Abstract:
A gusset ( 40 A-G) between two CMC walls ( 26, 28 ) has fibers ( 23 ) oriented to provide anisotropic strengthening of the wall intersection ( 34 ). The fibers ( 23 ) may be oriented diagonally to oppose in tension a wall-spreading moment of the walls ( 26, 28 ) about the intersection ( 34 ). Interlocking features ( 46, 48, 52, 56, 58 ) may be provided on the gusset to improve load sharing between the gusset and the walls. The gusset may have one or more diagonal edges ( 50, 51 ) that contact matching edges of a slot ( 42, 42 D,  43 D) to oppose wall-spreading (M 1 ) and wall-closing (M 2 ) bending of the walls ( 26, 28 ). The gusset may be installed in the slot after preparing the gusset and the walls to different temperatures. Then the assembly may be final-fired to produce differential shrinkage that causes compression of the gusset or the wall intersection.

Description:
FIELD OF THE INVENTION 
     This invention relates to ceramic matrix composite (CMC) structures with load-bearing CMC wall intersections, and particularly to means for strengthening such wall intersections against wall-spreading and wall closing moments of bending. 
     BACKGROUND OF THE INVENTION 
     Gussets have been used in metal components such as brackets to strengthen wall intersections. Providing gussets in load-bearing CMC wall intersections is difficult compared to metal. Ceramic matrix composites (CMC) are used for components in high temperature environments, such in gas turbine engines. CMC is formed by combining ceramic fibers with a ceramic matrix, and heating the combined material to a sintering temperature. The fibers add tensile strength in the directions of the fibers. The resulting material has a higher operating temperature range than metal, and can be optimized for strength by fiber orientations and layering. 
     CMC laminate fabrication from ceramic 2D broadloom fabrics is a standard industry practice. For such laminates, a limiting aspect is the interlaminar tensile and shear strength of the material. This is especially true for load-bearing structures, wherein loads are reacted through CMC wall intersections or flanges. CMC wall intersections tend to delaminate under cyclic mechanical and thermal stresses encountered in gas turbines. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in the following description in view of the drawings that show: 
         FIG. 1  is a side sectional view of a prior art CMC structure. 
         FIG. 2  is a sectional perspective view of a CMC structure according to aspects of the invention. 
         FIG. 3  is a side sectional view of the embodiment of  FIG. 2  with an insulation layer added. 
         FIG. 4  is a sectional perspective view of CMC structure according to further aspects of the invention. 
         FIG. 5  is a front view of the embodiment of  FIG. 4 . 
         FIG. 6  is a side sectional view of the embodiment of  FIG. 4 . 
         FIG. 7  is a side sectional view of an embodiment with a pin, and without tabs. 
         FIG. 8  is a side sectional view of an embodiment with a gusset plate that does not cut through the wall intersection. 
         FIG. 9  is a side sectional view of an embodiment as in  FIG. 9  without a tab. 
         FIG. 10  is a side sectional view of an embodiment with tabs formed by spreading the gusset plate fibers. 
         FIG. 11  is a sectional perspective view of the embodiment of  FIG. 10 . 
         FIG. 12  is a perspective view of a gusset with U-shaped overlay as an interlocking feature. 
         FIG. 13  is a perspective view of the U-shaped overlay of  FIG. 12 . 
         FIG. 14  is a perspective view of a U-shaped overlay on a gusset that does not cut through the wall intersection. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention teaches CMC gussets containing fibers that are discontinuous with the fibers of the first and second CMC walls, and are oriented to provide anisotropic strengthening to the intersection between the walls. Means for interlocking the gussets with the CMC wall structure for improved load transfer are described herein. 
       FIG. 1  illustrates a prior art CMC structure  20 , with ceramic fibers or fabric  22  impregnated with a ceramic matrix  24 , forming a first wall  26  and a second wall  28 . The two walls are joined at an intersection  34  with a rounded outer edge  30  and an inside fillet  32 . Relative bending moments between the two walls  26 ,  28  concentrate stresses in the intersection  34 , which tends to separate the fiber/matrix layers therein. In some gas turbine components, a ceramic thermal insulation layer  36  is applied to an outer surface  38  of a wall. For descriptive purposes, the vertical wall  28  will be considered a front wall herein. 
       FIG. 2  is a sectional perspective view of a CMC structure  20 A according to aspects of the invention. A gusset plate  40 A is inserted into a slot  42  across the intersection  34 . The gusset plate may be substantially orthogonal to both walls  26 ,  28 , and includes ceramic fibers  23 , at least some of which are oriented diagonally between the walls  26 ,  28  as shown. A diagonal fiber orientation maximizes tensile strength of the gusset in opposing separation of the walls  26 ,  28 . Herein “diagonal” means an angle between 30-60 degrees with respect to each wall—especially 45 degrees relative to both walls. For example, the fibers may be oriented 50 degrees relative to one wall and 40 degrees relative to the other wall. The fibers  23  of the gusset plate  40 A are discontinuous with the fibers  22  of the walls  26 ,  28 . 
     In this embodiment, the walls  26 ,  28  and the gusset plate  40 A may be formed separately. The walls  26 ,  28  may be prepared to a green-body or bisque-fired state, and the slots  42  may then be machined into the walls. Alternately, the slots  42  may be formed by a fugitive insert in the lay-up of the walls, and then removed after bisque firing. The gusset plate  40 A may also be prepared to a green-body or bisque-fired state, then inserted into the slot  42 . The walls  26 ,  28 , and gusset plate  40 A may then be co-fired to a final bonded state. Optionally, the gusset plate may be fired to a higher temperature than the wall structure  26 ,  28  before insertion, such that the walls  26 ,  28  shrink onto the gusset plate  40 A in final firing, producing a pre-compression that reduces the chance of bond separation.  FIG. 3  shows a side sectional view of the embodiment of  FIG. 2  with an added insulation layer  38 . 
