Abstract:
A method for joining fittings to a composite sandwich shell edge includes laying up an inner facesheet and positioning a wrapped flute mandrel on top; applying a layer of adhesive on the inner facesheet and positioning spacer-supported fittings on top; applying adhesive over the co-bonded fittings and laying up an outer facesheet forming an assembly; curing the assembly under heat and pressure; removing the fittings by first collapsing the spacers and removing the flute mandrel; placing an adhesive layer on the fittings with collapsible spacers inserted therein; reinserting the fittings between the inner and outer facesheets and effecting bonding and curing; and removing the spacers from the fittings by collapsing the spacers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to in its entirety U.S. application Ser. No. 12/950,144, filed Nov. 19, 2010, and entitled COMPOSITE SANDWICH SHELL EDGE JOINT. 
     TECHNICAL FIELD 
     The disclosure generally relates to joints for composite structures and more particularly, relates to methods for making highly weight-efficient, mechanically-fastened joint fittings and joining the fittings to composite sandwich shell edges. 
     BACKGROUND 
     Sandwich construction shells, in which facesheets are joined by a shear-carrying core, are the minimum weight solution for a wide range of structures. Composite materials such as graphite/epoxy may be used to make lighter structures than the metals that were formerly applied to most weight-critical structures. Joint designs have been lagging behind the development of the acreage areas of these structures. 
     Joining approaches that have been traditionally used for metal structures may not be applied unmodified to composite structures because of the lack of ductility in composites and limited bearing capability of thin composite facesheets. Sandwich structures may present further challenges since the facesheet-to-core bond may be compromised in combined shear and peel if subjected to concentrated loads. The state of the art attempts to avoid these problems by transitioning from sandwich construction to thick solid laminates at the edge of the shell and then using a metal joint member  15  which can be fastened conventionally by bolts  16 . This is shown in  FIG. 1  for the edge joint assembly  10 . However, the cost of mechanically attaching metal frames to solid laminate may be high due to the requirement to drill a large number of close tolerance holes in the composite, in addition to a severe weight penalty. 
     An additional shortcoming associated with many conventional sandwich edge joints is that the joints may transition to an asymmetrical flange configuration which may cause tension loads across the joint to put the adjacent shell in bending. To carry these secondary bending loads in addition to the primary in-plane loads, it may be necessary to locally reinforce the shell. This may add additional weight that may not be required in a joint structure with straighter load paths. 
     Coefficient of thermal expansion mismatch between metal joint elements and adjacent composites may result in additional stresses in the structure. For structures which are exposed to wide ranges of temperatures, such as launch vehicle components, substantial weight penalties may be imposed by the need to either reinforce the structure to carry these mismatch loads or soften the structure radially to mitigate strain mismatch. 
     Therefore, a method for making a highly weight-efficient, combination bonded and mechanically-fastened joint configuration for composite sandwich shell edges is needed. 
     SUMMARY 
     The disclosure is directed to a method for making a highly weight-efficient, combination bonded and mechanically-fastened composite sandwich shell edge joint. An illustrative embodiment of the method includes providing an outboard buildup pad, providing an inboard buildup pad that is spaced-apart and adjacent to the outboard buildup pad, bonding an outboard facesheet to the outboard buildup pad, bonding an inboard facesheet to the inboard buildup pad, providing bridging plies connecting the inboard buildup pad and the outboard buildup pad and mounting at least one barrel nut installed in the buildup pads. 
     The disclosure is further directed to a method for joining fittings to a composite sandwich shell edge. An illustrative embodiment of the method includes laying up an inner facesheet and positioning a wrapped flute mandrel on top; applying a layer of adhesive on the inner facesheet and positioning spacer-supported fittings on top; applying adhesive over the co-bonded fittings and laying up an outer facesheet forming an assembly; curing the assembly under heat and pressure; removing the fittings by first collapsing the spacers and removing the flute mandrel; placing an adhesive layer on the fittings with collapsible spacers inserted therein; reinserting the fittings between the inner and outer facesheets and effecting bonding and curing; and removing the spacers from the fittings by collapsing the spacers. 
