FIELD OF THE INVENTION 
     The present invention relates to adhesive fillets, and more particularly relates to methods and apparatus for making adhesive fillets for structural applications such as aircraft stiffeners. 
     BACKGROUND INFORMATION 
     Adhesive fillets are used in applications such as aircraft stiffeners. For example, stiffeners with hat-shaped cross sections are often attached to the interior surface of aircraft skins in order to provide improved mechanical properties. The adhesive fillets are positioned at interior corners where the stiffeners meet the aircraft skin. 
     Conventionally, such adhesive fillets have been manually formed into rod shapes by hand-rolling an adhesive sheet, followed by consolidation in a mandrel under heat and vacuum. Each adhesive rod takes several minutes to form, and the hand-rolled adhesive rods tend to have inconsistent shapes and quality. 
     The present invention has been developed in view of the foregoing and to address other deficiencies of the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention provides shaped adhesive fillets for applications such as stiffened panels for aircraft. The adhesive fillets are formed by stacking multiple layers of adhesive sheets, consolidating the stacked layers, cutting strips from the consolidated adhesive layers, and forming the cut strips into desired fillet shapes. The formed fillets may be used in stiffened aircraft panels, for example, at the interior corners of hat-shaped stiffening members. 
     An aspect of the present invention is to provide a method of making an adhesive fillet. The method comprises stacking multiple adhesive layers, and forming the adhesive layers into a fillet shape. 
     Another aspect of the present invention is to provide a formed adhesive fillet comprising multiple stacked adhesive layers. 
     A further aspect of the present invention is to provide an apparatus for making adhesive fillets comprising at least one roller structured and configured to form an adhesive strip into an adhesive fillet shape. 
     Another aspect of the present invention is to provide a stiffened panel including a stiffening member attached to the panel and at least one formed adhesive fillet attached to the panel and the stiffening member, wherein the at least one formed adhesive fillet comprises multiple stacked adhesive layers. 
     These and other aspects of the present invention will be more apparent from the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric sectional view of a stiffened panel including adhesive fillets in accordance with an embodiment of the present invention. 
         FIG. 2  is an isometric view of an adhesive sheet conventionally used to make adhesive fillets. 
         FIG. 3  is an isometric view of a rolled fillet rod made from an adhesive sheet as shown in  FIG. 2 . 
         FIG. 4  is an isometric view of a layup of multiple adhesive sheets utilized in accordance with an embodiment of the present invention. 
         FIG. 5  is a partially schematic side view of an assembly for consolidating the multi-layer adhesive layup shown in  FIG. 4 . 
         FIG. 6  is an isometric view of an adhesive bar cut from a consolidated multi-layer adhesive layup in accordance with an embodiment of the present invention. 
         FIG. 7  is a partially schematic side view of an adhesive fillet forming apparatus in accordance with an embodiment of the present invention. 
         FIG. 8  is a cross-sectional view of an adhesive fillet produced in accordance with an embodiment of the present invention. 
         FIG. 9  is a sectional end view illustrating the assembly of a hat-shaped stiffening member on a skin panel, including the use of adhesive fillets of the present invention. 
         FIG. 10  is a side sectional view of an opposing roller adhesive fillet forming apparatus in accordance with an embodiment of the present invention. 
         FIG. 11  is an end sectional view of the opposing roller adhesive fillet forming apparatus of  FIG. 10 . 
         FIG. 12  is an isometric view of a fillet catcher in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a stiffened structure  10  including a panel  12  and a stiffening member  14 . The panel  12  may be made of any suitable material such as metals or composites. For example, the panel may comprise a graphite/epoxy composite including unidirectional or woven graphite fibers in an epoxy resin matrix. In one embodiment, the panel  12  may comprise an aircraft skin having a typical thickness of from about 0.05 to about 0.2 inch or more. As a particular example, the aircraft skin may have a thickness of from 0.10 to 0.13 inch. The stiffening member  14  may be made of materials such as metals or composites, e.g., unidirectional or woven graphite fibers in an epoxy resin matrix, and may be dimensioned as desired for a particular application. For example, the stiffening member  14  may have a thickness of from about 0.05 to about 0.2 inch or more, a width of from about 1 to about 6 inches, a height of from about 0.05 to about 2 inches, and a length of from about 1 to about 8 feet or more. As a particular example, when used as an aircraft skin stiffener, the member  14  may have a typical thickness of from 0.11 to 0.126 inch, a width of from 2 to 4 inches, a height of from 0.7 to 1 inch, and a length of from 18 to 36 inches. The stiffening member  14  shown in  FIG. 1  has a hat-shaped cross section. However, other stiffening member shapes may be used in accordance with the present invention, such as L-shapes, I-shapes, J-shapes, T-shapes, Z-shapes and the like. 
