Patent Publication Number: US-7581301-B2

Title: Method for attaching a non-metal component to a metal component

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is a Divisional Application of U.S. Ser. No. 10/850,253 entitled “A FASTENER ASSEMBLY FOR ATTACHING A NON-METAL COMPONENT TO A METAL COMPONENT”, filed on May 20, 2004, and claims the benefit of the filing date thereof under 35 U.S.C. §120. The contents of the application are incorporated by reference herein. 

   The invention was made under a U.S. Government contract and the Government has rights herein. 

   BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The present invention relates to gas turbine engine fasteners and, more particularly, to gas turbine engine fasteners for attaching components fabricated from dissimilar materials. 
   2. Background Art 
   A typical gas turbine engine operates in an extremely harsh environment characterized by very high temperatures and vibrations. A conventional gas turbine engine includes a compressor for compressing entering air, a combustor for mixing and burning the compressed gases that emerge from the compressor with fuel, a turbine for expanding the hot gases to generate thrust to propel the engine, and an exhaust nozzle for allowing hot gases to exit the engine. Thus, the exhaust nozzle must accommodate extremely hot gases exiting the engine. 
   In military operations, design of planes to avoid detection by radar has become an important issue. The ability of the plane to remain undetected, also referred to as a signature of a plane, depends on the overall geometry of the plane and materials the plane is fabricated from. To minimize detection, it is preferable to eliminate gaps between engine parts and to achieve certain smoothness for the outer shape of the engine. Additionally, it is preferable to avoid use of metals on the outer surfaces of the engine. 
   Other considerations critical to engine design are avoiding air leakage and insulating certain engine components from exposure to hot gases. One type of a material that withstands hot temperatures is ceramic matrix composite (or CMC), material. However, it is difficult to attach the CMC material components to metal components. One obstacle to attaching the CMC material to the metal is different thermal expansions of the materials. In general, it is difficult to attach or join different materials in a gas turbine engine due to different thermal expansion properties. 
   SUMMARY OF THE INVENTION 
   According to the present invention, a fastener assembly for attaching a non-metal component to a metal component includes a fastener having a base portion and a protruding portion with the base portion having a shape complementary for mating with an attachment feature formed within the non-metal component such that the base portion is retained within the non-metal component. The protruding portion including a base end and a distal end with the distal end including a threaded portion. The fastener assembly also includes a tightening means for attaching onto the threaded portion of the protruding portion of the fastener such that once the base portion of the fastener is mated with the attachment feature of the non-metal component, thereby securing the fastener to the non-metal component, the protruding portion of the fastener extends from the metal component to allow the tightening means to be attached onto the threaded portion to secure the metal component between the non-metal component and the tightening means. 
   The fastener assembly allows attachment of components fabricated from dissimilar materials without forming through holes in one of the components. Additionally, the fastener assembly compensates for differences in thermal expansion rates between the non-metal component and the metal component while providing a tight attachment therebetween. 
   According to one embodiment of the present invention, the fastener assembly attaches a plow portion of a divergent flap of a gas turbine engine onto a backbone structure of the flap wherein the plow portion is fabricated from CMC material and the backbone structure is fabricated from metal. 
