Abstract:
The present invention is directed to a method of bonding at least two surfaces together. The methods step of the present invention include applying a strip of adhesive to a first surface along a predefined outer boundary of a bond area and thereby defining a remaining open area there within. A second surface, or gusset plate, is affixed onto the adhesive before the adhesive cures. The strip of adhesive is allowed to cure and then a second amount of adhesive is applied to cover the remaining open area and substantially fill a void between said first and second surfaces about said bond area. A stencil may be used to precisely apply the strip of adhesive. When the strip cures, it acts as a dam to prevent overflow of the subsequent application of adhesive to undesired areas. The method results in a precise bond area free of undesired shapes and of a preferred profile which eliminate the drawbacks of the prior art bonds.

Description:
ORIGIN OF INVENTION 
     The invention described herein was made by an employee of the United States Government, and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     RELATED APPLICATIONS 
     This application is based upon prior filed provisional patent application Ser. No. 60/979,109 filed Oct. 11, 2007 for “ADVANCED ADHESIVE BOND SHAPE TAILORING FOR LARGE COMPOSITE PRIMARY STRUCTURE SUBJECTED TO CRYOGENIC AND AMBIENT LOADING ENVIRONMENTS” the entire contents of which are herein incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to structurally bonding structural elements together using adhesive placed in a specific shape/profile, and more particularly to advanced adhesive bond shape tailoring suitable to large composite primary structures subjected to cryogenic and ambient loading environments. 
     2. Discussion of the Related Art 
     Bonding of primary structures to withstand cryogenic temperatures and high stress launch environment is a fundamental requirement of deploying cryogenic primary structures. Cryogenic primary structures may include space structures made of composite materials. Failure of the bonds between structural elements can result in significant damage to the primary structures in an environment where repair is unfeasible. 
     Conventional bonds utilize a gusset plate to bond together composite tubes. Previously adhesive was placed between the gusset plate and composite tubes by simply spreading a thin layer of adhesive to the tube and then affixing the gusset plate. However, prior art techniques to spread the adhesive often resulted in bond areas having profiles that often caused delamination or separation of the composite tubes as well as undesired stress placed on the surface of the tubes due to either the profile of the perimeter of the bond area, improper location of the bond area proximate the cut end of the tubes, or non-uniform and improper thickness of the adhesive. The adverse effects, of the aforementioned prior art techniques of applying the adhesives, are exacerbated by the extreme cryogenic temperatures reaching 27 degrees Kelvin. Failure to properly control the application of the adhesive and resultant bond area will result in a significant reduction in the strength of the bond, strip away the upper layers of composite materials or otherwise induce delamination/separation and induce undue stress on the tubes when the structure is exposed to extremely low temperatures. 
     Adhesive thickness should be minimal as thick layers of adhesive may strip away the upper layers of composite mater when exposed to extremely low temperatures due to the mismatch in coefficient of thermal expansion of the materials. Accurately and precisely tailoring the shape of the adhesive applied onto the primary structures such that these needs are met significantly increases bond strength and reduces susceptibility to cryogenic temperatures and high stress environments. 
     Additionally, stress applied onto the bond may be significantly reduced by accurately shaping the adhesive with a large radius, particularly in areas of high stress. Finally, accurately and precisely tailoring the shape of the applied adhesive may reduce or prevent the need for clean out of areas in the primary structure that have limited access during the assembly of the primary structure. 
