Patent Publication Number: US-2016245175-A1

Title: Compression molded fiber reinforced fan case ice panel

Description:
FIELD OF DISCLOSURE 
     The present disclosure generally relates to gas turbine engines, and more specifically, relates to impact-resistant ice panels for gas turbine engine fan cases. 
     BACKGROUND 
     Gas turbine engines, such as those used to provide thrust for an aircraft, typically include a fan surrounded by a fan case, a compressor section, a combustor or combustors, and a turbine section, each positioned sequentially in an upstream to downstream arrangement. In operation, air may be drawn into the engine and accelerated by the fan, and a fraction of the indrawn air may be directed through the compressor section, the combustor(s), and the turbine section. This air may first be pressurized in the compressor section and it may then be mixed with fuel and combusted in the combustor(s) to generate hot combustion gases. The hot combustion gases may subsequently expand through and drive the turbine section or sections which may, in turn, drive the compressor section or sections and the fan as all are mounted on one or more interconnecting shafts. After passing through the turbine section(s), the air may be exhausted through an exhaust nozzle to provide propulsive thrust to an associated aircraft or to provide power if used in land-based operations. 
     During operation, certain environmental conditions (e.g., high altitude, etc.), may cause ice to accumulate on the blades of the fan. When the ice reaches a certain size, it may be thrown radially outwardly off of the rotating fan blades and in a direction aft (or downstream) of the fan due to a combination of centrifugal force and airflow force. The released ice may then impinge upon an exposed inner flow path surface of the fan case known as the “ice impact zone”. In order to prevent the released ice from damaging the fan case, the “ice impact zone” may be covered with an impact-resistant ice panel which may be constructed of a material that is capable of deflecting the ice. The ice panel may consist of an underlying honeycomb core bonded to an exposed impact-resistant facesheet which may be formed from a lightweight and high-strength composite laminate. For example, the facesheet may consist of several layers (or plies) of cured “prepreg”, each consisting of a fiber reinforcement pre-impregnated with a resin matrix. 
     Current composite constructions for ice panel facesheets typically utilize a woven fabric of continuous fibers as the fiber reinforcement. For example, as described in U.S. Pat. No. 5,344,280, an ice panel facesheet may be formed from multiple plies of cured woven fiberglass in an epoxy resin matrix. Another common facesheet construction consists of multiple plies of cured pregpreg fabric, each consisting of woven aramid continuous fibers, such as Kevlar® yarn, in an epoxy resin matrix. However, while ice panel facesheets formed from woven aramid fiber/epoxy resin are effective at resisting damage, they may be time consuming and expensive to manufacture. In particular, because prepreg fabric formed from woven aramid fiber/epoxy resin may be relatively thin (about 0.2 mm to about 0.4 mm), the prepreg layup process may involve the manual (or automated) layup of as many as 25 layers of fabric or more to achieve a desired facesheet thickness. In addition, in order to allocate fiber reinforcement in multiple directions in the plane of the facesheet, the prepreg layup process may also involve the manual (or automatic) orientation of each layer of prepreg fabric in a different direction. Furthermore, because autoclaving is traditionally employed for curing plies of prepreg fabric, the removal of entrapped air after the layup of every few prepreg fabric plies may be necessary in some cases. Moreover, the woven aramid fiber/epoxy resin material may be expensive and material utilization may be less than optimal, as the autoclaving process typically involves the subsequent trimming and disposal of excess material. 
     Clearly, there is a need for more efficient and cost-effective approaches for the fabrication of high-performance ice panel facesheets for gas turbine engine fan cases. 
     SUMMARY 
     In accordance with one aspect of the present disclosure, an ice panel for a fan case of a gas turbine engine is disclosed. The ice panel may comprise a facesheet formed of a chopped prepreg tape that is cured. The chopped prepreg tape may comprise randomly oriented chips of fibers impregnated with a resin matrix. 
     In another refinement, the chopped prepreg tape may be cured by compression molding. 
     In another refinement, the facesheet may comprise a plurality of plies of the chopped prepreg tape. 
     In another refinement, the plurality of plies may further comprise at least one ply of a prepreg fabric. 
