Patent Publication Number: US-6713005-B2

Title: Method of forming a composite structure of expandable matrix and a reinforcing medium

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a divisional application of commonly assigned U.S. application Ser. No. 09/520,852 filed on Mar. 07, 2000, now U.S. Pat. No. 6,379,762, entitled Composite Structure of Expandable Matrix And A Reinforcing Medium. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to composite structures having a high strength-to-weight ratio and more particularly to a composite structure preferably formed with an expanding film/foam layer and a reinforcing member. 
     BACKGROUND OF THE INVENTION 
     Discussion 
     Composite panels formed from lightweight materials are commonly used throughout the aerospace industry for secondary structural applications, such as wing trailing edges, empenage close outs, wing-to-body fairings, strut fairings and access doors. Frequently, these panels are formed from sandwich constructions of relatively thin inner and outer layers of thermoset resin/fiber systems separated by a relatively thick layer of honeycomb core material. The skin materials provide strength and the core material provides stiffness to the composite panel. Although these structures are strong enough to support aerodynamic loading, they typically lack sufficient compression strength for the attachment of assembly details, such as access door hinges. Consequently, the core can crush if a fastener is mounted through it and the outer facesheets. 
     In such cases, the panels are selectively strengthened in the areas where increased compression strength is needed through the use of ‘hard points’. The usual method of forming ‘hard points’ into a panel is through the use of a monolithic cylinder of potting compound that is incorporated into the panel. A typical potting compound is Epocast 1625 A/B by Ciba Specialty Chemicals. 
     The use of potting compounds, however, has several drawbacks. A first drawback concerns the density of these compounds. Typically, the potting compounds used for the monolithic inserts have a density of about 0.75 to about 2.00 grams/cubic centimeter. In comparison, the density of the honeycomb material is about 0.07 to about 0.25 grams/cubic centimeter. Accordingly, a considerable weight penalty accompanies the use of such potting compounds. 
     In the aerospace industry, there are design requirements for structures and materials that exhibit energy absorption. Examples of such structures are aircraft tail skids, helicopter landing gear assemblies and landing strut assemblies of interplanetary spacecraft. Generally speaking, it is highly desirable that materials used for absorbing impact energy provide sustained crush resistance over a defined distance of travel or crush stroke and have a high crush strength-to-density ratio. Additionally, the materials used for absorbing impact energy are frequently required to limit the maximum deceleration force during an impact and as such, the initial peak compressive strength exhibited by the panel is highly significant. 
     Frequently, structures designed for impact energy absorption use a honeycomb core of a lightweight material, such as aluminum, to provide the impact energy absorption. Examples of these cores are Spiralgrid by Alcore Incorporated and Tube-Core by Hexcel Corporation. However, the ratio of crush strength-to-density for these materials is typically low, generally being on the order of approximately 180 psi per pounds per cubic foot. Furthermore, the initial peak compression strength of these materials is not readily adjustable. Generally speaking, where lower initial peak compression strengths are required, a technique of pre-failing is employed wherein the material is loaded to a point beyond the initial peak compression strength. 
     SUMMARY OF THE INVENTION 
     The present invention preferably provides a composite structure having a high strength-to-weight ratio that can be fabricated at a relatively low cost. A preferred composite structure uses an expanded syntactic film. 
     The present invention entails a method for forming a structure having an syntactic film layer and a reinforcing layer. 
     In one form, a preferred embodiment of the present invention provides a composite structure having first and second layers which are wrapped about one another to form a cylinder. The first layer is formed from an expandable film or foaming adhesive. The second layer is formed from a reinforcing member. The second layer is overlaid onto the first layer. The first and second layers are then rolled together in a jelly roll fashion. The roll is then cured to expand the film. The structure may thereafter be incorporated into a panel and drilled to receive a fastener. A method for forming a composite structure from an expandable film member and a reinforcing member is also provided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a perspective view of a composite structure constructed in accordance with the teachings of the present invention; 
     FIG. 2 is a side elevation view of a laminate member used in the construction of the composite structure of FIG. 1; 
     FIG. 3 is a side elevation view of the formation of the laminate roll-up used in the construction of the composite structure of FIG. 1; 
     FIG. 4 is a plan view of the formation of a laminate roll-up similar to that of FIG. 3; 
     FIG. 5 is a perspective view of the laminate roll-up of FIG. 3 being inserted into a mold for curing; 
     FIG. 6 is a perspective view of the laminate roll-up of FIG. 3 being cured in a mold; 
     FIG. 7 is a top view of a composite structure constructed in accordance with the teachings of a first alternate embodiment of the present invention; 
     FIG. 8 is a side elevation view of the composite structure of FIG. 7 illustrating the plurality of reinforcing fibers oriented in a first direction; 
     FIG. 9 is a side elevation view of the composite structure of FIG. 7 illustrating the plurality of reinforcing fibers oriented in a second direction; 
     FIG. 10 is a cross-sectional view of a composite structure constructed in accordance with the teachings of a second alternate embodiment of the present invention; 
     FIG. 11 is an exploded perspective view illustrating the use of several supplemental reinforcing members, each of which being rotated relative to the longitudinal axis of the composite structure; 
     FIG. 12 is a side elevation view of a laminate member used in the construction of a composite structure constructed in accordance with the teachings of a third alternate embodiment of the present invention; 
