Abstract:
A method is provided for securing a three-dimensional woven preform to a first composite laminate component, the preform having a base and at least one leg extending from the base. The preform may be used to attach a second component or may be used alone to stiffen the first component. The preform and the first component are uncured, whereas the second component is cured prior to assembly. The preform is positioned on the first component, adhesive optionally being located between the preform and the first component. Z-pins are driven through the base of the preform and into the first component, the pins extending into the base and the first component. The second component is attached to the leg of the preform. A vacuum bag and tooling are used while curing the first component and the preform and the preform to the first component. The second component may be bonded or fastened to the preform.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention generally relates to assembly of components using Z-pins and particularly relates to assembly of components into structural joints using Z-pins and woven preforms.  
           [0003]    2. Description of the Prior Art  
           [0004]    Typical methods known in the art for attaching a composite skin to a composite web include forming the web as an “I” or “C” shape, making them more complex and expensive to fabricate. The flanged sections are fastened to adjacent sections using methods similar to those used with metal components, for example, by using fasteners. However, use of the fasteners adds weight to the joints.  
           [0005]    These joints also have difficulty with standing out-of-plane loading. Typical remedies for this are thick laminate stack-ups using many layers of composite fabric and having large flange radii. While this reduces the tension forces between the layers of the flanged section, the result is a heavy joint, reducing the weight savings realized when using composites.  
           [0006]    Z-pins have been used to join two composite, laminate components in the prior art. For example, U.S. Pat. No. 5,868,886 to Alston, et al., discloses a method of installing composite patches on a composite surface. A precured patch is placed in a prepared opening, and an ultrasonic head induces localized melting in the patch and surface. The head then drives Z-pins into the layers oft he patch and into the layers of the surface, the Z-pins extending into both components to provide for greater strength in the joint. Likewise, U.S. Pat. No. 5,589,015 to Fusco discloses joining two laminate composites by using an ultrasonic head to drive Z-pins through a first component and into a second component. In the &#39;015 reference, the pins are held in a compressible carrier before being driven into the components, which may be cured or uncured.  
         SUMMARY OF THE INVENTION  
         [0007]    A method is provided for securing a three-dimensional woven preform to a first composite laminate component, the preform having a base and at least one leg extending from the base. The preform may be used to attach a second component or may be used alone to stiffen the first component. The preform and the first component are uncured, whereas the second component is cured prior to assembly. The preform is positioned on the first component, adhesive optionally being located between the preform and the first component. Over-wrap plies are optionally placed on the outer surfaces of preform, and Z-pins are driven through the over-wrap plies, through the base of the preform and into the first component, the pins extending into the base and the first component. The second component is attached to the leg of the preform. A vacuum bag and tooling are used while curing the first component and the preform and the preform to the first component. The second component may be bonded or fastened to the leg of the preform.  
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0008]    The novel features believed to be characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings.  
         [0009]    [0009]FIG. 1 is an exploded front view of an assembly using a Pi-shaped preform and cure tooling, the assembly being in accordance with the present invention.  
         [0010]    [0010]FIG. 2 is a front view of the assemble of FIG. 1 after installation and in accordance with the present invention.  
         [0011]    [0011]FIG. 3 is an exploded front view of an alternate embodiment of an assembly using a Pi-shaped preform, a sizing tool, and cure tooling, the assembly being in accordance with the present invention.  
         [0012]    [0012]FIG. 4 is a front view of the assembly of FIG. 3 before insertion of the second component and in accordance with the present invention.  
         [0013]    [0013]FIG. 5 is a front view of an assembly using a T-shaped preform to connect first and second components and in accordance with the present invention.  
         [0014]    [0014]FIG. 6 is a front view of a second embodiment of an assembly using a T-shaped preform to connect first and second components and in accordance with the present invention.  
         [0015]    [0015]FIG. 7 is a perspective view of a panel using T-shaped preforms to stiffen the panel and in accordance with the present invention.  
     
    
     DESCRIPTION OF THE INVENTION  
       [0016]    [0016]FIGS. 1 through 4 show a method for bonding two composite components using a woven preform and Z-pins and then co-curing the assembly. A three-dimensional (3-D), Pi-shaped, woven preform  11  is used to connect two composite parts  13 ,  15 , which may be, for example, a frame member  13  and a skin  15 , or other member. Parts  13 ,  15  may be any members of a substructure, including spars, ribs, longerons, etc. Preform  11 , frame  13 , and skin  15  are infused with a resin, for example, 977-3, available from Cytec Industries, Inc. of West Paterson, N.J. Preform  11  and skin  15  are not cured prior to assembly, whereas frame  13  is cured prior to assembly. Preform  11  may be woven from materials such as carbon fibers, Kevlar fibers, glass fibers, or other materials, or may be a combination of material types.  