     In an alternate embodiment, the method for fabricating a composite sandwich shell edge joint may include the use of fluorocarbon spacer bars in place of the fittings to form the fitting cavity during the cure of the facesheets and core. This embodiment has the advantage of reducing the risk of mark-off on the facesheets by placing fewer inches of edge against the facesheet during cure. The counterbalancing disadvantage is that the geometry of the fluorocarbon spacer bars must be carefully controlled and adequate caul strips provided at the joints between blocks to prevent a larger scale mark-off problem at the joints between blocks. 
     The features, functions, and advantages that have been discussed can be achieved independently in various embodiments of the present disclosure or may be combined in yet other embodiments further details of which can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE ILLUSTRATIONS 
         FIG. 1  is a perspective view of a conventional edge joint on a composite sandwich shell. 
         FIG. 2  is a perspective view of a composite cryotank in implementation of an illustrative embodiment of the composite sandwich shell edge joint. 
         FIG. 3  is an oblique view of a composite cryotank skirt edge. 
         FIG. 4  is a sectional view of an illustrative embodiment of the composite sandwich shell edge joint in which the entire joint buildup has been cured in place in a single co-bond step. 
         FIG. 4A  is an exploded perspective view of a joint body component of an illustrative embodiment of the composite sandwich shell edge joint. 
         FIG. 4B  is a sectional view of a sandwich shell edge joint with tapering buildup pads extending toward bridging plies to form a natural plenum for distributing flows between flutes. 
         FIG. 5  is a block diagram illustrating the process flow for fabricating a fitting for the sandwich shell edge joint. 
         FIG. 6  is an exploded view of the components of the tooling used in fabricating the fitting. 
         FIG. 7  is a series of perspective views illustrating the fabricating steps for the fitting. 
         FIG. 8  is a block diagram illustrating the process flow for joining fittings to a composite sandwich shell edge. 
         FIG. 9  is a series of perspective views illustrating the processing steps for joining fittings to a composite sandwich shell edge. 
         FIG. 10  is a partial, exploded perspective view of a completed composite sandwich shell edge joint. 
         FIG. 11  is a flow diagram of a spacecraft production and service methodology. 
         FIG. 12  is a block diagram of a spacecraft. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the described embodiments or the application and uses of the described embodiments. As used herein, the word “exemplary” or “illustrative” means “serving as an example, instance, or illustration.” Any implementation described herein as “exemplary” or “illustrative” is not necessarily to be construed as preferred or advantageous over other implementations. All of the implementations described below are exemplary implementations provided to enable persons skilled in the art to make or use the invention and are not intended to limit the scope of the invention, which is defined by the claims. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     Referring now to  FIGS. 2-4B , an illustrative embodiment of the composite sandwich shell edge joint, hereinafter joint, is generally indicated by reference numeral  1 . In some applications, the joint  1  may be applied as a skirt end joint on a composite cryotank  34  used for launch vehicles in the aerospace industry. However, it is to be understood that the joint  1  may be applicable to joining composite materials in any other type of structure and may serve as a minimum weight solution for a wide range of structures in various industries. The joint  1  may achieve greater efficiencies than conventional joints for weight-critical applications such as aerospace structures, for example and without limitation. The joint  1  may be an all-composite joint, thereby avoiding the manufacturing and stress problems rising from coefficient of thermal expansion mismatch in hybrid joints with metal rings attached to composite shells. 
     As shown in  FIG. 2 , the composite cryotank  34  may include a generally cylindrical tank wall  35  and a tank dome  36  on the tank wall  35 . As shown in  FIG. 4 , the tank wall  35  may include a fluted core  37 . Multiple joints  1  may be arranged in adjacent relationship to each other on the fluted core  37  and along the edge of the tank wall  35 . As illustrated in  FIG. 4 , each joint  1  may include a joint body  2  having an outboard tapered buildup pad  5 ; an inboard tapered buildup pad  6 ; and bridging plies  7  which connect the outboard tapered buildup pad  5  and the inboard tapered buildup pad  6 . An outboard facesheet  3  may be co-cured, co-bonded or bonded to the outboard tapered buildup pad  5 . An inboard facesheet  4  may be co-cured, co-bonded or bonded to the inboard tapered buildup pad  6 . 