     For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for apparatus and methods to bond primary structures such that the bond survives cryogenic temperatures and demanding, high stress launch vehicle environment. There is also a need for improved apparatus and methods to accurately and precisely tailor the shape of adhesive applied onto primary structures to effectively bond these primary structures using a gusset plate. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method of bonding at least two surfaces together. The methods step of the present invention include applying a strip of adhesive to a first surface along a predefined outer boundary of a bond area and thereby defining a remaining open area there within. A second surface or gusset plate is affixed onto the adhesive before the adhesive cures. The strip of adhesive is allowed to cure and then a second amount of adhesive is applied to cover the remaining open area and substantially fill a void between said first and second surfaces about the bond area. A stencil may be used to precisely apply the strip of adhesive. When the strip cures, it acts as a dam to prevent overflow of the subsequent application of adhesive to undesired areas and otherwise serve to precisely control the contour/shape of the overall bond area. The method results in a precise bond area free of undesired shapes and of a preferred profile which eliminates the drawbacks of the prior art bonds. 
     Apparatus, systems, and methods of varying scope are described herein. In addition to the aspects and advantages described in this summary, further aspects and advantages will become apparent by reference to the drawings and by reading the detailed description that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is cross section block diagram of an overview of a system to accurately and precisely tailor the shape of adhesive applied onto primary structures, to effectively bond the primary structures together using a gusset plate. 
         FIG. 2  is a cross section block diagram of an apparatus according to an aspect to accurately and precisely tailor the shape of adhesive applied onto primary structures. 
         FIG. 3  is a cross section block diagram of an apparatus according to an aspect to effectively bond the primary structures together using a gusset plate. 
         FIG. 4  is a flowchart of a method to accurately and precisely bond two or more surfaces by applying adhesive tailored to a predefined shape onto the surfaces according to an aspect. 
         FIG. 5  is a flowchart of a method to accurately and precisely bond two or more surfaces by applying adhesive tailored to a predefined shape with a controlled final bond line thickness onto the surfaces according to an aspect. 
         FIG. 6  is a flowchart of a method to accurately and precisely bond two or more composite tubes by applying adhesive tailored to a predefined shape with a controlled final bond line thickness onto the composite tubes, such that the bond survives cryogenic temperatures and demanding, high stress launch vehicle environment according to an aspect. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific aspects which may be practiced. These aspects are described in sufficient detail to enable those skilled in the art to practice the aspects, and it is to be understood that other aspects may be utilized and that logical, mechanical, electrical and other changes may be made without departing from the scope of the aspects. The following detailed description is, therefore, not to be taken in a limiting sense. 
     The detailed description is divided into four sections. In the first section, a system level overview is described. In the second section, apparatus of aspects are described. In the third section, aspects of methods are described. Finally, in the fourth section, a conclusion of the detailed description is provided. 
       FIG. 1  is cross section block diagram of an overview of a system to accurately and precisely tailor the shape of adhesive applied onto primary structures, to effectively bond the primary structures together using a gusset plate. System  100  solves the need in the art to accurately and precisely tailor the shape of adhesive applied onto the structural elements of a primary structure. 
     System  100  includes a flat sheet  102  impervious to adhesive, having a precisely uniform thickness, and a geometric cutout area  104  of a precisely defined shape outline. 
     Component  102  solves the need in the art to accurately and precisely tailor the shape of adhesive applied onto primary structures to effectively bond these primary structures by preventing excess adhesive from being applied onto the structural elements of the primary structure. 
     Component  104  solves the need in the art to accurately and precisely tailor the shape of adhesive applied onto primary structures to effectively bond these primary structures by precisely tailoring the shape of adhesive applied onto the structural elements of the primary structure. 
     The system level overview of the operation of an aspect is described in this section of the detailed description. 
     The stencil or stencil/template sheet  102  with a geometric cutout  104  is used to lay a bead of adhesive in the exact shape and location needed to bond structural elements of a primary structure to withstand cryogenic temperatures and high stress environments. The stencil/stencil/template is placed on the surface of the structural elements of the primary structure, adhesive is applied in the geometric cutout area, and excess adhesive is scraped off. The stencil/stencil/template sheet is removed leaving an outline of the desired adhesive shape profile. Once all adhesive outlines, or dams, are placed on composite tubes to be joined together, a gusset plate is affixed to bond the structural elements of the primary structure. 
