Abstract:
A protective hybrid composite for a rotor blade is based on the use of tape cast ceramic layers densified by pre-ceramic polymer infiltration methods and laminated together with polymer matrix composite prepregs, with or without an embedded metallic mesh, to form a conforming helicopter blade cladding that is laminated to the blade surface for added erosion protection. The hybrid composite is fabricated to net shape and laminated to the blade using either an adhesive or a polymer composite prepreg inner layer. Installation is accomplished by a standard composite fabrication method of vacuum bagging the blade while the system is laminated to its surface. Repair methods based on removal of ceramic tiles is facilitated by incorporation of a metallic mesh element laminated beneath the ceramic tiles that can be used to heat the tile and decrease its adhesion strength.

Description:
PRIORITY 
     This application claims priority under 35 U.S.C. §119(e) to U.S. Patent Application No. 61/220,033, which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a hybrid composite segment. Specifically, the present invention is for a hybrid composite segment for erosion resistant helicopter rotor blades. 
     BACKGROUND OF THE INVENTION 
     Impact from sand and other debris can be detrimental to the lifetime of rotating components such as helicopter blades. In a desert environment, blade leading edges are exposed to both rain and sand erosion. 
     One attempt at protecting blade leading edges is to use a leading edge metallic erosion strip consisting of nickel (Ni) on an outboard portion and titanium (Ti) on the inboard portion of the blade. The metallic strips are further protected by polymeric tapes or coating even though these are generally less effective in rain erosion conditions. 
     Other attempts at increasing the life of rotor blades can be found in U.S. patent application publication no. 2005/0169763, for example, which uses a strip of resilient polymer adhered to the leading edge of the blade. Others have simply placed a ceramic component onto the leading edge of the rotor blade, as disclosed in U.S. Pat. Nos. 6,447,254, 5,782,607, and 5,542,820. Still others have capped the leading edge with a nanoparticle-reinforced elastomer, as disclosed in U.S. Pat. No. 6,341,747. However, the above prior art attempts do not increase the time between maintenance of the rotor blades and decrease costs. 
     In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved erosion resistant helicopter rotor blade. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the attached drawings that form a part of this original disclosure: 
         FIG. 1  illustrates a plurality of hybrid segments in accordance with an embodiment of the present invention; 
         FIG. 2A  illustrates the plurality of hybrid segments disposed on a leading edge of a helicopter rotor blade in accordance with the embodiment of the present invention; 
         FIG. 2B  is a magnified view and partial cut-away of selected hybrid segments in  FIG. 2A ; 
         FIG. 3A  illustrates another embodiment of the present invention wherein the hybrid segments are disposed on a leading edge and include tiles that extend across an upper and lower surface of the blade; 
         FIG. 3B  is a magnified view and partial cut-away of selected hybrid segments in  FIG. 3A ; 
         FIG. 4A  is a partial cut-away and schematic of the embodiment shown in  FIGS. 3A and 3B ; 
         FIG. 4B  is a partial cut-away and schematic of another embodiment of the present invention wherein a gap is present at a tip of the blade; 
         FIG. 4C  is a partial cut-away and schematic of another embodiment of the present invention wherein tiles are staggered in position from one layer to the next layer; 
         FIG. 4D  is a partial cut-away and schematic of another embodiment of the present invention wherein tiles of varying thicknesses are in a staggered array; and 
         FIG. 5  is a partial cut-away and schematic of another embodiment of the present invention with an embedded metallic mesh. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIGS. 1-2 , a first embodiment of a hybrid composite  1  is shown generally at  1 . The hybrid composite  1  includes a plurality of hybrid segments  12  disposed on a leading edge  11  of a rotor blade  10 . The hybrid composite  1  protects areas of the blade  10  most prone to erosion damage. The hybrid segments  12  are disposed side by side on the blade  10  on a portion of the blade and are spaced apart at a predetermined distance. Thus, protection is provided at a lower cost because the hybrid segments  12  can be placed only in areas that experience high erosion. Each of the hybrid segments  12  includes multiple thin layers of erosion-resistant ceramic material, alternating with layers of tough fiber-reinforced polymer matrix composite  16 . 
