Abstract:
A blade containment structure for turbomachinery, such as a high bypass turbofan engine. The blade containment structure includes a first casing member having a wall that defines an inner containment shell that immediately surrounds the blades of the turbomachine, and a second casing member assembled to the first casing member. The second casing member has a wall that defines an outer containment shell that surrounds the inner containment shell, such that a cavity is defined by and between the inner and outer containment shells. The first casing member is formed from a high-toughness material such as steel or a nickel-base alloy, while the second casing member is formed from a lower-weight and potentially lower-toughness material as compared to the material of the first casing member.

Description:
FIELD OF THE INVENTION 
     The present invention relates to turbomachinery and blade containment structures for minimizing structural damage due to blade release. More particularly, this invention relates to a dual-wall containment casing for turbomachinery, such as a high bypass turbofan engine, in which the casing has a multi-component construction of different materials and is configured to benefit from the different mechanical and physical properties of the materials. 
     BACKGROUND OF THE INVENTION 
     High bypass turbofan engines are widely used for high performance aircraft that operate at subsonic speeds. These engines have a large fan placed at the front of the engine to produce greater thrust and reduce specific fuel consumption. The fan serves to compress incoming air, a portion of which is then delivered to the combustion chamber, with a larger portion being bypassed to the rear of the engine to generate additional engine thrust. The fan is circumscribed by a fan casing that must be capable of containing and minimizing damage to the engine from the remote event of a fan blade that is released from its hub during engine operation. For this reason, fan casings are equipped with specialized blade containment structures that serve to minimize structural damage to the engine as well as the aircraft to which the engine is mounted. 
     Various materials and configurations for blade containment structures have been proposed. Steel is well suited for blade containment on the basis of its mechanical properties, and particularly its toughness (strain to failure). However, a significant drawback to the use of steel in aerospace applications is its density. Consequently, thin steel containment structures coupled with a wrap formed of KEVLAR® or another fiber-reinforced polymer material have been developed. While reducing weight, these containment structures are characterized by significantly higher manufacturing costs. Containment structures formed of relatively lightweight metals such as aluminum alloys have also been used, though they do not provide the level of high toughness and other desirable mechanical properties possible with steel. 
     An additional consideration for blade containment structures is the natural frequency of the casing and the avoidance of blade/case interaction. The frequency of steel containment structures has typically been increased above blade/case interaction frequencies by the inclusion of rings that are integral with or bolted to the structure. Frequency-altering measures such as integral or bolt-on rings have also been required with aluminum containment structures. For steel containment structures coupled with fiber-reinforced polymer wraps, a honeycomb structure between the steel component and the wrap has been used to increase the natural frequency of the casing assembly. In addition to additional material, and manufacturing and assembly costs, each of the above modifications for addressing blade/case interaction incurs the penalty of unwanted weight, space and cost. 
     From the above, it can be seen that improvements in blade containment through material selection based on mechanical properties and the structural requirements for avoiding blade/case interaction have combined to increase the weight and cost of manufacturing high bypass turbofan engines. It would be desirable if a blade containment structure were available that exhibited the blade containment capabilities of steel casings but without the weight penalty associated with steel, while also maintaining fan casing natural frequencies at acceptable margins to avoid blade/case interactions. 
     BRIEF SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a blade containment structure for a fan casing of turbomachinery, such as a high bypass turbofan engine. The blade containment structure of this invention includes a first casing member having a wall that defines an inner containment shell immediately surrounding the blades of the turbomachine, and a second casing member assembled to the first casing member. The second casing member has a wall that defines an outer containment shell surrounding the inner containment shell of the first casing member, such that a cavity is defined by and between the inner and outer containment shells. According to the invention, the first casing member is formed from a relatively tough material such as a stainless steel or a nickel-base alloy, while the second casing member is formed from a material that is less dense than the material of the first casing member. The material of the second casing member may also have lower toughness as compared to the material of the first casing member. 
