Abstract:
A method uses a three-dimensional, adhesive-infused, woven preform to assemble two components, each component having z-pins extending from bonding surfaces. The components and preform are assembled with surfaces of the preform contacting surfaces of the components, the z-pins penetrating into the preform. The adhesive in the preform is then cured, adhering the preform to the components and retaining the z-pins within the preform. The adhesive may be cured at room temperature or through heat applied to the outer component. Alternatively, an electron-beam may be used to cure the adhesive. Use of z-pins in the bond area and an adhesive, instead of a resin, creates a stronger joint, especially with fiber-reinforcement of the adhesive. The thickness of the compressible, three-dimensional weave provides for a larger dimensional tolerance at each bond line.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention generally relates to assembly of components using woven preforms and particularly relates to assembly of components in closeout joints using adhesive-infused preforms.  
           [0003]    2. Description of the Prior Art  
           [0004]    Closeout panels can present problems for manufacturers, in that panels may attach to a substructure without access to the backside of the panel. In the past, these panels have been bolted to the substructure or attached using blind fasteners, such as pull rivets. These methods require expensive and time-consuming drilling and fastening operations and may weaken the structure. More recently, these panels have been co-bonded or secondarily bonded using resin or a thin layer of adhesive.  
           [0005]    Typically, laminating resins are used as the matrix material in woven textiles, this also being true for woven preforms used to connect components made of composites or other materials. An example of a commonly used laminating resin is 977-3, available from Cytec Industries, Inc., of West Paterson, N.J. The laminating resin is infused into a textile product and is cured to form a polymer matrix in the finished composite component. When assembling a typical joint using a preform, the preform may be co-cured along with uncured composite components or the components may be cured prior to assembly using an uncured preform. Because of the inferior bonding characteristics of laminating resins, a thin layer of adhesive is often placed between the preform and the components. Generally, an adhesive film is used, which is expensive and adds to fabrication time.  
           [0006]    To achieve proper bonding when using a thin layer of adhesive, such as an adhesive film, between pre-cured components, special attention must be paid to the interface at the adhesive layer. This bond line is critical, and, where two surfaces are brought together, the distance between the surfaces must be within a critical tolerance to ensure a proper bonding layer. The thickness of the adhesives is usually about 0.015″ thick with a bond layer tolerance of ±0.005″. Methods for ensuring proper bonding may include tools, such as molds or vacuum bags, but particular applications may prevent the use of tools due to the inaccessibility of one or both sides of the joint. An example of this type of application is a closeout panel, such as the skin of a wing being bonded to an internal spar.  
           [0007]    Z-pins have been used in joints connecting two composite, laminate components in the prior art. For example, U.S. Pat. Nos. 5,863,635, 5,968,639, and 5,980,665 to Childress discloses inserting z-pins into a first composite component to form stubble at a bonding face, then curing the first component. An uncured second component is then bonded to the first component with the stubble extending into and among the fibers of the second component and through the bond line.  
           [0008]    As shown in FIG. 1 and in the &#39;635, &#39;639, and &#39;665 patents, an additional prior-art method includes inserting a padup strip  11  between two cured components  13 ,  15 . Components  13 ,  15  are generally formed of plies of woven or unidirectional fibers and a resin matrix and are cured with a Z-pin stubble extending from surfaces  17 ,  19 . Padup strip  11 , which is typically formed of the same materials as components  13 ,  15  or formed of a pure adhesive material without fiber reinforcement, is uncured during assembly. Components are assembled with padup strip  11  between surfaces  17 ,  19 , the z-pin stubble fields extending into padup strip  11 . The resin in padup strip  11  is then cured to co-bond the components  13 ,  15  to padup strip  11 .  
