Patent Publication Number: US-2009232657-A1

Title: Blade

Description:
The present invention relates to blades for a turbomachine, and in particular the present invention relates to a fan blade for a turbofan gas turbine engine. 
     A turbofan gas turbine engine comprises a fan, which comprises a fan rotor and a number of circumferentially spaced radially outwardly extending fan blades secured to the fan rotor. The fan is surrounded by a fan casing, which defines a fan duct. The fan casing is also arranged to contain one or more of the fan blades in the unlikely event that a fan blade becomes detached from the fan rotor. 
     If a fan blade becomes detached from the fan rotor, for example due to impact with a large foreign body, e.g. a bird, the released fan blade strikes a main fan casing containment region. The fan blade generally progressively breaks up under a buckling action. The fan blade increases in strength from the tip to the root and at some position between the tip and the root the remaining portion of the fan blade, including the root, no longer buckles. The remaining portion of the fan blade has substantial mass and is accelerated to impact a rear fan containment region of the fan casing. 
     It is necessary to provide additional material to the rear fan containment region of the fan casing to contain the remaining portion of the fan blade. The additional material may be in the form of an increase in thickness, the provision of ribs, honeycomb liners etc, which dissipate the impact energy by plastic deformation of the material. However, these methods of protecting the rear fan containment region add weight to the turbofan gas turbine engine and so ideally should be avoided. 
     One approach taken to the above problem is defined in U.K. patent publication no. 2399866 (Rolls Royce). Apertures are provided in the root which extend through that root. The apertures create beneficial deflection upon impact such that there is a reduced load placed upon the rearward portions of the fan casing. It is therefore less necessary to provide additional reinforcement in the casing to resist remaining portions of the fan blade as described above. Unfortunately this approach requires use of intrusive machinery within the blade in order to form apertures which extend through the root generally to the blade cavity between the surfaces of that blade. Essentially by allowing deformation there is a reduction in the energy transferred to the casing by encouraging break up of the root fragment about the apertures, or at least flexing as described. 
     The problem with respect to machining processes such as drilling or otherwise to form the apertures is the associated risk of tool breakage in, and even by this stage, a relatively high value component. Furthermore, it will also be understood that break out of the aperture into the cavity formed in the blade is hard to design and control. Additionally, generally the cavity is no longer sealed by the aperture passing through the root to it and therefore generally a further operation is required in order to prevent fluid ingress to the cavity in use. Finally, it will be understood that if the cavity between the surfaces of the blade extends to a relatively low position in the root, that is to say the root is relatively thin so that introduction of apertures may create particular problems in this highly stressed region of the blade. 
     In accordance with the present invention there is provided a blade for a turbine engine, the blade comprising a super plastically formed sandwich comprising a concave wall and a convex wall with a cavity membrane therebetween, the walls and cavity member are secured together by bonding about their edges in order to define a cavity therebetween when subject to a super plastic deformation process, the blade characterised in that the membrane incorporates slots which extend towards the cavity once formed by super plastic deformation. 
     Generally, the slots are flat. Advantageously, the slots extend radially from a root base of the blade towards the cavity. Typically, the slots are of differing length. Generally, the slots extend below a release plane for the blade. Typically, the cavity is sealed. Possibly, the slots extend into open passage engagement with the cavity. Alternatively, the slots do not extend into open passage association with the cavity. 
     Also in accordance with the present invention there is provided a method of making a blade for a turbine engine, the method comprising taking two wall panels to be formed respectively as a concave wall and a convex wall in the blade and placing a membrane panel between them whereby superplastic deformation a blade is formed with a cavity between the walls, the method characterized in that prior to sandwiching the membrane panel between the walls, slots are formed in the membrane at positions and in configurations to extend towards the cavity once formed by the superplastic deformation process. 
     Typically, the walls and the membrane are secured together about their edges by a diffusion bonding process. 
     Generally, the slots are formed in the blade at a position whereby the slots facilitate deformation of a root portion of the blade. 
    
    
     
       Embodiments of the present invention will now be described by way of example and with reference to the accompanying drawings in which; 
         FIG. 1  is a schematic illustration of the panels utilised in order to form a blade by a superplastic formation and diffusion bonding process; 
         FIG. 2  is a schematic cross section through a blade root portion in accordance with the present invention; 
         FIG. 3  illustrates a schematic front view of a blade formed in accordance with the present invention; and 
         FIG. 4  provides a schematic cross section illustrating a blade root portion impact upon a fan casing containment within a turbine engine. 
     
