Abstract:
A structural, energy absorbing composite element for aircraft structures includes a metallic or nonmetallic skin shell with a porous filler material insert positioned within the skin shell. The porous filler material is a material having ligaments that collapse resulting in a densification of the porous filler material in response to impact loading or a compression force sufficient to cause failure of the combined assembly.

Description:
FIELD OF THE INVENTION 
     The present invention relates to stanchions, struts and other supports for aircraft and other vehicles. More particularly, the invention relates to a composite element for structural applications having an outer shell composed of a metallic or nonmetallic composite skin and a collapsible core material. The composite element is lightweight and has high-energy absorption properties under buckling and impact conditions. 
     BACKGROUND 
     Long structural elements such as columns are utilized to vertically support other members such as a floor and must be capable of withstanding compressive, buckling, and impact loads. These structural elements must support the load of the structure it supports and added loads such as vehicles and people. 
     The load bearing capabilities of these structural elements is determined by the shape of their cross-section, length and material. For lighter weight applications, C or L-channels and hollow structural beams such as round, square or rectangular tubing of aluminum or composite materials are often used. Although strong by design, one disadvantage of these conventional support columns and stanchions is their lower energy absorbing properties during a failure. In crash conditions where the structure may be in compression, composite materials often buckle suddenly and absorb little energy. Failure of these structural elements, known as compressive failure, occurs when the structural elements experience ultimate compressive stresses that are beyond what the material is capable of withstanding. Additionally, buckling failure may result from column instability as a function of the height and width of the structural element. 
     Composite materials having multiple layered materials are often used in structural applications due to their inherent strength properties. These materials also provide great design flexibility due to the large various material selections and the ability to create various shapes. Structural composite elements often are composed of sandwich-type structure having an outer skin and a filler or core material. The outer skin, or shell, defines the shape and structure of the element whereas the filler material supports the shell. 
     Various materials are used for both the skin and core of a structural element and are typically chosen based upon the application and environment of use. For example, weight is often a design consideration in aerospace applications. Structural elements such as support beams of wall structures often need to be both lightweight and capable of carrying a mechanical load. In these applications, lightweight metallic skins made of aluminum and nonmetallic skins of thin carbon/epoxy or graphite/epoxy skins are often utilized. Environmental factors such as temperature and corrosive conditions are also considered when selecting skin materials. 
     The filler material provides both structural support and maintains the shape of the composite element. A variety of filler materials are known and range from the simple, such as balsa wood or other metallic and nonmetallic stiffeners, to complex structures such as aluminum or other nonmetallic honeycomb cores. Core materials are selected based upon their material properties such as flexibility, stiffness and strength-to-weight ratios, energy and sound absorption properties and others. 
     Environment must also be considered when selecting filling materials. It is not uncommon for carbon/epoxy skinned materials to absorb water, especially in aircraft applications where the structure undergoes pressurizing and depressurizing and are constantly exposed to changing atmospheric conditions. Water trapped in a composite may degrade the filler materials. Water may vaporize in warm conditions and even freeze at low temperatures and high altitudes and when undergoing these phase changes may damage the filler structure especially honeycomb-style core fillers, or disbond the filler from the skin. 
     Accordingly, there is a need for a lightweight composite structural element that has low structural weight, high structural strength and is capable of absorbing impact or crash energy and stabilizing long-column buckling that does not suffer from the problems and limitations of the prior art. 
     SUMMARY 
     The present invention provides a structural composite element that provides the required strength and rigidity for support columns and stanchions and provides inherent energy absorbing properties. This is achieved by the configuration of porous filler material and outer shell. The outer shell may be a metallic material made of aluminum alloys or a composite skin such as carbon/epoxy, fiberglass, metal matrix composite or other suitable composite materials. 
     A structural composite member constructed in accordance with an embodiment of the invention may comprise a porous core material having high mechanical energy absorption properties in all directions. The porous filler material further allows for the wicking of moisture condensation away from the skin or shell thereby reducing the onset of corrosion and chances of filler material damage. 
     Another exemplary embodiment of the present invention provides a lightweight structural composite element for long-columns and stanchions that have a high strength-to-weight ratio. An embodiment of the structural composite element also provides increased stiffness with higher energy absorbing characteristics during failure by compression, impact, or buckling than conventional materials. 
