Patent Publication Number: US-8966754-B2

Title: Methods for reducing stress on composite structures

Description:
TECHNICAL FIELD 
     Embodiments described herein generally relate to methods for reducing stress on composite structures. More specifically, embodiments herein generally describe methods for reducing stress on composite structures by providing a mounting flange on the composite structure that delaminates or separates, yet remains intact, after maximum stress capacity has been exceeded. 
     BACKGROUND OF THE INVENTION 
     In gas turbine engines, such as aircraft engines, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel in a combustor. The mixture is then burned and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas expands through the turbine which in turn spins the shaft and provides power to the compressor. The hot exhaust gases are further expanded through nozzles at the back of the engine, generating powerful thrust, which drives the aircraft forward. 
     Because engines operate in a variety of conditions, foreign objects may undesirably enter the engine. More specifically, foreign objects, such as large birds, hailstones, sand and rain may be entrained in the inlet of the engine. As a result, these foreign objects may impact a fan blade and cause a portion of the impacted blade to be torn loose from the rotor, which is commonly known as fan blade out. The loose fan blade may then impact the interior of the fan casing causing a portion of the casing to bulge or deflect. This deformation of the casing may result in increased stresses along the entire circumference of the engine casing. 
     In recent years composite materials have become increasingly popular for use in a variety of aerospace applications because of their durability and relative lightweight. Although composite materials can provide superior strength and weight properties, and can lessen the extent of damage to the fan casing during impacts such as blade outs, designing flanges on structures fabricated from composite materials still remains a challenge. 
     Laminated composite structures generally have superior strength in-plane due to the presence of continuous reinforcing fibers. However, issues may arise when attaching a secondary structure to an interposing flange located about the body of the composite structure, as opposed to about an end of the composite structure. Such issues are due to a general lack of continuous fibers at the points of attachment, or joints, between the flange and primary composite structure. This, in addition to significant out-of-plane loads caused by the weight of the secondary structure, may result in a weak attachment joint that is susceptible to damage from increased stresses, such as those resulting from a fan blade out or those inherently present due to the weight of the secondary structure. 
     To address such weaknesses at the point of attachment, it may be desirable to provide supplementary reinforcement to the joints of the mounting flange, such as additional fibers or metal brackets. However, with the addition of these reinforcements, the weight-saving benefits provided by using composite structures can be significantly reduced. Moreover, even with additional reinforcements, the mounting flange may still not be strong enough to adequately support the weight of the attached secondary structure, with or without the additional stresses caused by a blade out. Ultimately, continuous stresses on the already weakened flange may result in catastrophic failure to one or more of the primary composite structure, the attached secondary structure, the engine or the aircraft. 
     Accordingly, there remains a need for methods for reducing stress on composite structures having mounting flanges that provide the desired attachment without the previously described failure issues. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Embodiments described herein generally relate to methods for reducing stress on composite structures comprising providing a primary composite structure having a circumference, providing at least one mounting flange operably connected to the primary composite structure about the circumference to form a joint, and providing a secondary structure operably connected to the primary composite structure at the mounting flange wherein when stresses on the primary composite structure exceed a maximum capacity level delamination of the mounting flange from the primary structure occurs at the joint and the secondary structure remains operably connected to the mounting flange. 
     Embodiments herein also generally relate to methods for reducing stress on composite structures comprising providing a primary composite structure having a circumference, providing at least one mounting flange operably connected to the primary composite structure about the circumference to form a joint, and providing a secondary structure operably connected to the primary composite structure at the mounting flange wherein when stresses on the primary composite structure exceed a maximum capacity level separation of the mounting flange from the primary structure occurs at the joint and the secondary structure remains operably connected to the mounting flange. 
     These and other features, aspects and advantages will become evident to those skilled in the art from the following disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       While the specification concludes with claims particularly pointing out and distinctly claiming the invention, it is believed that the embodiments set forth herein will be better understood from the following description in conjunction with the accompanying figures, in which like reference numerals identify like elements. 