    
    
     As indicated above, weight is an important factor with respect to turbine engines particularly when used in aircraft. Occasionally through excessive impacts blades will fracture and therefore the casing must be sufficiently robust in order to contain the blade fragments again so that the potential for aircraft wing or cabin puncture by the fragments is eliminated. By their nature, blades for turbine engines become more robust towards their root, particularly as blades will incorporate cavities themselves in order to reduce weight. Traditionally, blades have been cast or otherwise molded, but it is also possible to create blades by so called super plastic formation (SPF) and typically diffusion bonding about the edges of the component walls of the blade. The processes of superplastic formation are well known and generally will involve diffusion bonding about the edge of the wall panels and then through deformation in a mould, creation of a blade with a cavity between these wall panels results. Typically, the edges are finely finished in order to create the leading and trailing edges, etc. Generally between the wall panels a membrane will be provided which may incorporate a cavity or may be a flat sheet in order to provide additional material at the periphery of the blade for leading and trailing edges along with root formation as well as reinforcement. 
       FIGS. 1 and 2  show schematic depictions of in  FIG. 1  wall panels and a membrane panel to be associated in accordance with the present invention in order to create a blade, whilst  FIG. 2  shows a schematic cross section through a root portion of a formed blade. It will be appreciated that a blade will generally have a convex and a concave surface provided by the wall panels  1 ,  2  once formed. 
     In accordance with the present invention the membrane panel  3  incorporates slots  4  which extend in the portion of the eventual blade which will define the root for the blade. These slots  4  as described previously with respect to UK patent publication no. 2399866 will act to precipitate hinge and crack deformation by that root portion if it becomes detached and percussively impacts with a turbine engine casing. 
     In terms of the manufacturing process the walls  1 ,  2  will be arranged to sandwich the membrane panel  3  between them. Typically, the peripheral edges of the panels  1 ,  2 ,  3  will be secured together through a diffusion bonding process and then by outward expansion and super plastic forming, a cavity created in the central portion between the panels  1 ,  2 . Typically, the membrane as indicated above provides a so called line core which extends across the cavity in order to provide reinforcement in the blade. As indicated above, the slots  4  in the present invention will extend towards that formed cavity but may not actually enter into open passage engagement with the cavity. The purpose of the slots is to provide root fragmentation and deformation under impact in order to reduce the necessary robustness of the blade containment casing, and so provided that objective is achieved it is not necessary for the slots to be in open engagement with the cavity formed. 
       FIG. 2  illustrates a schematic cross section of a root portion of a blade in accordance with the present invention. In comparison with  FIG. 1  it will be noted that there are five slots  14 , rather than three as depicted in  FIG. 1 , but as indicated above the number of slots and their configuration is determined by the objective of creating root fragmentation and deformation for reduced impact problems with the containment casing, so that the number of slots can vary and their positioning altered in order to achieve that objective. As illustrated a blade  20  comprises a sandwich as illustrated previously of wall panels  21 ,  22  with a membrane panel  23  sandwiched between. It will be understood that in the root portion depicted in cross section  FIG. 2 , these panels  21 ,  22 ,  23  are all diffusion bonded together in order to form an essentially solid composite so that to illustrate this effect, broken lines  25  have been used. However, in reality such demarcation may be visually difficult to denote. 
     As can be seen, the slots  24  extend along the length of the root portion of the blade  20  in a generally radial band between an outer pressure surface  26  created by the wall panel  21  and an inner suction surface  27  created by the wall panel  22 . 
     The slots  24  as illustrated are utilised in order to create fragmentation and deformation under impact load so that it will be understood that the angular position  28  of each slot need not be regular but determined in order to create the desired propagation of fragmentation in the root section as well as deformation under impact. 
     It will be understood that the height or width  29  of each slot  24  is generally determined by the width of the membrane  23  sandwiched between the wall panels  21 ,  22  and diffusion bonded together. Again, this height or width  29  will be determined by operational requirements with respect to fragmentation of the root section under impact. 
     As indicated above, the slots are generally in a radial band between the wall panels  21 ,  22 . In such circumstances, as indicated above, the objective is to achieve root section fragmentation and deformation under impact so it will be understood that the angular extent  11  of the slots is not necessarily the same for all slots  24  within the root portion. Similarly, the angular separation  12  between slots  24  may be regular or irregular as required in order to achieve fragmentation and deformation under impact with a containment casing. 
     It will be appreciated in comparison with prior arrangements in which apertures were drilled or otherwise cut into a previously solid root portion, the present invention allows less stressful creation of slots in the root portion with less potential for tool failure in creation of the slots. Essentially no machining of the finish plate is required in order to form the slots. Consequently, material scrap losses are also greatly reduced. 
