Abstract:
A turbine rotor blade of the spar and shell construction, where a one piece shell is secured to a hollow spar using a plurality of chordwise extending shear ties that are cast into a space formed between the shell interior and the spar exterior. A fill pipe is inserted into the hollow spar and is used to deliver the liquid retainer material to the hard to reach slots formed in which the shear ties solidify.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     None. 
     GOVERNMENT LICENSE RIGHTS 
     None. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates generally to a turbine rotor blade of a gas turbine engine, and more specifically to a turbine rotor blade having a spar and shell construction that can be easily assembled and disassembled. 
     Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98 
     In a gas turbine engine, a hot gas stream is passed through a turbine to drive a fan or an electric generator to power an aircraft in an aero gas turbine engine or to produce electrical power in the case of an industrial gas turbine engine. An efficiency of the engine can be increased by passing a higher temperature gas into the turbine. However, the turbine inlet temperature (TIT) is limited to the material properties of the turbine such as the turbine stator vanes and the rotor blades. Internal cooling of the vanes and blades of the turbine is done to allow for gas stream temperatures higher than the melting point of the airfoils. A typical turbine airfoil (blade or vane) is made from a nickel alloy which has high temperature resistance as well as high strength for use in a turbine. A nickel alloy rotor blade is formed as a single solid piece with internal cooling passages formed from an investment casting process. Stator vanes are also formed from casting a nickel alloy but with impingement cooling inserts added after the vane is cast. However, turbine inlet temperatures have reached a limit using these nickel alloy materials with airfoil cooling. 
     Recently the turbine rotor blade is formed from a spar and shell construction in which a shell is secured to a spar to form the rotor blade. With the spar and shell construction, the shell can be made from a different material than the spar. Thus, the shell can be formed from a higher temperature resistant material than that of the spar. The spar takes all the load from the shell so that the shell can be made mostly for high temperature resistance than for high stress resistance. With the spar and shell construction, the shell can be formed from a ceramic material or even a refractory metal such as molybdenum. One major problem with the spar and shell constructed turbine rotor blade is in the assembly or disassembly of the shell from the spar and of retaining the shell to the spar against the high centrifugal loads due to the rotation of the rotor blade. 
     Single crystal metals have been used for turbine rotor blades because of their high stiffness against centrifugal loads. To continue to increase the turbine inlet temperature beyond the current single crystal material capability different materials systems needs to be utilized. Ceramic materials and refractory alloys have higher use temperatures but do not have the tensile strength to be self-supporting at the rotational speeds and temperatures that are desirable. These materials could be used as a closeout for the airfoil if they could be kept in a compressive state or in a partially compressive and mild tensile state. This can be achieved by restraining the shell at the outer edge of the airfoil via a shelf that is attached to the tensile member called the spar which is internal to the shell. There is a practical limit to the speed and consequently the pull exerted on the spar because the entire load of the shell must be taken out at the tip and then this load transitioned inboard to the blade attachment. 
     BRIEF SUMMARY OF THE INVENTION 
     A spar and shell turbine rotor blade where the spar and shell are fabricated separately of differing materials and joined and made inseparable by the use of cast in place shear ties. The shell is fabricated by forming a hollow casing with the outside contour conforming to the desired airfoil contour. The wall thickness of the shell is controlled in order to keep the resultant stresses in an acceptable range for the material but is kept as thin as possible. The material of the shell could be silicon nitride, titanium aluminide, nickel aluminide, or one of the refractory alloys. The shell has an integral tip cap which forms a cavity that is closed at the end. The exterior surface is smooth for aerodynamic reasons and the inside has features at the tip for locking the shell to the spar and also has channels spaced radially also on the inside surface. The protrusions from the tip and the wall channels will provide the means for shearing the compressive load in the shell into the support structure of the spar. Conventional fabrication methods are used to make the shell. 
     The spar is fabricated conventionally from a superalloy such as cast IN100 or CM247 or one of the single crystal alloys and will carry the entire radial load of the airfoil to below the platform and through the attachment into the disk. The tip section has recesses for trapping the protrusions at the tip of the spar and channels which match the location of the channels on the shell when the spar is fully inserted into the shell. The spar wall is tapered from narrow at the tip too thick at the root in order to carry the progressively increasing load sheared in from the shell. 
     After the spar is fabricated a pipe or sprue is inserted into tapered channels formed in the spar walls and leading into the outward facing channels. This sprue is for the purpose of introducing material into the closed regions formed by the shell and spar matching channels and the tip region mating locks. 