   The foregoing and other advantages of the present invention become more apparent in light of the following detailed description of the exemplary embodiments thereof, as illustrated in the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic depiction of a gas turbine engine; 
       FIG. 2  is a schematic side elevational view of a divergent flap and an external flap of the gas turbine engine of  FIG. 1 ; 
       FIG. 3  is a schematic side elevational view of the divergent flap of  FIG. 2  with a plow portion shown in cross-section; 
       FIG. 4  is a schematic top view of the divergent flap of  FIG. 3 ; 
       FIG. 5  is an enlarged, partial view of the divergent flap of  FIG. 3  showing the plow portion in cross-section during non-operational condition; 
       FIG. 6  is an enlarged partial view of the divergent flap of  FIG. 3  showing the plow portion in cross-section during operational condition; 
       FIG. 7  is a schematic, perspective, broken-away view of a plow fastener assembly attaching a hotsheet of the divergent flap to a bracket; 
       FIG. 8  is an exploded view of the plow fastener assembly of  FIG. 7  attaching the hotsheet and the bracket; 
       FIG. 9  is a schematic cross-sectional view of the plow fastener assembly of  FIG. 8  taken along line  9 - 9 ; 
       FIG. 10  is a schematic cross-sectional view of the plow fastener assembly of  FIG. 8  taken along line  10 - 10 ; 
       FIG. 11  is a cross-sectional view of an attachment fastener assembly securing a hotsheet and a backbone structure of the divergent flap of  FIGS. 3 and 4  with the attachment fastener assembly passing through a substantially round hole; and 
       FIG. 12  is a cross-sectional view of an attachment fastener assembly securing the hotsheet and the backbone structure of the divergent flap of  FIGS. 3 and 4 , with the attachment fastener assembly passing through an elongated slot. 
   

   DETAILED DESCRIPTION OF THE PRESENT INVENTION 
   Referring to  FIG.1 , a gas turbine engine  10  includes a compressor  12 , a combustor  14 , and a turbine  16  centered around a central axis  17 . Air  18  flows axially through the engine  10 . As is well known in the art, air  18  is compressed in the compressor  12 . Subsequently, the compressor air is mixed with fuel and burned in the combustor  14 . The hot gases expand generating thrust to propel the engine  10  and to drive the turbine  16 , which in turn drives the compressor  12 . The exhaust gases from the turbine  16  exit through the exhaust nozzle  20 . 
   Referring to  FIG. 2 , the exhaust nozzle  20  includes a plurality of external flaps  24  arranged circumferentially about the axis  17  and a plurality of divergent flaps  26  disposed radially inward from the external flaps. Each external flap  24  includes an external flap surface  28  having a particular geometry. Each divergent flap  26  includes a fore portion  30  and an aft portion  32 . The fore portion  30  includes a hinge assembly  36  for securing the divergent flap  26  to the gas turbine engine. The divergent flap  26  further comprises a hotsheet  38  extending the length of the flap  26  from the fore portion  30  to the aft portion  32 , a backbone structure  40  disposed radially outward of the hotsheet  38  and secured thereto by means for attachment  42 , and a plow portion  46  disposed in the aft portion  32  of the divergent flap  26  and secured to the backbone structure  40  by a plow fastener assembly  48 , shown in  FIGS. 3-10 . 
   Referring to  FIGS. 3 and 4 , in the preferred embodiment of the present invention, the hotsheet  38  comprises a substantially flat substrate fabricated from ceramic matrix composite (CMC) having a hotsheet inner side  50  exposed to the exhaust gases  18  and a hotsheet outer side  52  facing the backbone structure  40 . The hotsheet inner side  50  and the hotsheet outer side  52  extend from the fore portion  30  to the aft portion  32  and include a hinge edge  56  and a trailing edge  58 . In the preferred embodiment, the trailing edge is defined by a chamfered surface  60 . The hotsheet  38  also includes a plurality of attachment openings  61 , as best seen in  FIG. 4 . The openings  61  also include a countersink hole  62  formed within the hotsheet inner side  50 , as best seen in  FIG. 11 . 
   The backbone structure  40  extends the length of the hotsheet  38  and provides structure thereto. In the preferred embodiment, the backbone structure  40  is fabricated from metal. Additionally, in one embodiment of the present invention, the backbone structure  40  includes an aft support  63  extending into the aft portion  32  of the divergent flap  26 , as best seen in  FIG. 3 . The backbone structure  40  also includes a plurality of backbone openings  64 . 