     As further shown in  FIG. 4 , a barrel nut  14  may extend through the barrel nut opening  13  and may be engaged by bolt  12  which is inserted into an opening in the bridging plies  7 . The barrel nut  14  may be a standard fastener type which is well-suited to incorporation in the joint  1 . The barrel nut  14  may be positioned so that the interfacing fastener centerline is nominally located on the center surface of the sandwich shell which is defined by the outboard facesheet  3  and the inboard facesheet  4 . This placement of the barrel nut  14  may minimize bending loads being introduced into the tank wall  35  by limiting loading to mainly tension loads transmitted through the barrel nut. In some applications, if needed, additional joint strength may be obtained by using custom barrel nuts with a larger surface area in bearing against the bridging plies  7  and facesheets  3  and  4  and/or with radiused comers at the ends of the cylindrical nut body of the barrel nut. The number of joints  1 , hence barrel nuts  14 , which are used in a given application can be determined by the tensile line load that each joint  1  must carry. 
     The composite outboard tapered buildup pad  5  and inboard tapered buildup pad  6  of the joint body  2  may be configured to efficiently transfer load from the barrel nut  14  to the outboard facesheet  3  and the inboard facesheet  4 . Fabrication methods may provide good clamp-up pressure to the film adhesive bondlines between the buildup pads  5 ,  6  and facesheets  3 ,  4 . A thin, uniform, bondline is stronger than a thick bondline or one with varying thickness across the bond. 
     Overall pad width of each buildup pad  5 ,  6 , as shown in  FIG. 4 , may be chosen to minimize the unsupported length of the facesheet  3 ,  4  between pads  5 ,  6 . The width of the pads  5 ,  6  at their interface to the bridging plies  7  may be chosen to provide sufficient area to meet the joint compressive line load requirements. Pad, at right angles to the joint edge, may be dictated by the length of solid laminate required to transfer loads from the bridging plies  7  to the pads  5 ,  6  plus the length of tapered flange required to shear load into the facesheets  3 ,  4  without delamination. As shown in  FIG. 4A , the extensions  3   a ,  4   a  of the facesheets  3 ,  4 , respectively, past the bridging plies  7  may be chosen to match the fore-and-aft length  8  of the buildup pads  5 ,  6 , so that there may be a minimal length of unsupported face sheet  3 ,  4  between the bridging plies  7  and the pads  5 ,  6 . 
     Each buildup pad  5 ,  6  may be thickest in the area where the barrel nut  14  is installed and may taper toward the edges. The thin edges on the build-up pads  5 ,  6  may reduce shear peaking to maximize attainable bonded joint strength. Fluted cores, for example and without limitation, may be a good candidate for launch vehicle composite sandwich structures because of their suitability for pre-launch purging.  FIG. 4B  illustrates how the two tapering buildup pads  5 ,  6  extending toward the bridging plies  7  form a natural plenum  9  for distributing flows between flutes  11 . In this instance, purge requirements may lead to extending the longitudinal flanges of the buildup pads  5 ,  6  to increase the cross-sectional area of the plenum  9  they naturally form. Depending on whether purge flows are to be immediately vented overboard or collected for disposal at some distance from the launch pad, venting cutouts (not illustrated) may be added to the skirt ends between barrel nut installations or a closeout channel  13  may be added over the component-to-component interface surface to seal off the plenum  9 . 
     Solid laminate may be required across the section in which each barrel nut  14  is installed. This may be obtained by placing the bridging plies  7  between the two buildup pads  5 ,  6 . Since the bridging plies  7  may pick up only a small fraction of the load transmitted through the barrel nut  14 , the joints between the bridging plies  7  and the buildup pads  5 ,  6  may be less critical than the bonded joints between the buildup pads  5 ,  6  and the facesheets  3 ,  4 . 