         [0017]    As shown in the figures, preform  11  is Pi-shaped, having a base  17  on its lower portion that has a continuous, flat lower surface  19  and a pair of spaced-apart, planar legs  21  extending vertically upward from base  17 . Each leg  21  is at a position that is offset from, but near to, the center of base  17 . Legs  21  are shown as being parallel to each other and generally perpendicular to base  17 . In the installed position, inner surfaces  23  of legs  21  face each other for receiving frame member  13 , forming a clevis. A small, upward-facing surface  25  of base  17  lies between the lower ends of legs  21 . It is preferable for the outer surface of legs  21  and the upper surface of base  17  to be tapered at their outer edges, as shown, but the ends may be squared. Also, though not shown in the figures, legs  21  can be at other angles relative to each other and to base  17 , which provides for parts  13 ,  15  to be oriented at angles other than 90°.  
         [0018]    [0018]FIG. 1 is an exploded view of the components used to form the assembly. An adhesive film  27 , for example, AF191, available from 3 M of St. Paul, Minn., is placed between lower surface  19  of preform  11  and upper surface  29  of skin  15  for adhering preform  11  to skin  15 . Frame  13  and skin  15  each comprise a plurality of layers of composite material in this embodiment. Frame  13  has a cured resin matrix, but skin  15  remains uncured. Components  13 ,  15  are shown as flat planes, but skin  15  may be curved.  
         [0019]    Various resin systems are sold under the terms “laminating resins” and “adhesives,” though there is no “bright-line, ” industry-standard definition by which to distinguish one from the other. The term “adhesive” is used herein to mean a resin system that has a lower modulus of elasticity and/or a higher strain-to-failure than the resin forming the matrix oft he parts to be adhered. The combination of these characteristics is described as higher toughness, and adhesives have a higher toughness than laminating resins, which tend to be more brittle and have lower crack formation loads.  
         [0020]    Results from ASTM tests can be used to distinguish, generally, between laminating resins and adhesives. High-strength, structural laminating resins have a peel strength rating generally ranging from 0-15 pounds per linear inch, whereas the peel strength of adhesives is greater than  15  pounds per linear inch. For example, the Bell Peel test (ASTM D3167 “Standard Test Method for Floating Roller Peel Resistance of Adhesives”) shows that the peel strength of AF191 is 30-45 pounds per linear inch at room temperature, but the peel strength of 977-3, which is used to laminate the parts, is 0-6 pounds per linear inch. In addition, laminating resins generally have a tensile strength greater than 7500 pounds per square inch (psi) as tested using ASTM D638 (“Standard Test Method for Tensile Properties of Plastics”), with high-strength resins ranging to 10000 psi. Adhesives generally have tensile strengths less than 6500 psi. Thus, in the present application, “adhesives” also means resin systems with tensile strengths less than 6500 psi and a peel strength greater than 15 pounds per linear inch. “Laminating resins” is used to mean resin systems having tensile strengths greater than 7500 psi and a peel strength of less than 15 pounds per linear inch. Thus, when adhering two resin-infused components, an adhesive is used between the components to provide for a high bond strength.  
         [0021]    If necessary for load requirements, a resin-infused textile layer forms a shear or overwrap ply  31  and is laid on the outer surface of each leg  21  that extends across the upper surface of base  17 . Over-wrap plies  31  provide additional connective layers between preform  11  and skin  15 . Adhesive film  27  extends beyond the outermost edge of the lower portions of over-wrap plies  31 . Each over-wrap ply  31  extends upward to the upper edge of leg  21 .  
         [0022]    In order to provide for a stronger joint when preform  11  is adhered to skin  15 , Z-pins  33  are driven through over-wrap plies  31 , base  17  of preform  11 , through adhesive film  27 , and into skin  15  through surface  29 . Pins  33  are also driven through surface  25  of preform  11  and into skin  15 . Pins  33  push aside the fibers of preform  11 , plies  31  and skin  15  as pins  33  are inserted. Pins  33  are preferably formed from graphite or titanium and are initially held within a foam carrier  35 , pins  33  being vertically oriented and arranged in a matrix that provides for the desired a real density and pin locations after insertion of pins  33 . Pins  33  have very small diameters, typically around 0.02 inches.  
         [0023]    Pins  33  are inserted by using an ultra-sonic vibrating head (not shown) to drive them into skin  15 . A lower surface  37  of carrier  35  containing pins  33  is placed against an optional separator film  38 , which is placed on each over-wrap ply  31  over base  17 . Carrier  35  is located laterally on over-wrap ply  31  to position pins  33  over the desired insertion locations. The vibrating head is placed against an upper surface  39  of carrier  35  and driven downward while vibrating. Carrier  35  is made from a foam and collapses between the head and over-wrap ply  31  as the head moves downward. Because pins  33  are rigid, the vibrating head forces pins  33  downward once the upper ends of pins  33  come in contact with the lower surface oft he head. Pins  33  pass out of carrier  35 , through separator film  38 , through over-wrap ply  31 , through preform  11 , and through adhesive film  27 . Alternatively, over-wrap plies  31  may be laid on base  17  and leg  21  after pins  33  are inserted. The lower ends of pins  33  enter skin  15  at upper surface  29  and travel through a portion of the thickness of skin  15 . Pins  33  are pushed into skin  15 , preferably until the vibrating head is near the upper surface of over-wrap ply  31 . Additional pins  33  are driven through surface  25  in the clevis of preform  11 . The head is withdrawn, and carrier  35  is removed, leaving a small portion of the upper ends of pins  33  remaining above over-wrap ply  31  and surface  25 . If pins  33  are made from graphite, the exposed ends of pins  33  maybe removed to leave the upper ends of pins  33  flush with over-wrap ply  31  and surface  25 , as shown in FIG. 2. If pins  33  are titanium, the vibrating head is used to drive them downward until pins  33  are flush with over-wrap ply  31  or surface  25 . Pins  33  made from graphite may also be driven inward until flush.  
