Abstract:
A method of forming a sheath for a fan airfoil having a leading edge, a trailing edge, a tip, a root, a suction side and a pressure side includes electroplating a nano-structured material to form a sheath with a solid portion to wrap around the leading edge and first and second flanks to secure the solid portion to the pressure side and the suction side of the airfoil.

Description:
BACKGROUND 
       [0001]    When exposed to impact loading, for example, a bird strike, a blade is subject to cracking, delamination (if the blade is a composite laminate blade) and deformation. This cracking or delamination from an impact loading is usually initiated at the leading edge of an airfoil. The cracking and delamination can then spread to other sections of the blade, potentially resulting in catastrophic failure of the blade. Fan blades are also subject to significant rain and sand erosion, especially at the leading edge. 
         [0002]    Adding a protective sheath over the leading edge of a lightweight airfoil, such as one made of aluminum alloy or a composite, can give the blade substantially all the strength of a blade made entirely of a high strength metal such as a titanium alloy. This strength helps to protect against cracking, delamination and deformation. Additionally, the leading edge can be made thinner than the lower strength or lower strain capable material it protects. Thinner leading edges provide engine efficiency improvements. A high strength metallic material offers significant benefits in weight and efficiency by restricting the use of the heavier, higher-strength material to only the sheath. The overall shape of a sheath and the need for a thin, sharp leading edge makes fan blade sheaths difficult and expensive to machine. Typically, sheaths for composite turbofan engine fan blades are made of titanium. Titanium is used for most legacy fan blades and has good strength and impact characteristics. 
         [0003]    Electroformed sheaths have been used in certain propeller and helicopter blades to provide wear and erosion resistance. Propeller blades generally have large leading radii and blunt leading edges, making propeller blades able to resist cracking from impacts, leaving the sheath to resist erosion. However, the technology cannot be easily adapted to blades for use in a turbofan engine due to the need for sharp leading edges on thin blades, and the speed of foreign objects striking the blades. The electroformed sheaths for propeller blades, typically made by electroplating Nickel (“Ni”) or Nickel-cobalt (“Ni—Co”), do not have adequate strength and ductility, in thin sections to adequately protect a fan blade in a turboengine. 
       SUMMARY 
       [0004]    A method of forming a sheath for a fan airfoil having a leading edge, a trailing edge, a tip, a root, a suction side and a pressure side includes electroplating an ultra-fine grained or nano-structured material to form a sheath with a solid portion to wrap around the leading edge and first and second flanks to secure the solid portion to the pressure side and the suction side of the airfoil. 
         [0005]    A sheath for a fan airfoil having a leading edge, a trailing edge, a tip, a root, a suction side and a pressure side includes a solid portion to wrap around the airfoil leading edge; a first flank for attaching the solid portion to the pressure side; and a second flank for attaching the solid portion to the suction side, wherein the sheath is electroformed with an ultra fine grained or nano-structured material. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a cross-sectional view of a gas turbine engine. 
           [0007]      FIG. 2  is a perspective view of a composite blade with sheath according to the present invention. 
           [0008]      FIG. 2A  is a cross-sectional view of the composite blade with sheath of  FIG. 2  from line A-A. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]      FIG. 1  is a cross-sectional view of gas turbine engine  10 , which includes turbofan  12 , fan case  13 , compressor section  14 , combustion section  16  and turbine section  18 . Compressor section  14  includes low-pressure compressor  20  and high-pressure compressor  22 . Air is taken in through fan  12  as fan  12  spins in fan case  13 . A portion of the inlet air is directed to compressor section  14  where it is compressed by a series of rotating blades and vanes. The compressed air is mixed with fuel, and then ignited in combustor section  16 . The combustion exhaust is directed to turbine section  18 . Blades and vanes in turbine section  18  extract kinetic energy from the exhaust to turn shaft  24  and provide power output for engine  10 . 
         [0010]    The portion of inlet air which is taken in through fan  12  and not directed through compressor section  14  is bypass air. Bypass air is directed through bypass duct  26  by guide vanes  28 . Some of the bypass air flows through opening  29  to cool combustor section  16 , high pressure compressor  22  and turbine section  18 . Fan  12  includes a plurality of blades  30  which spin in fan case  13 . 
         [0011]      FIG. 2  illustrates blade  30  with electroformed nano-structure sheath  32 .  FIG. 2A  illustrates a cross-sectional view of blade  30  with sheath  32  taken along line A-A in  FIG. 2 . 
         [0012]    Blade  30  includes airfoil  34  with leading edge  36 , trailing edge  38 , tip  40 , root  42 , suction side  44  and pressure side  46 . Sheath  32  includes solid portion  48  covering leading edge  36 , and tapered flanks  50  extending from each side of solid portion  48 . The cross-sectional view of blade  30  with sheath  32  in  FIG. 2A  includes sheath solid portion  48  with tapered flanks  50 , and airfoil  34  with leading edge  36 , trailing edge  38 , suction side  44  and pressure side  46 . 