     As shown in  FIG. 1 , the stiffened structure  10  includes adhesive fillets  16  at the interior corners where the stiffening member  14  meets the surface of the panel  12 . In accordance with the present invention, the adhesive fillets  16  are made by a process of stacking, cutting and forming a selected number of multiple layers of adhesive material, as more fully described below. 
       FIGS. 2 and 3  illustrate a conventional method for making adhesive fillets. The process starts with an adhesive sheet or strip  20  as shown in  FIG. 2 . The adhesive strip  20  typically has an approximate thickness of about 0.014 inch, a width of about 1 inch, and a length of about 36 inches. As shown in  FIG. 3 , the adhesive strip  20  is hand rolled into a rod  22 . In accordance with conventional processes, the hand-rolled rod  22  may be placed in a V-shaped mandrel under vacuum and may be subjected to heat and manual pressure in order to at least partially consolidate the adhesive rod  22 . 
       FIG. 4  illustrates a multi-layer adhesive structure  30  of the present invention including multiple adhesive layers  30   a - 30   e . Five adhesive layers  30   a - 30   e  are shown in the embodiment of  FIG. 4 . However, other numbers of layers may be used depending on the particular application. For example, from 5 to 8 layers may be desirable. Each of the layers  30   a - 30   e  of the multi-layer adhesive structure  30  may be made of a material such as epoxy adhesive sold under the designation FM300 by Cytec, or the like. Materials such as scrim cloth, short chopped fibers, unidirectional graphite fibers and fillers may optionally be embedded in each layer  30   a - 30   e , or positioned between each layer. The thickness of each layer  30   a - 30   e  preferably ranges from about 0.01 to about 0.02 inch. A particularly preferred thickness is about 0.014 inch The total thickness of the multi-layer adhesive structure  30  typically ranges from about 0.05 to about 0.2 inch or more, more preferably from about 0.07 to about 0.112 inch. Any suitable width of the multi-layer adhesive structure  30  may be used, for example, from about 1 to about 3 feet. Similarly, any suitable length of the multi-layer adhesive structure  30  may be used, for example, from about 1 to about 8 feet, or as a continuous roll. 
     The multi-layer adhesive structure  30  as shown in  FIG. 4  may be consolidated in an assembly  40  including a vacuum table  41 , as shown in  FIG. 5 . The multi-layer adhesive structure  30  is sandwiched between gas permeable layers  42   a  and  42   b . The gas permeable layers  42   a  and  42   b  may comprise sheets of materials such as porous fluorinated ethylene propylene (FEP). Non-porous sheets  44   a  and  44   b  are positioned against the permeable sheets  42   a  and  42   b . The non-permeable sheets  44   a  and  44   b  may be made of any suitable material such as metal, rigid plastic, flexible plastic or the like. Metal plates  46   a  and  46   b  are positioned outside the non-permeable sheets  44   a  and  44   b . A vacuum bag  48  made of any suitable material such as nylon or silicone rubber is positioned over the various layers and sealed  49  to the vacuum table  41 . Vacuum applied by the vacuum table  41  creates a vacuum inside the sealed bag  48 . The adhesive layers  30   a - 30   e  may be further consolidated by heating, e.g., for 0.5 to 2 hours at a temperature of about 140° F. Without such consolidation at elevated temperature, the adhesive layers  30   a - 30   e  may delaminate during the subsequent forming process. 