     As indicated above, the super plastic formation and diffusion bonding process in order to create a blade  20  allows a cavity to be formed which is essentially closed and therefore remains sealed avoiding any ingress of fluids during the lifetime of the blade. Additionally, by ensuring that the cavity remains sealed, it is still possible to use conventional physical tap testing in order to determine through typical quality control processes the acceptability of the blade. 
     As indicated above, generally with superplastically formed and diffusion bonded blade manufacturing processes, a line core web in the form of a membrane between the forming walls is utilised in order to create reinforcement. Provision of slots in accordance with the present invention does not impinge upon those webs and gives a clean entry to the blade cavity if required. However, alternatively the slots may stop short of the cavity provided the desired disposition to fragmentation and deformation under impact loads by the root portion with a containment casing is achieved. 
     As the slots can essentially be punched or otherwise formed in the membrane prior to assembly, it will be appreciated that a far greater range of slot size, shape and position in the root portion can be achieved and possible through machining apertures as utilised with prior approaches. 
       FIG. 3  provides a schematic plan view of a blade  31  in accordance with the present invention with a cavity depicted by broken line  30  and slots by broken lines  34 . As can be seen, the cavity  30  is centrally positioned within the blade  31 , and as described previously is formed by super plastic formation and diffusion bonding between wall panels and through diffusion bonding about the periphery of the blade  31 . Essentially the cavity  30  expands laterally and proportionately in order to define the concave and convex wall surfaces of a blade for operational functionality. Edges  14 ,  15  of the blade  31  will generally be formed as the leading and trailing edges. 
     Internally within the blade the slots  34  as can be seen extend towards the cavity  30 . In the embodiment depicted in  FIG. 3 , these slots  34  extend into open passage engagement  35  with the cavity  30 , but it will be understood that slots may be provided in a pre-formed membrane which will not extend into the cavity provided these slots facilitate fragmentation in the root portion and deformation under impact. 
     As depicted in  FIG. 3 , generally the blade cavity  30  is sealed as a production pre requisite in order to prevent fluid ingress during use. 
     Generally, a root portion  36  of the blade will be shaped or otherwise formed in order to create means for root engagement with a mounting disc for the blade  31 . Thus, the root portion  36  may adopt a dovetail or firtree configuration for such engagement with the rotor disc mounting for the blade  31 . In such circumstances, as can be seen, the slots  34   a  extend below a containment line or release plane for the blade  31  in use. In any event, the slots  34  encourage root fragmentation, break up and deformation under impact with a containment casing. 
     It will be understood that the use of slots in a membrane with subsequent superplastic formation and diffusion bonding in order to create the blade, enables a far wider range of slot structures and distributions to be used. In such circumstances, rather than being constrained by the capabilities of drill or other cut machining processes, it is possible to achieve a slot and blade design which is tunable to the particular requirements of the blade in use. Such tuning may be through comparison of a situation where if the blade fragments easily then there is localised plastic strain reduction, but energy transfer is similar to present, but if the design is altered to increase the proportion of material that is plastically deformed, then energy transferred to the casing is reduced, but localised plastic strains are relatively unchanged. 
       FIG. 4  illustrates the impact regime of the root fragment  41  after fracture with a fan casing  42 . Thus, the fragment as can be seen generally comprises a convex surface  43  and a concave surface  44 . Such an approach leads to leading and trailing edge impact reactions  45  with the casing  42 . By provision of slots  46  in the fragment  41  it will be understood that the slots facilitate bending  47  and hinging resulting in flexing and deformation of the fragment  41  dissipating energy. This bending  47  causes flexing of the fragment  41  towards the casing  43  increasing the impact surface between the fragment  41  and the casing  42  increasing the impact contact surface. It is also possible as described previously for the fragment to further fragment through cracking about the slots  46 . In any event, the bending  47  leads the central portions of the fragment  41  to move in the direction of arrowhead  48  as indicated in order to increase the impact area upon the casing  42 . 
     Increased nominal stress due to the reduced load bearing area plus stress concentration due to the slots and increased compliance results in greater plastic deformation and cracking dissipating energy. Eventually as indicated through plastic hinge deformation about the slots  46 , the root fragment  41  becomes flat or at least becomes flatter increasing the impact contact area with the casing  42 . 
     Modifications and alterations to the present invention will be appreciated by those skilled in the art. Thus, as illustrated in the embodiments above the slots generally will comprise cut outs within the membrane sandwiched between the diffusion bonded wall bands to that membrane. However, alternatively these slots may comprise insert elements of materially different mechanical properties to thereby render the root essentially solid but through those insert materials having a different reaction under stress to create fragmentation and deformation for energy loss and therefore reduced necessity for a robust more weighty containment casing. Furthermore, in addition to the slots being shaped and distributed to facilitate desired fragmentation and deformation under impact load with the casing, it will also be understood that the edges of the slots may be particularly shaped to facilitate plastic deformation for energy dissipation and/or cracking in the root fragment of a blade.