         [0013]    Sheath  32  covers leading edge  36  of airfoil  34  with solid portion  48  by bonding tapered flanks  50  to suction side  44  and pressure side  46  of airfoil  34 . Tapered flanks  50  can be bonded to suction side  44  and pressure side  46  with various adhesives including, but not limited to, rubber, silicone or epoxy resin. Sheath  32  can be made of an ultra fine grained or nano-structured material, such as nano-Ni, nano-Co or a nano-Ni/Co alloy, which has sufficient stiffness and strength to withstand an impact load, such as a bird strike. The ultra fine grained or nano-structured material can have a grain size of about 10 nanometers to about 100 nanometers. Solid portion  48  of sheath  32  can vary in thickness to ensure that it covers the entire leading edge  36  of airfoil  34  and can be about 0.1 inches (2.54 mm) to about 0.2 inches (5.08 mm) thick. The length of solid portion  48  (extending out from leading edge  36 ) can vary widely, but must be sufficiently long to provide protection for leading edge  36  of blade  30 . 
         [0014]    Sheath  32  can be made by conventional electroplating of a nano-Ni material. This typically includes placing a tool formed corresponding to the desired sheath shape in a bath, hooking up a current to the tool, and allowing sufficient time for metal ions from the bath solution to plate directly onto the tool until the desired sheath thickness is reached. Once a desired thickness is reached, the tool is extracted from the bath, and the sheath is separated from the tool. The separation can be done by hand, by machine or a combination of both. The tool can then be reused. For a nano-Ni sheath, the tool can be made of titanium. 
         [0015]    Past leading edge sheaths were generally made by machining a piece of titanium into a desired shape corresponding with the blade to which the sheath was to be bonded. This process was very difficult due to the shape of a blade and the sharp edges required on the leading edge. This machining process also resulted in wasted material. The process of machining a sheath generally took about 30 hours, making it costly in terms of manufacturing personnel as well. By electroforming the sheath, no metal waste is generated. Additionally, the electroforming results in a sheath that is generally more uniform in thickness and shape and requires fewer man-hours to make. 
         [0016]    The use of nano-Ni in electroformed sheath  32  can increase the strength, toughness, hardness and ductility of sheath  32 . This leads to improved bird strike and erosion capability over conventional electroplated Ni sheaths. The ultra fine grained size results in improved strength and ductility over conventional coarse grained materials. Past electroformed sheaths were generally formed of Ni or Ni—Co. Electrofomed Ni—Co has good strength properties (ultimate tensile strength “UTS” of about 225 ksi and yield strength “YS” of about 137 ksi), but low ductility, for example 3% elongation ductility. Ni plating generally has good ductility properties, but lower strength. The use of nano-Ni, nano-Co or nano-Ni/Co alloy in electroforming results in sheath  32  having good strength due to the fine grain size in the material (UTS of about 180 ksi and YS of about 120 ksi) and good ductility, for example 7% elongation ductility. The high strength helps to resist cracking and deformation after impacts, and the high ductility allows the sheath to tolerate additional deflection instead of simply cracking when subject to an impact. 
         [0017]    Electroformed nano-Ni sheath  32  provides extra strength to blade  30 , allowing blade  30  to be made of lightweight materials, such as composites or aluminum (including aluminum alloys), and still maintain optimal performance and levels of aerodynamic efficiency under impact loading similar to the levels of prior art metal blades. Solid portion  48  of sheath  32  provides a layer of protection against impacts and erosion for leading edge  36  of airfoil  34 . Tapered flanks  50  bond solid portion  48  to airfoil to hold solid portion  48  in place. Tapered flanks  50  further provide extra stiffness to airfoil  48  and more surface area for a smooth load transfer during impacts to blade  30 . 
         [0018]    In summary, electroformed nano-structure sheath  32  provides strength, toughness, hardness and ductility to allow blade  30  to be made of lighter, more economical materials while maintaining similar resistance to erosion and impact cracking as past blades made of expensive material such as titanium. The use of electroplating makes the manufacture of sheath  32  more economical and more uniform than past methods of machining. The use of nano-structured materials, such as nano-Ni, allows for the use and benefits of electroforming while maintaining high levels of strength and ductility. 
         [0019]    Sheath  32  can be used to protect against delamination, deformation and cracking in any type of light weight blade (composite, carbon fiber, aluminum, etc.). While the means of securing sheath  32  to blade  30  is discussed as tapered flanks  50 , different means can be used to secure solid portion  48  of sheath  32  to airfoil  34 . 
         [0020]    While a general process was discussed in relation to electroforming sheath  32 , this was for example purposes only and any electroforming process generally known in the art can be used to electroform nano-structured sheath  32 . While the invention has been described mainly in relation to nano-Ni sheaths, sheath  32  can be formed of other ultra-fine grained materials. 
         [0021]    While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. For example, sheath could be formed in a different shape such as extending over the tip of blade as well as the leading edge. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.