     In some aspects, shims are placed in the center of the bond area to control final bond line thickness and then a gusset plate is placed onto the adhesive outline on the structural elements of the primary structure. Once the adhesive cures, the shims are removed leaving an adhesive dam. 
     In other aspects, the gusset plate has a hole pattern on its face. Once the adhesive cures, the shims are removed leaving an adhesive dam. Additional adhesive is then injected through the hole pattern in the gusset plate from the interior edge toward the outer edge, thus completely filling the area without voids. 
     In yet other aspects, the stencil/stencil/template has alignment features, cut out width and thickness details that precisely place an exact adhesive profile. The details may be varied depending on the desired bond line thickness and bond area placement. 
     While the system  100  is not limited to any particular flat sheet impervious to adhesive and any particular geometric cutout area, for sake of clarity a simplified flat sheet impervious to adhesive and a simplified geometric cutout area are described. 
     In the previous section, a system level overview of the operation of an embodiment was described. In this section, the particular apparatus of such an embodiment are described by reference to a series of diagrams. 
       FIG. 2  is a cross section block diagram of apparatus  200  according to an aspect to accurately and precisely tailor the shape of adhesive applied onto primary structures. Apparatus  200  solves the need in the art to accurately and precisely tailor the shape of adhesive applied onto the structural elements of a primary structure. 
     Apparatus  200  includes a flat sheet  202  impervious to adhesive, having a precisely uniform thickness, and a geometric cutout area  204  of a precisely defined shape outline. In some aspects, the geometric cutout area  204  includes the outline of a circular pattern  206 . In other aspects, the radius of the circular pattern is proportional to the expected stress on the surfaces of the structural elements to be bonded. In yet other aspects, the sheet metal cut out stencil/stencil/template has a uniform thickness of 0.04 inches to minimize the adhesive thickness applied onto the surfaces s of the structural elements to be bonded. 
       FIG. 3  is a cross section block diagram of apparatus  300  according to an aspect to effectively bond the primary structures together using a gusset plate. 
     Apparatus  300  includes a first diagonal composite tube element  302 , a vertical composite tube element  304 , a second diagonal composite tube element  306 , a gusset plate  308 , a layer of adhesive  310  laid out in a precisely defined shape profile between the composite tube element and the gusset plate, and a hole pattern  312  on the face of the gusset plate. In some aspects, two or more surfaces are bonded with a predefined shape profile which includes applying adhesive in the shape of an outline of a predefined shape profile on two or more surfaces to be bonded, affixing a gusset plate onto the adhesive on the two or more surfaces, and applying additional adhesive to completely fill the interior of the adhesive shape profile. In other aspects, affixing the gusset plate includes affixing a gusset plate with a hole pattern on its face onto the two or more surfaces. In yet other aspects, applying additional adhesive to completely fill the interior of the adhesive shape profile includes allowing the adhesive to cure, thereby forming an adhesive dam, and injecting additional adhesive through the hole pattern on the face of the gusset plate to completely fill the interior of the adhesive shape profile. 
     In the previous section, apparatus of the operation of an embodiment was described. In this section, the particular methods performed by proper usage of such an embodiment are described by reference to a series of flowcharts. 
       FIG. 4  is a flowchart of a method  400  to accurately and precisely bond two or more surfaces by applying adhesive tailored to a predefined shape onto the surfaces according to an aspect. Method  400  solves the need in the art to bond primary structures such that the bond survives cryogenic temperatures and demanding, high stress launch vehicle environment. 
     Method  400  includes two or more un-bonded surfaces  402 , applying adhesive in the outline of a predefined shape profile onto two or more un-bonded surfaces  404 , affixing a gusset plate  406 , applying additional adhesive to fill the interior of the shape profile  408 , and yields two or more bonded surfaces  410 . 
