FORMED THEREWITH, AND METHODS OF PRODUCING

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 61 /345,402, filed May 17, 2010, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention generally relates to ceramic matrix composite (CMC) materials. More particularly, this invention relates to ceramic matrix composite sandwich structures, gas turbine engine components comprising such structures, and processes of making such structures.

[0003] The growth in size of new aircraft engines, coupled with the increasing cost and scarcity of exotic raw materials used in high temperature metal alloys, is driving the utilization of new materials for making components suitable for use in intermediate to high temperature environments. One alternative to metal alloys for making engine components for such environments is ceramic matrix composites (CMCs).

[0004] Generally, a CMC is a material that includes a second ceramic phase embedded in a ceramic matrix. The second ceramic phase imparts different thermal, elastic, and structural properties to the ceramic matrix thereby making the resulting CMC tougher, in other words, the CMC is not as susceptible to surface and bulk flaws as the ceramic matrix alone. This second ceramic phase can include a variety of forms including particulates, whiskers, chopped fibers, fiber tows, fiber cloths, and any combination thereof. CMCs may also include laminates of different materials as long as those laminates contain at least one ceramic layer. Though there are a variety of CMCs available, the aviation industry typically turns to CMCs comprising continuous fiber tows, either in tape or cloth forms, due to the higher strains-to-failure that these forms of reinforcements are capable of imparting to a ceramic matrix.

[0005] High grades of continuous ceramic fiber utilized by the aviation industry can be expensive. Thus, the use of large quantities of these materials is not cost-effective. Moreover, backside cooling is often necessary or desirable, in which case CMCs may experience thermal stresses due to their relatively low thermal conductivities. Often, thousands of small holes are drilled throughout a CMC, or fugitive threads are incorporated into a CMC, to provide needed cooling to the finished component. This can further increase the cost of the material.

[0006] Another consideration when looking to alternate materials is engine noise. The ability to produce a quieter engine is becoming a differentiating factor for airframers when selecting turbine engines for next generation aircraft. Noise from a turbine engine can be attributed to numerous sources, including the fan, turbine, combustor, aft-turbine engine component vibration, and high-speed exhaust gases. While a variety of alternatives have been explored to address such noise issues, these alternatives can often result in added weight and/or require cooling air, both of which can decrease the efficiency of the engine.

[0007] Accordingly, there remains a need for material systems suitable for use in intermediate to high temperature turbine engine applications that can provide strains-to- failure comparable to continuous fiber reinforced CMCs, while reducing both the quantity of ceramic reinforcement needed, as well as the affect of through-thickness thermal stresses. In addition, it would be desirable that such systems also be capable of providing noise damping benefits.

BRIEF DESCRIPTION OF THE INVENTION

[0008] The present invention provides ceramic matrix composite sandwich structures, gas turbine engine components that comprise such sandwich structures, and methods of producing such sandwich structures. [0009] According, to a first aspect of the invention, a gas turbine engine component comprises a ceramic matrix composite sandwich structure that includes a core having oppositely-disposed first and second surfaces, a first facesheet bonded to the first surface, and a member bonded to the second surface so that the core is between the first facesheet and the member. The first facesheet and the core comprise, respectively, first and second ceramic matrix composite materials, and the second ceramic matrix composite material of the core comprises a ceramic reinforcement material in a ceramic matrix material.

[0010] According to a second aspect of the invention, a ceramic matrix composite sandwich structure comprises a core having oppositely-disposed first and second surfaces, a first facesheet bonded to the first surface, and a member bonded to the second surface so that the core is between the first facesheet and the member. The first facesheet and the core comprise, respectively, first and second ceramic matrix composite materials, and the second ceramic matrix composite material of the core has a ceramic reinforcement material in the form of a felt or an open weave fabric.

[0011 ] Other aspects of the invention include methods of producing the component and ceramic matrix composite sandwich structure comprising their respective elements described above.