   Referring to  FIGS. 5 and 6 , the plow portion  46  includes a plow body  66  having a plow outer surface  68  and a plow inner surface  70 , as well as a plow outward edge  72  and a plow inward edge  74 . The plow outer and inner surfaces  68 ,  70  have a contour to minimize plane signature and to provide optimal aerodynamic characteristics. In non-operating condition of the engine, the plow  46  is not in register with the trailing edge  58  of the hotsheet  38 , as seen in  FIG. 5 . Rather, the plow  46  is disposed axially inward from the chamfered surface  60  and forms an offset  75  between the plow outer surface  68  and the chamfered surface  60 . A gap  76  is also formed between the hotsheet outer surface  52  and the plow inward edge  74 . In the preferred embodiment, the plow portion  46  is fabricated from CMC. 
   Referring to  FIGS. 6-10 , the plow  46  is attached to the backbone structure by means of the plow fastener assembly  48 . In the preferred embodiment, the plow  46  includes attachment features  77  for attaching the plow portion onto the backbone structure comprising a dovetail slot  80  formed therein and a recess  82  also formed within the plow portion, as best seen in  FIGS. 8 and 9 . The recess  82  includes a substantially flat recess surface  83  and a recess wall  84 . The dovetail slot  80  includes a bottom slot surface  85  and wedge slot surfaces  86 . 
   Referring to  FIGS. 7 and 8 , the plow fastener assembly  48  includes a plow fastener  94 , a nut  96 , and a bracket  98 . The plow fastener  94  includes a base portion  104  and a protruding portion  106  extending from the base portion. The protruding portion  106  includes a distal end  108  and a base end  110  with threads  114  formed on the distal end  108 . The base portion  104  has a substantially trapezoidal shape adapted to fit into the dovetail slot  80  of the plow  46 . The fastener includes a radius  116  formed at the base end  110  of the protruding portion  106  of the fastener  94 , as best seen in  FIGS. 9 and 10 . The nut  96  is adapted to be fastened onto the threads  114  of the protruding portion  106  of the fastener  94 . The bracket  98  includes a first side  118  and a second side  120  with ribs  124  formed thereon. The ribs  124  are formed to fit into the recess  82  of the plow  46 , as best seen in  FIGS. 8-9 , and in the preferred embodiment, are formed on opposite sides of an opening  126  formed within the bracket  98 . The opening  126  is adapted to allow the protruding portion  106  of the fastener  94  to fit therethrough. A Belleville washer  128  can optionally be placed between the bracket  98  and the nut  96 . 
   Referring to  FIGS. 8-10 , as the plow fastener  94  is inserted into the dove-tail slot  80 , a gap  130  is formed between the base portion  104  of the fastener  94  and the dovetail slot  80 , as best seen in  FIGS. 9 and 10 . The gap  130  and the radius  116  allow for thermal expansion of the fastener  94  and minimize loading of the CMC material of the plow. As the plow fastener  94  fits into the attachment features  77  of the plow, the ribs  124  fit into the recess  82 . The recess  82  includes the substantially flat recess surface  83  to accommodate the ribs  124 . The recess  82  and the ribs  124  ensure retention of the plow fastener  94  within the plow  46 . The Belleville washer  128  maintains the preload if components grow thermally. Although one Belleville washer  128  is shown, a plurality of washers can also be used. 
   Referring to  FIGS. 3 ,  4 , 11  and  12 , means for attachment  42  of the CMC hotsheet  38  to the backbone structure  40  includes a fastener  134 , a washer  136 , a spacer  138 , at least one Belleville washer  140 , and a nut  142 . The fastener  134  includes a head portion  146  and a body portion  148  with the body portion including a plurality of threads  150 . The fastener  134  passes through the attachment opening  61  and the countersink hole  62  formed within the CMC hotsheet  38 . The fastener head portion  146  fits into the countersink hole  61 . The washer  136  is sandwiched between the hotsheet  38  and the backbone structure  40  and supports the spacer  138 . The spacer  138  includes a cylindrical portion  154  and a ring portion  156  extending outwardly from the cylindrical portion. The cylindrical portion  154  of the spacer is substantially adjacent to the fastener body  148  and the ring portion  156  extends radially outward from the backbone structure  40  defining a spacer gap  158  therebetween, as best seen in  FIGS. 6-12 . The length of the cylindrical portion  154  of the spacer  138  is greater than the thickness of the backbone structure  40  disposed therein to define the gap  158 . At least one Belleville washer  140  is disposed radially outward from the spacer  138  with the nut  142  tightened to clamp all the components together against the hotsheet  38  to a predetermined preload condition for a relatively tight fit without any looseness between the hotsheet  38  and other components. 