         FIG. 1  is a schematic cross-sectional view of one embodiment of a gas turbine engine; 
         FIG. 2  is a schematic view of one embodiment of a fan casing having a mounting flange; 
         FIG. 3  is a schematic perspective view of one embodiment of a composite structure forming tool; 
         FIG. 4  is a schematic cross-sectional view of one embodiment of a mounting flange operably connected to a fan casing; 
         FIG. 5  is a schematic view of one embodiment of a fan casing having mounting flanges and attached secondary structure; 
         FIG. 6  is a schematic representation of one embodiment of a process for fabricating a mounting flange; 
         FIG. 7  is a schematic cross-sectional view of one embodiment of a flange shoe; 
         FIG. 8  is a schematic cross-sectional view of one embodiment of an extended flange shoe; 
         FIG. 9  is a schematic cross-sectional view of one embodiment of tooling used during final cure of a composite fan casing having two mounting flanges; and 
         FIG. 10  is a portion of a schematic cross-sectional view of one embodiment of tooling used during final cure of a composite fan casing having two adjacent mounting flanges. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Integral Composite Mounting Flanges 
     Embodiments described herein generally relate to methods for reducing stress on a composite structure that can reduce the occurrence of severe part damage to both a primary composite structure and an attached secondary structure, while concurrently helping to eliminate catastrophic part failure. While embodiments herein may generally focus on integral mounting flanges on composite fan casings of gas turbine engines, it will be understood by those skilled in the art that the description should not be limited to such. Indeed, as the following description explains, the integral mounting flange described herein may be utilized on any generally cylindrically-shaped composite structure. 
     Turning to the figures,  FIG. 1  is a schematic representation of one embodiment of a gas turbine engine  10  that generally includes a fan assembly  12  and a core engine  14 . Fan assembly  12  may include a fan casing  16  and an array of fan blades  18  extending radially outwardly from a rotor disc  20 . Core engine  14  may include a high-pressure compressor  22 , a combustor  24 , a high-pressure turbine  26  and a low-pressure turbine  28 . Engine  10  has an intake side  30  and an exhaust side  32 . Fan assembly  12  and low-pressure turbine  28  may be coupled by a first rotor shaft  34  while high-pressure compressor  22  and high-pressure turbine  26  may be coupled by a second rotor shaft  36 . 
       FIG. 2 . illustrates one embodiment of an acceptable primary composite structure  38 . As used herein, “composite structure (preform)” refers to any component, or preform thereof, fabricated from composite materials. Composite structure  38  may comprise a generally cylindrical member, such as fan casing  16 . Henceforth, the cylindrical member will be referred to as fan casing  16 , though it should not be limited to such. Fan casing  16  may be generally cylindrical in shape and may be fabricated from any acceptable material. In one embodiment, however, fan casing  16  may be fabricated from a composite material, such as, but not limited to, glass fibers, graphite fibers, carbon fibers, ceramic fibers, aromatic polyamide fibers such as poly(p-phenylene terephthalamide) fibers (i.e. KEVLAR®), and combinations thereof. In one embodiment, the composite material may comprise carbon fibers. Additionally, fan casing  16  may be fabricated using any acceptable fabrication method known to those skilled in the art. See, for example, U.S. Patent Application No. 2006/0201135 to Xie et al. 
     Fan casing  16  may generally comprise a body  40  having a forward end  42  and an aft end  44 . As used herein, “fan casing” is used to refer to both pre- and post-cure composite fan casings. Those skilled in the art will understand which stage is being referenced from the present description. Fan casing  16  may also comprise at least one integral composite mounting flange  46 . As used herein, “mounting flange” refers to any flange interposed circumferentially about body  40  of fan casing  16 , or other primary composite structure, that may be used to operably connect a secondary structure to the primary structure, as described herein below. By “interposed” it is meant that mounting flange  46  may be located circumferentially about body  40  of fan casing  16 , as opposed to about either of forward end  42  or aft end  44 . 
     Fan casing  16  may also be fabricated using any tool known to those skilled in the art. See, for example, U.S. Patent Application No. 2006/0134251 to Blanton et al. In one embodiment, as shown in  FIG. 3 , composite structure forming tool  37  may have a circumference, a generally cylindrically shaped core  33 , and comprise a first endplate  72  and a second endplate  84  that may be removeably attached to core  33  of tool  37 . 