     FIG. 13 is a plan view of the formation of a laminate roll-up used in the construction of a composite structure constructed in accordance with the teachings of a fourth alternate embodiment of the present invention; and 
     FIG. 14 is a perspective view of a composite structure constructed in accordance with the teachings of the forth alternate embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1 of the drawings, a composite structure constructed in accordance with the teachings of the present invention is generally indicated by reference numeral  10 . Composite structure  10  is shown to include a first layer  12  formed from an expandable film or foam (hereinafter film), such as a syntactic film  14 , and a second layer  16  formed from a reinforcing member  18 . 
     The syntactic film  14  is preferably a layer of Synspand® syntactic film by Dexter Corporation. A preferred film has a thickness of about 25 mils to about 200 mils and preferably has a thickness of about 50 mills to about 100 mils. Alternatively, the first layer  12  may be formed from an expandable foam such as FM 490A or FM 390 foaming adhesives by Cytec Industries, preferably having a thickness of about 25 to mills about 100 mils, and more preferably a thickness of about 25 mils to about 50 mils. Other factors which are relevant to the selection of the material for the syntactic film  14  include chemical compatibility with the reinforcing member  18 , the amount by which the material will expand during curing and the mechanical properties of the material, including adhesive strength and shear strength, which will permit applied loads to be transferred to the reinforcing member  18 . Preferably, the reinforcing member  18  is a tape having a plurality of oriented reinforcing fibers  20  (shown in FIG.  2 ), such as T6C190-12-F263-32 epoxy preimpregnated carbon tape by Hexcel Corporation having graphite fibers. Alternatively, the reinforcing member  18  may be any tape, fabric or yarn having graphite, fiberglass or other fibers which perform a reinforcing function. Those skilled in the art will understand that the reinforcing member  18  may be of a single layer or a series of sub-layers wherein the material forming the reinforcing member is layered upon itself. 
     With additional reference to FIGS. 2 through 6, the first and second layers  12  and  16  are initially flat and superimposed to one another. The syntactic film  14  is sufficiently tacky in the uncured state to adhere to the reinforcing member  18 . The reinforcing member  18  is next overlaid onto the syntactic film  14  to form a laminate member  30 . The reinforcing member  18  is preferably oriented to the syntactic film  14  such that the plurality of oriented reinforcing fibers  20  are oriented perpendicular to the longitudinal axis  32  of the laminate member  30  as shown in FIG.  4 . 
     Alternatively, the reinforcing member  18  may be oriented at a predetermined angle relative to the syntactic film  14  to alter the physical characteristics (e.g., compressive strength) of composite structure  10 . One example of this is illustrated in FIG. 3 where the reinforcing member  18  is oriented relative to the syntactic film  14  such that the plurality of oriented reinforcing fibers  20  are parallel to the longitudinal axis  32  of the laminate member  30 . 
     The laminate member  30  is next rolled or wrapped onto itself along its longitudinal axis  32  to form a laminate roll-up  40  having a longitudinal axis  41 . To initiate the wrapping process, a narrow strip of the syntactic film  14  may be placed along the width of the laminate member  30  at the end of the laminate member that will correspond to the center of the laminate roll-up  40 . The resulting laminate roll-up  40  is generally cylindrical in shape, the end of which having a swirled appearance similar to that of a cinnamon or jelly roll. The laminate roll-up  40  is placed into a mold cavity  42  in a mold  44  (FIG. 5) and cured to expand the syntactic foam adhesive at an elevated temperature (FIG.  6 ). Due to the expanding nature of the syntactic film  14 , it is important that air be permitted to escape from the mold  44  during the curing step. Depending upon the type of material used, it may also be beneficial to evacuate air from the mold  44  via a vacuum line  46 , as when the FM 490A foaming adhesive by Cytec Industries is used for the syntactic film  14 . 
     After the curing step has been completed, the composite structure  10  may be processed through conventional machining operations to prepare the composite structure  10  for a desired application. For example, the composite structure  10  may be cut to a desired length, inserted into and bonded to a predetermined location in a honeycomb core (not shown) and formed with the honeycomb core into a composite panel (also not shown). The panel may then be drilled through its depth at positions which have been reinforced with the composite structure  10  to receive a fastener (not shown). In isolation, the composite structure  10  will appear as shown in FIG. 1 with a clearance hole  50 . 
     An alternate method for incorporating composite structure  10  into a panel includes the forming of the laminate roll-up  40  to predetermined dimensions and placing the un-cured laminate roll-up  40  into a cavity formed in the honeycomb core of a panel (not shown). The honeycomb core subassembly is then incorporated into a sandwich panel assembly with top and bottom plies of conventional composite resin preimpregnated materials. The entire assembly is then cured under conditions of heat and pressure. In this manner, the laminate roll-up  40 , is co-cured with the resin preimpregnated materials, saving the processing step of curing separately in a mold. Subsequent operations of drilling to receive a fastener are similar in both methods of forming honeycomb core reinforcement. 
     The density of the composite structure  10  will vary depending the mold fill ratio (i.e., the ratio of the volume of the uncured laminate roll-up  40  to the volume of the mold cavity  42 ). Mold fill ratios ranging between about 60 percent to about 85 percent have provided composite structures  10  with densities ranging between about 0.40 grams/cubic centimeter to about 0.90 grams/cubic centimeter. Table 1, below, details several of the various physical properties of various configurations of the composite structure  10 , as well as for various types of potting compounds. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Physical Properties of Inserts 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Density 
                 Compressive 
               