   Referring back to  FIG. 4 , the backbone structure  40  includes the plurality of backbone openings  64  to allow attachment of the backbone structure  40  onto the hotsheet  38 . The backbone openings  64  proximate to the hinge assembly  36  are substantially round and sized to accommodate the body portion  148  of the fasteners  134 , as seen in  FIG. 11 . The remaining backbone openings are formed as elongated slots to allow for movement of the backbone structure  40  relative to the hotsheet  38 , as seen in  FIG. 12 . Thus, the backbone structure  40  is fixedly attached to the hotsheet at the fore portion  30  of the flap  26 . However, the backbone structure  40  is free to translate axially as a result of thermal expansion toward the aft portion  32  of the flap  26 . 
   In operation, once the engine  10  begins to operate, the temperature of the engine quickly rises from the ground ambient temperature to extreme high temperatures. The temperature of the gases  18  passing through the engine also rises resulting in extremely high temperatures and creates harsh environment for a majority of the gas turbine components. More specifically, as the engine  10  begins to operate, the hot gases  18  are exhausted through the exhaust nozzle  20  causing the divergent flap  26  to heat to very high temperatures. The hotsheet  38  is in contact with the exhaust gases  18  exiting the engine. The hotsheet  38  is specifically designed to withstand the hot temperatures. Although the CMC hotsheet is subjected to extremely high temperatures, the hotsheet does not expand a great deal due to the material properties of CMC. However, the metal backbone structure  40  is subject to greater thermal expansion. Therefore, as the backbone structure  40  expands, the plow  46 , secured to the backbone structure, moves aft toward the trailing edge of the hotsheet  38 . As the plow  46  shifts relative to the trailing edge  58  of the hotsheet  38 , the offset  75  is bridged and is substantially eliminated. As the backbone structure expands, the plow outer surface  68  becomes substantially flush with the chamfered surface  60  and the external flap outer surface  28 , as best seen in  FIGS. 2 and 6 . The extremely hot temperatures also cause the aft portion  32  of the hotsheet  38  to warp and deflect. The aft support  63  of the backbone structure  40  minimizes the deflection of the trailing edge  58  of the hotsheet  38 . By minimizing deflection, contact between the plow inward edge  74  and the hotsheet  38  is also minimized. 
   In the plow fastener assembly  48 , the dovetail slot  80  retains the fastener  94  therein. The recess  82  provides a locking feature to prevent rotation and translation of the fastener  94  with respect to the CMC sheet. The gap between the base portion of the fastener  94  and the dovetail slot  80  allows for thermal growth of the metal fastener without loading the CMC material. The Belleville washer can be placed between the nut and the feature to maintain preload when the parts thermally expand and to reduce the stiffness of the fastener assembly to minimize CMC stresses than can occur because of thermally induced tightening of the assembly. 
   The plow fastener assembly  48  allows attachment of a CMC sheet onto a metal structure without forming a through hole opening in the CMC sheet. Such feature is particularly critical in stealth plane design where the outer surface of the plane must be fabricated from particular materials and must not include metal fasteners on the surface thereof. Additionally, this unique attachment provides a connection between the CMC material and metal structure without leakage since a need for holes or openings is eliminated. Furthermore, the fastener  94  is insulated from the hot side  50  of the CMC sheet  38 , thereby maintaining integrity of the fastener. The plow fastener assembly  48  can be used to join any CMC material with metal structure. In one embodiment of the present invention, the plow  46  is attached to the backbone structure via the bracket  98 , as shown in  FIGS. 5-8 . The bracket  98  is fabricated from metal and can be easily attached to the backbone structure  40  subsequently by various conventional fastening means  160 , such as rivets or bolts, as shown in  FIGS. 5 and 6 . Thus, the bracket  98  provides a bridge between the CMC sheet and other components to which the bracket can be attached by use of conventional fastening techniques. However, in this particular case, the plow can be directly attached to the backbone structure  40 . 