       FIG. 5  is a flowchart of a method  500  to accurately and precisely bond two or more surfaces by applying adhesive tailored to a predefined shape with a controlled final bond line thickness onto the surfaces according to an aspect. Method  500  solves the need in the art to bond primary structures such that the bond survives cryogenic temperatures and demanding, high stress launch vehicle environment. 
     Method  500  includes two or more un-bonded surfaces  502 , applying adhesive in the outline of a predefined shape profile onto two or more un-bonded surfaces  504 , placing shims at the center of the bond area  506 , affixing a gusset plate  508 , removing the shims from the center of the bond area  510 , applying additional adhesive to fill the interior of the shape profile  512 , and yields two or more bonded surfaces  514 . 
     In some aspects, applying the adhesive in the shape of an outline of a predefined shape profile includes placing a stencil/stencil/template cutout sheet of the outline of the predefined adhesive shape profile on the two or more surfaces, applying adhesive within the boundary of the stencil/stencil/template cutout sheet, and removing the stencil/stencil/template cutout sheet. 
     In other aspects, affixing a gusset plate onto the adhesive on the surface of two or more surfaces further includes affixing a gusset plate with a hole pattern on its face onto the adhesive on the two or more surfaces. 
     In yet other aspects, applying additional adhesive to completely fill the interior of the adhesive shape profile includes allowing the adhesive to cure, thereby forming an adhesive dam, and injecting additional adhesive through the hole pattern on the face of the gusset plate to completely fill the interior of the adhesive shape profile. 
     In yet other aspects, placing a stencil/template cutout sheet of the outline of a predefined adhesive shape profile includes placing a sheet metal cut out stencil/template of the outline of a circular pattern on two or more surfaces. In yet other aspects, placing a stencil/template cutout sheet of the outline of a predefined adhesive shape profile includes placing a sheet metal cut out stencil/template of the outline of a circular pattern on two or more surfaces, in which the radius of the circular pattern is proportional to the expected stress on the two or more surfaces. In yet other aspects, the radius of the circular pattern is the full radius of the bond width. In yet other aspects, placing a stencil/template cutout sheet of the outline of a predefined adhesive shape profile includes placing a sheet metal cut out stencil/template of the outline of a circular pattern on two or more surfaces, in which the sheet metal cut out stencil/template has a uniform thickness of 0.04 inches. 
     In other aspects, applying adhesive in the shape of an outline of a predefined shape profile further includes placing a stencil/template sheet cut out such that the adhesive is kept from bonding all the way to the cut edge of the two or more surfaces being bonded. In yet other aspects, the two or more surfaces being bonded include two or more composite tubes. 
       FIG. 6  is a flowchart of a method  600  to accurately and precisely bond two or more composite by applying adhesive tailored to a predefined shape with a controlled final bond line thickness onto the composite tubes, such that the bond survives cryogenic temperatures and demanding, high stress launch vehicle environment according to an aspect. Method  600  solves the need in the art to bond primary structures such that the bond survives cryogenic temperatures and demanding, high stress launch vehicle environment. 
     Method  600  includes two or more un-bonded composite tubes  602 , placing a stencil/template cut out sheet of the outline of a predefined shape profile onto the two or more composite tubes  604 , applying adhesive within the boundary of the stencil/template cut out sheet  606 , removing the stencil/template cut out sheet  608 , affixing a gusset plate onto the composite tubes  610 , and applying addition adhesive to fill the interior of the shape profile  612 , and yields two or more bonded composite tubes  614 . 
     In some aspects, a method to bond two or more composite tubes together to withstand cryogenic temperatures and space shuttle launch condition includes applying a thin uniform layer of adhesive in a predefined shape profile on the surface of the two or more composite tubes, and affixing a gusset plate onto the adhesive on the two or more composite tubes. In other aspects, applying a thin uniform layer of adhesive on the surface of the two or more composite tubes includes placing a stencil/template sheet of the outline of the predefined adhesive shape profile on the two or more composite tubes, applying adhesive within the boundary of the stencil/template sheet, and removing the stencil/template sheet. 