               
                   
                 Insert Construction 
                 (g/cm 3 ) 
                 Strength (ksi) 
               
               
                   
                   
               
               
                   
                 Synspand (.050) with a double 
                 0.89 
                 24.2 
               
               
                   
                 layer of carbon tape epoxy 
               
               
                   
                 Synspand (.050) with a single 
                 0.80 
                 20.0 
               
               
                   
                 layer of carbon tape epoxy 
               
               
                   
                 Synspand (.100) with a single 
                 0.70 
                 12.1 
               
               
                   
                 layer of carbon tape epoxy 
               
               
                   
                 Type 27 Potting Compound 
                 2.00 
                 20.0 
               
               
                   
                 Type 25 Potting Compound 
                 2.00 
                  8.5 
               
               
                   
                   
               
            
           
         
       
     
     While the composite structure  10  has been described thus far as including a single reinforcing member which has been wrapped internally in a spiral manner, those skilled in the art will appreciate that the invention, in its broader aspects, may be constructed somewhat differently. A first example of an alternate construction is illustrated in FIGS. 7 through 9. Composite structure  10 ′ is illustrated to be identical to composite structure  10  except for a second reinforcing member  60  which circles its perimeter. 
     Like reinforcing member  18 , the second reinforcing member  60  is preferably formed from a tape having a plurality of oriented reinforcing fibers  62  (FIGS.  8  and  9 ), such as T6C190-12-F263-32 epoxy preimpregnated carbon tape by Hexcel Corporation having graphite (carbon) fibers. Alternatively, the reinforcing member  18  may be any tape, fabric or yarn having graphite, fiberglass or other fibers which perform a reinforcing function. Accordingly, those skilled in the art will understand that second reinforcing member  60  may be a separate and discreet from reinforcing member  18 , or may be fixedly coupled to reinforcing member  18 . Those skilled in the art will also understand that the second reinforcing member  60  may be of a single layer or a series of sub-layers wherein the material forming the second reinforcing member is layered upon itself. 
     The process for forming composite structure  10 ′ differs from that of composite structure  10  in that before the laminate roll-up  40 ′ is placed into the mold  44 , the second reinforcing member  60  is wrapped around the perimeter of the laminate roll-up  40 ′. The second reinforcing member  60  is preferably oriented to the laminate roll-up  40 ′ such that the plurality of oriented reinforcing fibers  62  are oriented perpendicular to the longitudinal axis  41 ′ of the laminate roll-up  40 ′ as shown in FIG.  8 . Construction in this manner provides a composite structure  10 ′ that is particularly well suited for applications involving impact energy absorption. The outside wrap localizes the initial failure to a narrow zone at one end of structure  10 ′. The remaining part of the structure remains in tact and continues to support load. As load is applied through the crush stroke, the fracture zone moves up the length of structure  10 ′ as the outside wrap incrementally fractures. The specimen has an “unzipped” appearance after loading. 
     Alternatively, the second reinforcing member  60  may be oriented at a predetermined angle relative to the laminate roll-up  40 ′ to alter the physical characteristics (e.g., compressive strength) of composite structure  10 ′. One example of this is illustrated in FIG. 9 where the second reinforcing member  60  is oriented relative to the laminate roll-up  40 ′ such that the plurality of oriented reinforcing fibers  62  are parallel to the longitudinal axis  41 ′ of the laminate roll-up  40 ′. Construction in this manner provides a composite structure  10 ′ that is particularly well suited for applications requiring compression strength. 