[0012] A technical effect of the invention is the ability of the ceramic matrix composite sandwich structure to exhibit desirable properties at intermediate to high temperatures, such as those existing in gas turbine engines. The ceramic matrix composite sandwich structure achieves such properties in part due to the ceramic reinforcement material within its core. Preferred constructions for the ceramic reinforcement material serve to minimize the quantity of reinforcement material required by the core while enabling the sandwich structure to exhibit desired properties, for example, an elastic modulus of greater than 70 GPa and a strain-to-failure of greater than about 0.2% at a temperature of at least 425°C. Other potential benefits of the sandwich structure include reduced through-thickness thermal stresses and noise damping effects. [0013] Other aspects and advantages of this invention will be better appreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG. 1 schematically represents a cross-sectional view of a high-bypass turbofan engine.

[0015] FIG. 2 schematically represents a cross-sectional view of a CMC sandwich structure in accordance with an embodiment of the invention.

[0016] FIGS. 3, 4 and 5 are scanned images showing ceramic fabrics suitable for use in facesheets of the sandwich structure represented in FIG. 2.

[0017] FIG. 6 schematically represents a cross-sectional view of a CMC sandwich structure in accordance with another embodiment of the invention.

DETAI LED DESCRIPTION OF THE INVENTION

[0018] Embodiments described herein generally relate to ceramic matrix composite sandwich structures, gas turbine engine components comprising the same, and methods for making the same. Such structures are suitable for use in intermediate to high service operating temperatures and are capable of exhibiting strain-to-failure properties that are comparable to conventional continuous fiber reinforced CMCs, while also having the potential for reducing the quantity of ceramic reinforcement needed and/or the adverse effects of through-thickness thermal stresses. In addition, sandwich structures described herein are capable of exhibiting desirable acoustic damping characteristics. While various applications are foreseeable and possible, applications of particular interest include high temperature applications, for example, components of gas turbines, including land-based and aircraft gas turbine engines. While specific references will be made to certain components for use within the turbine sections of gas turbine engines, those skilled in the art will appreciate that the teachings of this invention are applicable to a variety of components.

[0019] For illustrative purposes, FIG. 1 schematically represents a high-bypass turbofan engine 10 of a type known in the art. The engine 10 is schematically represented as including a fan assembly 12 and a core engine 14. The fan assembly 12 is shown as including a composite fan casing 16 and a spinner nose 20 projecting forwardly from an array of fan blades 1 8. Both the spinner nose 20 and fan blades 18 are supported by a fan disc (not shown). The core engine 14 is represented as including a high-pressure compressor 22, a combustor 24, a high-pressure turbine 26 and a low-pressure turbine 28. A large portion of the air that enters the fan assembly 12 is bypassed to the rear of the engine 10 to generate additional engine thrust. The bypassed air passes through an annular-shaped bypass duct 30 and exits the duct through a fan nozzle 32. The fan blades 1 8 are surrounded by a fan cowling or nacelle that defines an inlet duct 34 to the engine 10, as well as the fan nozzle 32. An exhaust nozzle 36 and a centerbody 38 extend aftward from the core engine 14. The centerbody 38 is concentrically aligned with the exhaust nozzle 36 along the centerline of the core engine 14. The centerbody 38 defines a convergent-divergent path through the nozzle 36, such that the inner surface of the exhaust nozzle 36 defines an outer boundary of the engine exhaust flowpath and the outer surface of the centerbody 38 defines an inner boundary of the exhaust flowpath.

[0020] As previously described, embodiments herein generally relate to CMC sandwich structures suitable for use in producing components that are capable of operating in intermediate and high temperature engine environments, including the turbine section of the engine 10 of FIG. 1 . As used herein, "intermediate temperature" refers to temperatures of about 800°F (about 425°C) to about 1500°F (about 815°C), while "high temperature" refers to temperatures greater than about 1 500°F (81 5°C). CMC sandwich structures of this invention generally comprise a core that is "sandwiched" between two facesheets. With reference to FIG. 2, a sandwich structure 40 is represented as a multi- layered structure that generally comprises first and second facesheets 42 and 44 separated by a core 46 and attached to first and second surfaces 48 and 50, respectively, of the core 46. The facesheets 42 and 44, as well as the core 46, may comprise different combinations of CMC materials, as will be explained below. Mechanically, such sandwich structure 40 is similar to an I-beam in which the bending stiffness of the structure 40 is highly dependent on the modulus of each facesheet 42 and 44 and the distances of the facesheets 42 and 44 from the center of the core 46 in a balanced I-beam system.