     These and other important aspects of the present invention are described more fully in the detailed description below. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       Embodiments of the present invention are described in detail below with reference to the attached drawing figures, wherein: 
         FIG. 1  is a fragmentary isometric cut-away view of a structural composite element with a porous filler material constructed in accordance with an embodiment of the invention; 
         FIG. 2  is a cross-sectional view of the porous filler material of the structural composite element of  FIG. 1 ; 
         FIG. 3  is a fragmentary isometric cut-away view of the structural composite element of  FIG. 1  attached to an attachment fitting; 
         FIG. 4  is a fragmentary side elevation view of the structural composite element with a skin shown in phantom to provide a view of the porous filler material and the attachment fitting of  FIG. 3 ; and 
         FIG. 5  is a fragmentary, cross-sectional side elevation view of the structural composite element and the attachment fitting of  FIG. 3 . 
     
    
    
     The drawing figures do not limit the present invention to the specific embodiments disclosed and described herein. The drawing figures are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the invention. 
     DETAILED DESCRIPTION 
     The following detailed description of the invention references the accompanying drawings that illustrate specific embodiments in which the invention can be practiced. The embodiments are intended to describe aspects of the invention in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments can be utilized and changes can be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense. The scope of the present invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled. 
       FIG. 1  illustrates a structural composite element  10  constructed in accordance with an embodiment of the invention and having a skin  12  and an porous filler material  14 . The structural composite element may be used as or incorporated in to a stanchion, strut or other support element of an aircraft, vehicle, or building. In one embodiment, skin  12  may be composed of structural, load bearing materials such as aluminum alloys and other metallic materials such as nickel, Inconel or the like. Alternatively, skin  12  may be composed of a composite such as carbon/epoxy, boron/epoxy, metal matrix composite, or other advanced materials. Carbon/epoxy materials are often advantageous due to the unique structural properties of carbon fiber orientation providing excellent strength properties. The thickness of skin  12  depends upon the material selection and strength criteria. For example, thin skinned components may have skins as thin as a few 0.2 inches or for greater load carrying structures having thickness of 1 to 4 inches. 
     As shown in  FIG. 1 , skin  12  of structural composite element  10  may be formed as a rectangular, hollow beam that encases porous filler material  14 . Porous filler material  14  may be composed of various metallic and nonmetallic filler materials such as porous cores. One embodiment utilizes an aluminum metallic porous filler material such as Duocel® aluminum foam manufactured by ERG Materials and Aerospace, Inc. or alternatively, stabilized aluminum foam, (“SAF”) manufactured by Cymat, Co. of Canada. These porous filler materials of aluminum foam cores provide a metal skeletal structure wherein the foam contains a matrix of cells and ligaments that are regular and uniform throughout the foam. Various densities of foam, number of pores per inch, are available with each density providing different strength characteristics. Alternatively, metallic porous filler material manufactured by Recemat International of the Netherlands may be used. Recemat International produces porous filler manufactured from alternative metallic materials such as copper, nickel, and a corrosion resistant nickel-chromium alloy. Please note that the materials described above are merely examples, and equivalent materials may be produced by other manufacturers not listed herein without departing from the scope of the invention. 
     These porous filler materials or metallic foam cores provide ease of assembly since they may be cut, milled, ground, lapped, drilled and rolled similar to metal. Likewise, metallic porous filler material may be anodized, coated or metal plated for corrosion resistance. The metallic porous filler material can also be brazed to the skin material or adhesively bonded. 
       FIG. 2  is a schematic representation of a cross-section of aluminum foam of porous filler material  14  showing open cell  16  and ligament structure  18 . Ligament structure  18  creates multiple supports for skin  12 . As skin  12  deforms under a load or impact, the load or impact energy is transferred to the ligament structure  18  of porous filler material  14 . Under over load conditions or impact, ligament structure  18  fails or crushes resulting in the ligament structure  18  filling open cells  16 . This densification process absorbs energy that would otherwise be redistributed to surrounding structure. The collapsing of the cells  16  of porous filler material  14  absorbs the impact energy and prevents or reduces the rebound of composite structural element  10  after compaction. Unlike conventional honeycomb-style filler materials, porous filler material  14  can absorb energy from impacts or compression in any direction. Similarly, under crash conditions that create compressive forces which typically result in buckling of the structure, ligament structure  18  absorbs the energy by collapsing and thereby stops the transfer of energy along skin  12  and reduces the severity of buckling. This advantage increases the crash worthiness of the structure. 