     In yet other aspects, affixing a gusset plate onto the adhesive on the two or more composite tube includes placing shims in the center of the bond area to control the final bond line thickness, affixing a gusset plate with a hole pattern on its face onto the adhesive on the two or more composite tubes, allowing the adhesive to cure thereby forming an adhesive dam, removing the shims, and injecting additional adhesive through the hole pattern on the face of the gusset plate to completely fill the interior of the adhesive shape profile. 
     In some aspects, applying adhesive within the boundary of the stencil/template cut out sheet includes placing the stencil/template cut out sheet such that the adhesive is kept from bonding all the way to the cut edge of each of the two or more composite tubes being bonded, applying adhesive within the boundary of the stencil/template sheet cut out, and scraping off any excess adhesive. 
     In other aspects, applying adhesive within the boundary of the stencil/template cut out sheet includes shaping the adhesive with a generous radius in areas on the two or more composite tubes of high expected stress. In yet other aspects, the radius of the circular pattern of the adhesive profile shape is the full radius of the bond width. 
     In yet other aspects, applying adhesive within the boundary of the stencil/template cut out sheet includes placing the stencil/template cut out sheet such that the need for adhesive clean up during assembly of areas that have no access is minimized, and applying adhesive within the boundary of the stencil/template sheet cut out, and scraping off any excess adhesive. 
     The appendix attached hereto includes structural details and test analysis data of a structural member and associated bond joints according to one embodiment of the present invention the entire contents of which are hereby incorporated herein by reference. A summary of the detailed discussion regarding the structural and test data will now be given with reference to  FIG. 3 . 
     The structure members  302 ,  404 , &amp;  306  are square tube carbon fiber laminates having a wall thickness of 2.42 mm. The gusset plate  308  is QI laminate having a thickness of 2.28 mm. Tubes  302  &amp;  306  are angled relatively to the tube  304 . Preferably no acute angle exists between structural members bridged by a gusset plate. Members  302  &amp;  306  are positioned at a 45 degree angle relative to tube  304 . Members  302  &amp;  306  are positioned at a 45 degree angle relative to tube  304 . Preferably the smallest angle between adjacent joined tube members is 30 degrees for using the full radius bond profile of 25 mm. The bond areas shown as  310  preferably has a smooth continuous outer periphery with no small curves less than a radius of curvature of 12.5 millimeters proximate the cute end of tube for angles less than 30 degrees. A workable adhesive is EA9309.3NA and preferably has a thickness of the order of 0.010-0.04 inches. The bond areas is also preferably sufficiently offset from the ends or sides of the tube by at least 0.25 inches which has been sufficiently demonstrated to avoid delaminating the carbon fiber laminate tubes adjacent the cut end of the tube. 
     The structural assembly is assembled by laying out pr positioning each structural member  302 ,  204 , &amp;  306  in a desired position relative to one another. A stencil  102  is positioned on top of an exposed surface of each of the structural tubes. A first amount of adhesive is applied over the opening of the stencil and the adhesive spread to sufficiently force the adhesive through the opening of the stencil and contact the surface of the tubes. Excess adhesive is then removed in any commonly known methods in the art. The stencil is fabricated with a predetermined desired shape to define the outer boundary of the bond area. However, the shape of the stencil opening is preferably limited to only a strip to define the outermost boundaries of the bond area. The pattern of the strip and the placement of the stencil is such to ensure that the bond area remains clear of the edges of the tube, maximizes the smallest radius of curvature and avoids small changes in dimensions. Preferably the strip transitions from a straight line along its edges to the a maximum radius of curvature to transition from one side of the tube to the other while maintaining a ¼ inch clearance from terminal cut end of the tube. The same procedure is repeated for each tube  302 - 206 . It is noted the either the same stencil or different stencils may be used to deposit the initial strip of adhesive. A similar stencil may be used for tubes  302  &amp;  306  to accommodate the angled cut ends and a different stencil for tube  304 . The geometry of the tube ends and dimensions of the tubes will dictate the shape of the strip and bond area while striving to avoid small radii of curvature and stay ¼ inch distance from end of the tube. 