[0021] Each facesheet 42 and 44 comprises a CMC composition that contains a ceramic reinforcement material, preferably in the form of one or more ceramic fabrics (not shown). The fabrics may be formed of a variety of known fabric materials used in CMC structures. For example, in one embodiment each facesheet 42 and 44 may have an oxide-oxide CMC composition, in which case each facesheet 42 and 44 has a reinforcing fabric, cloth, or paper that is formed of oxide-based material and is contained within a matrix formed of an oxide-based material. The oxide-based materials may be, for example, aluminum oxide (AI2O3), silicon dioxide (S 1O2), aluminosilicates, and mixtures thereof. Nonlimiting examples of aluminosilicates include crystalline materials such as mullite (3Al2C 2Si02), as well as glassy aluminosilicates. Other nonlimiting examples of suitable ceramic materials for the facesheets 42 and 44 include silicon carbide reinforcing fabrics, cloths or papers contained in an oxide-based material, for example, a SiC-oxide CMC composition.

[0022] Ceramic fabrics suitable for use in the facesheets 42 and 44 include fabrics having a tight weave or an open weave. As used herein, a "tight weave" refers to fabrics in which there is contact between adjacent tows in the final CMC sandwich structure, as illustrated in FIG. 3, whereas the term "open weave" refers to fabrics having a visible space between the tows in the final CMC sandwich structure, as illustrated in FIGS. 4 and 5. Nonlimiting examples of tight weave fabrics suitable for use in the facesheets 42 and 44 include oxide-fiber cloths such as AF- 10, BF-20, XN-513 and DF- 1 1 (commercially available from the 3 M Company), as well as SiC-fiber cloths such as PN-S 15H 16PX and TM-517E08PX (commercially available from UBE America, Inc.). Nonlimiting examples of open weave ceramic fabrics suitable for use in the facesheets 42 and 44 include AF-8 (commercially available from the 3M Company), as well as specialty woven cloths such as a 14x 14 plain weave using 800 filament Nextel™720 tows that are individually twisted 0.5 times/inch (about 0.2 times/cm), a 10x 10 plain weave using two 800 Nextel™720 filament tows that are twisted together 1 .5 times/inch (about 0.6 times/cm), and a 8x8 plain weave using four 750 Nextel™440 filament tows that are twisted together 1 .5 times/inch (about 0.6 times/cm). The facesheets 42 and 44 may comprise the same ceramic materials, or the facesheets 42 and 44 may be formed of different ceramic materials. Moreover, in some instances it may be desirable for one of the facesheets 42 or 44 to comprise a tight weave fabric and the other to comprise an open weave fabric, for example, to tailor the acoustic damping characteristics of the sandwich structure 40, as will be discussed below. The ceramic reinforcement material typically constitutes about 5 to about 45 volume percent of each facesheet 42 and 44, depending on the type of reinforcement. Tight weave reinforcement materials preferably constitute about 35 to about 45 volume percent of a facesheet 42 or 44, while open weave and paper reinforcement materials preferably constitute about 5 to about 20 volume percent of a facesheet 42 or 44.

[0023] The ceramic matrix of the facesheets 42 and 44 can be formed from a ceramic slurry, in which case the facesheets 42 and 44 are initially in the form of a prepreg. As used herein, a "ceramic slurry" refers to any fluid material containing a mixture of one or more types of polymer materials and one or more types of ceramic particles that are mixed in a solvent to form a substantially uniform mixture that is capable of being dispersed around the ceramic fabrics of the facesheets 42 and 44, and then subsequently transformed into a ceramic material through the application of heat. As an example, the polymer materials can be converted to form a ceramic material during a sintering operation performed on the facesheets 42 and 44 prior to assembly with the core 46, or during a sintering operation performed on the sandwich structure 40 during which the facesheets 42 and 44 can be bonded to the core 46 for high temperature applications. US Patent 5,601 ,674 provides a nonlimiting description of a ceramic slurry of a type that can be used with the present invention.

