Abstract:
A columnar structure comprises a generally hollow laminate core, an outer composite skin, and a sleeve-like reinforcement. The sleeve-like reinforcement surrounds the laminate core and is sandwiched between the laminate core and the outer composite skin for reacting compressive loads imposed on the columnar structure.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is related to co-pending U.S. patent application Ser. No. 13/288,491 filed Nov. 3, 2011, which is incorporated by reference herein in its entirety. 
     BACKGROUND INFORMATION 
     1. Field 
     The present disclosure generally relates to composite columnar structures, and deals more particularly with a hybrid composite tubular strut internally reinforced to better resist axial compression loads. 
     2. Background 
     Columnar structures formed of composites are used in a variety of applications because of their favorable strength-to-weight ratio. For example, composite tubular struts may be used in the aerospace industry as a support or brace for transferring loads in either direction along the longitudinal axis of the strut, thus placing the strut in either compression or tension. Fittings on the ends of the strut provide additional strength at the points of attachment of the strut to a structure. 
     The tubular struts mentioned above may be fabricated from fiber reinforced resin laminates. Such laminates may exhibit greater load carrying ability when placed in tension than when placed in compression. This is because the compressive strength of the resin is generally less than its tensile strength. Consequently, in order to meet performance specifications, it may be necessary to over-size the strut to carry a specified level of compression loading. Over-sizing the strut, however, may add cost and/or undesired weight to a vehicle or other structure to which the strut is attached. 
     Accordingly, there is a need for a composite columnar structure that exhibits improved ability to carry compression loads. There is also a need for a cost effective method of making a columnar structure with improved compression load carrying ability that adds little or no weight to the structure. 
     SUMMARY 
     The disclosed embodiments provide a composite columnar structure such as a tubular strut that exhibits an improved ability to resist axial compression loads while adding little or no weight to the structure. Improved compression load capability is achieved by incorporating a sleeve-like reinforcement around laminated plies forming a core of the strut. The reinforcement allows composite tubular struts and similar columnar structures to be designed that are “right-sized” to meet both compression and tension load carrying specifications while minimizing the weight of the strut. 
     According to one disclosed embodiment, a columnar structure is provided comprising a generally hollow laminate core, an outer composite skin, and a reinforcement. The reinforcement surrounds the laminate core and is sandwiched between the laminate core and the outer composite skin for reacting compressive loads imposed on the columnar structure. The laminate core may be substantially tubular and the reinforcement may include a sleeve-like layer of material extending substantially completely around the laminate core. The sleeve-like layer of material may be one of a metal such as without limitation, titanium, a precured fiber reinforced composite or a ceramic, and the laminate core may be a fiber reinforced resin such as a carbon fiber reinforced plastic. The reinforcement may comprise first and second halves that are seamed together in a direction parallel to a longitudinal axis of the laminate core. In one embodiment, the reinforcement may include corrugations on the inside wall thereof which may control wrinkling of underlying laminate plies of the laminate core during consolidation and curing of the laminate core. 
     According to another embodiment, a strut comprises a generally tubular, fiber reinforced resin core, and a sleeve-like reinforcement around the fiber reinforced resin core having a compressive strength greater than the compressive strength of the fiber reinforced resin core. The sleeve-like reinforcement may be a corrugated metal, and may include first and second halves assembled together along seams extending in a longitudinal direction of the fiber reinforced resin core. The strut may further comprise a pair of spaced apart end fittings including a pair of attachment pins adapted to attach the strut to a structure. The pins lie substantially in a first plane, and the seams lie substantially in a second plane generally perpendicular to the first plane. In one variation, the sleeve-like reinforcement is a ceramic. In another variation, the sleeve-like reinforcement is titanium, and the fiber reinforced resin core is carbon fiber reinforced plastic. The sleeve-like reinforcement is co-bonded to the fiber reinforced resin core and to an outer skin. 
     According to still another embodiment, a method is provided of making a strut, comprising fabricating a composite laminate core, fabricating a sleeve-like reinforcement, assembling the sleeve-like reinforcement over the composite laminate core, and fabricating an outer skin over the sleeve-like reinforcement. The method may further comprise co-bonding the sleeve-like reinforcement to the composite laminate core and to the outer skin. Fabricating the sleeve-like reinforcement may include forming corrugations on an inside face of a member. Fabricating the composite laminate core includes laying up plies of a fiber reinforced resin, and assembling the sleeve-like reinforcement over the composite laminate core includes placing the member on the composite laminate core with the corrugations against the plies of the composite laminate core. The method may further comprise consolidating and curing the composite laminate core, and using the corrugations on the member to control wrinkling of the plies during the consolidation. 