     The overlapping portions of the first and second casing members provide for a multi-component containment structure whose ability to contain a released blade benefits from the toughness of the first casing member immediately adjacent the blades. Simultaneously, the containment structure benefits from the relatively low weight of the second casing member, which preferably forms the balance of the containment structure such that the overall weight of the fan casing is significantly lower than that possible if the entire containment structure was formed of the material of the first casing member. The overlapping portions of the first and second casing members also provide a load path if the first casing member is torn on blade impact, and the cavity defined by the overlapping portions can be sized and configured to accommodate a variety of devices that provide positive damping between the casing members to react any case vibrational interaction with the blades. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 represent partial cross-sectional views of fan casings of a high bypass turbofan engine equipped with blade containment structures in accordance with two embodiments of this invention. 
     FIGS. 3 and 4 are alternative damping devices illustrated with the blade containment structure of FIG.  1 . 
     FIG. 5 is a damping device illustrated with the blade containment structure of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides blade containment structures for use in turbomachinery, and particularly high bypass turbofan engines. As represented in FIG. 1, an annular-shaped blade containment casing  10  is shown as having a two-piece construction that surrounds fan blades  12  of the fan section of an engine. The casing  10  is composed of annular-shaped forward and aft casings  14  and  18 , respectively, having wall portions that overlap each other in the vicinity of the blades  12 . The overlap provides a dual-wall construction that promotes the ability of the casing  10  to withstand a localized maximum impact from a fan blade  12  released from its hub (not shown) by distributing the impact of the blade  12 , so as to reduce the likelihood of engine damage. 
     In FIG. 1, the overlapping portions of the forward and aft casings  14  and  18  are designated as inner and outer shells  16  and  20 , respectively, which define an annular-shaped cavity  22  therebetween. Because of the proximity of the blade tips to the inner shell  16 , the fan blades  12  may rub the inner shell  16  during hard aircraft landings or maneuvers. As is generally conventional, a portion of the inner shell  16  immediately adjacent the blade tips is provided with an abradable material  38 , such that the abradable material  38  will sacrificially abrade away when rubbed by the fan blades  12 . The inner shell  16  is generally delineated from an axially forward section  24  of the forward casing  14  by a flange  26 , to which the aft casing  18  is secured by any suitable means, such as fasteners (not shown). The aft end of the forward casing  14  is shown as terminating with a friction damper  30  composed of an annular rim or flange  32  that is preloaded against an annular shoulder  34  of the aft casing  18 , beyond which an aft end  28  of the aft casing  18  extends. The degree to which the flange  32  is deformed to generate a desirable level of friction damping with the shoulder  34  will depend on the natural frequencies of the blades  12  and the containment casing  10 , as well as geometric blade/casing interfaces which generate high radial loading, and can generally be appropriately estimated by those skilled in the art. 
     As seen from FIG. 1, the overlapping inner and outer shells  16  and  20  surround a region  40  within the casing  10  that a blade is typically propelled if released. Consequently, the dual-wall section formed by the inner and outer shells  16  and  20  must be capable of withstanding impact from a released blade  12 . A key aspect of this invention is that the inner and outer shells  16  and  20  are formed of different materials, with the inner shell  16  being formed of a relatively tough material, while the outer shell  20  is formed of a lighter-weight material that can have lower toughness as compared to the inner shell  16 . Suitable high-toughness materials for the inner shell  16  include steel alloys such as 304SS stainless steel that preferably exhibit a modulus of toughness on the order of about 36 ksi (about 250 MPa). In contrast, suitable materials for the outer shell  20  include aluminum and its alloys, such as 2029 or 6061, whose densities are significantly less than steel and have a modulus of toughness of about 6 ksi (about 40 MPa). In the embodiment of FIG. 1, the inner and outer shells  16  and  20  are formed as integral parts of the forward and aft casings  14  and  18 , such that the entire forward casing  14  is formed of the same high-toughness material and the entire aft casing  18  is formed of the same lightweight material. Notably, the thicknesses of the inner and outer shells  16  and  20  are shown as being approximately equal, generally on the order of about 4 to about 9 mm in thickness, more preferably about 5 to about 6 mm in thickness. Generally, the thickness of the inner shell  16  may be equivalent to that of a prior steel casing, while the thickness of the outer shell  20  may be 85% or less of that required for prior aluminum casings. As a result, the present invention significantly reduces the overall weight of the casing  10  as compared to all-aluminum casings, and also incurs lower costs as compared to all-steel casings since a lower cost material can be substituted for steel for the aft casing  18 . 
