaped fiber layer structures that are nested together and form at least a portion of the trailing end of the airfoil wall. Each of the wishbone-shaped fiber layer structures has a pair of arms that merge into a single leg. The pair of arms are formed by first and second fiber layers that are each comprised of a network of fiber tows. The first and second fiber layers merging to form the single leg. The single leg includes fiber tows from each of the first and second fiber layers that are interwoven. 
     In a further embodiment of any of the foregoing embodiments, the series includes at least three of the wishbone-shaped fiber layer structures. 
     In a further embodiment of any of the foregoing embodiments, the first and second fiber layers are S-shaped. 
     In a further embodiment of any of the foregoing embodiments, the airfoil wall includes one or more filler fiber plies between consecutive ones of the wishbone-shaped fiber layer structures. 
     In a further embodiment of any of the foregoing embodiments, the series of wishbone-shaped fiber layer structures are situated between an exterior pressure side face skin fiber layer and an exterior suction side face skin fiber layer. 
     In a further embodiment of any of the foregoing embodiments, the one or more filler fiber plies, the exterior pressure side face skin fiber layer, the exterior suction side face skin fiber layer, and the first and second fiber layers are of equivalent compositions. 
     In a further embodiment of any of the foregoing embodiments, the first and second fiber layers define, respectively, thicknesses t 1  and t 2 , the single leg defines a thickness t 3 , and t 3  is less than or equal to the sum of t 1  and t 2 . 
     A gas turbine engine according to an example of the present disclosure includes a compressor section, a combustor in fluid communication with the compressor section, and a turbine section in fluid communication with the combustor. The turbine section has airfoils disposed about a central axis of the gas turbine engine. Each of the airfoils has an airfoil wall defining a leading end, a trailing end, and pressure and suction sides that join the leading end and the trailing end. The airfoil wall has a wishbone-shaped fiber layer structure. The wishbone-shaped fiber layer structure has a pair of arms that merge into a single leg. The pair of arms are formed by first and second S-shaped fiber layers each of which is comprised of a network of fiber tows. The first and second S-shaped fiber layers merging to form the single leg. The single leg includes fiber tows from each of the first and second S-shaped fiber layers that are interwoven. The single leg forms at least a portion of the trailing end of the airfoil wall. 
     In a further embodiment of any of the foregoing embodiments, the wishbone-shaped fiber layer structure is situated between an exterior pressure side face skin fiber layer and an exterior suction side face skin fiber layer, there is filler fiber material between the single leg of the wishbone-shaped fiber layer structure and the exterior pressure side face skin fiber layer and between the single leg and the exterior suction side face skin fiber layer, and the pair of arms define an airfoil cavity there between. 
     In a further embodiment of any of the foregoing embodiments, the filler fiber material, the exterior pressure side face skin fiber layer, the exterior suction side face skin fiber layer, the first and second S-shaped fiber layers are of the equivalent ceramic matrix composite compositions, the first and second S-shaped fiber layers define, respectively, thicknesses t 1  and t 2 , the single leg defines a thickness t 3 , and t 3  is less than or equal to the sum of t 1  and t 2 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various features and advantages of the present disclosure will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows. 
         FIG. 1  illustrates a gas turbine engine. 
         FIG. 2  illustrates a sectioned view of an airfoil of the engine. 
         FIG. 3  illustrates the trailing end of the airfoil of  FIG. 2 . 
         FIG. 4  illustrates a representative view of a fiber layer. 
         FIG. 5A  illustrates a wishbone-shaped fiber structure. 
         FIG. 5B  illustrates another example wishbone-shaped fiber structure. 
         FIG. 6  illustrates another example airfoil in which a 3-D fiber filler is used. 
         FIG. 7  illustrates nested wishbone-shaped fiber structures. 
         FIG. 8  illustrates nested wishbone-shaped fiber structures with fiber filler plies in between the fiber structures. 