     The hybrid composite  1  comprises multiple thin layers of segmented, erosion-resistant ceramic material, alternating with continuous layers of fiber reinforced polymer matrix composite  16 . The number of layers is preferably 2 to 10 layers, for example, that provides an overall thickness of the composite  1  in the range of 1 to 3 mm, for example. The hybrid composite  1  is tolerant to damage, while presenting a ceramic surface with high hardness and erosion resistance. When disposed on the blade  10 , the hybrid composite  1  takes the form of a net shaped cladding. Thus, the present invention advantageously provides a hybrid composite  1  formed as a net shaped cladding that is laminated directly to the surface of the blade  10  using the polymer matrix composite  16  or another adhesive. In addition, the curved ceramic segments, which are formed to net shape, are laminated with polymer composite prepreg layers. The ability to produce the ceramic elements to net shape using thin flexible tape-cast layers that can be molded in their green state is beneficial. 
     The erosion-resistant ceramic material comprises a hard ceramic containing, for example, Al 2 O 3 , SiC, Si 3 N 4  and B 4 C. In the embodiment shown in  FIGS. 1-2 , the exterior ceramic material constitutes an exterior shell  14 , and the interior ceramic material constitutes an interior shell  18 . A layer of the polymer matrix composite  16  is disposed between the exterior shell  14  and the interior shell  18  as well as between the interior shell  18  and the leading edge  11 . 
     The exterior shell  14  protects against erosion from sand and rain. Furthermore, the multilayer structure of the hybrid composite  1  protects against unusually excessive erosion that may eventually penetrate the exterior shell  14 . The polymer matrix composite  16  includes optimal reinforcement architectures to reduce crack opening displacements of the ceramic shells  14 ,  18  or tiles  20  in the event of fracture and to bond strongly to the ceramic material to prevent the loss of broken fragments. 
     The hybrid segments  12  are bent around the leading edge  11  so as to generally take the form of the leading edge  11 . That is, the hybrid segments  12  can be C-shaped or U-shaped. Specifically, the hybrid segments  12  are formed such that an exterior surface follows the outer-mold line profile of the blade  10  including the leading edge or blade tip  11  curvatures. In other words, the exterior shell  14  is disposed on a layer of the polymer matrix  16  positioned on the interior shell  18  such that the exterior shell  14  continues or matches an exterior contour of the blade  10 . The interior shell  18  is directly adhered to the leading edge  11  of the blade  10  using the polymer matrix composite  16  or an adhesive. 
     In use, a plurality of the hybrid segments  12  is disposed on the blade  10 . Preferably, the segments  12  are spaced apart at a predetermined distance. The predetermined distance can affect the stiffness and is determined to allow deflection matching to the underlying blade  10 . In addition, the lateral dimensions of the hybrid segments  12  can affect stiffness and are determined or adjusted accordingly. 
     Various hybrid segments  12  can be replaced as they become worn without having to replace all of the segments  12 . That is, if a portion of the blade  10  experiences more wear than other portions, the corresponding hybrid segments  12  with higher wear can be replaced. In addition, the composition of the hybrid segments  12  can be varied to tailor stronger segments for those portions of the blade  10  that consistently have more wear than other portions. 
     Unlike prior art components that are formed as a single piece along the length of the leading edge, the hybrid segments  12  are not monolithic. Specifically, the hybrid segments  12  are made as replaceable pieces that are disposed on the leading edge  11  and are individually removable as wear occurs. This advantageously allows failure in a non-catastrophic manner. That is, the hybrid segments  12 , in their segmented geometry, can be replaced without replacing an entire, one-piece component that covers the entire length of the leading edge. 
     Indeed, the use of the hybrid segments  12  advantageously allows field repair by replacement of individual hybrid segments  12  or individual exterior shells  14 . As explained in detail below, the hybrid composite  1  can include a metallic mesh  22  to aid in removal of the hybrid segments  12  or merely the exterior shell  14 . 
