Abstract:
A turbomachine including securing means that extend between the pressure wall and the suction surface and which includes an energy absorbing portion for absorbing energy after impact to the blade by a foreign object. The energy absorbing portion has a catch that provides the blade with an improved resistance to bursting.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is entitled to the benefit of British Patent Application No. GB 0708377.7 filed on May 1, 2007. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a turbomachine blade, for example, a compressor blade for a gas turbine engine and in particular to a fan blade for a gas turbine engine. 
       BACKGROUND OF THE INVENTION 
       [0003]    A turbofan gas turbine engine  10 , as shown schematically in  FIG. 1 , comprises in axial flow series an inlet  12 , a fan section  14 , a compressor section  16 , a combustion section  18 , a turbine section  20  and an exhaust  22 . The fan section  14  comprises a fan rotor  24  carrying a plurality of equi-angularly-spaced radially outwardly extending fan blades  26 . A fan casing  28  that defines a fan duct  30  surrounds the fan blades  26  and the fan duct  30  has an outlet  32 . The fan casing  28  is supported from a core engine casing  34  by a plurality of radially extending fan outlet guide vanes  36 . 
         [0004]    The turbine section  20  comprises one or more turbine stages to drive the compressor section  18  via one or more shafts (not shown). The turbine section  20  also comprises one or more turbine stages to drive the fan rotor  24  of the fan section  14  via a shaft (not shown). 
         [0005]    One known wide chord fan blade is disclosed in US2004/0018091 to the present applicant and is depicted in  FIGS. 2 and 3 . The blade  26  comprises a root portion  40  and an aerofoil portion  42 . The root portion  40  comprises a dovetail root, a firtree root, or other suitably shaped root for fitting in a correspondingly shaped slot in the fan rotor, or for mounting to a disk to form a blisk by linear friction welding or other appropriate method. The aerofoil portion  42  has a leading edge  44 , a trailing edge  46  and a tip  48 . The aerofoil portion  42  comprises a concave wall  50 , which extends from the leading edge  44  to the trailing edge  46 , and a convex wall  52  that extends from the leading edge  44  to the trailing edge  46 . The concave and convex walls  50  and  52  respectively comprise a metal for example a titanium alloy. The aerofoil portion  42  has an interior surface  54  and at least a portion, preferably the whole, of the hollow interior  54  of the aerofoil portion  42  is filled with a vibration damping system  56 . 
         [0006]    The damping material  56  is a relatively low shear modulus material having viscoelasticity. Viscoelasticity is a property of a solid or liquid which when deformed exhibits both viscous and elastic behaviour through the simultaneous dissipation and storage of mechanical energy. Suitable materials comprise a polymer blend, a structural epoxy resin and liquid crystal siloxane polymer. 
         [0007]    One particular and preferred polymer blend comprises, per 100 grams: 62.6% Bisphenol A-Epochlorohydrin (Epophen resin EL5 available from Borden Chemicals, UK); 17.2 grams Amine hardener (Laromin C260 available from Bayer, Germany); 20.2 grams of branched polyurethane (Desmocap 11 available from Bayer, Germany). This polymer blend is then mixed in a mass ratio of 1:1 with a structural epoxy resin, preferably Bisphenol A-Epochlorohydrin mixed with an amine-terminated polymer (e.g. Adhesive 2216 available from 3M). 
         [0008]    A fan is susceptible to Foreign Object Damage, or FOD. Composite blades are not as robust as metal blades but offer advantages in terms of reduced mass. Where a hollow blade is provided there is a risk that the blade may burst when impacted by a large object. The use of a viscoelastic filler or core offers damping but also offers a secondary advantage in that the sides of the blade are held together to resist bursting, particularly busting at the trailing edge tip. Blade robustness may be improved through the provision of an internal warren truss arrangement as shown in  FIG. 4  where metal girders  60  extend between the concave face  50  and convex face  52  of the aerofoil. The viscoelastic damping material extends around the girders  60   
         [0009]    The girders inhibit bursting of the blade upon impact by foreign objects but provide a pathway for the transmittal of vibrational loads through the damping material which can render such damping material obsolete. 
       SUMMARY OF THE INVENTION 
       [0010]    Accordingly, the present invention seeks to provide a novel turbomachine blade that addresses, and preferably overcomes, the above mentioned problems. 
         [0011]    According to the invention, there is provided a turbomachine blade comprising a root portion and an aerofoil portion, the aerofoil portion having a leading edge, a trailing edge, a wall for forming a pressure surface extending from the leading edge to the trailing edge and wall for forming a suction wall extending from the leading edge to the trailing edge, wherein the aerofoil portion includes securing means extending between the pressure wall and the suction surface, wherein the securing means and comprising a first extension extending from the suction wall and a second extension extending from the pressure wall, the securing means having an energy absorbing portion comprising a first catch element provided on the first extension and a second catch element provided on the second extension and wherein the first catch element is arranged to engage with the second catch element for absorbing energy after impact to the blade by the foreign object. 
         [0012]    Preferably, the pressure wall is concave. The suction wall may be convex. 
         [0013]    Preferably, the first catch element and the second catch element are separated from each other by a volume containing a viscoelastic damper. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  depicts a first embodiment of a blade provided by the present invention in operational condition. 
           [0015]      FIG. 2  is a simplified schematic illustration of a known wide chord saw blade. 
           [0016]      FIG. 3  is a second schematic view of the fan blade of  FIG. 2 . 
           [0017]      FIG. 4  is a simplified schematic illustration, in section, of a known fan blade having an internal warner truss. 
           [0018]      FIG. 5  depicts a cross-sectional view of a blade having an internal spring. 