[0024] Nonlimiting examples of suitable polymers for use in the ceramic slurry include polymers that harden and convert to silica, and more preferably polymers that have a conversion efficiency to silica of at least 30 weight percent. Nonlimiting examples of suitable polymers include silicone resins, for example, methylsesquisiloxane mixtures of the polysiloxane family commercially available from sources such as General Electric Silicone Products Div. (for example, SR350 and SR355) and Dow Corning® (for example, 249 silicone resin). Nonlimiting examples of ceramic particle constituents that are suitable for use in the ceramic slurry include oxides of such elements as Al, Si, B, and combinations thereof, including such commercially available materials as AI2O3, S1O2 _, B2O3 and 3Al ₂ 0 ₃ ^« 2Si0 ₂ . Typically, preferred ceramic particles sizes are believed to have diameters of less than one micrometer, though the use of larger particles is also foreseeable. Nonlimiting examples of suitable solvents for use in the ceramic slurry include liquids that are capable of dissolving the polymer(s) and uniformly distributing the ceramic particles and dissolved polymer(s) around the ceramic fabrics of the facesheets 42 and 44. Nonlimiting examples of suitable solvents organic solvents such as ethyl alcohol, isopropyl alcohol and acetone.

[0025] While the relative quantities of the polymer(s), ceramic particles, and solvent can vary depending on the solubility and saturation limit of the polymer and the desired viscosity of the slurry, an example of a suitable composition for the ceramic slurry comprises, but weight, about 30% to about 60% ceramic particles, about 10% to about 60% polymer, and about 25% to about 50% solvent. More preferred ranges for the constituents of the ceramic slurry are believed to be, by weight, about 35% to about 40% ceramic particles, about 15% to about 20% polymer, and about 30% to about 45% solvent.

[0026] The core 46 of the sandwich structure 40 represented in FIG. 2 is a CMC composition that contains a ceramic reinforcement material, preferably in the form of a ceramic felt material. As will be discussed below in reference to FIG. 6, another type of sandwich structure 140 contains a core 146 having a different type of CMC compositions, in which the ceramic reinforcement material is in the form of a fabric. In either case, the ceramic reinforcement material is dependent upon the particular application but preferably does not constitute more than 20 volume percent of the core 46/146. More particularly, the ceramic reinforcement material preferably constitute at least 5 to about 20 volume percent of the core 46/146, and more preferably about 7 to about 15 volume percent of the core 46/146. Suitable thicknesses for the cores 46 and 146 will vary depending on the particular application, though core thicknesses in a range of about 3 mm to about 37 mm, more preferably about 12 mm to about 25 mm, are believed to be particularly well suited for use in gas turbine applications.

[0027] In the embodiment of FIG. 2, in which the ceramic reinforcement material of the core 46 is a felt material, the core 46 can be made by first suspending and blending ceramic fibers, optional glass fibers, and one or more fugitive bulking materials in a soapy water solution to make a core suspension. The core suspension may include varying combinations of materials, a nonlimiting example of which is, by weight, about 50% to about 83% ceramic fibers, about 0% to about 30% glass fibers, and about 10% to about 35% fugitive material(s).

[0028] Preferred ceramic fibers for the felt core 46 of FIG. 2 are capable of promoting the strain properties of the core 46. Notable examples include silicon-containing CMC fibers such as silicon carbide, silicon nitride, silicon oxycarbides, silicon oxynitrides, and mixtures thereof. Other notable examples include oxide materials previously noted as suitable for use in the facesheets 42 and 44. The ceramic fibers of the felt core 46 can have various lengths, typically ranging from about 4 mm (about 1 /8 inch) to about 38 mm (about 1 .5 inch), for example, about 20 mm (about 0.75 inch).