     Once the initial adhesive is applied using the stencil, the stencil is carefully removed from each tube, the gusset plate is positioned and simply placed on the strip of adhesive and pressed down slightly to compress/squeeze the adhesive to achieve a controlled spreading and placement of the perimeter bond strip, and to guarantee a continuous and uninterrupted bond strip that once cured, will completely control the flow of subsequently injected adhesive. As previously mentioned, shims may be used to maintain a desired thickness of the bond. Preferably the thickness of the stencil will be about four times the thickness of the shims to achieve a controlled spreading of the adhesive dam shape, and to guarantee a continuous and uninterrupted adhesive strip with final thickness about equal to the thickness of the shims. The assembly is simply left alone until the strips of adhesive have cured establishing an initial bond between each tube and the gusset plate and thereby forming an initial connection between the tubes. 
     The gusset plate has a series of holes located atop the bond area to facilitate applying a second amount of adhesive to fill the bond area. Preferably the holes are positioned in a line extending just inside the outermost boundary of the bond area adjacent the cut terminal end of the tube ( 304 ) to the opposite end of the bond are located inward the tube. Because the strip of adhesive has cured it will act as a dam as subsequent adhesive is injected into the holes. The adhesive is preferably first injected into the hole closest to the radius end of the dam. The fluid adhesive fills the void between the gusset plate  306  and the tube  305 . The dam causes the adhesive to force out any air or other gases in the void/bond area. Preferably sufficient adhesive is introduced into the first hole until the adhesive fills the void past the second hole. Adhesive is then subsequently injected into the second hole until the adhesive reaches the next hole. This process is repeated until the entire void is filled and the bond area completed. This process ensures a complete fill and forces out any air or other contaminates. Once the entire bond area has been filled for tube, the assembly is left to cure. The resultant bonded joints have been shown to have excellent strength and exceed design criteria down to 19 degrees Kelvin. 
     A bond joint similar to that of tube members  304  &amp;  306  of  FIG. 3  was performed and shown to exceed design criteria. Specifically as shown in the Appendix, adjacent tubular members were subjected to five temperature cycles between room temperature and 19 degrees Kelvin. The bond arrangement was tested for transverse shear (0.41 kips nominal) and axial tension tested (2.9 Kips nominal). The assembly was also tested to failure/axial tension failure (7.01-9.41 Kips) which exceeds a threshold requirement of 5.68 kips. Further details regarding the specific dimensions and structural composition of the assembly and associated test data results are identified in the Appendix attached hereto. 
     An apparatus and method for advanced adhesive bond shape tailoring for large composite primary structures subject to cryogenic and ambient loading environments is described. Although specific aspects are illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific aspects shown. This application is intended to cover any adaptations or variations. For example, although described in sheet metal cut out stencil terms, one of ordinary skill in the art will appreciate that implementations can be made in plastic cut out stencil or any other stencil/template that provides the required function. 
     In particular, one of skill in the art will readily appreciate that the names of the methods and apparatus are not intended to limit aspects. Furthermore, additional methods and apparatus can be added to the components, functions can be rearranged among the components, and new components to correspond to future enhancements and physical devices used in aspects can be introduced without departing from the scope of embodiments. One of skill in the art will readily recognize that aspects are applicable to future composite structures, and different composite structures that do not necessarily operate in cryogenic environments. 
     The terminology used in this application is meant to include all stencil cut out sheets, methods to tailor the adhesive profile shape by using a stencil cut out sheet and all cryogenic and high stress environments and alternate technologies which provide the same functionality as described herein.