Abstract:
A simple ceramic matrix composite fastening system that is utilized for attaching components of dissimilar materials, particularly, ceramic matrix composites (CMCs) and metallic engine components. The system is comprised of a detachable subassembly bracket fabricated from metal. The bracket has a metallic engine component attached to one end and a CMC component attached to the other end. The bracket releasably secures the CMC and the metallic component together using rivets or pins, which are inserted into holes through the CMC to securely fasten the adjoining parts.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus used to fasten ceramic matrix composites (CMCs) to metallic components. 
     2. Description of Related Art 
     Conventional gas turbine engines operate at harsh environmental conditions characterized by high temperatures, high pressures and intense mechanical and acoustic vibrations. Engine manufacturers are in search of new advanced materials that are capable of providing improved durability, greater thrust, longer life, and superior overall performance to replace current state of the art nickel based superalloys. Those skilled in the art of manufacturing engines have identified ceramic matrix composites (CMCs) as having qualities that far surpass the performance capabilities of nickel based superalloys. CMCs can withstand higher temperature conditions, have greater weight reduction capabilities and improved durability over other state of the art materials. CMCs have especially good vibrational damping capabilities and a low coefficient of thermal expansion. 
     While CMCs do have many advantages, they also present design challenges, especially in their application to hot section engine components. These limitations make it difficult to design fastening systems to attach CMCs to metallic engine components. Most traditional CMCs fastening systems are unable to withstand or dissipate heavy loads and their design often leads to space constraints on the rest of the engine system. One such fastening system uses a combination of screw and rivet technology. This fastening method unavoidably leaves machined holes in the CMC. These holes can result in stress concentrations and increase the likelihood of CMC fracture. 
     Another method of fastening CMCs to metallic engine components is a CMC self-sealing approach where oxygen entering the engine is consumed in the CMC microcracks. This method prevents access to the carbon matrix interface creating a sealcoat but the sealcoat is prone to degradation. This fastening system does have a high degree of damage tolerance however, it is not enough to sustain the heavy loads and high temperatures that exist during engine assembly. 
     Accordingly, there is a need for a fastening apparatus that can overcome, alleviate, and/or mitigate one or more of the aforementioned and other deleterious effects of prior art. A novel apparatus is needed that will reduce space constraints, dampen mechanical and acoustic vibrations, compensate for the mismatch in thermal expansion between CMC and metal, and be able to sustain and/or dissipate extreme acoustic, thermal and weight bearing loads that are often not withstandable using traditional apparatuses. 
     SUMMARY OF THE INVENTION 
     The present invention provides a simple CMC fastening system that connects CMCs to a non-CMC component. The system has a detachable subassembly bracket that has a slotted configuration with a plurality of holes therethrough. A plurality of fasteners are received through the holes to hold the bracket in place. One end of the bracket is secured to the CMCs via the fasteners and the other end is secured to a non-CMC component. 
     The above-described and other features and advantages of the present disclosure will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a perspective view of the first embodiment of the simple CMC fastening system design of the present invention; 
         FIG. 2  illustrates a perspective view of the second embodiment of the simple CMC fastening system design of the present invention using a compliant bracket; and 
         FIG. 3  illustrates a cross sectional view of the simple CMC fastening system of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings and in particular  FIG. 1 , there is shown the first embodiment of the CMC fastening system of the present invention, generally represented by reference numeral  10 . Fastening system  10  has a rigid, slotted bracket  14  that tightly straddles a CMC  18 . An opposite end  30  of the bracket is attached to a bridge clamp  12 . Bridge clamp  12  is secured to bracket  14  using a nut  28  attached to a threaded post on top of bracket  14 . Bracket  14  is preferably fabricated from metal. Bracket  14  may be fastened to CMC  18  using either a single point attachment or a multi-point attachment. 
     Bracket  14  and CMC  18  have a plurality of apertures  32  and  20 , respectively, through which a plurality of rivets  26  are inserted to function as fasteners. Apertures  32  and  20  are elongated in shape which allows for axial expansion of the system overall. 