       FIG. 5  provides a schematic process flow for a preferred method of making the fitting. Since a full scale sandwich panel of the fitting may be in the neighborhood of 1.5″ thick, the fitting may consist of a couple of hundred specially orientated and shaped rectangular blanks. Therefore, it is suggested that an ultrasonic cutout/pick and place machine that is commercially available be used to size and stack these laminate layers. A spacer insert is required when all the lower tapered plies are in place to support and define edge periphery during cure. Once the insert is in place, the top tapered plies can be placed. To insure tight dimensional control the fitting is made in a compression picture frame die using relatively high cure pressures (100 to 500 psi) to insure that the material squeezes down to the desired thickness and that the part is free from porosity. 
     The fabrication process for the fittings used in the present disclosure composite sandwich shell edge joint is shown in  FIGS. 5 ,  6  and  7 .  FIG. 5  is a block diagram illustrating the process flow for the fabrication of the fittings. For instance, in the process steps  501 ,  502 ,  503 ,  504 , and  505 , the forming tool, which includes the compactor tool  520 , the collar tool  550  and the cavity tool  570 , is first prepared by cleaning and applying a release coating. The prepreg tapes are then removed from the freezer and ultrasonically cut to variable rectangular sizes. This includes, as shown in  FIG. 6 , the outboard ply stack  530 , the bridge ply stack  540  and the inboard ply stack  560 . After a layer of peel ply is placed in the tool  570 , a first stack half of rectangular plies stack  560  is placed manually or by a stack machine. In the next process steps of  506 ,  507 ,  508  and  509 , a spacer of a collar tool  550  is inserted into the cavity tool  570 . A second stack half of rectangular plies, or the bridge ply stack  540  is then manually or automatically placed in the die. After a layer of a peel ply is placed on the laminate stack formed by the inboard ply stack  560 , the bridge ply stack  540  and the outboard ply stack  530 , the compactor tool  520  is then closed and the part is cured. In the final steps of  510 ,  511 ,  512 ,  513 ,  514  and  515 , the fitting after cured is removed from the tool and the periphery of the fitting is de-flashed. A co-bond/co-cure collar is then inserted while the peel ply is removed and a film adhesive layer is applied, the fittings are then inserted into a sandwich lay-up. It should be noted that steps  512 - 515  involve the mounting of the fittings into a sandwich lay-up. 
     The various processing steps for forming the fitting are further shown in  FIG. 7  in six steps of  7   a ,  7   b ,  7   c ,  7   d ,  7   e  and  7   f . For instance,  7   a  shows the inboard ply stack is placed in the cavity tool  570 ,  7   b  shows the collar tool  550  is placed on top of inboard ply stack  560 ,  7   c  shows that the bridge ply stack  540  is placed in the pocket  552  of collar tool  550 ,  7   d  shows that the outboard ply stack  530  is placed in the cavity tool  570 ,  7   e  shows that the compactor tool  520  is placed on top of the outboard ply stack  530  and cured, and  7   f  shows a cured fitting  580  removed from the tooling. 
       FIG. 8  is a block diagram illustrating the process flow for joining fittings to a composite sandwich shell edge. In general, the fittings must be supported by either hard or collapsible/expandable spacers. The major flow steps, as shown in  FIG. 9 , include placing the inner skin  902 , placing the flutes  904  and fitting/spacers  906  on the inner skin  902 , and then placing the outer skin  912  on the flutes  904  and spacers  906 . These steps are shown as steps  801 ,  802 ,  803 ,  804 ,  805 ,  806 ,  807 ,  808 ,  809 , and  810  in  FIG. 8 . On one side of the part, the fittings  906  can be co-bonded in place with adhesive, but on the other side the fittings must be initially placed with a release film and peel ply and secondarily bonded with film adhesive. The bonded side fitting must be temporarily removed after initial cure to allow for flute mandrel extraction. These steps are shown in  FIG. 8  by the steps of  811 ,  812 ,  813  and  814 . Later they can be reinserted into the sandwich panel, with film adhesive, to facilitate a high temperature bond (250° or 350° F.). Removable spacer tooling must be used to support fittings that are undergoing co-bond or bond. But solid spacer tools can be used when the fittings are subsequently removed to allow for flute mandrel removal. Fittings can be located on sandwich panel skins using a laser projection system, tooling pins at the barrel nut locations, or a system where the spacers pin to the lay-up mandrel. In so doing, the fittings are bonded to the sandwich panel to exacting dimensions (i.e., ±0.005″). Moreover, unlike other metallic and composite end ring solutions, the fittings described in this disclosure require no mechanical fasteners to facilitate joining to sandwich panel face skins. These steps are represented in  FIG. 8  by steps  815 ,  816 ,  817 ,  818 ,  819 ,  820 ,  821  and  822 . 