[0029| Chopped glass fibers are desirable in the core 46 for having the ability to bond the ceramic fibers together within the felt core 46. The glass fibers do so by softening, yet not melting completely, when heated, for example, during the above-noted sintering operation performed to yield the sandwich structure 40 represented in FIG. 2. While the component use temperature can help determine specific chopped glass fibers suitable for use in a particular application, the chopped glass fibers can be generally selected from a variety of commercially-available fiber glass materials, including but not limited to E- glass, M-glass, S-glass and Pink ^® insulation (Owens-Corning ^® ). Suitable lengths for the chopped glass fibers will typically range from about 4 mm (about 1 /8 inch) to about 25 mm (about 1 inch), for example, about 20 mm (about 0.75 inch). In a particular example, fiberglass insulation can be cut into pieces of about l xl inch (about 25x25 mm), so that its glass fibers are dispersed during blending with the ceramic fibers and fugitive material.

[0030] One or more fugitive materials are preferably chosen on the basis of providing a bulking effect to the felt core 46 and later creating porosity during sintering of the core 46. While various different types of fugitive materials can be used, preferred fugitive materials decompose when heated, for example, during the above-noted sintering operation performed on the sandwich structure 40. As an example, cellulose fillers can be used as a fugitive material to promote ceramic slurry migration during prepregging or infiltrating the felt core 46, as will be described below. Alternatively or in addition, chopped rayon fibers or bulk aramid can be used as a fugitive material, with the benefit of minimizing the final density of the core 46. Still other notable fugitive materials include aramid particles, for example, having particles diameters of about 4 mm ( 1 /8 inch).

[0031 ] Core suspensions comprising blends of ceramic fibers, glass fibers and fugitive material can be poured into a conventional felt-making machine to consolidate the fibers and extract any excess water solution. The resulting wet felt core can then be placed in a felt dryer to remove any residual water and yield a dried felt core. A ceramic slurry, as previously defined, may then be applied to the dried felt core using a prepregging process or vacuum-assisted infiltration to yield a preliminary core in the form of a slurry- impregnated core. Suitable prepreg and infiltration processes for use with this invention are known in the art and therefore will not be discussed in any detail here. The ceramic slurry may have the identical composition as that described for use in the facesheets 42 and 44, or it may comprise a different combination of polymer and ceramic particles than that used in the facesheets 42 and 44.

[0032] In general, lay-up of the sandwich structure 40 can entail individually applying the facesheets 42 and 44 to the surfaces of the slurry-impregnated preliminary core, which upon sintering will yield the structure 40 represented in FIG. 2. As previously noted, the facesheets 42 and 44 can be sintered prior to application to the preliminary core, or the facesheets 42 and 44 and the preliminary core can undergo sintering together by applying the facesheets 42 and 44 to the preliminary core while still in the form of prepregs. Another alternative is to apply the facesheets 42 and 44 to the preliminary core after being heated to cure the polymers of their respective ceramic slurries without converting the polymers to a ceramic. If the facesheets 42 and 44 are applied to the preliminary core in the form of prepregs, sintering of the resulting structure can be carried out so that the ceramic slurries of the facesheets 42 and 44 and the slurry- impregnated preliminary core bond the facesheets 42 and 44 to the core 46. Optionally, prepregs of the facesheets 42 and 44 may be bonded to the preliminary core with a suitable adhesive. On the other hand, if the facesheets 42 and 44 are applied to the preliminary core in a cured or presintered condition, the use of an adhesive will typically be necessary to bond the facesheets 42 and 44 to the core 46, in which case further curing and/or sintering steps may be carried out as necessary.