         [0024]    Once pins  33  are driven into the assembly, a sheet of adhesive film  41 , preferably AF191, is placed against inner surface  23  of each leg  21  for adhering frame  13  within the clevis formed by legs  21 . Semi-rigid over-presses  43  are used to distribute force applied to over-presses  43  across the width and height of preform  11 , surfaces  45 ,  47  being in contact with over-wrap plies  31 . The distribution of force causes more consistent bonding at the interface of skin  15  and preform  11  and a more consistent bonding within the clevis of legs  21  to frame  13 . Also, rigid tool  49  is placed under skin  15  to form the desired shape of skin  15 . The assembly and tooling are placed within a vacuum bag (not shown) from which the air is drawn, allowing outside air pressure to apply force to over-presses  43  and rigid tool  49 . This urges base  17  toward skin  15  and forces legs  21  toward frame  13 , causing preform  11  to conform to the desired shape. The assembly is preferably placed into an autoclave to cure preform  11  and skin  15  and to cure adhesive film  27 ,  41 . Pins  33  are secured within the cured resin matrix of preform  11  and skin  15 .  
         [0025]    [0025]FIG. 2 shows a completed, cured assembly after tooling  43 ,  49  (FIG. 1) has been removed. Frame  13  is adhered between legs  21 , legs  21  having been cured in a vertical orientation to frame  13 . Base  17  is adhered to skin  15 , and Z-pins  33  extend through base  17  into skin  15 .  
         [0026]    An alternative method of assembly is depicted in FIGS. 3 and 4. As described above, preform  11  is affixed to surface  29  of skin  15  and pins  33  are inserted. A sizing tool  51  and a nonstick peel ply  53  are then inserted within the clevis of preform  11 , and preform  11  and skin  15  are cured with the tool in place of frame  13  (FIGS. 1 and 2). Tool  51  has a width that is larger than frame  13 , and over-presses  43  ensure that legs  21  conform to the shape and size of tool  51  during curing. Peel ply  53  allows for minimum force to be used when removing tool  51  after preform  11  has been cured. As shown FIG. 4, tool  51  is removed, leaving an oversized slot between inner surfaces  23  of legs  21 . Though not shown in the figures, a paste or film adhesive is introduced into the clevis, and frame  13  is then inserted into the clevis and adhered to preform  11  by the adhesive.  
         [0027]    Woven preforms may also have other 3-D shapes, for example, a T-shaped preform  55  having only one leg  57 . As shown in FIGS. 5 and 6, preform  55  maybe used in connecting parts  59 ,  61  in a substructure. FIG. 5 shows a T-shaped preform being used as a connector, base  63  of preform  55  being placed against a first, uncured, composite component  59  and Z-pinned as described above. Leg  57  of preform  55  is cured at an angle relative to base  63 , though leg  57  will typically be perpendicular to base  63 . After curing, a second component  61  is affixed to leg  57  using, for example, adhesives or fasteners  65 .  
         [0028]    An alternative use of T-shaped preform  55  as a connector is shown in FIG. 6. Overwrap plies  67  are laid against the outside surfaces of preform  55 , over-wrap plies  67  extending beyond the height of leg  57 . Component  59 , preform  55 , and over-wrap plies  67  are cured together. Using various tooling (not shown), the upper portion of over-wrap plies  67  can be cured in desired configurations, for example, as a straight web or as the top portion of an I-beam, as shown. Overwrap plies are cured to form connecting surfaces for receiving second component  61 , which can be mounted to the flanged section of over-wrap plies  67  to complete the substructure.  
         [0029]    [0029]FIG. 7 illustrates the use of T-shaped preforms  55  used as stiffeners for large surfaces. Preforms  55  can be Z-pinned to a skin  69  with optional over-wrap plies (not shown), as described above, then cured with leg  57  being generally perpendicular to base  63 . The rigid leg provides for a higher moment of inertia, resisting bending of skin  69 .  
         [0030]    The present invention provides for several advantages. The Z-pins provide for a stronger joining of the preform and skin. Inserting Z-pins through the preform eliminates the problem of having to “bed down” the preform on previously installed Z-pins, a problem requiring shorter exposed portions of the Z-pins. Also, the problems of Z-pin breakage when removing peel plies and limitations on a real density of Z-pins are eliminated.  
         [0031]    While the invention has been shown in only some of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit thereof.