           [0009]    An alternative method of assembly using z-pins is disclosed in U.S. Pat. Nos. 5,876,540, 5,876,832, 5,935,698 to Pannell and shown in FIG. 2. A pre-cured strip  21  is formed of a plurality of plies of fibers and a resin matrix, a plurality of z-pins  23  extending from opposite sides of strip  21 . Components  25 ,  27  are also formed of composites and may be cured or partially cured. To assemble partially cured components  25 ,  27 , strip  21  is positioned between components  25 ,  27 , then z-pins  23  are inserted into adjacent surfaces  29 ,  31 . The resin in components  25 ,  27  is cured to co-bond surfaces  29 ,  31  and to retain z-pins  23  within components  25 ,  27 . Alternatively, if components  25 ,  27  are pre-cured, padup strips  33  are used between strip  21  and surfaces  29 ,  31 . Padup strips  33 , like padup strip  11  in FIG. 1, are typically formed of the same materials as components  25 ,  27  or formed of a pure adhesive material without fiber reinforcement.  
           [0010]    Several currently pending applications are related to the present invention, these applications disclosing inventions using preforms having fibers in a three-dimensional weave to create structural joints. U.S. patent application Ser. Nos. 09/898,633 and 10/028,613, filed Jul. 2, 2001, and Dec. 21, 2001, respectively, disclose adhesive-infused preforms and methods of joint assembly, the joints lacking z-pin reinforcement. U.S. patent application Ser. No. 09/946,627, filed Aug. 31, 2001, and U.S. patent application Ser. No. 09/973,208, filed Oct. 9, 2001, disclose z-pin reinforced joints and methods of assembly using resin-infused preforms formed from a three-dimensional weave pattern.  
           [0011]    A need exists for an improved method that reduces the steps in assembly and provides for a strong joint when joining components using a woven preform. A further need exists for a method of joining components in a structural joint that provides for a larger dimensional tolerance between components when using an adhesive at the bond line.  
         SUMMARY OF THE INVENTION  
         [0012]    A method uses a three-dimensional, adhesive-infused, woven preform to assemble two components, each component having z-pins extending from bonding surfaces. The components and preform are assembled with surfaces of the preform contacting surfaces of the components, the z-pins penetrating into the preform. The adhesive in the preform is then cured, adhering the preform to the components and retaining the z-pins within the preform. The adhesive may be cured at room temperature or through heat applied to the outer component. Alternatively, an electron-beam may be used to cure the adhesive. Use of z-pins in the bond area and an adhesive, instead of a resin, creates a stronger joint, especially with fiber-reinforcement of the adhesive. The thickness of the compressible, three-dimensional weave provides for a larger dimensional tolerance at each bond line. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    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.  
         [0014]    [0014]FIG. 1 is an exploded, perspective view of a prior-art assembly using a padup strip and components having z-pin stubble.  
         [0015]    [0015]FIG. 2 is a front view of a prior-art assembly formed using a pre-cured strip to connect components, the pre-cured strip having z-pins extending from opposite sides.  
         [0016]    [0016]FIG. 3 is an exploded, front view of an assembly of the present invention.  
         [0017]    [0017]FIG. 4 is an exploded, front view of a second embodiment of an assembly of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    [0018]FIGS. 3 and 4 illustrate preferred embodiments of assemblies using an adhesive-infused, three-dimensional (3-D), woven textile preform used for assembling parts into structural joints. The preferred adhesive is FM® 300, also available from Cytec Industries, Inc., but other adhesives will work, providing the adhesive can be infused in a way that properly “wets out,” or saturates, the fiber bundles in the preform.  
         [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,” as used herein, is meant as a resin system that has a lower modulus of elasticity and/or a higher strain-to-failure than the resin forming the matrix of the 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 up to 15 pounds per linear inch, whereas the peel strength of adhesives are 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 FM® 300 adhesive is 23-29 pounds per linear inch at room temperature, but the peel strength of 977-3 laminating resin, which is used to laminate the parts, is up to 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 12000 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.  