     In another refinement, the prepreg fabric may comprise continuous woven aramid fibers in an epoxy resin. 
     In another refinement, the chopped prepreg tape may have a thickness of at least about 0.5 mm. 
     In another refinement, the fibers in the chopped prepreg tape may be non-continuous unidirectional fibers. 
     In another refinement, the non-continuous unidirectional fibers may be carbon fibers. 
     In another refinement, the non-continuous unidirectional fibers may comprise fibers selected from the group consisting of carbon fibers, glass fibers, aramid fibers, boron fibers, ceramic fibers, and polymeric fibers. 
     In another refinement, the resin matrix in the chopped prepreg tape may be a thermoset resin. 
     In another refinement, the thermoset resin may be an epoxy resin. 
     In another refinement, the ice panel may further comprise a honeycomb core beneath the facesheet, and the honeycomb core and the facesheet may be bonded to the inner surface of the fan case with an adhesive. 
     In accordance with another aspect of the present disclosure, a gas turbine engine is disclosed. The gas turbine engine may comprise a fan, a fan case surrounding the fan, a compressor section located downstream of the fan, a combustor located downstream of the compressor section, and a turbine section located downstream of the combustor. The gas turbine engine may further comprise an ice panel located aft of the fan on an exposed inner surface of the fan case and the ice panel may comprise a facesheet comprising a chopped prepreg tape that is cured. The chopped prepreg tape may comprise randomly oriented chips of fibers impregnated with a resin matrix. 
     In another refinement, the chopped prepreg tape may be cured by compression molding. 
     In another refinement, the facesheet may comprise a plurality of plies of the chopped prepreg tape. 
     In another refinement, the plurality of plies may further comprise at least one ply of a prepreg fabric. 
     In another refinement, the fibers in the chopped prepreg tape may comprise non-continuous unidirectional fibers. 
     In another refinement, the non-continuous unidirectional fibers may comprise fibers selected from the group consisting of carbon fibers, glass fibers, aramid fibers, boron fibers, ceramic fibers, and polymeric fibers. 
     In accordance with another aspect of the present disclosure, a method for fabricating a facesheet for an ice panel for a fan case of a gas turbine engine is disclosed. The method may comprise: 1) stacking a plurality of plies of a chopped prepreg tape to a desired thickness, 2) placing the plurality of plies between two mold surfaces, and 3) curing the plurality of plies into a shape of the facesheet by compression molding. 
     These and other aspects and features of the present disclosure will be more readily understood when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a gas turbine engine having a fan case and an ice panel on an inner surface of the fan case, constructed in accordance with the present disclosure. 
         FIG. 2  is a cross-sectional view of the fan case and the ice panel of  FIG. 1  shown in isolation. 
         FIG. 3  is a top perspective view of a chopped prepreg tape, constructed in accordance with the present disclosure. 
         FIG. 4  is a perspective view of a rolled-up sheet of the chopped prepreg tape of  FIG. 3 , constructed in accordance with the present disclosure. 
         FIG. 5  is a flowchart depicting a sequence of steps which may be involved in fabricating the chopped prepreg tape, in accordance with a method of the present disclosure. 
         FIG. 6  is a flowchart depicting a sequence of steps which may be involved in fabricating a facesheet of the ice panel from the chopped prepreg tape, in accordance with a method of the present disclosure. 
         FIG. 7  is a schematic illustration of some of the steps of  FIG. 6 . 
         FIG. 8  is a flowchart depicting a sequence of steps for fabricating an ice panel facesheet according to current methods (prior art). 
     
    
    
     It should be understood that the drawings are not necessarily drawn to scale and that the disclosed embodiments are sometimes illustrated diagrammatically and in partial views. In certain instances, details which are not necessary for an understanding of this disclosure or which render other details difficult to perceive may have been omitted. It should be understood, of course, that this disclosure is not limited to the particular embodiments disclosed herein. 