     After the multi-layer adhesive structure  30  has been consolidated, it may be cut into strips.  FIG. 6  illustrates an adhesive strip  32  having a height H corresponding to the height of the consolidated multi-layer adhesive structure  30  shown in  FIG. 5 . The adhesive strip  32  has a width W and a length L. The height H typically is from about 0.05 to about 0.2 inch, the width W is typically from about 0.1 to about 0.5 inch, and the length L is typically from about 1 to about 8 feet or more. For example, H may range from 0.07 to 0.112 inch, W may range from 0.17 to 0.25 inch, and L may range from 30 to 36 inches. The strip  32  may be formed by any suitable cutting operation. For example, a conventional Gerber cutter may be used to automatically cut the consolidated multi-layer adhesive structure  30  into the strips  32 . The cutter may utilize a blade oriented at a selected angle, for example, an angle of from about 45 to about 60 degrees measured from the plane of the multi-layer adhesive structure  30 , preferably at an angle of from about 50 to about 55 degrees. The blade thickness should be sufficient to minimize the possibility of blade fracture during the adhesive cutting process. A blade thickness of 0.04 inch has been found to be suitable. 
     As shown in  FIG. 7 , the adhesive strip  32  may be formed into the shape of a fillet  34  by feeding the strip  32  through opposing rollers  36  and  37 . Although the rollers  36  and  37  shown in  FIG. 7  are opposed to each other, the rollers may alternatively be offset from each other. One or both of the rollers  36  and  37  may be driven by a conventional motor (not shown). In accordance with an embodiment of the present invention, a catcher  38  may be positioned adjacent to at least one of the rollers  36  and  37  in order to facilitate ejection of the adhesive fillet  34  from the rollers  36  and  37 . For example, the catcher  38  may be used to prevent the adhesive fillet  34  from sticking to the roller  37  after it has been formed. 
       FIG. 8  illustrates a cross section of the adhesive fillet  34 . The adhesive fillet  34  has a base angle A which may range from about 90 to about 120°, preferably from about 100 to about 110°. For example, an angle A of about 107° may be suitable. An arm of the adhesive fillet  34  may have a dimension B of from about 0.1 to about 0.5 inch, preferably from about 0.2 to about 0.3 inch. For example, the dimension B may be from about 0.22 to about 0.25 inch. The adhesive fillet  34  also has a typical chord distance C of from about 0.2 to about 0.8 inch, preferably from about 0.35 to about 0.5 inch. For example, the chord distance C may be about 0.36 inch. The adhesive fillet  34  may also be concave, with a curved surface having a radius R of from about 0.1 to about 2 inch or more, preferably from about 0.2 to about 0.5 inch. For example, the radius R may be about 0.3 inch. 
       FIG. 9  is a sectional end view illustrating the assembly of a hat-shaped stiffening member  14  on a skin panel  12 , including the use of adhesive fillets  16  of the present invention. The adhesive fillets  16  are attached at interior corners  18  of the stiffening member  14 . A mandrel  50  is positioned on the skin panel  12 , and the stiffening member  14  and adhesive fillets  16  are pressed against the mandrel  50  and skin panel  12 . The stiffening member  14  and adhesive fillets  16 , mandrel  50  and skin panel  12  are then placed under a vacuum to ensure intimate contact of the assembly. The assembly is then cured in an autoclave or oven. After the stiffening member  14  and adhesive fillets  16  are secured to the skin panel  12 , the mandrel  50  is removed by sliding it out from the stiffening member  14 . 
       FIGS. 10 and 11  illustrate an opposing roller adhesive fillet forming apparatus  52  in accordance with an embodiment of the present invention. The apparatus  52  includes a lower frame  54 , which rotatably supports a drive shaft  56 . A motor  58  is connected to the drive shaft  56 . The lower roller  37  is mounted on the drive shaft  56 . The motor  58  rotates the drive shaft  56  and the lower roller  37  at a desired speed, such as 33 rpm. An upper frame  64  supports a rotatable shaft  66  upon which the upper roller  36  is mounted. A rotatable handle  68  permits vertical movement of the upper roller  36  in relation to the lower roller  37 . In the embodiment shown in  FIGS. 10 and 11 , the lower roller  37  has a recessed roller surface in the form of a generally V-shaped groove, while the upper roller  36  has an extended roller surface that fits into the recess of the lower roller  37 . 