     Rivets  26  are inserted through bracket apertures  32  and CMC apertures  20 . Rivets  26  function as fasteners that securely connect bracket  14  to CMC  18  and hold bracket  14  in position. Preferably, rivets  26  are flared end rivets to minimize the stress induced in the bracket and CMC apertures that would occur if a regular rivet were used. A regular rivet would expand after installation to fill the hole, and damage the CMC. The flared end rivet functions more like a pin, and secures the hardware without adding the extra stress that a traditional rivet would. Alternatively, rivets  26  may be substituted with pins that would also function to securely connect bracket  14  to CMC  18  and hold bracket  14  in position. 
     Rivets  26  may be installed inside of a plurality of optional sleeves  22  before being inserted into bracket apertures  32  and CMC apertures  20 . The function of sleeve  22  is to prevent any stress or damage from being induced in the edges of CMC apertures  20 . 
     A leaf spring  16  is inserted at the point where bracket  14  and CMC piece  18  converge. The purpose of leaf spring  16  is to dampen mechanical vibrations and to compensate for slack induced due to clearance between the mating parts. The leaf spring  16  impinges directly upon the CMC piece and the bracket  70  (see  FIG. 3 ). 
       FIG. 2  illustrates a second embodiment of the present invention generally shown by reference numeral  46 . Elements of the first embodiment are substantially identical to the second embodiment except where indicated. The second embodiment of the CMC fastening system has an alternative bracket design  36 . Bracket  36  is a compliant bracket that has a vertical gap  42  which allows bracket  36  to flex. The flexibility allows a plurality of apertures  38  in the CMC and a plurality of apertures  48  in the bracket to line up when the parts are hot. Compliant bracket  36  reduces the impact of the differences in coefficients of thermal expansion of the CMC and the metal bracket. If the metal bracket and the CMC are at the same temperature, the distance between the apertures in the bracket will increase more than the distance between the apertures in the CMC and therefore, can induce stress into the CMC. 
     If the bracket is very compliant, apertures  38  in the CMC and apertures  48  in the bracket can be round in shape. It the bracket is moderately compliant, apertures  38  in the CMC and apertures  48  in the bracket can be elongated as in the first embodiment, but the degree of elongation will be less than in the first embodiment because of the compliant design of bracket  36 . 
       FIG. 3  illustrates a cross sectional view of the CMC fastening system of the present invention, generally represented by reference numeral  70 . Fastening system  70  has a first sleeve  80  and a second sleeve  72  that capture a single flare end rivet  78 . Alternatively, rivet  78  may be a standard rivet or a double countersunk rivet. A Belleville washer  74  and a washer/shim  76  may be used on one or both sides of a CMC rib  82  to maintain a tight fit during engine operation. 
     Both the first and second embodiments of the CMC fastening system may require additional parts if there is a substantial discrepancy between the coefficient of thermal expansion of the CMC and the metallic engine component attachment. The fastening system can achieve thermal expansion using a spring if necessary. Any such discrepancy upon expansion of the metal when the CMC does not expand along the length of the rivet can be compensated for using additional springs, such as a Belleville washer(s) or wave springs. The Belleville washer can be placed between the nut and the feature to maintain when the parts thermally expand. The washer can serve the additional purpose of reducing the stiffness of the fastener assembly to minimize CMC stress that tends to build because of thermally induced tightening of the assembly. 
     Both embodiments of the CMC fastening system and of the current invention may use either a single or multi-point attachment, although a single point attachment would not use a compliant bracket. Single point attachment is preferred where the load bearing capability of the material is above the applied load. In the case of a multi-point attachment, a design feature can be added that allows compliance as needed. If a single point attachment is utilized and rotational freedom is required, the springs may be adjusted in size or eliminated entirely, depending on the specification requirements. 
     While the present disclosure has been described with reference to one or more exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular embodiment(s) disclosed as the best mode contemplated, but that the disclosure will include all embodiments falling within the scope of the appended claims.