         FIG. 9  illustrates another example trailing end of an airfoil. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically illustrates a gas turbine engine  20 . The gas turbine engine  20  is disclosed herein as a two-spool turbofan that generally incorporates a fan section  22 , a compressor section  24 , a combustor section  26  and a turbine section  28 . The fan section  22  drives air along a bypass flow path B in a bypass duct defined within a housing  15  such as a fan case or nacelle, and also drives air along a core flow path C for compression and communication into the combustor section  26  then expansion through the turbine section  28 . Although depicted as a two-spool turbofan gas turbine engine in the disclosed non-limiting embodiment, it should be understood that the concepts described herein are not limited to use with two-spool turbofans as the teachings may be applied to other types of turbine engines including three-spool architectures. 
     The exemplary engine  20  generally includes a low speed spool  30  and a high speed spool  32  mounted for rotation about an engine central longitudinal axis A relative to an engine static structure  36  via several bearing systems  38 . It should be understood that various bearing systems  38  at various locations may alternatively or additionally be provided, and the location of bearing systems  38  may be varied as appropriate to the application. 
     The low speed spool  30  generally includes an inner shaft  40  that interconnects, a first (or low) pressure compressor  44  and a first (or low) pressure turbine  46 . The inner shaft  40  is connected to the fan  42  through a speed change mechanism, which in exemplary gas turbine engine  20  is illustrated as a geared architecture  48  to drive a fan  42  at a lower speed than the low speed spool  30 . The high speed spool  32  includes an outer shaft  50  that interconnects a second (or high) pressure compressor  52  and a second (or high) pressure turbine  54 . A combustor  56  is arranged in exemplary gas turbine  20  between the high pressure compressor  52  and the high pressure turbine  54 . A mid-turbine frame  57  of the engine static structure  36  may be arranged generally between the high pressure turbine  54  and the low pressure turbine  46 . The mid-turbine frame  57  further supports bearing systems  38  in the turbine section  28 . The inner shaft  40  and the outer shaft  50  are concentric and rotate via bearing systems  38  about the engine central longitudinal axis A which is collinear with their longitudinal axes. 
     The core airflow is compressed by the low pressure compressor  44  then the high pressure compressor  52 , mixed and burned with fuel in the combustor  56 , then expanded through the high pressure turbine  54  and low pressure turbine  46 . The mid-turbine frame  57  includes airfoils  59  which are in the core airflow path C. The turbines  46 ,  54  rotationally drive the respective low speed spool  30  and high speed spool  32  in response to the expansion. It will be appreciated that each of the positions of the fan section  22 , compressor section  24 , combustor section  26 , turbine section  28 , and fan drive gear system  48  may be varied. For example, gear system  48  may be located aft of the low pressure compressor, or aft of the combustor section  26  or even aft of turbine section  28 , and fan  42  may be positioned forward or aft of the location of gear system  48 . 
     The engine  20  in one example is a high-bypass geared aircraft engine. In a further example, the engine  20  bypass ratio is greater than about six (6), with an example embodiment being greater than about ten (10), the geared architecture  48  is an epicyclic gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.3 and the low pressure turbine  46  has a pressure ratio that is greater than about five. In one disclosed embodiment, the engine  20  bypass ratio is greater than about ten (10:1), the fan diameter is significantly larger than that of the low pressure compressor  44 , and the low pressure turbine  46  has a pressure ratio that is greater than about five 5:1. Low pressure turbine  46  pressure ratio is pressure measured prior to inlet of low pressure turbine  46  as related to the pressure at the outlet of the low pressure turbine  46  prior to an exhaust nozzle. The geared architecture  48  may be an epicycle gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.3:1 and less than about 5:1. It should be understood, however, that the above parameters are only exemplary of one embodiment of a geared architecture engine and that the present invention is applicable to other gas turbine engines including direct drive turbofans. 