     Turning to  FIG. 4 , integral composite mounting flange  46  may generally include at least one core fiber  52 , though in one embodiment mounting flange  46  may comprise a plurality of core fibers  52 . Core fibers  52  may be circumferentially oriented about fan casing  16 . By “circumferentially oriented” it is meant that core fibers  52 , whether fiber tows, textile preforms or a combination thereof, generally circumscribe fan casing  16  and are continuous in the circumferential direction. Mounting flange  46  may also generally comprise at least one layer of multidirectional attachment fibers  54  that may operably connect core fibers  52  to fan casing  16  as described herein below. As used herein, “multidirectional” refers to textile preforms comprising the attachment fibers that have fiber tows oriented in more than one direction. 
     As will be understood by those skilled in the art, core fibers  52  may be fabricated in different ways. In one embodiment, core fibers  52  may be fabricated from a plurality of continuous, unidirectional fiber tows bundled and bonded together. In another embodiment, core fibers  52  may comprise textile preforms, such as a flattened biaxial braid sleeve, having a majority of fiber tows that are continuous in the circumferential direction, and the remaining fibers either continuous or non-continuous in the non-circumferential direction. It is this general circumferential orientation of core fibers  52  that can provide added strength to the flange in the circumferential direction as explained herein below. Regardless of the particular assembly utilized, core fibers  52  may comprise a first core side  56  and a second core side  58 . 
     Fiber tows of core fibers  52  may be comprised of any suitable reinforcing fiber known to those skilled in the art, including, but not limited to, glass fibers, graphite fibers, carbon fibers, ceramic fibers, aromatic polyamide fibers such as poly(p-phenylene terephthalamide) fibers (i.e. KEVLAR®), and combinations thereof. Additionally, while any number of fiber tows may be used to construct core fibers  52 , in one embodiment there may be from about 100 to about 5000 fiber tows used to construct core fibers  52 . Moreover, each fiber tow may comprise from about 3000 to about 24,000 fiber filaments. In general, when assembled, core fibers  52  may constitute about half of the overall thickness T of mounting flange  46 . While the thickness of mounting flange  46  may vary according to application, in one embodiment, mounting flange  46  may have a thickness of from about 0.5 inches (1.27 cm) to about 1 inch (2.54 cm). 
     As explained previously, in addition to circumferential core fibers  52 , each mounting flange  46  may also include at least one layer of attachment fibers  54  operably connecting each of first core side  56  and second core side  58  of core fibers  52  to fan casing  16 . Unlike core fibers  52 , attachment fibers  54  may be constructed of multidirectional textile preforms, such as weaves or braids, that need not have a majority of fiber tows oriented circumferentially. In this way, attachment fibers  54  can display a generally uniform strength distribution throughout. As with the core fibers, each fiber tow of attachment fibers  54  may comprise from about 3000 to about 24,000 fiber filaments. Generally, when assembled, attachment fibers  54  may constitute the remaining half of the overall thickness of flange  46 . 
     As illustrated in  FIG. 5 , mounting flange  46 , once cured, may be used to operably connect at least one secondary structure  48  to fan casing  16  and thus, flange  46  may be located in a variety of locations along the length of body  40  of fan casing  16 . In some instances, it may be desirable to include more than one mounting flange  46 . As shown in  FIG. 5 , in one embodiment, secondary structure  48  may be, for example, an accessory gear box  50  that can be mounted to fan casing  16  using the mounting flanges  46  and any attachment method known to those skilled in the art, such as bolts. Other possible secondary structures may include, but are not limited to, an oil tank, oil and fuel monitoring modules, other engine externals and combinations thereof. It will be understood that “engine externals” refers to any accessory, module or component that may be connected to the outside of the engine. Such secondary structures may be constructed of any acceptable material known to those skilled in art such as, for example, aluminum, and as described previously may weigh significantly more than the corresponding fan casing to which they are attached. For example, in one embodiment, fan casing  16  may weight about 200 pounds while accessory gear box  50  may weigh about 300 pounds. 