     Structural composite element  10  has greater inherent strength provided by porous filler material  14  over hollow beam type structures or those utilizing honeycomb cores. The same or increased strength properties can be realized using lighter weight skins, such as a decreased steel gauge or by material substitution such as aluminum or titanium. Thus, an advantage of the present invention is lighter weight structures having equivalent or increased strength properties. 
     Another advantage of open cells  16  of porous filler material  14  is that open cells  16  allow any entrapped moisture to wick away from skin  12 , and travel out of the structure. This reduces the risk or effect of environmental corrosion and prolongs the service life of the element. 
     Porous filler material  14  also may assist in the manufacturing of the structural composite element. When skin  12  is an epoxy matrix material such as carbon/epoxy, porous filler material  14  may be used as the lay-up tool thereby eliminating the need for a mandrel. Metallic porous filler material  14  may be machined to shape and the carbon/epoxy laid on top of porous filler material  14  for a matched fit. Composite skin  12  may be adhesively bonded after cure or alternatively, adhesive may be applied to porous filler material  14  and composite skin  12  lay-up positioned on porous filler material  14  and the materials co-cured. 
       FIGS. 3-5  illustrate an exemplary embodiment of the structural composite element  10 , wherein at least one end of the porous filer material  14  may be substantially tapered to a point or a rounded-off nub and may connect with an attachment fitting  20 . The attachment fitting  20  may be a clevis fitting as shown, comprising a first flange portion  22 , a second flange portion  24 , and an interface portion  26  connecting the two flange portions  22 , 24  in a substantially U-shaped configuration and interfacing with the composite skin  12  and the porous filler material  14 . 
     The first and second flange portions  22 , 24  may each have a hole formed therethrough and may be provided with further structural support by at least one brace portion  28 , 30 , connected with at least one of the flange portions  22 , 24 . Additionally, the attachment fitting  20  may comprise a shoulder portion  31  which may be positioned proximate and/or adjacent the brace portions  28 , 30  between the flange portions  22 , 24  and the interface portion  26 . For example, the shoulder portion  31  may have a chamfered face, such as a 45-degree chamfered face, laterally swept around the attachment fitting  20  which may interface with and/or contact an end portion of the composite skin  12 . 
     The interface portion  26  may be positioned inward of the composite skin  12  and may cover at least a portion of the tapered end of the porous filler material  14 . For example, the interface portion  26  may be shaped to form a cavity  32  at an end opposite the first and second flange portions  22 , 24 , and at least a portion of the tapered end of the porous filler material  14  may be positioned within the cavity  32 . 
     The interface portion  26  may also have a hole formed therethrough into which a fuse pin  34  may be inserted for connecting the attachment fitting to the composite skin  12 . The fuse pin  34  may be a notched fuse pin or any type of fuse pin known in the art and may be inserted through both the composite skin  12  and the interface portion  26  and secured in place. 
     The fuse pin  34  may shear off when a predetermined critical load value is applied. Once this happens, the attachment fitting  20  may begin to slide down the structural composite element  10 , compressing it. The porous filler material  14  may stabilize the structural composite element  10  from buckling and crippling. Then the brace portions  28 , 30  and/or the shoulder portion  31  of the attachment fitting  20  may induce a local shear force, pressing into the composite skin  12  and beginning to shred the composite skin  12  as the attachment fitting  20  compresses the structural composite element  10 . For example, the chamfered configuration of the shoulder portion  31  may be operable to force the porous filler material  14  to compress to full densification along the structural composite element  10 . 
     Although the invention has been described with reference to the embodiments illustrated in the attached drawings, it is noted that equivalents may be employed and substitutions made herein without departing from the scope of the invention as recited in the claims.