     DETAILED DESCRIPTION 
     Referring now to the drawings, and with specific reference to  FIG. 1 , a gas turbine engine  10  in accordance with the present disclosure is depicted. The gas turbine engine  10  may be effective for powering an associated aircraft or for otherwise providing power if used in land-based operations. In an upstream to downstream arrangement, the gas turbine engine  10  may consist of: 1) a fan  12  which may consist of a plurality of blades  14  connected to a hub  16  and which may be circumferentially surrounded by a fan case  18 , 2) a compressor section  20  (which may include a low pressure compressor and a high pressure compressor), 3) an annular combustor  22  (although a series of circumferentially spaced can combustors may also be used), and 4) a turbine section  23  which may include a low pressure turbine  24  and a high pressure turbine  25  tied to the low pressure compressor and the high pressure compressor, respectively, by one or more concentrically mounted shafts  26 . All of the aforementioned parts may then be surrounded by a nacelle  27 , as shown. 
     During normal operation, air  28  may be drawn into the engine  10  and accelerated through the engine  10  by the rotation of the blades  14  of the fan  12 . After passing the fan  12 , a fraction of the indrawn air may be routed through the compressor section  20 , the combustor(s)  22 , and the turbine section  23 . More specifically, the air  28  may first be compressed and pressurized in the compressor section  20 , and it may then enter the combustor  22  where it may be mixed with fuel and combusted to generate hot combustion gases. The hot combustion gases may then expand through and drive the rotation of the turbine section  23  which may, in turn, drive the rotation of the compressor section  20  and the fan  12  by driving the rotation of the interconnecting shaft(s)  26 . The gases may then be exhausted through an exhaust nozzle  32  to provide propulsive thrust to an associated aircraft or to otherwise provide power for other applications. 
     Under certain environmental conditions, such as high altitude, ice may accumulate on the blades  14  of the fan  12  and, once the ice accumulates to a sufficient level, it may be shed radially outwardly and aft of the fan  12  as shown by direction  33  due to a combination of centrifugal force (caused by the rotation of the blades  14 ) as well as the force of the airflow acting upon it. The released ice may then impinge upon an ice impact zone  35  of the fan case  18 . The impact zone  35  may be an inner surface of the fan case  18  that is susceptible to damage from ice impact and it may be located aft (or downstream) of the fan  12 , as best shown in  FIG. 1 . 
     In order to prevent the released ice from damaging the fan case  18 , the impact zone  35  may be covered with an impact-resistant ice panel  36 , as best shown in  FIG. 2 . The ice panel  36  may be annular in shape and it may consist of an impact-resistant facesheet  38  and an underlying honeycomb core  40 . The facesheet  38  and the honeycomb core  40  may be bonded to an inner surface of the fan case  18  with a suitable adhesive to form a sandwich-like structure, as shown. 
     The facesheet  38  of the present disclosure may be formed from one or more layers (or plies) of a chopped prepreg tape  42  (see  FIG. 3 ) that is shaped and cured by compression molding or another suitable method. As an alternative arrangement, the facesheet  38  may be formed from one or more plies of the chopped prepreg tape  42  and one or more plies of a prepreg fabric, such as woven aramid fiber/epoxy resin prepreg tape or another type of prepreg fabric. As detailed below, the use of the chopped prepreg tape  42  for forming the facesheet  38  may offer a number of manufacturing advantages over current methods and materials for ice panel facesheets, without significantly compromising the performance of the facesheet  38 . Such manufacturing advantages may include increased ease of production, increased production rates, increased material utilization (i.e., reduced waste), and reduced production costs. 
     As shown in  FIG. 3 , the chopped prepreg tape  42  may consist of a plurality of randomly oriented chips  44  and each chip  44  may consist of a fiber reinforcement impregnated with a resin matrix. Each of the chips  44  in the chopped prepreg tape  42  may have a length of about 200 mm and a width of about 50 mm, or other dimensions, and they may lay relatively flat to form a mat-like structure, as shown. According to one arrangement, the fiber reinforcement in the chips  44  may be non-continuous unidirectional (i.e., aligned in parallel) fibers, although they may be non-continuous woven fibers or non-continuous randomly oriented fibers in some circumstances. “Non-continuous fibers”, according to the present disclosure, may refer to short fibers having an aspect ratio (ratio of fiber length to fiber diameter) of less than about  200 . 