         [0021]    To provide higher strain-to-failure characteristics, epoxy-based adhesives usually have rubber modifiers added to them. The higher strain capability improves load distribution through the preform, reducing the crack formation at the outer edges of the bond lines and in the weave that can lead to catastrophic failure of the joint at loads less than those which would cause failure of the parts. Also, adhesives usually have a higher viscosity than laminating resins. Laminating resins easily saturate woven components, whereas adhesives require an infusion process to wet-out the fiber bundles.  
         [0022]    The preforms can be infused with adhesive in many ways. For example, one method is by hot-melt infusion, in which adhesive films are laid adjacent to the preform, and heat is applied to cause adhesive to wick into preform. Another method involves drawing preforms through a tank containing adhesive dissolved in a solvent, usually acetone or toluene. The preforms are immersed in the solution, then removed from the tank. The solvent is allowed to evaporate, or “flash off,” leaving the adhesive in the preform. To completely wet-out the preforms, this process may be repeated several times. The preform is saturated with the adhesive and is laid up while uncured. The parts, or components, to be joined may be formed from cured or partially cured composites or may be formed from other materials, e.g., plastics, metals, etc. Additional methods of infusion include resin-transfer molding (RTM) and vacuum-assist resin-transfer molding (VARTM).  
         [0023]    Referring to the figures, FIG. 3 shows an exploded assembly for connecting components  33 ,  35 , such as planar closeout panel  33 , which may be, for example, an outer skin of an aircraft wing, and planar spar  35 . Panel  33  is a cured, fiber-reinforced composite having a plurality of z-pins  37  inserted through bonding surface  39  prior to curing of panel  33 . Z-pins  37  are inserted using any appropriate technique and are arranged in a selected pattern, z-pins  37  preferably being normal to surface  39 . The number of z-pins  37  is selected to provide a desired areal density of z-pins  37  relative to the area of surface  39 . Curing of panel  33  affixes z-pins  37  in the matrix of panel  33 . Spar  35  may be formed of any rigid material, such as composites or metal.  
         [0024]    A cured, pi-shaped, woven preform  41  is bonded to spar  35 , preform being woven from fibers using a three-dimensional (3-D) weave pattern. Preform  41  preferably has a matrix formed form laminating resin. Preform  41  has a base  43  having a continuous bonding surface  45 , and a pair of spaced-apart legs  47  extend vertically from base  43 . Each leg  47  is at a position that is offset from, but near to, the center of base  43 . In this embodiment, legs  43  are parallel to each other and generally perpendicular to base  43 . In the installed position, the inner surfaces of legs  47  face each other to form a slot  49  for receiving spar  35 . A plurality of z-pins  51  are inserted into base  43  through bonding surface  45  prior to curing of preform  41 . The pattern and areal density of z-pins  51  are preferably approximately the same as those for z-pins  37  in panel  33 . Preform  41  may be secondarily bonded to spar  35  after curing of preform  41  or may be co-bonded to spar  35 . Alternatively, if spar  35  is formed from composites, spar  35  and preform  41  may be co-cured.  
         [0025]    An adhesive-infused, woven preform  53  has a rectangular cross-section and opposed bonding surfaces  55 ,  57  and is woven using a 3-D weave pattern to have a selected thickness t. Preform  53  is used to connect panel  33  to preform  41  by bonding surface  39  of panel  33  to surface  55  and surface  45  of preform  41  to surface  57 . Preform  53  preferably has at least two warp-fiber layers and thickness t of about 0.050″ or may have additional layers, providing an increased thickness t.  