[0033] Adhesives used to bond the facesheets 42 and 44 to the core 46 can comprise a mixture of a polymer and ceramic particles processed with a solvent to form a substantially uniform mixture that does not readily infiltrate any open porosity of the slurry-impregnated preliminary core or core 46. Particularly suitable polymers include those capable of being converted to a ceramic material that is compatible with the ceramic materials of the facesheets 42 and 44 and core 46. Nonlimiting examples of suitable polymers include those mentioned above for use in the ceramic slurries of the facecoats 42 and 44, including the SR350 and SR35 silicone resins commercially available from General Electric Silicone Products Div. and the 249 silicone resin commercially available from Dow Corning®, in which case the polymer has a conversion efficiency to silica of at least 30 weight percent. Particularly suitable compositions for the solvent and ceramic particles of the adhesive include those mentioned above for use in the ceramic slurries of the facecoats 42 and 44. Suitable particle sizes for the ceramic particles are on the order of -325mesh (less than 44 micrometers). Suitable amounts of solvent in the adhesive are generally on the order of about 8 to about 20 weight percent. Additionally, up to about 10 weight percent chopped oxide or silicon carbide ceramic fibers can be added to the adhesive to reduce the incidence of shrinkage cracks that might occur within the ceramic formed by the adhesive if the sandwich structure 40 is subjected to temperatures of greater than about 815°C (1500°F).

[0034] The ratio of ceramic particles to polymer within the adhesive may depend on the ceramic (e.g., silica) yield of the polymer. For example, the SR350 silicone resin has a conversion efficiency of about 83wt% S1O2 while the SR355 and 249 silicone resins have conversion efficiencies of about 60wt% Si0 ₂ . However, suitable results are believed to be achieved with an adhesive that comprises about 3 to 4 parts by weight of ceramic particles for every 1 part by weight of the silica yield from the polymer.

|0035] In embodiments in which prepregs of the facecoats 42 and 44 are applied to a prepreg of the core 46, the prepregs can be placed in a mold and cured and sintered together to yield the structure 40 of FIG. 2. This approach can be advantageous for fabricating large, complex ceramic sandwich structures, in that machining of the core 46 can be avoided or minimized. In one embodiment, the prepreg stack (with or without an adhesive) is bagged under vacuum, placed in an oven, and heated at a temperature of about 250°F (about 120°C) for approximately two hours to cure the polymers of the prepregs and, if present, the adhesive. The cured structure can then be sintered by heating the cured structure to a temperature at least about 74°C and up to a temperature of between about 600°C and 1000°C for about two hours to convert the matrix materials of the facesheets 42 and 44 and core 46 to their respective ceramic materials and produce the final sandwich structure 40 represented in FIG. 2. The temperature employed during sintering will determine in part whether the structure 40 is suitable for use in an intermediate or high temperature engine environment.

[0036] In the embodiment of FIG. 6 utilizing a fabric core material, a sandwich structure 140 is formed that can comprise the above-described facesheets 42 and 44 bonded to a fabric core 146. In this embodiment, a ply of an open weave fabric (as described above in reference to the facesheets 42 and 44) can be prepregged with a ceramic slurry of a type previously described for prepregging or infiltrating the felt core material used to form the felt core 46 of FIG. 2. The continuous tows of an open weave fabric contribute strength to the core 146 and sandwich structure 140, while the open interstices between tows of the weave permit air migration and cooling. Plies of the prepregged open weave fabric can be laid-up to a desired thickness, depending on the particular requirements of the application, to yield a preliminary core. The final fabric core 146 can comprise a plurality of layers of the same open weave fabric, or a plurality of layers of different open weave fabrics. The facesheets 42 and 44, which may be produced with tight weave or open weave fabrics that have been prepregged with a ceramic slurry (for example, as described above) can then be applied to the preliminary core, preferably bonded with the use of an adhesive (for example, as described above), after which the prepreg stack can be cured and sintered as previously described to yield the sandwich structure 140.

[0037] Sandwich structures 40 and 140 of the types described above can be used to manufacture an entire engine component, or can be produced as discreet strips that can be selectively placed within or on a component during the fabrication thereof to provide tailored density, cooling, and/or acoustic properties. In addition, sandwich structures 40 and 140 of the types described above can be fabricated directly on a metal or ceramic component such that the component can serve as one of the two facesheets 42 or 44 of the structure 40 or 140. This approach can reduce the use of costly tooling, even for components having complex geometries, and yield a sandwich structure 40 or 140 that form-fits to the component, thereby reducing variations in the thickness of the adhesive and improving attachment between the component and the sandwich structure 40 or 140. The structure 40 or 140 can be adhesively bonded or mechanically attached to the metal or ceramic component by heating to a temperature that will cure the polymer of the ceramic slurry (or polymers of the ceramic slurries), after which the structure 40 or 140 can be sintered to convert the polymer to a ceramic material.