     Embodiments of the mounting flange described herein can provide several benefits over existing attachment mechanisms. In particular, the integral mounting flange can reduce the occurrence of severe part damage to both the primary composite structure, as well as the attached secondary structure, while concurrently helping to eliminate catastrophic part failure. Without intending to be limited by theory, it is believed that, in general, fiber-reinforced composite structures, such as the mounting flanges herein, can have relatively weak interfaces between fiber layers and, therefore, have relatively weak through-thickness strength compared to their in-plane strength. If stresses on the composite structure exceed a defined maximum capacity level, these fiber layers can have a tendency to delaminate, or separate, prior to actual fiber breakage occurring. This delamination or separation can reduce the load and stress on the attachment joint where the mounting flange connects to the primary structure. As will be understood by those skilled in the art the maximum stress capacity level of the primary composite structure can vary depending on such factors as materials of fabrication, method of fabrication and the like. 
     Embodiments set forth herein are designed take advantage of the previously described phenomenon. More specifically, the integral mounting flange may be fabricated to permit delamination, or even separation, of the flange from the primary composite structure at the joint under excessive stresses, such as those caused by a fan blade out or by the weight of an attached secondary structure. However, because the core fibers of the flange can be constructed from continuous, circumferentially oriented fibers, even after delaminating or separating the flange can remain a movable yet intact ring about the primary structure. Thus, even if the integral mounting flange delaminates or separates from the primary composite structure, it generally remains in place with all secondary structures attached. This can allow stresses on both the primary composite structure and the mounting flange to be reduced while maintaining the attached secondary structure in the same general placement as originally intended. Because of this, the delamination or separation can reduce damage to both the primary and secondary structures, as well as help to prevent catastrophic part failure. 
     Methods of Fabricating Integral Composite Mounting Flange 
     Fabricating a mounting flange as set forth herein may generally comprise applying core fibers about the primary composite structure, followed by applying attachment fibers to operably connect the core fibers to the fan casing, or other primary composite structure. More specifically, as shown in  FIG. 6  step  100 , the fabrication of a mounting flange may begin with providing a primary composite structure having a circumference, such as fan casing  16 . In one embodiment, the primary composite structure may be complete except for final cure. In step  102 , a correspondingly shaped guide  60  may then be placed about body  40  of fan casing  16  in each location where a mounting flange is desired. Guide  60  may be removably held in place by shrink tape, for example. In one embodiment, guide  60  may be comprised of discrete arcuate members, each spanning about 180 degrees of body  40  of fan casing  16 . The arcs of guide  60  may be releaseably connected together for easy placement and adjustment about fan casing  16 . It will be understood, however, that guide  60  may be comprised of any number of pieces and have any shape that corresponds to the shape of the primary composite structure. Guide  60  can serve as a support for the later application of both the core fibers and the attachment fibers, as explained herein below. As previously mentioned, guide  60  may be circumferential and have an L-shaped cross-section as shown, and may be constructed from any rigid, lightweight material such as, for example, aluminum or composite. 
     In step  104 , once all guides  60  have been placed in the desired locations about body  40  of fan casing  16 , the application of core fibers  52  may be initiated. As previously discussed, core fibers  52  may comprise either unidirectional, circumferentially oriented fiber tows bundled and bonded together or textile preforms, such as a flattened biaxial braid sleeve, having a majority of continuous, circumferentially oriented fiber tows. 
     If unidirectional fiber tows are used to construct core fibers  52 , the tows may comprise fiber filaments that can be wound about the fan casing  16 . In general, a single tackified fiber tow can be precisely placed in the desired position about the fan casing and this process can be repeated until core fibers  52  have the desired size and shape. A debulking step may then be carried out to consolidate core fibers  52 , as described herein below. Alternately, if textile preforms are used to construct core fibers  52 , the textile layers can be layed-up and tackified on a flat, non-porous surface, such as a table or a tool. More specifically, the tackified textile layers can be stacked to form the core fibers  52  desired thickness and height, while still being long enough to circumscribe the fan casing. After debulking, as set forth below, the consolidated textile layers remain flexible enough to allow the layers to be manually or mechanically shaped into the proper radius to fit the fan casing, or other primary composite structure. Regardless of which type of fibers are used, finished core fibers  52  may have first core side  56  and second core side  58 . 