         [0026]    As surfaces  39 ,  55  and  45 ,  57  are moved toward each other, Z-pins  37 ,  51  penetrate preform  53  until surfaces  39 ,  55  and  45 ,  57  contact each other, the length of z-pins  37 ,  51  being less than thickness t of preform  53 . Bond layers form at the interfaces of surfaces  39 ,  55  and  45 ,  57 , connecting panel  33  to preform  41 , which is bonded to spar  35 . Because the adhesive is infused in preform  53  having selected thickness t, the bond layer dimensional tolerance is increased, preform  53  allowing for a larger variation in distance between surfaces  39 ,  45 . Without preform  53 , the distance between surfaces  39 ,  45  must be within a critical tolerance to ensure a proper bonding layer. Additionally, use of preform  53  allows for some misalignment of panel  33  in relation to preform  41  when bonding and can accommodate dimensional variations in surfaces  39 ,  45 .  
         [0027]    Mechanical pressure is all that is required to push panel  33  towards preform  41  during curing, compressing preform  53  and ensuring continuous bondlines between surfaces  39 ,  55  and  45 ,  57 . If the adhesive is a heat-cured adhesive, heat is applied to the outer surface of panel  33  to cause the rapid curing of the adhesive. Alternatively, adhesives used in preform  53  may be cured by other types of cure mechanisms, for example, electron-beam curing.  
         [0028]    During assembly, panel  33  and preform  41  are fabricated to desired dimensions and shapes, then z-pins  37 ,  51  are inserted prior to curing of panel  33  and preform  41 . Preform  41  may be bonded to spar  35  during or after curing of preform  41 . Preform  53  is fabricated to have a selected thickness t, then infused with an adhesive. Preform  53  is positioned between panel  33  and base  43  of preform  41 , then panel  33  is moved toward preform  41 , with z-pins  37 ,  51  penetrating preform  53 . Panel  33  is moved toward preform  41  until surface  39  contacts surface  55  of preform  53  and surface  45  contacts surface  57 , then mechanical pressure is applied to the outer surface of panel  33  for compressing preform  53  during curing of the adhesive.  
         [0029]    When fabricating preform  53 , thickness t may be increased to ¼″ or beyond and may involve the use of thicker fibers. However, the weight of the extra adhesive used in a thicker preform would likely mean that thicker preforms would be reserved for applications where minimization of weight is not a primary concern, for example, in construction of boats.  
         [0030]    [0030]FIG. 4 illustrates a second assembly using preform  53  to connect panel  33  to a cured woven preform  59 . T-shaped preform  59  has a base  61  and a generally perpendicular leg  63  extending from base  61 . Preform  59  is connected to spar  35  with fastener  65 , which may be of any appropriate type, or preform  59  may be bonded to spar  35 . Prior to curing of preform  59 , z-pins  67  are inserted into base  61  through bonding surface  69  in a desired pattern having a selected areal density, the pattern and density preferably being approximately the same as those for z-pins  37  in panel  33 . As in the previously described assembly, the multi-layered, rectangular cross-section of preform  53  allows for a larger dimensional tolerance between panel  33  and preform  59 .  
         [0031]    During assembly, panel  33  and preform  59  are fabricated to desired dimensions and shapes, then z-pins  37 ,  67  are inserted prior to curing of panel  33  and preform  59 . Preform  59  is fastened to spar  35  using fastener  65 . Preform  53  is fabricated to have a selected thickness t, then infused with an adhesive. Preform  53  is positioned between panel  33  and base  61 , then panel  33  is moved toward preform  59 , with z-pins  37 ,  67  penetrating preform  53 . Panel  33  is moved toward preform  59  until surface  39  contacts surface  55  of preform  53  and surface  69  contacts surface  57 , then mechanical pressure is applied to the outer surface of panel  33  for compressing preform  53 .  
         [0032]    The advantages of the present invention include the increased strength from the addition of z-pins in the bond area and using an adhesive, rather than a resin, within a 3-D woven preform used to connect components. Another advantage is the reduction of steps needed to complete the assembly. By infusing the adhesive into preforms, pieces can be joined without the need for a separate adhesive film being inserted between a resin-infused connector and the pieces to be joined. Also, the thickness of the preform allows for a larger dimensional tolerance at the bond line, while providing the strength of fiber-reinforced adhesive.  
         [0033]    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.