[0038] Nonlimiting examples of components that can be manufactured using the CMC sandwich structure technology described herein include the core engine exhaust nozzle 36 and the exhaust centerbody 38 of FIG. 1 . For example, the sandwich structures 40 and 140 of FIGS. 2 and 6 can be used to form an interior skin of the nozzle 36 and/or an exterior skin of the centerbody 38 to provide these components with desirable properties, for example, structural, thermal, and acoustic properties desired for the nozzle 36 and/or centerbody 38 at the high temperatures of the exhaust gas flowpath. Either sandwich structure 40 or 140 can be laid up on a tool to obtain the desired geometries for the nozzle 36 and/or centerbody 38. Alternately, the sandwich structures 40 and 140 can be laid up directly on the nozzle 36 or centerbody 38 and subsequently adhered thereto. Acoustic damping can be realized with the nozzle 36 and centerbody 38 though the use of the sandwich structure 140 comprising an open weave fabric, as described previously.

[0039] Sandwich structures of the types described above can be constructed to have a lower density as compared to corresponding metal components. For example, a sandwich structure having a thickness of about one-half inch (about 13 mm) and fabricated as described above is capable of having densities of about 0.3 g/cc (about 19 lbs/ft ³ ) and less. In addition, sandwich structures of the types described above are capable of an elastic modulus of about 10 Msi (about 70 GPa) or higher and a strain-to-failure of about 0.2% or higher at intermediate or high temperatures. High strain-to-failure properties are achieved in part through the use of appropriate adhesives, for example, of the types described above, which are capable of maintaining the bond between the facesheets 42 and 44 and core 46 and 146 in intermediate and high temperature engine environments of operation. Moreover, the use of fabric cores 146 of the type described in reference to FIG. 6 can yield a sandwich structure 140 that exhibits an air flow capability of about 7x l 0 ^"4 lbs/sec per square inch (about 5x10 ^"5 kg/s per square centimeter) at a P/P _A ratio of 1 .2, where P is the applied cooling pressure and PA is the atmospheric pressure. This can allow components made from such sandwich structures to attain desired internal cooling needed to reduce thermal stress to acceptable levels without the need to drill holes or use fugitive threads, as is currently done. Those skilled in the art will understand that the effectiveness of the cooling can relate to both the tow count and the P/PA ratio, as such factors can influence how much time air interacts with the internal ceramic structure and therefore, the extent of internal cooling.

[0040] The use of fabric cores 146 of the type described in reference to FIG. 6 can also yield a sandwich structure 140 that provides acoustic benefits. More particularly, the low density and modulus of the fabric core 146 can be utilized to damp engine noise. For example, the fabric core 146 can be fabricated to contain multiple open-weave fabrics, and the thickness of each fabric layer differ from other fabric layers to modify the density and modulus of the core 146, which in turn modifies the acoustic properties of the core 146 and the structure 140 as a whole.

[0041 ] The use of felt cores 46 of the type described in reference to FIG. 2 can also provide acoustic benefits. For example, the orientation of the fibers within the core 40 can be modified to alter the impedance of the core 40 in a specific direction to match acoustic design needs. In particular, each layer of felt can be planar in the draw-down thickness with some interlocking between the fibers in the perpendicular direction. The felt can be cut into the desired dimension, and layers of the felt can be stacked to re-orient the planes to provide the desired properties. Additionally, lowering the tow count of one or both facesheets 42 and 44 can alter its impedance.

[0042] While the invention has been described in terms of particular embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the physical configurations of the sandwich structures 40 and 140 schematically represented in FIGS. 2 and 3 could differ from those shown, and materials and processes other than those noted could be used. Therefore, the scope of the invention is to be limited only by the following claims.