Abstract:
A material which exhibits auxetic characteristics and control of thermal expansion characteristics while experiencing significant stress reduction is disclosed. The material has a repeating pattern of void structures along both lateral symmetry lines and longitudinal symmetry lines.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 61/798,965, filed Mar. 15, 2013, the contents of which are hereby incorporated in their entirety. 
    
    
     FIELD OF TECHNOLOGY 
     Gas turbine combustors, turbine liners, and other components are subjected to thermal expansion and experience significant stress. An improved material for use in components that benefit from control of thermal expansion characteristics and that experiences significant stress reduction is disclosed. 
     BACKGROUND 
     Gas turbine engines operate at temperatures that are disruptive to the natural characteristics of metal and other engineering materials such as ceramics and composites. Such conditions cause material to fatigue, stress and fail. It is desired to provide stress-relief features by providing slots and various other geometric configurations in the surface of a material, such as a material for use in a turbine liner, and as they appear in auxetic structures. 
     While virtually all materials undergo a transverse contraction when stretched in one direction and a transverse expansion when compressed, auxetic materials do not. The magnitude of the transverse deformation exhibited by materials upon compression or stretching is expressed by a quantity known as Poisson&#39;s ratio. In ordinary materials that contract laterally when stretched and expand laterally when compressed, Poisson&#39;s ratio is defined as a positive number. However, some materials, when stretched, become thicker in the direction perpendicular to which they are being stretched. Such materials have a negative Poisson&#39;s ratio, and are referred to as auxetic materials. 
     The structure of a material may be altered in such a way that the material exhibits auxetic behavior. One way in which this may be done is by disposing an exemplary pattern of elliptical holes within and extending through the plane of the material. However, materials that are modified to exhibit auxetic behavior in this manner may exhibit stress concentrations at the edges of the minor radii of the holes. The stress concentrations may lead to cracking and, in severe cases, component failure. A need exists for a material that exhibits auxetic properties, and that will not be subject to stress concentrations and cracking. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       While the claims are not limited to a specific illustration, an appreciation of the various aspects is best gained through a discussion of various examples thereof. Referring now to the drawings, exemplary illustrations are shown in detail. Although the drawings represent the illustrations, the drawings are not necessarily to scale and certain features may be exaggerated to better illustrate and explain an innovative aspect of an example. Further, the exemplary illustrations described herein are not intended to be exhaustive or otherwise limiting or restricted to the precise form and configuration shown in the drawings and disclosed in the following detailed description. Exemplary illustrations are described in detail by referring to the drawings as follows: 
         FIG. 1  illustrates a schematic view of a gas turbine; 
         FIG. 2  illustrates a combustor for a gas turbine engine with a shell of the improved material; 
         FIG. 3  illustrates a material having a configuration of void structures that allows the material to exhibit auxetic properties with stress relief holes, where the void structures are barbell shaped; 
         FIG. 4  illustrates a material having the configuration of void structures of  FIG. 3 , showing the forces acting on the material when compression is applied; 
         FIG. 5A  illustrates the shape of the void structures in  FIG. 3 , in which the void structures are shaped as a traditional barbell; 
         FIG. 5B  illustrates an alternative shape for the void structure of  FIG. 3 , in which the void structures are shaped as a compressed barbell; 
         FIG. 5C  illustrates another alternative shape for the void structures of  FIG. 3 , in which the void structures are shaped as a pince-nez; 
         FIG. 5D  illustrates another alternative shape for the void structures of  FIG. 3 , in which the void structures are shaped as a reverse pince-nez; 
         FIG. 6A  illustrates another alternative shape for the void structure of  FIG. 3 , in which the void structures are shaped as a question mark; 
         FIG. 6B  illustrates another alternative shape for the void structures of  FIG. 3 , in which the void structures are shaped as a compressed question mark; 
         FIG. 6C  illustrates another alternative shape for the void structures of  FIG. 3 , in which the void structures are hook-shaped; 
         FIG. 6D  illustrates another alternative shape for the void structures of  FIG. 3 , in which the void structures are reverse hook-shaped; 
         FIG. 6E  illustrates another alternative shape for the void structures of  FIG. 3 , in which the void structures are J-hook shaped; 
         FIG. 7  illustrates a material having void structures in a hook-shaped configuration, as shown in  FIG. 3 , that allows the material to exhibit auxetic properties with areas of minimal stress; 
         FIG. 8  illustrates a material having the configuration of hook-shaped void structures shown in  FIG. 7 , showing the forces acting on the material when compression is applied; 
         FIG. 9  illustrates a material having J-shaped void structures; 
         FIG. 10  illustrates a material having the J-shaped void structures shown in  FIG. 9 , and showing the forces acting on the material when compression is applied; and 
         FIG. 11  illustrates a laser cutting path for manufacturing the barbell slot profile. 