     The hybrid composite  1  is assembled as a single blade cover or cladding with at least one continuous polymer matrix composite  16  layer holding the hybrid segments  12  in place. The design and manufacture of the hybrid composite  1  facilitates assembly while not precluding subsequent replacement of individual damaged hybrid segments  12 . The ceramic tiles  20  and polymer composite matrix  16  layers are assembled and formed onto the blade  10  by vacuum molding in a tool that defines the outer mold line shape. Once assembled, the laminate stacks are vacuum bagged and warm pressed to form a final configuration. 
     In the embodiment of the hybrid composite  1  shown in  FIG. 3 , each of the hybrid segments  12  further include a plurality of tiles  20 . Specifically, the hybrid segments  12  are extended across the blade  10  using multiple thin layers of erosion-resistant ceramic material, in the form of tile  20 , alternating with layers of fiber-reinforced polymer matrix composite  16 . The tiles  20  are placed sequentially and are substantially aligned with one another as the plurality of tiles  20  extend across the top and bottom surfaces of the blade  10 . The erosion-resistant ceramic material of the tile  20  comprises a ceramic containing, for example, Al 2 O 3 , SiC, Si 3 N 4  and B 4 C. In the embodiment of  FIG. 3 , the exterior ceramic material bent around the leading edge  11  and in the form of exterior tiles  20  constitutes the exterior shell  14 , and the interior ceramic material bent around the leading edge  11  and in the form of interior tiles  20  constitutes the interior shell  18 . The number of layers is preferably 2 to 10 layers, for example, that provides an overall thickness of the composite  1  in the range of 1 to 3 mm, for example. 
       FIGS. 4A-D  are schematics illustrating examples of hybrid composites  1  with various laminate stacks.  FIG. 4A  is the stack used in the embodiment of  FIG. 3  and has uniform thickness ceramic tiles  20  that overlie one another in alignment. The embodiment in  FIG. 4B  also has ceramic tiles  20  that overlie one another in alignment but also have hybrid segments  12  that are in two sections. In other words, a gap is present at the tip of the blade  10  and a layer of the polymer matrix composite is disposed therein. The embodiment of  FIG. 4C  includes ceramic tiles  20  having a uniform thickness that are staggered in position from one layer to the next.  FIG. 4D  also illustrates tiles  20  that are staggered from one layer to the next. However, the tiles  20  are of varying thickness and a thick, single-layer hybrid segment  12  is disposed at the blade tip  11 . 
     A method of making the hybrid composite  1  is provided herein. Basically, the hybrid segments  12  and tiles  20  are made by tape casting a slurry of ceramic materials in thin sheets and densifying to net shape without applied pressure. Thus, the hybrid segments  12  can be formed as curved ceramic segments. The thin sheets preferably take the form of a tile and are laminated with polymer composite prepreg layers. The inventive method of making the hybrid segments  12  advantageously produces ceramic elements to net shape using thin flexible tape-cast layers that can be molded in their green state. As cast, the green thin sheets (tapes) contain an organic binder that renders them flexible. In this state, the green thin sheets can be placed within ceramic tooling and shaped. Upon heat-treatment, the binder burns out and the sheets partially sinter and become rigid. In this way, the hybrid segments  12  are formed such that its exterior surface follows the outer-mold line profile of the blade  11  including the blade tip curvatures. The binder phase in the tape facilitates lamination of green tapes at room temperature. Therefore, thin green tapes can be stacked together to increase the thickness of the ceramic tiles or to produce tiles with a tapered thickness. The tile thickness can also be tapered by ply drop-off during lamination rather than by costly machining. Thicker surface tiles can be used in highly impacted areas and thinner tiles in areas that experience less severe erosion in service. Furthermore, the ability to use tape lamination to build complex shaped ceramic tiles will allow consideration of a large number of protective cover designs without the restriction of a processing cost penalty. 