     The non-continuous fibers in the chips  44  may be carbon fibers or other types of fibers such as, but not limited to, glass fibers, aramid fibers, polymeric fibers, boron fibers, and ceramic fibers. The resin matrix in the chips  44  may be a thermoset resin such as an epoxy resin, but bismaleimide (BMI) and phenolic resin are other non-limiting possibilities. Alternatively, the resin matrix may be a thermoplastic resin such as, but not limited to, polyetherimide (PEI) and polyether ether ketone (PEEK). In addition, the fiber content in the resin matrix may be in the range of about 40% to about 60% by volume, but may also deviate from this range depending on a number of factors. Prior to curing, the resin matrix in the chopped prepreg tape  42  may exist in a partially cured state such that it may be sufficiently pliable and tacky to be bent, cut, and layered/stacked. Furthermore, due to the random orientation of the chips  44  (and, therefore the fibers) in the chopped prepreg tape  42 , the strength of the chopped prepreg tape  42  may be at least approximately the same in all directions in the plane of the chopped prepreg tape  42 . 
     As shown in  FIG. 4 , the chopped prepreg tape  42  may be provided as a rolled-up sheet  46  with a sheet thickness of between about 0.5 mm to about 3 mm, although other sheet thicknesses are also possible. Furthermore, in some circumstances, the chopped prepreg tape  42  may be provided in bulk form (i.e., as loose chips  44 ), rather than as a sheet. In any event, the chopped prepreg tape  42  may be commercially available as chopped prepreg material sold under the trade name HexMC® from Hexcel Corporation, or it may be purchased as another type of chopped prepreg material from Hexcel Corporation or another commercial source. Alternatively, it may be fabricated by various possible methods, such as the method depicted in  FIG. 5 . In particular,  FIG. 5  shows a series of possible steps for fabricating the chopped prepreg tape  42  having chips  44  consisting of unidirectional non-continuous fibers. As shown by a first block  50 , the method may first involve preparing unidirectional prepreg tape having continuous fibers (i.e., long fibers having an aspect ratio of about 200 or more) aligned parallel and embedded in a desired resin matrix, as will be understood by those with ordinary skill in the art. Alternatively, the unidirectional prepreg tape may be purchased from a commercial supplier. The unidirectional prepreg tape may then be chopped into pieces of a desired length and width to provide the chips  44  having unidirectional non-continuous fibers, according to a block  52 . According to a next block  54 , the chips  44  may then be distributed in a random orientation on a sheet to provide the chopped prepreg tape  42 . Notably, the randomly distributed chips  44  may spontaneously bond together on the sheet due to the adhesive properties/stickiness of the partially cured resin matrix. Optionally, the resulting chopped prepreg tape  42  may then be rolled up as a rolled-up sheet, if desired. 
     Referring now to  FIGS. 6 and 7 , a method for fabricating the facesheet  38  using the chopped prepreg tape  42  is depicted. Beginning with a first block  60 , plies  67  (or layers) of the chopped prepreg tape  42  having desired dimensions may be cut from the rolled-up sheet  46  or from another stock material. Alternatively, the plies  67  may be purchased in a pre-cut form in desired dimensions from a commercial supplier. According to a next block  62 , the plies  67  may be stacked to a desired thickness according to a manual or automated prepreg layup process to provide a pre-mold stack  68 , as will be apparent to those skilled in the art. If desired, one or more plies of prepreg fabric, such as woven aramid fiber/epoxy resin prepregs or another type of prepreg fabric, may be incorporated into the pre-mold stack  68  during the block  62 . 
     Importantly, because the chopped prepreg tape  42  may be much thicker than traditional prepreg fabric (i.e., woven aramid/epoxy resin prepreg), substantially fewer plies  67  of the chopped prepreg tape  42  may be required to achieve a given facesheet thickness. For example, if the chopped prepreg tape  42  is about 1 mm thick, only five plies  67  will be required in the pre-mold stack  68  to achieve a thickness of 5 mm, whereas  25  plies would be required to achieve the same thickness with prepreg fabric having a typical thickness of about 0.2 mm. In this regard, the efficiency of the prepreg layup process may be substantially improved by using the chopped prepreg tape  42 . Moreover, unlike conventional prepreg fabric (i.e., woven aramid fiber in epoxy resin), the fiber strength may be the same in every direction in the plane of the chopped prepreg tape  42 , such that each ply  67  may not need to be oriented in a different direction during the block  62 . This may further improve the ease and efficiency of the prepreg layup process compared with traditional methods (see additional details below). 