           [0019]      FIG. 6   a  schematically depicts a cross-sectional view of the blade of  FIG. 5  in normal use. 
           [0020]      FIG. 6   b  depicts a cross-sectional view of the blade of  FIG. 5  following an impact when the distance between the pressure and suction flanks increase significantly. 
           [0021]      FIG. 7   a  depicts a first embodiment of a blade provided by the present invention in operation. 
           [0022]      FIG. 7   b  is a simplified illustration of the blade of  FIG. 7   a  after impact from a foreign object. 
           [0023]      FIG. 8  depicts a cross-sectional view of a blade in accordance with a second embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    A blade  51  of  FIG. 5  has an internal spring section  70  that extends between the concave face  50  and convex face  52 . The spring is non-linear and enables vibrations at low strain levels to be accommodated in the viscoelastic damping material  56 . At least one spring element is provided, but where multiple spring elements are present these are provided at equal spacing across the span of the aerofoil that extends between the leading edge  44  and the trailing edge  46  and may be tessellated or interlinked. The springs are aligned with a length that extends generally parallel to the midline  80  of the blade and the majority of the springs are positioned in a region that is towards the tip end as this is the region most prone to failure in the case of soft body foreign object damage. 
         [0025]    The damping material  57  is a relatively low shear modulus material having viscoelasticity. Viscoelasticity is a property of a solid or liquid which when deformed exhibits both viscous and elastic behaviour through the simultaneous dissipation and storage of mechanical energy. Suitable materials for the damping layer  57  comprise a polymer blend, a structural epoxy resin and liquid crystal siloxane polymer. 
         [0026]    One particular and preferred polymer blend comprises, per 100 grams: 62.6% Bisphenol A-Epochlorohydrin (Epophen resin EL5 available from Borden Chemicals, UK); 17.2 grams Amine hardener (Laromin C260 available from Bayer, Germany); 20.2 grams of branched polyurethane (Desmocap 11 available from Bayer, Germany). This polymer blend is then mixed in a mass ratio of 1:1 with a structural epoxy resin, preferably Bisphenol A-Epochlorohydrin mixed with an amine-terminated polymer (e.g. Adhesive 2216 available from 3M). 
         [0027]    It is desirable for the damping material to have a modulus of elasticity in the range 0.5-100 MPa. 
         [0028]    The viscoelastic material allows the component to withstand high levels of vibration. The spring element  70  is formed integrally with the convex and concave surfaces and has a thickness  74  of about between 30 □m to 1 mm for an aero-fan blade. 
         [0029]    The spring element is formed during manufacture of the blade by powder fed laser deposition where a laser is directed at surface of the blade with sufficient power and focus to form a melt pool thereon into which a powder is supplied and melted. The laser translates across the surface and consequently the melt pool also translates across the surface. As the laser moves from an area to which powder has been added the added powder solidifies to form a deposit having a height. By making repeated passes over an area it is possible to add layers to previously added deposits thereby increasing the overall height of the deposit. 
         [0030]    As an alternative the springs may be formed using HIPping using an internal structure or a leachable or etchable support media. 
         [0031]    The spring may have other forms as embodied in  FIG. 6 . The spring in this embodiment comprises a tubular element, which is secured between the convex face and concave face by flanges  82 . The tube is oval in cross-section with the major axis  84  of the cross section lying substantially parallel to the concave and convex surfaces. Each flange is relatively thin to minimise the transmittal of vibrations. 
         [0032]    The tubes preferably run generally radially between the root and the tip though both the length and major axis can be orientated in other directions depending on the damping requirements and/or requirements on structural support.  FIG. 6(   a ) shows the arrangement in normal use. 
         [0033]    Upon impact of foreign objects, the blade may burst or deform with drastic changes to the cross-sectional width of the blade, i.e., the distance between the pressure flank and suction flank increases significantly as depicted in  FIG. 6(   b ). Upon such an impact the tubular member is stretched to absorb energy and retain the convex and concave surfaces preventing their separation. 
         [0034]    In an alternative aspect to the invention, the spring element is replaced with catches. The catches are not connected in normal use and consequently the vibrational transmit path is minimised. 
         [0035]      FIG. 7(   a ) depicts a first embodiment of a blade provided by the present invention in operational condition. The inside face of the concave surface is provided with a series of integral “T” arms  90  that are interleaved with a series of “T” arms  92  on the inside face of the convex surface. The top bars of the “T” arms overlap the top bars of the interleaved bars such that the underside faces of each bar opposes an underside face of an adjacent, interleaved bar. 
         [0036]      FIG. 7(   b ) is a simplified illustration of the blade of  FIG. 7(   a ) after impact from a foreign object. Upon failure of the blade because of impact, the convex face and concave face move apart and the underside of the “T” arms engage to retain the convex and concave surfaces and prevent their separation. 
         [0037]    The viscoelastic filler is added by pouring, under a slight positive pressure, the material into the internal cavity of the blade. 
         [0038]    In an alternative embodiment of the present invention shown in  FIG. 8 , the “T” arms are replaced with interlocking elongate structures with a “mushroom” form cross-section. The head and stalk of opposing “mushrooms” are shaped to provide a constant thickness of viscoelastic damper between them. 
         [0039]    The thickness of damping material is related to the damping modulus and has a thickness of between 500 and 1000 m. 
         [0040]    Although the invention has been described with reference to a fan blade  26 , it is equally applicable to a compressor blade. 
         [0041]    Although the invention has been described with reference to titanium alloy blades, it is equally applicable to other metal alloy, metal or intermetallic blades.