     The use of pressureless sintering to full or nearly full density advantageously provides a large decrease in cost relative to hot-pressed materials. Previously, high density was achieved by using expensive hot pressing. The method of making the hybrid composite  1  further includes the addition of a pre-ceramic polymer infiltrations rather than pressure to aid densification. After pressureless sintering, the relative density of the ceramic hybrid segments will be at least 65%, for example. The density is increased by infiltration of the connected porosity with a pre-ceramic polymer or precursor slurry that can be converted to ceramic through an additional heat-treatment. High-yield slurries and precursors are used routinely by those skilled in the art to densify alumina and silicon carbide fiber reinforced composites. These composites are infiltrated and heated to a temperature suitable for ceramic conversion of the precursor but below the densification temperature of the polymer composite matrix several times prior to heating (without pressure) to the final sintering temperature. Ultimately, the final density will depend on the number of infiltration cycles used. In this way, polymer composite matrix densities greater than approximately 90%, for example can be achieved. 
     The use of segmented ceramic layers in the hybrid composite  1 , which simplifies production of conformal structures, allows field repair by replacement of individual tiles after damage, contributes to high damage tolerance of the composite under impact and bending loads, and contributes to the ability to strain-match the blade  10 . 
     The present invention also advantageously provides the ability to select fiber volume fraction and lay-up within the continuous polymer matrix composite  16  layer, which allows the stiffness in various loading directions to be controlled. The present invention further provides the ability to attach hybrid laminates with a hard, dense ceramic strike face (the exterior surface of exterior shell  18 ) under ambient temperature conditions that will not damage the blade  10 . That is, the hybrid composite  1  including the hybrid segments  12  with tiles  20  is field removable and replaceable. 
     The composite  1  can also be configured to account for thermal conductivity and dielectric requirements established to ensure that a deicing system installed in the blades  10  remains functional. Through selection of materials for the ceramic strike face and the fiber reinforcement, conductivity can be 12 W/mK or 0.20 cal/cm sec K, for example. Thus, the present invention provides the protection described above without hindering the deicing system of the blade  10 . 
     Referring to  FIG. 5 , the hybrid composite  1  may include a conductive metallic mesh  22  comprised of wires embedded beneath a layer of ceramic material. The metallic mesh  22  is laminated directly beneath a ceramic layer of the hybrid segment  12 . The metallic mesh  22  can be used in any of the embodiments described herein. In the embodiment shown in  FIG. 5 , the metallic mesh  22  is directly beneath the exterior shell  14 . However, the metallic mesh  22  may be placed beneath each layer of ceramic material, as conditions require. The metallic mesh  22  is used to heat the polymer matrix composite  16  beneath a damaged segment  12  or individual tile  20 . The tile  20 , for example, is removed and a new tile  20  (and possibly a new prepreg layer) is laminated in its place. 
     To remove a tile  20 , the mesh  22  beneath it is heated by passing a current through the wires of the mesh  22  or by using handheld RF or microwave generators (such as those used as medical devices) to reduce the adhesive strength of the polymer matrix composite  16  locally. The replacement tile  20  is placed into the gap left by the removed tile  20 . A vacuum bag and heating pad is then placed locally over the new tile  20  to affix it to the erosion resistant cladding. Additionally, the embedded metallic mesh  22  could serve a dual role by providing lightening strike protection. 
     In understanding the scope of the present invention, the term “comprising” and its derivatives, as used herein, are intended to be open ended terms that specify the presence of the stated features, elements, components, groups, integers, and/or steps, but do not exclude the presence of other unstated features, elements, components, groups, integers and/or steps. The foregoing also applies to words having similar meanings such as the terms, “including”, “having” and their derivatives. The terms of degree such as “substantially”, “about” and “approximate” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. For example, these terms can be construed as including a deviation of at least ±5% of the modified term if this deviation would not negate the meaning of the word it modifies. 
     While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention. For example, the size, shape, location or orientation of the various components can be changed as needed and/or desired. Components that are shown directly connected or contacting each other can have intermediate structures disposed between them. The functions of one element can be performed by two, and vice versa. It is not necessary for all advantages to be present in a particular embodiment at the same time. Thus, the foregoing description of the embodiments according to the present invention is provided for illustration only, and not for the purpose of limiting the invention.