     Having positioned core fibers  52  in the desired location about fan casing  16 , attachment fibers  54  may be applied to each of first core side  56  and second core side  58  of core fibers  52 , as well as to fan casing  16  to operably connect core fibers  52  to fan casing  16 . In step  106 , guide  60  can be left in place while attachment fibers  54  are applied to, for example, first core side  56  of core fibers  52 . As previously described, attachment fibers  54  may comprise multidirectional textile preform layers, such as weaves or braids. Layers of attachment fibers  54  may be wrapped against both first core side  56  of core fibers  52  and fan casing  16  until the desired thickness is obtained. More specifically, a liquid resin, such as an epoxy, may be applied to core fibers  52  and fan casing  16  to provide a tacky layer to which attachment fibers  54  may be applied. Next, a layer of attachment fibers  54  may be applied over the liquid resin. This process can be repeated until the desired thickness of attachment fibers  54  is achieved. Though attachment fibers  54  may have any thickness, in one embodiment, the thickness of attachment fibers may be from about 0.125 inches (about 0.3 cm) to about 0.25 inches (about 0.6 cm). 
     Once attachment fibers  54  have been applied to first core side  56  of core fibers  52  a debulk may again be performed to consolidate the construction thus far. In particular, reinforcing fibers, such as core fibers  52  and attachment fibers  54 , may inherently have a substantial amount of bulk. In order to help prevent wrinkles and/or voids during the final cure of the composite, and to utilize near net shape tooling during the final cure, the fibers of the composite can be consolidated, or compressed, into a dimension that is closer to the desired final cured thickness. This consolidation occurs during debulk. 
     Debulk can be carried out using any common method known to those skilled in the art, such as, for example, by applying pressure to the composite fibers with either a vacuum bag, shrink tape, or other mechanical means. Resin applied to the fibers before debulk can help “tack,” or lock, the fibers in place once the pressure is applied. If the tackified fibers cannot be consolidated as desired at room temperature, then heat may be applied to lower the viscosity of the resin. The resin may then better infiltrate the composite fibers and allowing the consolidation to be carried out to the desired degree. In one embodiment, the guide may be left in place during the debulk process to provide support during fabrication. 
     After debulk, guide  60  may be repositioned adjacent to the completed side of the flange for the application of attachment fibers  54  to the opposing side of the flange as shown in step  108 . The previously described application and debulk of attachment fibers  54  may then be repeated on, for example, second core side  58  of core fibers  52 , to obtain an integral composite mounting flange perform  61  in step  110 . 
     Optionally, in one embodiment shown in step  112 , additional individual fiber tows  62  may be applied to attachment fibers  54  of mounting flange preform  61  prior to final cure to provide additional hoop strength. Such fiber tows will not affect the final cure of the composite structure. However, to avoid limiting the weight-saving benefits provided by using composite materials, it may be desirable to minimize the use of additional individual fiber tows  62 . 
     Once core fibers  52 , attachment fibers  54 , and optionally individual fiber tows  62 , have been layed-up and debulked, each guide  60  can be removed and the final cure tooling can be placed about fan casing  16 , including any flange performs, to serve as a mold during the curing process. As will be understood by those skilled in the art, the final cure tooling and process may vary according to such factors as resin used, part geometry, and equipment capability. However, in one embodiment, the tooling may comprise near net shape tooling, which not only helps prevent waste of raw material and machining time, but also eliminates having to machine into the attachment fibers, which could result in breaking the fibers and introducing weak points in the flange. 