     The features, functions, and advantages can be achieved independently in various embodiments of the present disclosure or may be combined in yet other embodiments in which further details can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features believed characteristic of the illustrative embodiments are set forth in the appended claims. The illustrative embodiments, however, as well as a preferred mode of use, further objectives and descriptions thereof, will best be understood by reference to the following detailed description of an illustrative embodiment of the present disclosure when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is an illustration of a perspective view of a hybrid composite tubular strut exhibiting an improved ability to resist axial compression loads according to one disclosed embodiment. 
         FIG. 2  is an illustration of a sectional view taken along the line  2 - 2  in  FIG. 1 . 
         FIG. 3  is an illustration of a perspective view of the strut shown in  FIG. 1  in an intermediate stage of fabrication in which two halves of a reinforcement are being installed on a laminate core. 
         FIG. 4  is an illustration similar to  FIG. 3 , but showing the two halves of the reinforcement having been installed. 
         FIG. 5  is an illustration similar to  FIG. 4  but showing an alternate embodiment of the reinforcement having corrugations. 
         FIG. 6  is an illustration of a perspective view of the corrugated reinforcement, in the area shown as  6 - 6  in  FIG. 5 . 
         FIG. 7  is an illustration of the area designated as  FIG. 7  in  FIG. 2 , but illustrating use of the corrugated form of the reinforcement. 
         FIG. 8  is an illustration of a cross sectional view of another form of the reinforcement. 
         FIG. 9  is an illustration of a flow diagram of a method of fabricating a hybrid composite columnar structure according to the disclosed embodiments. 
         FIG. 10  is an illustration of a flow diagram of aircraft production and service methodology. 
         FIG. 11  is an illustration of a block diagram of an aircraft. 
     
    
    
     DETAILED DESCRIPTION 
     Referring first to  FIG. 1 , a composite columnar structure illustrated as an elongate strut  20  includes a generally cylindrical, tubular body  22  and a pair of end fittings  24  for attaching the strut  20  to a structure (not shown). The strut  20  may function to transfer compression loads along the longitudinal axis  25  of the tubular body  22 , and may also transfer loads that place the tubular body  22  in tension. Each of the end fittings  24  may be made of a metal such as aluminum or titanium, or a composite or other suitable materials. The end fittings  24  may be fabricated by casting, machining, or other common manufacturing techniques. In applications where the end fittings  24  are formed of composite materials, they may include metallic inserts and/or metallic bushings (not shown). 
     Each of the end fittings  24  may include a clevis  26  having a central opening  28  aligned along an axis  32  for receiving a clevis pin  30  that attaches the strut  20  to the structure. The axes  32  of the clevis pins  30  lie substantially in the same plane  35 . The clevis pins  30  along with clevis  26 , form pivotal connections between the strut  20  and the structure to which it is attached. The strut  20  may be employed, for example and without limitation, as a brace between an aircraft engine (not shown) and an airframe (not shown). Any of a variety of other types of end fittings  24  are possible, depending on the intended use of the strut  20 . Also, as previously mentioned, the strut  20  may function to transfer axial loads biaxially along the longitudinal axis  25  of the strut  20  so that the strut  20  may be placed either in tension or compression or both in an alternating fashion along the longitudinal axis  25 . In some applications, the strut  20  may also experience limited torsional loading. In the illustrated example, the cross sectional shape of the tubular body  22  is substantially round and constant along its length, however other cross sectional shapes are possible, such as, without limitation, square, triangular, hexagonal or pentagonal shapes. Also, the tubular body  22  may have one or more tapers along its length. 
     Referring now to  FIG. 2 , the tubular body  22  broadly comprises a generally cylindrical, sleeve-like reinforcement  36  sandwiched between a cylindrical core  34  and an outer skin  38 . The sleeve-like reinforcement  36  increases the compressive strength of the tubular body  22 . The core  34  may comprise multiple plies  48  ( FIG. 7 ) of a suitable fiber reinforced resin, such as, without limitation, carbon fiber reinforced plastic (CFRP) that may be laid up over a removable mandrel (not shown) by manual or conventional automated layup techniques. The outer skin forms a protective covering over the sleeve-like reinforcement  36  and may also comprise multiple laminated plies of a fiber reinforced resin. The plies of the outer skin  38  also hold the sleeve-like reinforcement  36  in place and may enable the reinforcement  36  to better resist compressive loading. 