     Following the block  62 , the pre-mold stack  68  may be cured and shaped into the facesheet  38  by compression molding, according to a next block  64 . More specifically, the block  64  may involve placing the pre-mold stack  68  between two mold surfaces  70  in the shape of the desired facesheet  38  and applying pressure as well as heat from a hot press  72 , as schematically depicted in  FIG. 7 . During compression molding, the pre-mold stack  68  may flow and fill the void between the mold surfaces  70 , and the resin matrix may be cured to a solid state. Importantly, the compression molding process may yield a net molded shape, such that trimming of excess material from the resulting facesheet  38  may not be necessary. Accordingly, material utilization may be improved compared with traditional fabrication methods which often involve the trimming of excess material (see additional details below). Furthermore, the compression molding process may assist in removing any entrapped air in the pre-mold stack  68 , such that steps involving the removal of entrapped air during the prepreg layup process, as used in traditional facesheet fabrication methods, may be eliminated. 
     The increased efficiency for ice panel facesheet fabrication according to the methods of the present disclosure may be further appreciated by reference to  FIG. 8 , which illustrates a series of steps which may be used to fabricate an ice panel facesheet from plies of prepreg fabric (e.g., woven aramid fiber in epoxy resin) by a traditional method (prior art). In particular, the traditional facesheet fabrication process using prepreg fabric may first involve cutting plies of desired dimensions from a stock of the prepreg fabric (e.g, a rolled-up sheet, etc.) (block  74 ), followed by stacking some of the prepreg plies with each ply being oriented in a different direction to leverage fiber strength in all directions in the plane of the prepreg fabric (block  76 ). After some of the plies are stacked, entrapped air may be carefully removed from the stack according to a block  78 . As shown, blocks  76  and  78  may be repeated as needed to build up the desired thickness of the stack of prepreg fabric plies. Furthermore, as explained above, because the prepreg fabric may be several times thinner than the chopped prepreg tape  42 , many more plies may be required to achieve a desired thickness by the traditional fabrication method. Once a desired thickness is achieved, the stacked prepreg fabric plies may be cured and shaped by autoclaving, according to a block  80 , as shown. In particular, the autoclaving process may involve positioning the stacked prepreg fabric plies between a mold and a vacuum bag and applying heat, vacuum, and pressure. Following the block  80 , excess material may be trimmed and discarded according to the block  82  to provide the final ice panel facesheet. 
     INDUSTRIAL APPLICABILITY 
     In general, it can therefore be seen that the technology disclosed herein may have industrial applicability in a variety of settings including, but not limited to, gas turbine engine construction. The present disclosure introduces a new ice panel for a fan case of a gas turbine engine having a facesheet fabricated from a composite material consisting of cured chopped prepreg tape. As disclosed herein, this material construction may provide numerous manufacturing advantages over the traditional composite materials (e.g., woven aramid/epoxy resin composites) such as increased ease and efficiency of production, increased material utilization, and reduced manufacturing costs, without compromising the performance of the resulting facesheet. More specifically, the material construction of the facesheet as disclosed herein may eliminate some cumbersome and/or wasteful steps often used in traditional facesheet fabrication methods, such as the manual or automated orientation of each prepreg layer in a different direction during the prepreg layup process, the removal of entrapped air between prepreg plies during the prepreg layup step, and the trimming of excess material from the facesheet following curing. Furthermore, because the chopped prepreg tape may be several times thicker than traditional prepreg fabrics, the prepreg layup process may be greatly simplified, as fewer prepreg plies may be required to achieve a given facesheet thickness. Moreover, the chopped prepreg tape may be compatible with compression molding which may provide more directional control in the molding process compared with traditional autoclaving processes. It is expected that the technology disclosed herein may find wide industrial applicability in areas such as, but not limited to, aerospace and power generation applications.