       FIGS. 9A-9F  represent graphical representation of the process steps shown in  FIG. 8 . For instance,  FIG. 9A  shows an inner facesheet  902  is first laid up, and then wrapped flute mandrels  904  are placed on top.  FIG. 9B  shows that fittings  906  each with supporting, inflatable collars  908  and a release film  910  are then placed on top of the inner facesheet  902 . In the next step of the process shown in  FIG. 9C , an outer facesheet  912  is laid up over the flutes  904  and the fittings  906 .  FIG. 9D  shows that fittings  906  and collars  908  are removed prior to the removal of the flute mandrels  914 . The process then continues in  FIG. 9E  wherein fittings  906  and supporting inflatable collars  908  are replaced into the cavities formed between the inner facesheet  902  and the outer facesheet  912 . The fittings  906  are bonded using film adhesive. In the final step of the process, shown in  FIG. 9F , the inflatable collars  908  are removed to complete the skirt-end joint layup. 
     As shown in  FIG. 10 , the mating side of the joint  1  may incorporate a male threaded fastener to engage the barrel nut  14 . A variety of conventional joint types may be suitable for the purpose. In some applications, for example, a finger ring  30  having “mouse holes”  31  may be used for tins purpose. The finger ring  30  may be fastened to each joint  1  by extending a bolt  28  through a bolt opening (not shown) in the finger ring  30  and inserting the bolt  28  into the barrel nut installed in the joint body  2  of the joint  1 . The opening in the cryotank skirt joint that allows the bolt to reach the barrel nut is shown as  38  in  FIG. 3 . 
     Referring next to  FIGS. 11 and 12 , embodiments of the disclosure may be used in the context of a spacecraft manufacturing and service method  100  as shown in  FIG. 11  and a spacecraft  200  as shown in  FIG. 12 . During pre-production, exemplary method  100  may include specification and design  102  of the spacecraft  200  and material procurement  104 . During production, component and subassembly manufacturing  106  and system integration  108  of the spacecraft  200  takes place. Thereafter, the spacecraft  200  may go through certification and delivery  110  in order to be placed in service  112 . While in service by a customer, the spacecraft  200  may be scheduled for routine maintenance and service  114  (which may also include modification, reconfiguration, refurbishment, and so on). 
     Each of the processes of method  100  may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include without limitation any number of aircraft manufacturers and major-system subcontractors; a third party may include without limitation any number of vendors, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on. 
     As shown in  FIG. 12 , the spacecraft  200  produced by exemplary method  100  may include a structure  202  with a plurality of systems  204  and an interior  206 . Examples of high-level systems  204  include one or more of a propulsion system  208 , an electrical system  210 , a hydraulic system  212 , and an environmental system  214 . Any number of other systems may be included. Although an aerospace example is shown, the principles of the invention may be applied to other industries, such as the automotive industry. 
     The apparatus embodied herein may be employed during any one or more of the stages of the production and service method  100 . For example, components or subassemblies corresponding to production process  106  may be fabricated or manufactured in a manner similar to components or subassemblies produced while the spacecraft  200  is in service. Also one or more apparatus embodiments may be utilized during the production stages  106  and  108 , for example, by substantially expediting assembly of or reducing the cost of a spacecraft  200 . Similarly, one or more apparatus embodiments may be utilized while the spacecraft  200  is in service, for example and without limitation, to maintenance and service  114 . 
     Although the embodiments of this disclosure have been described with respect to certain exemplary embodiments, it is to be understood that the specific embodiments are for purposes of illustration and not limitation, as other variations will occur to those of skill in the art.