     After the spar is fully inserted into the shell and secured, the locking shear material is poured into the sprue and by means of either positive pressure, vibration or centrifugal load caused to flow and fill all of the channels for purposes of locking the spar and shell together. This locking material can be a number of different viscous materials which could be caused to flow into and fill these cavities. The preferred material is a mixture of high temperature ceramics reinforced with chopped alumina or carbon fiber. A molten alloy could be used but would introduce the complication of elevating the temperature of the parts while pouring. After solidification of the shear ties the sprue will be removed along with excess lock material in order to keep the structure as light as possible. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  shows a cross section through the turbine rotor blade of the present invention. 
         FIG. 2  shows a cross section of the spar with a fill pipe or sprue of the turbine rotor blade of the present invention. 
         FIG. 3  shows a cross section of the shell of the turbine rotor blade of the present invention. 
         FIG. 4  shows a cross section of the spar and the fill pipe or sprue being inserted into the hollow shell of the turbine rotor blade of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is turbine rotor blade for use in a gas turbine engine where the rotor blade is formed from a spar and shell construction so that the shell can be formed from a high temperature resistant material. A one piece shell is secured to a one piece spar using shear ties that are cast in place when the shell is positioned over the spar. A fill pipe or sprue is used to channel the shear tie material from outside of the blade to spaces formed between the shell and the spar that form the shear ties when the material has solidified. 
       FIG. 1  shows a cross section of the turbine rotor blade of the present invention with the shell positioned over the spar. The shell  11  is fabricated by forming a hollow casing with the outside contour conforming to the desired airfoil contour. The wall thickness of the shell  11  is controlled in order to keep the resultant stresses in an acceptable range for the material but is kept as thin as possible. The shell  11  is a one-piece shell made from a relatively high temperature resistant material such as a silicon nitride, titanium aluminide, nickel aluminide, or one of the refractory alloys such as molybdenum. The spar  13  is a hollow spar that can be made from a lower temperature resistant material than the shell such as a superalloy like cast IN100 or CM247 or one of the single crystal alloys that is capable of withstanding the entire radial load of the shell. The shell  11  can include an integral tip cap  12  or one that is formed separately and then secured to the tip end of the hollow shell  11 . The shell  11  is secured to the spar  13  using cast in place shear ties  14 . The tip end of the spar  13  and tip cap  12  are also secured together by the cast in place material that forms the shear ties  14 . 
       FIG. 2  shows the spar  13  with a tip end  16  having a number of holes or tip locking holes  17  that open into the hollow section of the hollow spar  13 . An external side of the spar  13  includes a plurality of chordwise extending channels or slots  15  that form the spaces for the cast in place shear ties  14 . A fill pipe or sprue  21  includes a main channel with a number of branches  22  that open into the spaces formed for the cast in place shear ties  14 . The branches have openings on the ends that are aligned with holes in the spar that open into the channels  15  formed in the spar  13 . In the embodiment of  FIG. 2 , the spar is secured to the spar using four cast in place shear ties  14  extending on the pressure side and the suction side of the blade. 
       FIG. 3  shows an inside view of the shell  11  with four chordwise extending channels  24  that aligned with the four chordwise extending channels  15  in the spar  13  to form the spaces for the cast in place shear ties  14 . The tip cap  12  includes a plurality of tip locking projections  23  that extend from a bottom side of the tip cap and pass through the tip locking holes  17  in the tip end  16  of the spar  13 . When the spar  13  is positioned in place in the shell  11  and the locking material is injected into the space, the locking material will solidify around the projections  23  extending out from the bottom of the holes  17  and lock the tip ends of the spar  13  and shell  11  together. 
       FIG. 4  shows the fill pipe or sprue  21  positioned in place within the spar  13  with the spar being inserted into the hollow section of the shell  11 . The fill pipe  21  is used to channel a material that is injected into the spaces formed by the channels  15  and  24  that form the spaces for the shear ties  14 . The material is passed through the main channel  21  of the fill pipe and flows through the side branches  22  and then into the spaces formed by the channels to form the shear ties  14 . The space formed between the tip cap and the tip end of the spar  13  is also supplied with the material through the top end of the main branch  21  to fill the space around the tip locking projections  23  extending from the tip cap  12 . The material flows through the tip locking holes  17  formed in the tip end of the spar  13  to fill this space by the use of either pressure, vibration or centrifugal load. When the material that forms the shear ties  14  has solidified, the fill pipe  21  can be broken free and removed from the blade or left in place. 
     The spar  13  is tapered from narrow at the tip end to thicker at the root end in order to carry the progressively increasing load sheared in from the shell  11 . The locking material for the shear ties  14  can be a mixture of high temperature ceramics reinforced with chopped alumina or carbon fibers. A molten alloy could also be used but would introduce the complication of elevating the temperature of the parts while pouring. After solidification of the shear tie material, the sprue or fill pipe can be removed along with excess locking material in order to keep the blade as light as possible.