     
    
    
     DETAILED DESCRIPTION 
     This present improvement provides enhanced material structure stress relief. A sheet of metal or other material such as a ceramic or a composite containing a pattern of elliptical holes or slots will exhibit auxetic behavior when loaded in the plane of the sheet, but will also exhibit stress concentrations at the minor radii. In a highly loaded component, this may lead to cracking and component failure. 
     A combustor liner with sheet metal walls could employ round effusion cooling holes. Several new hole configurations are proposed to reduce the stress concentration. The exemplary embodiments herein replace the conventional round effusion cooling holes with a pattern of slots forming an auxetic structure which can be referred to as an auxetic metamaterial. 
       FIG. 1  illustrates a gas turbine engine  10 , which includes a fan  12 , a low pressure compressor and a high pressure compressor,  14  and  16 , a combustor  18 , and a high pressure turbine and low pressure turbine,  20  and  22 , respectively. The high pressure compressor  16  is connected to a first rotor shaft  24  while the low pressure compressor  14  is connected to a second rotor shaft  26 . The shafts extend axially and are parallel to a longitudinal center line axis  28 . 
     Ambient air  30  enters the fan  12  and is directed across a fan rotor  32  in an annular duct  34 , which in part is circumscribed by fan case  36 . The bypass airflow  38  provides engine thrust while the primary gas stream  40  is directed to the combustor  18  and the high pressure turbine  20 . The gas turbine engine  10  includes an improved combustor  18  having a shell  42  made of improved material. It will be appreciated that the improved material could be used in other machinery and is not therefor limited to gas turbine engine environments. 
       FIG. 2  illustrates one example of the improved material being used in a combustor  18  of a gas turbine engine  10 . The combustor  18  has an outer liner  44  and an inner liner  46  made of metal. The inner liner  46  is made of the improved material  48 . The improved material  48  may exhibit auxetic properties, and may also be more resistant to stress concentrations and failure. 
       FIG. 3  illustrates an exemplary configuration for void structures  50  in the improved material  48 , that includes patterns that consist of horizontal and vertical void structures  50  arranged on horizontal and vertical symmetry lines  52 ,  54  in a way that the symmetry lines  52 ,  54  are equally spaced in both dimensions. The centers of the void structures  50  are on the crossing point  56  of the symmetry lines. Vertical and horizontal void structures  50  alternate on the vertical and horizontal symmetry lines  52 ,  54 . Any vertical void structures  50  are surrounded by horizontal void structures  50  along the horizontal symmetry lines  54 , and any horizontal void structures  50  are surrounded by vertical void structures  50  along the vertical symmetry lines  52 . The shapes of void structures  50  may include but are not limited to, S-shaped, hook-shaped, J-shaped, and barbell-shaped. 
     The slot configurations illustrated and described herein, when used as the building blocks of an auxetic structure, exhibit less stress at the tips of the slots than would be present in elliptical holes or narrow oblong slots. This allows either longer life with the same porosity or reduced porosity with the same life, as compared to an auxetic component with elliptical or oblong slots. 
     The improved material  48  could be comprised of a sheet of material that had void structures  50  disposed therein while the sheet was in its relaxed state. The void structures  50  that are shown in the surface of material  48 , may be formed via laser cutting, stamping, water jet cutting, electron beam cutting, or another manufacturing process. This process could also be used in other materials, such as rubber, foam, metal, or some other material for other applications, where auxetic properties and resistance to stress concentrations are desired. 
