Abstract:
A gas turbine engine includes a fan including a plurality of circumferentially spaced rotatable blades, and a fan casing for containing fragments of fan blades in the event of blade release, the fan casing having a shell surrounding the blades and circumscribing a containment zone of the fan. The shell is made of a fiber reinforced polymer composite material which includes nanoparticles.

Description:
TECHNICAL FIELD 
       [0001]    The application relates generally to composite casings for rotating blades and, more particularly, to such a casing for a fan blade of a gas turbine engine. 
       BACKGROUND 
       [0002]    An aircraft turbine engine fan casing is required to contain released fan blades in the event of blade failure. Fiber reinforced polymer composite materials have been used in fan casings because of their high strength to density ratio when compared to metal alloys. 
         [0003]    When fiber reinforced polymer composite material is subjected to a high energy impact, such as in a fan casing during blade release, the impact energy is generally absorbed by fiber breaking, fiber pull out, resin cracks, and ply delamination. A fan casing usually has to absorb impact energy, resist puncture, and maintain structural integrity for safe engine shutdown. 
         [0004]    A known theory to improve the energy absorption capability of fiber reinforced polymer composite materials is to promote fiber pull out. Fiber pull out generally absorbs energy via the creation of new surfaces between the fibers and the resin due to the frictional force that pulls and separates the fibers from the resin. However, fiber pull out usually reduces the post impact carrying load capability and structural integrity of the composite material, as pulled out fibers can no longer transfer loads to other fibers through the resin. 
         [0005]    Accordingly, improvements are desirable. 
       SUMMARY 
       [0006]    In one aspect, there is provided a gas turbine engine comprising a fan including a plurality of circumferentially spaced rotatable blades, and a casing for containing fragments of the blades in the event of blade release, the casing including a shell surrounding the blades and circumscribing a containment zone of the fan, the shell being made of a fiber reinforced polymer composite material including nanoparticles. 
         [0007]    In another aspect, there is provided a casing for surrounding rotating blades and containing blade fragments thereof in the event of blade release, the casing comprising a shell made of a composite material including a polymer resin, reinforcing fibers and nanoparticles, the fibers forming a first bond with the resin resisting separation up to a first mean impact energy threshold, the nanoparticles forming a second bond with the resin resisting separation up to a second mean impact energy threshold, the first mean impact energy threshold being substantially greater than the second mean impact energy threshold, such that upon impact of blade fragments with the casing, separation of the nanoparticles from the resin absorbs a portion of the impact energy. 
         [0008]    In another aspect, there is provided a method of improving post-impact structural integrity of a fan casing after a high speed, high energy impact from a released blade or blade portion, the casing being made of a fiber reinforced polymer resin composite material, the method comprising adding nanoparticles in the polymer resin during manufacture of the casing, wherein separation of the nanoparticles from the resin during the impact absorbs a portion of the impact energy and reduces pull out and breaking of the fibers. 
         [0009]    In a further aspect, there is provided a method of manufacturing a fan casing having improved blade containment capability, the method comprising forming the fan casing from a composite material including a polymer resin, reinforcing fibers and nanoparticles, the nanoparticles having an area of resin interface per volume substantially greater than that of the reinforcing fibers, an average impact energy threshold necessary for causing separation of any one of the nanoparticles from the resin being substantially lower than that for causing separation or breaking of any one of the fibers in the resin, such that during impact of a blade or a blade portion on the casing, the lower impact energy threshold of the nanoparticles causes separation of the nanoparticles from the resin to predominate over separation of the fibers from the resin or breaking of the fibers in the resin, thus limiting a reduction of a structural integrity of the casing caused by the impact, and the greater area of interface per volume of the nanoparticles limits an area of damage caused by the impact. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0010]    Reference is now made to the accompanying figures in which: 
           [0011]      FIG. 1  is a schematic cross-sectional view of a gas turbine engine, including a fan casing according to a particular embodiment of the present invention; and 
           [0012]      FIG. 2  is a schematic cross-sectional view of a gas turbine engine, including a fan casing according to an alternate embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]      FIG. 1  illustrates a gas turbine engine  10  of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan  12  through which ambient air is propelled, a compressor section  14  for pressurizing the air, a combustor  16  in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section  18  for extracting energy from the combustion gases. 
         [0014]    The fan assembly  12  includes an array of fan blades  22  extending radially outward from a rotor disc  24 . A fan casing  30  surrounds the fan assembly  12 . The fan casing  30  is a containment system and includes a shell  32  which has a longitudinal length that is approximately equal to a fan assembly length. More specifically, the length is selected so that the shell circumscribes a containment zone  34  of the fan assembly  12 . Containment zone as used herein is defined as a zone extending both axially and circumferentially around the fan assembly  12  where a fan blade or blade fragment is most likely to be ejected from the fan assembly  12 . 