     A significant amount of thrust is provided by the bypass flow B due to the high bypass ratio. The fan section  22  of the engine  20  is designed for a particular flight condition—typically cruise at about 0.8 Mach and about 35,000 feet (10,668 meters). The flight condition of 0.8 Mach and 35,000 ft (10,668 meters), with the engine at its best fuel consumption—also known as “bucket cruise Thrust Specific Fuel Consumption (′TSFC)”—is the industry standard parameter of lbm of fuel being burned divided by lbf of thrust the engine produces at that minimum point. “Low fan pressure ratio” is the pressure ratio across the fan blade alone, without a Fan Exit Guide Vane (“FEGV”) system. The low fan pressure ratio as disclosed herein according to one non-limiting embodiment is less than about 1.45. “Low corrected fan tip speed” is the actual fan tip speed in ft/sec divided by an industry standard temperature correction of [(Tram ° R)/(518.7° R)] 0.5 . The “Low corrected fan tip speed” as disclosed herein according to one non-limiting embodiment is less than about 1150 ft/second (350.5 meters/second). 
       FIG. 2  illustrates a sectioned view of an example airfoil  60  from the turbine section  28  of the engine  20  (see also  FIG. 1 ). For example, the airfoil  60  may be a blade or a vane. It is to be understood that although the examples herein are discussed in context of a turbine airfoil, the examples can be applied to airfoils in other sections of the engine  20 . 
     The aerodynamic profile of the airfoil  60  is formed by an airfoil wall  62 . In this regard, the airfoil wall  62  defines a leading end  62   a , a trailing end  62   b , a pressure side  62   c , and a suction side  62   d  of the airfoil  60 . The airfoil wall  62  circumscribes an interior cavity  64 , to which cooling air (e.g., from the compressor section  24 ) may be provided. 
     The airfoil wall  62  is formed of a ceramic matrix composite (CMC), an organic matrix composite (OMC), or a metal matrix composite (MMC). For instance, a CMC is formed of ceramic fibers that are disposed in a ceramic matrix. The CMC may be, but is not limited to, a SiC/SiC ceramic matrix composite in which SiC fibers are disposed within a SiC matrix. Example organic matrix composites include, but are not limited to, glass fiber, carbon fiber, and/or aramid fibers disposed in a polymer matrix, such as epoxy. Example metal matrix composites include, but are not limited to, boron carbide fibers and/or alumina fibers disposed in a metal matrix, such as aluminum. 
     Due to bending, thermal gradients, and pressure loading, a composite material in a trailing end of an airfoil is subjected stresses. Stresses on a composite material that is formed of fiber plies can cause the plies to delaminate from each other along the interfaces of the plies. Additionally, many composites use non-structural filler materials in regions between plies or groups of plies. Such fillers have different properties than the adjacent composite layup, which can exacerbate stresses on adjacent plies. As will be described below, the airfoil  60  includes features to facilitate reduction in stresses and, therefore, enhance durability. 
     One such feature is a wishbone-shaped fiber layer structure.  FIG. 3  illustrates a view of the trailing end of the airfoil  60  in which the airfoil  60  includes a wishbone-shaped fiber layer structure  66  (hereafter “structure  66 ”). The term “wishbone-shaped” refers to the resemblance of the structure  66  to a wishbone that has a leg and two arms that extend from an end of the leg to form a “V.” Here, the structure  66  includes a pair of arms  66   a  that merge in a transition region  66   b  into a single leg  66   c . In this example, the arms  66   a  are formed by first and second S-shaped fiber layers  68   a / 68   b . Each of the fiber layers  68   a / 68   b  is comprised of a network (e.g., a weave) of fiber tows  70  that are disposed in a matrix material  72  (both represented schematically). As an example, a representative portion of the fiber layers  68   a / 68   b  is depicted in  FIG. 4 . It is to be understood that although the network of fiber tows  70  is shown with a particular weave pattern, that other weave patterns may be used. Referring again to  FIG. 3 , the fiber layers  68   a / 68   b  merge in the transition region  66   b  to form the single leg  66   c . The single leg  66   c  forms at least a portion of the trailing end  62   b  of the airfoil wall  62 . As shown, the fiber layers  68   a / 68   b  may optionally include one or more additional layers  63   
       FIG. 5A  shows a representative view of a portion of the structure  66 . The woven fiber tows  70  of the fiber layers  68   a / 68   b  merge together such that the fiber tows  70  from each of the first and second fiber layers  68   a / 68   b  are interwoven in the single leg  66   c . Thus, once merged, the fiber layers  68   a / 68   b  cease to be distinct from each other and there are no interlaminar interfaces. In one further example that represents the merging, the fiber layers  68   a / 68   b  define, respectively, thicknesses t 1  and t 2 , the single leg  66   c  defines a thickness t 3 , and t 3  is less than or equal to the sum of t 1  and t 2 . That is, the two layers combine to form a single layer of equal or lesser thickness than the two layers combined. 