     In one embodiment, the sleeve-like reinforcement is cylindrical in shape and may comprise a layer of material  42  formed as semi-circular first and second reinforcement halves  36   a ,  36   b  that extend substantially the entire length of the tubular body  22 . In other embodiments, the layer of material  42  may comprise a single member or more than two members. The layer  42  may comprise a suitable material that exhibits the desired degree of compression strength, such as a metal foil or a ceramic, and is compatible with the material forming the core  34 . For example, where the core  34  is formed of CFRP, the layer of material  42  forming the reinforcement  36  may comprise titanium. The layer  42  may also comprise a precured resin that contains unidirectional reinforcement fibers such as, without limitation, steel fibers which resist axial compression loads applied to the strut  20 . The compressive strength of the sleeve-like reinforcement  36  is greater than that of the resin forming the core  34  in order to increase the overall compressive strength of the strut  20 . 
     In the illustrated example employing a two-piece reinforcement  36 , the halves  36   a ,  36   b  may be preformed and then assembled around the core  34 , forming diametrically opposite joint lines or seams  44 . The reinforcement halves  36   a ,  36   b  may or may not be mechanically joined along the seams  44 . In one embodiment, although not shown in the Figures, the two halves  36   a ,  36   b  may overlap each other along the seams  44  in order to allow the halves  36   a ,  36   b  to slip relative to each other and collapse slightly as the underlying core  34  shrinks during consolidation and curing of the core  34 . The thickness “T” of the layer of material  42  may vary with the application, depending upon the amount of compressive strength that is desired to be added to the strut  20 . While only a single cylindrical reinforcement  36  is shown in the illustrated example, the strut  20  may include multiple axially concentric reinforcements  36  (not shown) embedded in the tubular body  22 . In still other embodiments, the reinforcement  36  and/or the core  34  may taper from a thin cross section portion to a thicker cross section portion along the length of the tubular body  22 , while the outer cylindrical shape of the tubular body  22  remains substantially constant. 
     Referring to  FIG. 3 , strut  20  may be assembled by laying up plies  48  ( FIG. 7 ) of the core  34  over end fittings  24 , however other methods of attaching the end fittings  24  to the core  34  are possible. The two halves  35   a ,  36   b  of the sleeve-like reinforcement  36  may be preformed by any suitable process, and then assembled over the core  34 . Depending of the thickness “T” ( FIG. 2 ) of the reinforcement  36 , the reinforcement  36  may be formed-to-shape by forming a layer of material  42  over the core  34 , using the core  34  as a mandrel.  FIG. 4  illustrates the two halves  36   a ,  36   b  having been assembled over the core  34  and depicts one of the seams  44 , which, as previously mentioned, may represent a mechanical joint line attachment of the two halves  36   a ,  36   b . The circumferential location of the seams  44  may be chosen so as to optimize the buckling strength of the tubular body  22 . For example, in the illustrated embodiment, the seams  44  may be located circumferentially such that they lie in or near a plane  37  ( FIGS. 1 and 2 ) that is substantially perpendicular to the plane  35  of the clevis pins  30 . Orienting the seams  44  generally perpendicular to the axes of the pins  30  in this manner may better enable the reinforcement  36  to resist bending moments in a plane near or substantially parallel to or within the plane  35  and thereby improve the bucking strength of the strut  20 . However, it should be noted that the benefits provided by the disclosed embodiments may be realized even when the seams  44  are not located at circumferential positions that optimize the buckling strength of the strut  20 . 
       FIG. 5  illustrates an alternate embodiment of the strut  20  that includes a two-piece sleeve-like cylindrical reinforcement  36  having corrugations  46 . Referring to  FIG. 6 , the corrugations  46  include circumferentially spaced, longitudinally extending corrugation ridges  46   a  on the inside face  45  of the reinforcement  36 . The corrugations  46  may be formed by any of a variety of processes that are suited to the material from which the reinforcement  36  is made. Referring to  FIG. 7 , it can be seen that the ridges  46   a  of the corrugation  46  extend down into and are compressed against the laminated plies  48  of the core  34 . During consolidation and curing of the strut  20 , the core shrinks and the corrugation ridges  46   a  are compacted against the core  34 , tending to control wrinkle formation in the plies  48  of the core  30 . This wrinkle control is achieved as a result of the corrugation ridges  46   a  depressing and lengthening portions of the plies  48  around the ridges  46   a  in order to tighten and/or absorb the shrinkage of the plies  48  during consolidation/curing. 