       FIGS. 3 and 4  illustrate one example of a configuration for void structures  50  in an improved material  48  in which the shape of the void structures  50  may lead to reduced stresses in the material  48 . Further, the configuration of the void structures  50  enables the improved material  48  to exhibit auxetic properties. As shown in  FIG. 3 , void structures  50  extend both laterally in rows and longitudinally in columns in the material  48  in a repeating pattern, with the rows and columns generally perpendicular to one another. 
     Each of the void structures  50  shown in  FIGS. 3 and 4  has a slot portion  60  and two holes  62 . Each of the holes  62  is disposed on an opposite end of the slot portion  60  of the void structure  50 , and serves to reduce stress concentrations at the ends of void structures  50 .  FIG. 4  shows the stresses on the material  48  of  FIG. 3  when compression is applied to one side of the material, and how the “traditional barbell” shaped configuration  64  of the void structures  50  in a material results in auxetic behavior. 
     The “traditional barbell” configuration  64  shown in  FIGS. 3 and 4  minimizes the propagation of cracks by disposing a hole at both ends of the slot portion  60  to relieve the stress concentration found there. Furthermore, the configuration of void structures  50  shown in  FIGS. 3 and 4  removes less material than is removed when using conventional elliptical slots. This results in a material structure which is less likely to crack and is less porous. Reduced porosity is desirable for applications such as a combustor liner that requires a controlled level of porosity to control the flow of air through the combustor liner wall. Reduced porosity is also desirable in other gas turbine applications such as turbine seal segments or blade tracks, or any component whose functions include maintaining a pressure differential or metering air flow. 
     The configurations for “barbell” void structures  50  shown in  FIGS. 5A to 5D  depict variations for void structures in which the void structures are formed with slot portions connecting pairs of round or oblong holes, rather than the elliptical or oblong slots typically used in the manufacture of materials that exhibit auxetic characteristics. The slot portion may be straight or curved. The length and width of the slot portion, the diameter of the holes, the shape of the holes (i.e. round, oblong, elliptical, other variations) and the spacing between slots can be varied to achieve the desired combination of auxetic behavior, stress reduction, and porosity. One embodiment presents a slot  60  having a width and holes  62  having a diameter. The diameter is several times great than the width. 
       FIG. 5A  shows the traditional barbell configuration shown in  FIGS. 3 and 4 .  FIG. 5B  shows a compressed barbell  68  configuration. The compressed barbell  68  is similar to the traditional barbell configuration  64  shown in  FIG. 5A , but with oblong holes  66  rather than round holes  62 . 
       FIG. 5C  illustrates a pince-nez  70  variation on the barbell configuration. In the pince-nez configuration  70 , the holes  72  are both disposed on the same side of the slot portion  60 .  FIG. 5D  illustrates a reverse pince-nez  74  configuration for a void structure  50  in which the holes  76  are each disposed on an opposite side of the slot portion  60 . It will be appreciated that holes  72 ,  76  of both the pince-nez  70  and the reverse pince-nez  74  configurations may be either round holes, as shown in the barbell configuration  64 , or oblong holes, as shown in the compressed barbell configuration  68 . The void structures  50  illustrated in  FIGS. 5A through 5D  may all be used in similar applications. The inside portion of the holes  72  may have all of their material removed. 
     An alternative to the barbell void structure configurations is a slot with hooks at each end, as shown in  FIGS. 6A through 6E . The “double hook” void structures  50  illustrated in  FIGS. 6A through 6E  have the same advantages over traditional auxetic materials as the variations shown in  FIGS. 3 through 5D , in that more material is retained, they have lower stress concentrations, and they exhibit auxetic behavior. However, there are several advantages to using the double hook void structures rather than the barbell void structure configurations. 