         [0015]    The shell  32  is made of a fiber reinforced polymer composite material, with nanoparticles added during the manufacture of the shell  32 , e.g. prior to molding. In a particular embodiment, the shell  32  is made of carbon fiber reinforced epoxy. Alternate fiber reinforced polymer composite materials include aromatic polyamide (aramid) fibers such as Kevlar® and Twaron®, ultra high molecular weight polyethylene fibers such as Spectra® and Dyneema®, poly(diimidazo pyridinylene (dihydroxy) phenylene) fibers such as M5®, and poly(p-phenylene-2-6-benzobisoxazole) fibers such as Zylon®, and the like, bonded with an appropriate thermoset or thermoplastic. 
         [0016]    In a particular embodiment, the nanoparticles are clay or clay based reinforcements (e.g. montmorillonite), particle dispersions (e.g. magnetic particles, e.g. Cobalt iron oxide), molecules such as hyper-branched polymers, nano-spheres (e.g. ceramic powders e.g. SiO 2 , TiO 2 ), elements (e.g. carbon, carbon nano-tubes), nanolarge pendant groups grafted on the end of the polymer chain, or any adequate combination thereof. 
         [0017]    The nanoparticles are preferably provided with a content of at least 2 phr. In a particular embodiment, the nanoparticles are provided up to a content of 10 phr. 
         [0018]    The difference between the fracture surface of a resin with and without nanoparticles included therein is significant. For example, in test conducted, the fracture surface of a resin with 6 phr of nano-clay particles was shown to undergo much more deformation before fracture than the fracture surface of a resin without nanoparticles, which had a smooth featureless brittle fracture surface. 
         [0019]    Tests of samples of carbon fiber reinforced epoxy without nanoparticles and with 2 phr of nano-clay particles have shown that when compared to the composite without nanoparticles, the composite with 2 phr nano-clay had a Mode I interlaminar fracture toughness approximately 52% greater; a flexural strength approximately 38% greater; and a modulus of elasticity approximately 37% greater. 
         [0020]    As the surface area of a nanoparticle is much smaller than that of a continuous fiber strand, fracture by resin-nanoparticle separation predominates over fiber-resin separation, or fiber pull-out, and over fiber break, because nano-size fractures are much easier to create than millimeter-sized fractures. In other words, the bond between each fiber and the resin resists separation up to a first mean impact energy threshold, while the bond between each nanoparticle and the resin resists separation up to a second mean impact energy threshold, with the first mean impact energy threshold being substantially greater than the second mean impact energy threshold. As such, upon high speed, high energy impact of blade fragments with the casing shell  32 , a portion of the impact energy is absorbed through separation of the nanoparticles from the resin, with only the portion of the impact energy not absorbed by resin-nanoparticle separation being left for potentially causing fiber pull-out and fiber break. 
         [0021]    As one gram of nanoparticles has a surface area that can be over 200 and even over 1200 square meters, minute additions creates a large amount of resin-nanoparticle interface in a small volume of material. Thus with the large amount of resin-nanoparticle interface per unit volume, and with a portion of the blade impact energy being absorbed via resin-nanoparticle separation, the damage caused by the blade impact is distributed in a much smaller area and volume of the casing shell  32  when compared to the damage that would be caused if the same impact energy was completely absorbed by resin-fiber separation and fiber break; the addition of nanoparticles in the casing shell  32  thus advantageously reduces the area of impact damage. 
         [0022]    As the energy absorbed by the resin-nanoparticle separation significantly reduces the amount of fiber pull-out produced by the impact, as well as the amount of energy remaining for causing other types of damages such as fiber break and resin damage, and as the nanoparticles also significantly reduce the area and volume of damage caused by the impact, the residual structural strength and structural integrity of the casing shell  32  after impact are thus improved. In addition, the undamaged area of the shell  32  continues to benefit from the enhanced material properties brought by the presence of the nanoparticles. 
         [0023]    Referring to  FIG. 2 , an alternate embodiment is shown, where an engine  110  includes a fan casing  130  with a first shell  132  circumscribing the containment zone  34  of the fan assembly  12 , and a second shell  133  surrounding the first shell  132 . As in the previous embodiment, the first shell  132  is made of fiber reinforced polymer composite material including nanoparticles. The second shell  133  is also made of a fiber reinforced polymer composite material including nanoparticles, and may have a higher ratio of continuous fiber to resin content than that of the first shell  132 . In a particular embodiment, the first and second shells  132 ,  133  are made of the same fiber reinforced polymer composite material, and have the same type of nanoparticles included therein. Alternately, the two shells  132 ,  133  can be made of different materials. In a particular embodiment, the first shell  132  is molded, and the second shell  133  is then molded onto the first shell  132 . 
         [0024]    The second shell  133  benefits from energy absorbed by the first shell, thus further lessening the amount of fiber pull-out and fiber break caused by the impact as well as the size of the area and volume of impact damage. As such, the post impact structural integrity of the fan casing  130  is further improved by the presence of the second shell  133 . In addition, a higher fiber content in the second shell  133  further improves post impact structural strength and structural integrity of the fan casing  130 . 
         [0025]    The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. For example, the fiber reinforced polymer composite casings  30 ,  130  including nanoparticles can be used around other rotating equipment which have a risk of producing fragments which must be contained, for example around turbine rotors in a gas turbine engine or elsewhere. Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.