     With the blade containment casing  10  described above, the high-toughness inner shell  16  will bear the brunt of impact in the event that a fan blade  12  is released. A portion of the impact load is distributed to the lighter-weight outer shell  20  through the flange  26  and the friction damper  30 , the latter of which will also absorb and dissipate some of the impact force to further minimize damage to the casing  10  and engine. Consequently, the casing it is configured to benefit from the toughness of the material used to form the inner shell  16 , yet also has the advantage of reduced weight as a result of the lighter-weight material used to form the aft casing  18 . As a result of being shielded by the inner shell  16 , the toughness of the aft casing  18  is less critical, permitting the use of materials whose toughness can be significantly less than that of the inner shell  16 . 
     FIG. 2 illustrates a blade containment casing  110  having a three-piece construction in accordance with a second embodiment of the invention. As with the casing  10  of FIG. 1, the casing  110  surrounds the fan blades  112  of a turbofan engine fan section, and is composed of forward and aft casings  114  and  118 , respectively, which define overlapping inner and outer shells  116  and  120 . The blades  112  are surrounded by the dual-wall construction provided by the inner and outer shells  116  and  120 , which also define an annular-shaped cavity  122 . As with the embodiment of FIG. 1, the inner shell  116  is provided with a sacrificial abradable material  138  for rub encounters with the fan blades  112 . Also similar to the embodiment of FIG. 1, the casing  110  includes a friction damper  130  composed of an annular rim or flange  132  that is preloaded against an annular shoulder  134  of the aft casing  118 , beyond which an aft end  128  of the aft casing  118  extends. 
     The casing  110  differs from that of FIG. 1 by the configuration of the forward casing  114  and the inclusion of a separate forward casing end  124  in lieu of the integral forward section  24  of FIG.  1 . The forward casing end  124  has a flange  126  that is secured by fasteners (not shown) or other suitable means to the aft casing  118 , and is also secured to the fore end of the forward casing  114  by an anti-rotation device  136 , such as axial-oriented shear pins. A key advantage of this embodiment is that the weight of the casing  110  can be further reduced by forming the forward casing end  124  of the same or similar lighter-weight material used to form the aft casing  118 , e.g., aluminum alloy, while the forward casing  114  is again formed of a relatively high-toughness material such as steel. The thicknesses of the inner and outer shells  116  and  120  are again shown to have approximately equal thicknesses. 
     FIGS. 3,  4  and  5  serve to illustrate alternative or supplemental damping devices that can be employed with the casings  10  and  110  of this invention. The damping devices of FIGS. 3 and 4 are illustrated with the blade containment casing  10  of FIG. 1, while the damping device of FIG. 5 is illustrated with the casing  110  of FIG. 2, though any one or more of the damping devices could be adapted for use with either casing  10  and  110 . 
     In FIG. 3, a damping device  42  is shown as including an elastomeric pad  44  within the cavity  22  and radially abutting a stepped surface  46  of the inner shell  16 . The casing  10  is also equipped with an elastomeric pad  48  between the flange  32  of the inner shell  16  that is preloaded against the shoulder  34  of the aft casing  18 . In FIG. 4, elastomeric damping is achieved with a damping device  50  composed of a pin  52  mounted to the flange  32  of the inner shell  16  and abutting the shoulder  34  of the aft casing  18 . A spring  54  is mounted on the pin  52  between the flange  32  and the head  56  of the pin  52  to provide damping between the forward and aft casings  14  and  18 . 
     Finally, FIG. 5 illustrates the casing  110  equipped with the friction damping device  130  of FIG. 2, and further equipped with an elastomeric damping device  142  composed of an elastomeric bumper  144  mounted on an adjustable pin  146  within the cavity  122 . The bumper  144  abuts the outward surface of the inner shell  116 , and absorbs vibration as well as helps distribute impact loads during blade release. 
     While the invention has been described in terms of certain embodiments, it is apparent that other forms could be adopted by one skilled in the art. In addition, it is foreseeable that the invention could be adapted for use in turbomachinery other than high bypass turbofan engines. Accordingly, the scope of the invention is to be limited only by the following claims.