     Conceptually, the double hook void structure may be an improvement over the barbell configurations  64 ,  68 ,  70 , and  74 . The hook-shaped configuration distributes the stress across a larger area in the same way as the barbell configurations shown and described in  FIGS. 5A through 5D , but without removing material from the interior of the holes. As with the barbell and related configurations  64 ,  68 ,  70 ,  74 , several parameters may be varied to achieve the desired properties. These include the length, width and angular orientation of the straight section  78 , the shape of the hooks, and the spacing between slots. The double hook configurations exhibit lower porosity than the barbell configurations, which is advantageous when the material is used for a combustor liner, seal arm, or other component where control of air leakage is required. 
     A variety of double hook configurations for void structures  50  are shown in  FIGS. 6A through 6E . The hooks  80  in the “question mark” configuration  82  shown in  FIG. 6A  are configured as portions of a circle. The hooks  84  in the “compressed question mark” configuration  86  shown in  FIG. 6B  are configured as elongated circle portions.  FIG. 6C  shows a “hook” configuration  88  in which rounded hooks  90  each extend from an opposite side of the slot portion  78 , while the rounded hooks  92  of the “reverse hook” configuration  94 , shown in  FIG. 6D , each extend from the same side of slot portion  78 . The hooks  96  of the “J-hook” configuration  98 , shown in  FIG. 6E , are flatter than the hooks  90 ,  92  in the “hook”  88  and the “reverse hook”  94  configurations. It will be understood that the configurations for the void structures  50  shown and described are only a few of the possible variations that are encompassed within this disclosure. 
       FIG. 7  illustrates an example of a material having void structures  50  in the “hook” configuration  88  shown in  FIG. 6C . The void structures in  FIGS. 6A through 6E  remove less material from the base material than is removed when preparing conventional auxetic materials, and also less material than the barbell void structures shown in  FIGS. 5A through 5D . The rounded hooks  80 ,  84 ,  90 ,  92 , and  98  at the ends of the slot portions  78  of the void structures  50  retain the advantage of lower stress concentrations at the ends of the slot portions  78  than exhibited in conventional auxetic materials. 
       FIG. 8  illustrates a material having the auxetic structure shown in  FIG. 7 , showing forces within material  48  when the material is compressed on one side. As a compressive force is applied, the material  48  contracts in the direction in which the force is applied, and also in a direction perpendicular to the direction of the compressive force. Similarly, when tension is applied, the material  48  expands in the direction in which the force is applied, and also in a direction perpendicular to the direction of the tensile force. 
       FIG. 9  illustrates a material having auxetic properties with void structures  50  in a J-hook configuration  98 . The J-hooks extend in a repeating pattern along a longitudinal axis and along lateral axes, and the longitudinal axes cross the lateral axes in a generally orthogonal direction.  FIG. 10  illustrates the auxetic structure shown in  FIG. 9 , showing forces within the material when the material is compressed on one side. Similar to the material shown in  FIG. 8 , the material  48  compresses in the direction in which a compressive force is applied, and also compresses in a direction perpendicular to the direction of the compressive force. 
     Both the barbell and the double-hook configurations can be manufactured by laser cutting in a single operation, although other conventional means of cutting the material may be used. Laser cutting eliminates the possibility of misalignment when performing multiple operations which could, in turn, lead to the creation of stress risers. One proposed tool path for cutting the barbell-shaped slot is shown in  FIG. 11 . Six separate steps are shown in this proposed tool path. The initial cut may be made within what will become the center of one of the circles at an end of the “barbell.” Since there may be some cracking and imperfections associated with the initial cut of a laser, it may be beneficial to make the initial cut in an area that will not be associated with high stress levels. In this case, the initial cut is made in an area that will become waste material and will be discarded. Once the initial cut is made, the cutting continues to complete the first circle on the first end of the barbell. Then the slot portion is cut, and the second circle is cut, again completing the cutting in a waste area. Once the cutting is complete, the material in the interior of the circles is removed and discarded. 
     It will be appreciated that the aforementioned method and devices may be modified to have some components and steps removed, or may have additional components and steps added, all of which are deemed to be within the spirit of the present disclosure. Even though the present disclosure has been described in detail with reference to specific embodiments, it will be appreciated that the various modifications and changes can be made to these embodiments without departing from the scope of the present disclosure as set forth in the claims. The specification and the drawings are to be regarded as an illustrative thought instead of merely restrictive thought.