     In general, the final cure tooling  64  may comprise various combinations of flange shoes and extended flange shoes. Flange shoes  66  may comprise any number of pieces that when coupled together may be positioned circumferentially about fan casing  16 , and optionally mounting flange performs  61 , and may comprise a substantially L-shaped cross-section, as shown in  FIG. 7 . Extended flange shoes  68 , shown in  FIG. 8 , may have a first side  69  and a second side  71  and may also comprise any number of pieces that when coupled together may be positioned circumferentially about fan casing  16 , and optionally mounting flange performs  61 . Extended flange shoes  68  may comprise a substantially U-shaped cross-section, as shown in  FIG. 8 . Both flange shoes  66  and extended flange shoes  68  may be constructed of any material having a greater thermal coefficient of expansion than the fan casing preform. In one embodiment, flange shoes  66  and extended flange shoes  68  may be constructed from metals, alloys or combinations thereof, such as aluminum or steel. Additionally, as explained herein below, either or both of flange shoes  66  and extended flange shoes  68  may comprise a flange cavity to accommodate an end flange preform or a mounting flange preform. 
     More particularly, as shown in  FIG. 9 , a first extended flange shoe  70  may be placed about fan casing  16  such that first side  69  of first extended flange shoe  70  is adjacent to a first endplate  72  of the composite structure-forming tool  74  upon which fan casing  16  is fabricated. First extended flange shoe  70  may be removeably coupled to the first endplate  72  using any attachment method known to those skilled in the art, such as, for example, bolts. Once positioned, first extended flange shoe  70  may overlay any first end flange preform  76  present, and continue along body  40  of fan casing  16  to a first mounting flange preform  78 , as shown in  FIG. 9 . A first flange shoe  80  may then be positioned about fan casing  16 , adjacent to second side  71  of first extended flange shoe  70  and the two may be removeably coupled together about first mounting flange preform  78 . In this way, first extended flange shoe  70  can serve as an endplate to first flange shoe  80  and provide the support necessary to help ensure first flange shoe  80  remains in position such that first mounting flange preform  78  retains its desired shape and orientation about fan casing  16  during final cure. 
     As also shown in  FIG. 9 , a second extended flange shoe  82  may be placed about fan casing  16  such that a first side  69  of second extended flange shoe  82  is adjacent to a second endplate  84  of composite structure-forming tool  74 . Second extended flange shoe  82  may be removeably coupled to the second endplate  84  in the same manner provided above for first extended flange shoe  70 . Again, second extended flange shoe  82  may overlay any second end flange preform  86  present, and continue along body  40  of fan casing  16  to a second mounting flange preform  88 , as shown in  FIG. 9 . A second flange shoe  90  may then be positioned about fan casing  16  adjacent to second side  71  of second extended flange shoe  82  and the two may be removeably coupled together about second mounting flange preform  88 . As before, second extended flange shoe  82  can provide support to second flange shoe  90  and help ensure second flange shoe  90  remains in position such that second mounting flange preform  88  retains its desired shape and orientation about fan casing  16  during final cure. 
     For each coupling of an extended flange shoe and a flange shoe, there may also be a flange-shaped cavity formed to accommodate any mounting flange preform. It will be understood by those skilled in the art that cavity may be formed in a flange shoe, an extended flange shoe, or a combination thereof. For example, in  FIG. 9 , first extended flange shoe  70  contains a cavity  92  to accommodate first mounting flange preform  78  while second flange shoe  90  contains a cavity  92  to accommodate second mounting flange preform  88 . Additional cavities  92  may be included to account for end flanges if present. 
     As shown in  FIG. 10 , if adjacent mounting flange performs are present, a first extended flange shoe  70 , having a first side  69  and second side  71 , may be removeably coupled to a second extended flange shoe  82 , having a first side  69  and second side  71 , to form a cavity  92  therebetween about a first mounting flange preform  78 . A first flange shoe  80  may then be removeably coupled to second side  71  of second extended flange shoe  82  in the manner described above to form another cavity  92  about a second mounting flange preform  88 . Indeed, any number of extended flange shoes may be coupled together in this manner to accommodate a fan casing having multiple adjacent mounting flange performs to help ensure the flange performs have the support needed to remain properly positioned and proportioned. 
     Once all flange shoes and extended flange shoes have been coupled together about the fan casing and the mounting flange performs, the final cure of the fan casing may commence. Those skilled in the art will understand how to determine the proper final cure parameters based on such factors as part size and resin utilized. At the end of the final cure, the tooling may be removed and an article including a composite structure having at least one mounting flange is obtained and any desired secondary structure may then be attached thereto. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.