       FIG. 4  is a perspective sectional view of CMC structure  20 B according to further aspects of the invention. It is similar to embodiment  20 A with added interlocking features in the form of a pin  46  and tabs  48 . These features enhance load transfer between the gusset plate  40 B and the walls  26 ,  28 . The tabs  48  may be inserted into the gusset plate  40 B or formed thereon either before or after inserting the gusset plate into the slot  42 . The pin  46  is inserted after the gusset plate is inserted into the slot. The tabs  48  contact outer surfaces  38 ,  39  of the first and/or second walls  26 ,  28 , to oppose a wall-spreading moment of bending about the intersection. The size of the interlocking features  46 ,  48  may be engineered to balance interlaminar shear, in-plane shear, and in-plane tensile strengths of the materials and fiber orientations used for the parts  26 ,  28 ,  40 B,  46 , and  48 .  FIG. 5  is a front view of the embodiment of  FIG. 4 .  FIG. 6  is a side sectional view of the embodiment of  FIG. 4 . 
     In the embodiment of  FIGS. 4-6 , a green or bisque-fired gusset plate  40 B may be inserted into a wall structure  26 ,  28  that has been fired to a higher temperature, such that upon final firing of the assembly, the gusset plate  40 B shrinks more than the wall structure. This tightens the interlocking features  46 ,  48  against the wall structure  26 ,  28 , providing pre-compression of the plies  22 ,  24  in the intersection  34 . 
       FIG. 7  illustrates a CMC structure  20 C with a gusset plat  40 C and pin  46 , but without the tabs of  FIG. 4 . Bending moments M 1  that would separate the walls  26 ,  28  are opposed by the diagonal ridge  50  of the gusset plate contacting the diagonal surfaces of the slot  42 , due to its diagonal angle A. The intersection  34  can be considered an origin  35  for relative bending moments between the walls  26 ,  28 . The pin  46  prevents the gusset plate  40 C from separating from the slot  42  under the forces M 1 , and compresses the fillet  32  under such forces. 
       FIG. 8  illustrates a CMC structure  20 D with a gusset plate  40 D in a two-part slot  42 D,  43 D that spans the intersection  34 , but does not cut through it. The slot has a first hole  42 D in the first wall  26  and a second hole  43 D in the second wall  28 . The gusset plate may have a tab  52  at one or both ends to interlock against outer surfaces  38 ,  39  of one or both walls  26 ,  28 . 
       FIG. 9  illustrates a CMC structure  20 H as in  FIG. 8  without a tab  52 . This embodiment has two diagonal edges  50 ,  51  on the gusset plate  40 H contacting diagonal surfaces of the holes  42 D,  43 D to oppose both a wall-spreading moment M 1  and a wall-closing moment M 2  about the intersection  34 . 
       FIGS. 10 and 11  illustrate a CMC structure  20 E in which the gusset plate  40 E has tabs  56  formed by spreading fibers  23  of the gusset plate at ends of the plate during wet lay-up. The tabs  56  interlock against outer surfaces  38 ,  39  of one or both walls  26 ,  28  to oppose the wall-spreading moment M 1  previously described. This gusset plate  40 E and the tabs  56  thereon may be formed separately from the wall structure  26 ,  28  and inserted into the slot  40 E as previously described. Alternately, the gusset plate  40 E may be formed by wet lay-up within the slot  42 E, and one or both ends of the gusset plate  40 E may be spread against the respective wall  26 ,  28  to form the tabs  56 . A pin  46  may be provided through the gusset plate  20 E as previously described. 
       FIG. 12  shows a U-shaped CMC overlay  58  bonded to a gusset plate  40 F that works in slots  42  as in  FIGS. 2-4 . The overlay  58  widens the portion of the gusset plate spanning between the walls  26 ,  28  on the inner side of the intersection  34 . The overlay  58  has surfaces  60 ,  61  that contact the inner surfaces of both walls  26 ,  28  respectively, thus opposing the wall-closing moment M 2 . The diagonal ridge  50  of the gusset plate  40 F contacts diagonal surfaces of the slot  42  to oppose the spreading moment M 1  as previously described. Optionally, tabs  48 ,  52  as previously described can further oppose the wall-spreading moment M 1 . The U-shaped CMC overlay  58  reinforces the ridge  50  of the gusset plate  40 F against buckling, allowing the gusset plate  40 F to be thinner for a given strength requirement. 
       FIG. 13  shows the U-shaped CMC overlay  58  of  FIG. 12  separately. It can be formed and bisque-fired separately, and then slipped over the gusset plate  40 F after insertion of the gusset plate  40 F into the slot  42  of the CMC structure. Alternately, the overlay  58  can be wrapped over the ridge  50  of the gusset plate  40 F in wet lay-up after insertion of the gusset plate  40 F into the slot  42   e.    
       FIG. 14  shows a U-shaped CMC overlay  58  bonded to a gusset plate  40 G that works in holes  42 D,  43 D as in  FIG. 9 . The gusset plate  40 G has first and second diagonal edges  50  and  51  that contact diagonal surfaces of the holes  42 D,  43 D to oppose both bending moments M 1  and M 2 . This gusset plate  40 G spans the intersection  34  without cutting it, as previously described. This embodiment has advantages of both  FIGS. 9 and 12 . It opposes wall-separating and wall-closing moments, does not cut the intersection  34 , and may be flush with the outer surfaces  38 ,  39  of the walls  26 ,  28 . 
     While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.