     As shown most clearly in  FIG. 10 , a feed channel  70  extends from the lower and upper frames  54  and  64  inwardly toward the rollers  37  and  36 . The catcher  38  is mounted adjacent the rollers  37  and  36  on a support member  71 , and includes an exit channel  72  through which the adhesive fillet travels after it has been formed by the rollers  37  and  36 . 
       FIG. 12  is an isometric view of a fillet catcher  38  in accordance with an embodiment of the present invention. The exit channel  72  extends through the fillet catcher  38 . The fillet catcher  38  includes contoured surfaces  74   a ,  74   b  and  76 , which substantially conform with the surface of the lower roller  37 . The fillet catcher  38  also includes contoured surfaces  78   a  and  78   b , which substantially conform with the surface of the upper roller  36 . As an adhesive fillet is formed by the rollers  37  and  36 , it is removed from the rollers by the fillet catcher  38 . The formed adhesive fillet contacts the contoured surface  76  of the fillet catcher  38 , which extends into the cavity of the lower roller  37 . The formed adhesive fillet then travels through the exit channel  72  of the fillet catcher  38 , and exits from the back end  79  of the fillet catcher  38 . 
     The following example is intended to illustrate various aspects of the present invention, and is not intended to limit the scope of the invention. 
     EXAMPLE 
     Five layers of adhesive sheets were stacked as shown in  FIG. 4 . Each sheet had a thickness of 0.014 inch, and was made of FM300 supported with scrim cloth. The width of the sheets was 0.23 inch, and the length of the sheets was 34 inches. The stacked adhesive sheets were consolidated in an assembly similar to that shown in  FIG. 5 . The sheets were sandwiched between gas permeable layers made of fluorinated ethylene propylene Teflon film (FEP) sold under the designation Pin Hole A5000. The gas permeable layers were sandwiched between non-permeable sheets made of nonporous Armalon material. Metal plates having thicknesses of 0.125 inch were positioned as shown in  FIG. 5 , and the assembly was covered with a sheet of porous polyester sold under the designation Air Weave and a nylon vacuum bag. The bag was sealed to a vacuum table by a seal made of nylon bagging material sold under the designation Airtech. The assembly was heated to a temperature of 140° F. under a vacuum of 26 inches of mercury and held for 1 hour. The pressure applied by the plates was about 12 psi. At temperatures above about 140° F., the adhesive layers may not sufficiently consolidate, while at temperatures above 140° F. the adhesive may begin to cure, making subsequent forming difficult. The consolidated adhesive layers were then cooled to room temperature and stored under vacuum. 
     Upon removal from the assembly, the consolidated multi-layer adhesive structure had a thickness of about 0.075 inch. It was then cut in a Gerber cutter with a cutting blade angle of 55° to form multi-layer adhesive strips having heights of 0.075 inch, widths of 0.23 inch and lengths of 38 inches. 
     Each strip was then formed into an adhesive fillet shape by passing the strips through a roll-former as illustrated in  FIGS. 10 and 11 . The roll-former was equipped with a catcher as illustrated in  FIG. 12 , which prevented the formed adhesive fillet from sticking to the lower roller. 
     The formed adhesive fillet had a cross section similar to that shown in  FIG. 8 , having an angle A of 107.14°, an arm dimension B of 0.226 inch, a chord length C of 0.363 inch, and a radius R of 0.306 inch. During the roll-forming operation, the cross sectional area of the adhesive fillet was reduced approximately 60 percent in comparison with the initial cross sectional area of the multi-layer adhesive strip, resulting in compaction, deformation and elongation of the original strip by about 20 percent. If the cross sectional area of the adhesive fillet is reduced by 200 percent or more, it has been found that the adhesive fillet elongates over 30 percent and forms an irregular shaped fillet. After formation, the adhesive fillet may be sandwiched between a mandrel hat and skip panel, e.g., as shown in  FIG. 9 . 
     Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims.
Methods of forming shaped adhesive fillets are disclosed. The adhesive fillets may be made by stacking multiple layers of adhesive sheets, consolidating the stacked layers, cutting strips from the consolidated layers, and forming the strips into adhesive fillets having desired cross sectional shapes. The fillets may be formed by feeding the strips through an apparatus comprising opposing rollers and a catcher, which facilitates release of the adhesive fillets from the rollers. The formed adhesive fillets are useful for applications such as stiffened panels for aircraft.