       FIG. 5B  shows another example wishbone-shaped fiber layer structure  166  that can alternatively be used. In the prior example the fiber layers  68   a / 68   b  are woven into 2D single leg  66   c  layer. In this example, however, the fiber layers  168   a / 168   b  are woven into a 3D single leg  166   c . It is to be appreciated that the illustrated fiber architectures are non-limiting examples and that other fiber architectures may alternatively be used in the wishbone structure. 
     Referring again to  FIG. 3 , the first and second fiber layers  68   a / 68   b  are each S-shaped. An “S-shape” as used herein refers to the resemblance of the shape of the first and second fiber layers  68   a / 68   b  to the letter “S.” In this regard, each of the first and second fiber layers  68   a / 68   b  includes two inflection points P 1  and P 2  at which the curvature of the respective fiber layers  68   a / 68   b  changes from concave to convex and vice versa. The inflection points P 1  and P 2  of the S-shape serve to provide two locations for the structure  66  to flex when under a bending moment. The allowance of the structure  66  to flex at these two locations distributes the stress rather than having it concentrated at one location, thereby reducing the peak stress at any single location. Further examples of the S-shaped geometry are discussed below. 
     In between the inflection points P 1  and P 2  the fiber layers  68   a / 68   b  may be entirely curved. However, in the illustrated example, the fiber layers  68   a / 68   b  each have a region  76  over which they are straight. The straight region  76  serves to enable lengthening or shortening the S-shape of the fiber layers  68   a / 68   b . For instance, air pressure in the internal cavity  64  tends to spread the fiber layers  68   a / 68   b  apart, thereby subjecting the fiber layers  68   a / 68   b  to bending moments. For relatively higher pressures/stresses a longer straight region  76  may be used for high stress reduction, while for relatively lower pressures/stresses in the interior cavity  64  a shorter straight region  76  may be selected since the pressure/stress is lower. That is, the geometry of the S-shape can be easily tailored in the design stage to the expected stress and pressure conditions in the airfoil  60 . Additionally, the distance between the transition region  66   b  and the inflection points P 2  may be adjusted in the design stage to tailor the size of the interior cavity and the stresses in the trailing end  62   b.    
     The airfoil wall  62  further includes an exterior pressure side face skin fiber layer  72   a  and an exterior suction side face skin fiber layer  72   b . The structure  66  is situated between the face skin fiber layers  72   a / 72   b . There is filler material  74  between the structure  66  and each of the face skin fiber layers  72   a / 72   b . In this example, the filler material  74  includes one or more CMC fiber plies  74   a  that are stacked to a desired thickness to fill the space between the structure  66  and the face skin fiber layers  72   a / 72   b . As shown, the fiber plies  74   a  are stacked such that the shortest plies are on the inside against the structure  66 . Alternatively, however, the stack may be inverse, such that the longest plies are against the structure  66 . The filler material  74  may be densified with the structure  66 , partially pre-densified prior to densification of the structure  66 , or fully densified prior to densification of the structure  66 . In the illustrated example, the thickness of the filler fiber material  74  increases along the fiber layers  68   a / 68   b  (in a direction toward the tip of the trailing end  62   b ). The thickness is maximum along the transition region  66   b  and then decreases along the single leg  66   c.    
     In one example, the filler material  74  and the face skin fiber layers  72   a / 72   b  are of equivalent compositions. For instance, the filler material  74  and the face skin fiber layers  72   a / 72   b  are composed of CMCs that nominally have the same composition fibers, the same composition matrix, and the same volume amounts of fibers and matrix. In one further example, the fiber layers  68   a / 68   b  are also of the same, equivalent composition as the filler fiber material  74  and the face skin fiber layers  72   a / 72   b . This eliminates a difference in properties between different materials that can otherwise exacerbate stresses. 