     Accordingly, those skilled in the art will understand that reinforcing member  18  and second reinforcing member  60  will cooperately define the physical properties of structural insert  10 ′. As such, the orientations of reinforcing member  18  and second reinforcing member  60  need not be identical. Moreover, since second reinforcing member  60  may be formed from a plurality of sub-layers of the reinforcing material, each sub-layer need not be continuous with the previous sub-layer. By including sub-layers of multiple orientations, it is possible to further alter the physical properties of composite structure  10 ′ to suit a given set of design criteria. 
     A second example of an alternate construction is illustrated in FIGS. 10 and 11. Composite structure  10 ″ is shown to be constructed similarly to composite structure  10  except for the inclusion of one or more reinforcing discs  80 . Each of the reinforcing discs  80  are preferably formed from a tape having a plurality of oriented reinforcing fibers  82 , such as T6C190-12-F263-32 epoxy pre-impregnated carbon tape by Hexcel Corporation having graphite fibers. 
     The process for forming composite structure  10 ″ differs from that of composite structure  10  in that before the laminate roll-up  40 ″ is placed into the mold  44 , the reinforcing disc  80  is placed to an end  84  of the laminate roll-up  40 ″. After curing, the reinforcing disc  80  provides the composite structure  10 ″ with characteristics that are similar to a washer disbursing axially directed point loads across a larger surface. These properties may be enhanced by the use of several reinforcing discs  80 , with each disc  80  being rotated relative to an adjacent disc  80  to rotate the orientation of the plurality of oriented reinforcing fibers  82  (FIG.  11 ). 
     A third example of an alternate construction is illustrated in FIG.  12 . In this embodiment, the laminate member  30  is replaced with laminate member  30 ′. Laminate member  30 ′ is similar to laminate member  30  except that an additional reinforcing layer  18 ′ is coupled to a second side of the syntactic film  14 . Reinforcing layer  18 ′ is preferably a tape having a plurality of oriented reinforcing fibers  20 ′, such as T6C190-12-F263-32 epoxy preimpregnated carbon tape by Hexcel Corporation having graphite fibers. Alternatively, the reinforcing member  18 ′ may be any tape, fabric or yarn having graphite, fiberglass or other fibers which perform a reinforcing function. Those skilled in the art will understand that the reinforcing member  18 ′ may be of a single layer or a series of sub-layers wherein the material forming the reinforcing member is layered upon itself. Those skilled in the art will also understand that reinforcing layer  18 ′ may be oriented relative to the syntactic film  14  at an angle that is different from reinforcing layer  18 . 
     A fourth example of an alternate construction is illustrated in FIGS. 13 and 14. In this embodiment, composite structure  10   b  is similar to composite structure  10 ′ except that laminate member  30  is replaced with laminate member  30 ″. Laminate member  30 ″ is similar to laminate member  30  except that reinforcing member  18  has a width that is narrower than that of syntactic film  14 . In this embodiment, reinforcing member  18  and syntactic film  14  are aligned to one another such that they are aligned on one side (e.g., the lower side) and offset to one another on the other side (the upper side). Laminate member  30 ″ is then rolled to form laminate roll-up  40   b , wrapped with a second reinforcing member  60  and cured as described above. Construction in this manner is desirable where it is necessary to limit or control the initial peak compression load in an impact energy absorbing design. By offsetting reinforcing member  18  while maintaining the width of the second reinforcing member  60  at the full height of the composite structure  10   b , failure of the composite structure  10   b  initiates at a relatively lower load, thereby lowering the maximum deceleration force during an impact while maintaining the same energy absorbing crush strength over the crush stroke. Those skilled in the art will understand that the amount by which the failure load is lowered will be dependent upon the magnitude of the offset. 
     While the invention has been described in the specification and illustrated in the drawings with reference to a preferred embodiment, 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 invention as defined in the claims. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention, but that the invention will include any embodiments falling within the foregoing description and the appended claims.