   Although in a preferred embodiment the bracket  98  is included between the plow portion and the backbone structure, the plow  46  could be directly attached to the backbone structure  46 . However, the bracket  98  provides a bridge between the plow portion and the backbone structure, thereby enabling the plow via the bracket to be attached to any structure by various means of a conventional attachment. Additionally, in the preferred embodiment, the plow fastener assembly  48  is disposed in one plane to accommodate for thermal expansion and minimize thermal stresses. The fastening means  160  attaching the bracket  98  to the backbone structure  40  are also disposed in such a plane to minimize any thermal stress. The backbone openings  64  formed as elongated slots and disposed within the backbone structure also allow the backbone structure  40  to thermally expand relative to the hotsheet  38 , thereby moving the plow  46  toward the trailing edge  56  of the hotsheet. 
   The means for attachment  42  allows attachment of the CMC material to other types of materials without damaging the CMC material while applying significant tightening force to the assembly. As the nut  142  tightened onto the fastener  134 , the metal of the backbone structure  46  is trapped between the spacer  138  and the washer  134  and all components are clamped together against the hotsheet  38  to a set preload for a tight fit without looseness between the CMC material and other components. The elongated slots  64  formed within the backbone structure  40  allow movement of the backbone structure relative to the hotsheet without introducing looseness to the attachment assembly  42 . The Belleville washer  140  maintains preload and reduces the stiffness of the fastener assembly to minimize CMC stresses than can occur because of thermally induced tightening of the assembly. The spacer allows for the thermal growth of the backbone structure while maintaining a tight attachment of the assembly. 
   One advantage of the present invention is that the plow  46  bridges the gap between the hotsheet  38  and the external flap  24 . This feature ensures a smooth overall contour of the engine to minimize detection of the plane. An additional advantage of the present invention is that the plow moves relative to the hotsheet  38  to bridge the offset  75  during the hot condition to further minimize detection of the plane. Another advantage of the present invention is that the plow portion  46  is fabricated from the CMC material. The plow portion, fabricated from the CMC material, minimizes signature of the plane. The features of the present invention also accommodate different rates of thermal expansion of CMC and metal components. For example, the backbone openings  64  allow relative movement between the backbone structure  40  and the hotsheet  38 , thereby accommodating different rates of thermal expansion of metal and CMC material and also allowing the plow  46  to be moved toward the trailing edge  56  of the hotsheet  38  to minimize even small gaps for further improving signature of a plane. Another advantage of the present invention is that the aft portion  63  minimizes deflection of the hotsheet  38 . A further advantage of the present invention is that the bracket  98  allows attachment of the CMC sheet onto any material. 
   Additionally, the present invention overcomes the difficulty of fastening a CMC plow portion onto metal components. The plow fastener assembly  48  eliminates a need for forming through openings in the external surface of the engine and also compensates for different rates of thermal expansion between metal and CMC. 
   One advantage of the means for attachment  42  is that the CMC panel can be tightened with significant force and still allow sliding motion between the CMC panel and the metal structure. Additionally, any rattling of the components within the opening is eliminated, therefore, minimizing degradation of the material and extending service life of the components. This fastening scheme not only attaches the CMC component to a dissimilar material component, but also accommodates any thermal growth mismatch and secures the CMC component under positive and negative pressure conditions. The fastening scheme permits sliding of the structure relative to the CMC panel to eliminate thermally induced stresses. 
   While the present invention has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this invention may be made without departing from the spirit and scope of the present invention.