     The ability of the sleeve-like reinforcement  36  to control wrinkling of the underlying plies  48  during the consolidation process may be achieved using other forms of the reinforcement  36 . For example, referring to  FIG. 8 , in lieu of corrugating the layer of material  42  comprising the reinforcement  36  as described above, longitudinally extending, spaced apart raised strips  47  of any suitable material may be applied by a suitable technique to the inside face  45  of the layer of material  42 , either before or after the layer of material  42  has been formed into the desired shape. 
     Attention is now directed to  FIG. 9  which illustrates the overall steps of a method of fabricating the composite tubular strut  20  described previously. Beginning at  50 , laminated core  30  is fabricated by laying up composite plies  48  over a suitable mandrel (not shown), which may be for example, an inflatable or ablative mandrel. Next, at  52 , the reinforcement  36  may be fabricated either by preforming one or more layers of material  42  into halves  36   a ,  36   b  of the desire cross sectional shape, or by forming the material over the core  30 , using the core  30  as a mandrel. At step  54 , a suitable adhesive is applied over the core  30 , following which at  56 , the reinforcement  36  is assembled over the core  30 . The seams  44  between the reinforcement halves  36   a ,  36   b  may be located such that they lie substantially in a plane  37  that is substantially perpendicular to the plane  35  of the clevis pin  30  axes  32  in order to better resist bending forces, however, the seams  44  may be located at other points, depending on the construction and geometry of the end fittings  24 . At step  58  a suitable adhesive is applied over the reinforcement  36 . At step  60 , outer skin is applied over the reinforcement  36  by laying up additional composite plies over the reinforcement  36 . At step  62 , the strut  20  is debulked, compacted and cured, thereby co-bonding the reinforcement  36  to the core  30  and the outer skin  38 . Finally, at step  64 , the mandrel on which the core  30  is laid up may be removed. 
     Embodiments of the disclosure may find use in a variety of potential applications, particularly in the transportation industry, including for example, aerospace, marine, automotive applications and other application where automated layup equipment may be used. Thus, referring now to  FIGS. 10 and 11 , embodiments of the disclosure may be used in the context of an aircraft manufacturing and service method  70  as shown in  FIG. 10  and an aircraft  72  as shown in  FIG. 11 . Aircraft applications of the disclosed embodiments may include, for example, without limitation, load transferring members such as struts, supports, connecting rods and similar columnar structures. During pre-production, exemplary method  70  may include specification and design  74  of the aircraft  72  and material procurement  76 . During production, component and subassembly manufacturing  78  and system integration  80  of the aircraft  72  takes place. Thereafter, the aircraft  72  may go through certification and delivery  82  in order to be placed in service  84 . While in service by a customer, the aircraft  72  is scheduled for routine maintenance and service  86 , which may also include modification, reconfiguration, refurbishment, and so on. 
     Each of the processes of method  70  may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include without limitation any number of aircraft manufacturers and major-system subcontractors; a third party may include without limitation any number of vendors, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on. 
     As shown in  FIG. 11 , the aircraft  72  produced by exemplary method  70  may include an airframe  88  with a plurality of systems  90  and an interior  92 . Examples of high-level systems  90  include one or more of a propulsion system  94 , an electrical system  96 , a hydraulic system  98 , and an environmental system  100 . Any number of other systems may be included. Although an aerospace example is shown, the principles of the disclosure may be applied to other industries, such as the marine and automotive industries. 
     Systems and methods embodied herein may be employed during any one or more of the stages of the production and service method  70 . For example, components or subassemblies corresponding to production process  78  may be fabricated or manufactured in a manner similar to components or subassemblies produced while the aircraft  72  is in service. Also, one or more apparatus embodiments, method embodiments, or a combination thereof may be utilized during the production stages  78  and  80 , for example, by substantially expediting assembly of or reducing the cost of an aircraft  72 . Similarly, one or more of apparatus embodiments, method embodiments, or a combination thereof may be utilized while the aircraft  72  is in service, for example and without limitation, to maintenance and service  86 . 
     The description of the different advantageous embodiments has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the embodiments in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. Further, different advantageous embodiments may provide different advantages as compared to other advantageous embodiments. The embodiment or embodiments selected are chosen and described in order to best explain the principles of the embodiments, the practical application, and to enable others of ordinary skill in the art to understand the disclosure for various embodiments with various modifications as are suited to the particular use contemplated.