       FIG. 6  illustrates a modified example of the airfoil  60  that is the same as shown in  FIG. 3  except that instead of filler material  74  there is filler material  174 . For instance, the filler material  174  is a 3-D woven fabric that is shaped to a desired geometry and thickness to fill the space between the single leg  66   c  and the face skin fiber layers  72   a / 72   b . Some types of airfoils, such as turbine vanes, are relatively small and a 3-D woven fabric may not be feasibly manufactured to such a small size. Therefore, the filler fiber material  174  may be better suited to relatively larger airfoils. In one alternative, the filler material  174  is a monolithic ceramic, such as SiC or Al 2 O 3 . In further examples, the filler material  174  may be a combination of full or partially densified CMC plies and monolithic structure. 
       FIG. 7  illustrates another example that can be incorporated into the trailing end  62   b  of the airfoil  60 . Here, rather than a single wishbone-shaped fiber layer structure  66  as in the prior examples, there is a series of structures  66  that are nested together to form a chevron pattern. For instance, each structure  66  nests into the “V” of the next structure  66  to in essence form a stack of structures  66 . As further shown in  FIG. 8 , one or more filler fiber plies  78 , which may be the same as the plies used above in the filler fiber material  74 , can be provided between consecutive ones of the structures  66 . The filler fiber plies  78  serve to strengthen the overall structure and also take up space in order to achieve the desired shape of the trailing end  62   c . The filler fiber plies  78  may be 2D fiber layers or 3D fiber pieces. As shown, the filler fiber plies  78  are discrete pieces between each structure  66 , however, the filler fiber plies  78  may alternatively be continuous V-shaped pieces that nest into the “V” of the structure  66 . 
       FIG. 9  illustrates a further example that can be applied with any of the aforementioned examples. As depicted, the only the peripheral outlines of the various fiber layers are shown, although it is understood that the nominal structure is as shown in  FIG. 3 , or alternatively as shown in  FIG. 6 . In this example, there is a height “h 1 ” defined as the distance taken perpendicularly from the surface of the face skin  72   b  to the inside surface of the fiber layer  68   b  and intersecting a point “a” at which the fiber layer  68   b  begins to bend into a convex curvature. There is a second height “h 2 ” defined as the distance taken perpendicularly from the surface of the face skin  72   b  to the point “b” at which the fiber layers  68   a / 68   b  initially merge. There is a length “L” that is defined as the distance taken perpendicularly from the height h 1  to the height h 2 . Each inflection point P 1  and P 2  also have associated radii of curvature R 1  and R 2 , respectively. Optionally, if there a straight portion  76 , it has a length “G.” It is to be understood that the fiber layer  68   a  has the same above attributes as fiber layer  68   b  with respect to face skin  72   a.    
     In the illustrated example, the various heights and lengths above are selected to evenly distribute bending stresses due to pressure in the cavity  64 . For example: 
       2h1≤L≤4h1;
 
       5h1≤R1≤8h1;
 
       5h1≤R2≤8h1;
 
       1.1h1≤h2≤3h1; and
 
       0≤G≤2h1
 
     When the cavity  64  is pressurized, the pressure tends to spread the fiber layers  68   a / 68   b  apart, thereby applying a bending stress on the fiber layers  68   a / 68   b . The attributes above serve to facilitate an even distribution of that stress along the lengths of fiber layers  68   a / 68   b  from the location at “a” to the location at “b.” For instance, the attributes above facilitate the establishment of each fiber layer  68   a / 68   b  as a constant strength beam such that for a given bending moment applied at the location of h 1 , the stress along the respective fiber layers  68   a / 68   b  from “a” to “b” is substantially constant. Thus, the rather than the bending stress being concentrated at a location, which would cause a relatively high stress peak at that location, the stress is distributed and thus lowered below the peak stress. That is, the gradual, constant increase in height from h 1  to h 2  provided by the S-shaped geometry serves to facilitate distributed, relatively lower stress. 
     Although a combination of features is shown in the illustrated examples, not all of them need to be combined to realize the benefits of various embodiments of this disclosure. In other words, a system designed according to an embodiment of this disclosure will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments. 
     The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims.