Abstract:
A structural member such as a strut includes a composite material tube having metal end fittings that are attached to the tube by co-bonded, double shear joints. The double shear bond joint construction reduces the residual stress on the bonds that result from mismatch of the coefficients of thermal expansion of the composite tube and the metal end fittings. The ends of the fittings that are bonded to the tube may include a stepped profile that functions to limit the peak stresses in the bonds.

Description:
TECHNICAL FIELD 
     This disclosure broadly relates to composite structural members, and deals more particularly with a composite tube having co-bonded metal end fittings, and a method for making the tube. 
     BACKGROUND 
     Structural members formed from both composite and metallic materials are used in a variety of applications in the aerospace industry. For example, structural members such as a strut may be formed from a composite material tube having metallic end fittings that attach the strut to other structure in an aerospace vehicle, such as a commercial aircraft. The strut may act as either a support or a connecting member, transferring force in either direction along the longitudinal axis of the strut. Thus, the strut may be subjected to either compressive or tension loading. The use of a composite tube normally provides a weight advantage over a metallic tube, while the use of metallic end fittings provides additional strength at points of attachment. 
     In some cases, the metallic end fittings may be attached to the composite tube using fasteners that pass through the tube and the fitting. This attachment technique may result in stress concentrations in the tube in the area around the fasteners, and therefore requires that the tube have a greater thickness in order to accommodate these localized stresses. This additional tube thickness increases both the weight of the structural member, and the cost of materials. 
     The use of fasteners may be obviated by bonding the end fittings directly to the composite tube. In order to form the attachment bond, a cylindrical section of the end fitting may be inserted into an open end of the tube and a bond is formed at the overlapping, contacting areas between the interior wall of the tube and the exterior wall of the end fitting. The axial length of the bond must be sufficient to withstand shear forces produced by the compression and/or tension loads which the structural member is designed to transfer. Higher loading therefore requires a longer bond length between the end fitting and the tube. Longer bond lengths create a problem, however, due to the difference in the coefficients of thermal expansion (CTE) of the composite tube compared to metal end fittings. This problem is due, in part to the process used to produce the bond. The bonding process involves curing the composite materials forming the tube at elevated temperature while the metal fitting is attached to the tube. In some cases, the metal fitting may be bonded to a prefabricated tube. In either case, the metal fitting expands a greater amount than the tube during the curing process, since the CTE of metal is higher than that of the composite material. Subsequent cooling of the metal and composite material results in the metal and the composite material contracting at different rates, producing residual stresses in the bond area. The residual stresses may be exacerbated as a result of the bond being subjected to thermal cycling and tension and/or compression loading during in-flight service. Thermal cycling may occur during typical aircraft operations when aircraft components are exposed to temperatures of about 90° F. or more on the ground to as low as about −60° F. or lower at typical flight altitudes. 
     Accordingly, there is a need for a bond construction that overcomes the problems mentioned above. Embodiments of the disclosure are directed toward satisfying this need. 
     SUMMARY 
     An embodiment of the disclosure may include a method for making a structural member, such as a strut. The method may include the steps of: forming an inner composite tube wall portion; placing at least one fitting over an end of the inner tube wall portion; forming an outer composite tube wall portion over the inner tube wall portion and the fitting; and, co-bonding both the inner tube wall portion and the outer tube wall portion to the fitting. The inner composite tube wall portion may preferably be made by forming a lay-up of composite material over a mandrel, curing the lay-up after the fitting has been placed over the end of the tube wall portion, and then removing the mandrel from the lay-up after the lay-up has been cured. The lay-up of the inner composite tube wall portion is preferably compacted or debulked before the fitting is placed on the end of the tube wall portion. The method may further include inserting the mandrel into a mandrel mold and expanding the mandrel to form the mandrel into a desired mandrel shape. The lay-up that forms the inner tube wall portion may be made by wrapping plies of composite material at least partially around the mandrel so that the lay-up can expand during any subsequent processing step. 
     In accordance with another embodiment, a method may be provided for making a composite aircraft strut having a metal end fitting. The method may include the steps of: forming a first lay-up of composite material defining an inner tube wall portion; placing at least a section of a fitting over an end of the inner tube wall portion, forming a second lay-up of composite material over the first lay-up and the fitting section, the second lay-up defining an outer tube portion covering the first tube wall portion and the fitting section; and, co-bonding both the inner and outer tube wall portions to the fitting section. The first lay-up may be formed by laying up plies of composite material partially around a mandrel, and then debulking the lay-up. The second lay-up may be formed by wrapping uncured plies of a fiber reinforced polymer material over the first lay-up and over the fitting section. Co-bonding of the metal fitting with the inner and outer tube wall portions may result in a double shear bond that is relatively short in length. 
     According to another embodiment, a structural member may include: a composite material tube having co-bonded inner and outer tube wall portions; and, a metal fitting having at least a section disposed between and co-bonded to the inner and outer tube wall portions. The fitting section forms a first bond joint with the inner tube wall portion and a second bond joint with the outer tube wall portion, providing a double shear bond. In one embodiment, the bond joints may be scarf joints, while in another embodiment, the joint may have steps of decreasing thickness in an axial direction. The double shear bond joint may reduce stress on the bond resulting from the mismatch of the coefficients of thermal expansion of the metal fitting and the composite tube. Co-bonding of the fitting with the composite tube results in a bond strength that may satisfy design load requirements, without the need for fasteners, although fasteners may also be used. 
     The co-bonded double shear joint of at least one embodiment may also reduce the residual stresses present in the bond to acceptable levels, and may also peel stresses in the joint, especially at the ends of the joint. The double shear joint construction is also advantageous in that the eccentricity of the components forming the joint may be reduced. 
     These and further features, aspects and advantages of the embodiments will become better understood with reference to the following illustrations, description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE ILLUSTRATIONS 
         FIG. 1  is a perspective view of a strut having a composite material tube and metallic end fittings, according to an embodiment. 
         FIG. 2  is a longitudinal section illustration of an end of the strut depicted in  FIG. 1 , showing the use of a scarf joint according to one embodiment. 
         FIG. 3  is a perspective illustration of one end of the strut shown in  FIG. 1 , a portion of the outer tube wall portion having been broken away to reveal the inner tube wall. 
         FIG. 4  is a longitudinal sectional illustration taken through the end of the strut shown in  FIG. 1 , but depicting a stepped bond joint which forms another embodiment. 
         FIG. 4A  is a fragmentary, longitudinal section illustration taken through the end of the strut shown in  FIG. 1 , but showing an alternate lay-up arrangement. 
         FIG. 4B  is a fragmentary, longitudinal section illustration taken through the end of the strut shown in  FIG. 1 , but showing another lay-up arrangement. 
         FIG. 5  is an enlarged illustration of a section of the stepped bond joint shown in  FIG. 4 , designated as “A”. 
         FIG. 6  is an end illustration of the end fitting shown in  FIG. 3 . 
         FIG. 7  is a sectional illustration taken along the line  7 - 7  in  FIG. 3 . 
         FIG. 8  is a plan illustration of the end fitting shown in  FIG. 6 . 
         FIG. 9  is a side illustration of the end fitting shown in  FIG. 6 . 
         FIG. 10  is a side illustration of a mandrel rod having an expandable mandrel shown in a deflated condition. 
         FIG. 11  is a longitudinal sectional illustration of a female mandrel mold into which mandrel rod depicted in  FIG. 10  has been inserted. 
         FIG. 12  is an illustration similar to  FIG. 11 , but showing the mandrel having been inflated. 
         FIG. 13  is a side illustration of the mandrel wrapped with multiple plies of fiber reinforced material to form an inner tube wall portion. 
         FIG. 14  is an illustration similar to  FIG. 13 , but showing the first lay-up having been debulked and end fittings having been installed over the inner tube wall portion. 
         FIG. 15  is an illustration similar to  FIG. 14  but showing the first lay-up having been placed in a lay-up mold for compaction and curing. 
         FIG. 16  a side illustration showing the second lay-up having been applied over sections of the end fittings and the first lay-up to form an outer tube wall portion. 
         FIG. 17  is a simplified flow diagram showing the steps for making the composite tube having co-bonded end fittings according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Referring first to  FIGS. 1 and 2 , a structural member in the form of a strut  20  may comprise a cylindrical tube  22  and a pair of end fittings  24  secured to the opposite ends of tube  22  by double shear bonds. The tube  22  may comprise, but is not limited to a composite material, such as multiple laminated plies of a fiber reinforced polymer resin. An example of multiple plies of a fiber reinforced polymer resin may be carbon fiber reinforced epoxy. The tube  22  may include an inner tube wall portion  32 , and an outer tube wall portion  34  which are co-bonded, as shown in  FIG. 2  as a cylinder. Cylindrical tube  22  may have other cross sectional shapes such as, but not limited to square, triangle, hexagon, or pentagon. 
     Each of the end fittings  24  may be, but is not limited to a metal such as aluminum or titanium, or a composite end fitting. A metallic end fitting may be formed by casting, machining or other common manufacturing techniques. A composite end fitting may include metallic inserts and/or metallic bushings. Each of the end fittings  24  may include a clevis  30  provided with aligned openings  26 . While a double tab  31  configuration is shown, a single tab or triple tab configuration or more than 3 tab configurations are within the scope of the embodiments of the disclosure. The openings  26  may allow the strut  20  to be connected by pins (not shown) or other pivoting and/or fastening means to structural components, such as in an aircraft. Depending upon the particular application, strut  20  may function to transfer axial loads bi-directionally, so that the strut  20  may be either placed in tension or compression, or both in alternating fashion, along its central axis. Each of the end fittings  24  may include an axial opening  28  that is aligned with the central axis of the tube  22  for purposes which will become apparent later. 
     As best seen in  FIG. 2 , each of the end fittings  24  may include an interior area  35  that is generally hollow in order to reduce the weight of the end fitting  24 , and a cylindrical end section  36 , although configurations other than cylindrical are contemplated. The cylindrical end section  36  may have a tapered cross section that is disposed between and co-bonded to the inner and outer tube wall portions  32 ,  34 , respectively. As will be discussed later, the inner and outer tube wall portions  32 ,  34 , may be formed from laminates having tapered profiles that complementary match the tapered cross section of the cylindrical end section  36  so as to define a double scarf joint  37 . The inner and outer tube wall portions  32 ,  34 , respectively form, in combination with the cylindrical end section  36 , an overlapping, double shear bond at the scarf joint  37 . 
     While not shown, a coupling means, such as, but not limited to a fastener may couple wall portions  32  and  34  to cylindrical end section  36 . A coupling means may work with co-bonding or singularly without co-bonding. 
     Reference is now made to  FIGS. 3-9  which depict an alternate construction of the composite tube  22  having co-bonded end fittings  24 . The cylindrical end section  36  of each of the end fittings  24  may be provided with a plurality of inner and outer, circumferential steps  38  such that the thickness of cylindrical end section  36  progressively decreases in the direction away from axial opening  28 . As can be seen in  FIGS. 4 and 5 , the inner and outer tube wall portions  32 ,  34  each may comprise a plurality of plies of composite material, such as, but not limited to a fiber reinforced polymer resin which may be fabricated using techniques described later below. The laminated plies  42  ( FIG. 5 ) may be arranged in groups  40  having progressively greater lengths in the direction of the end fitting  24 . Each ply group  40  terminates at an end of one of the steps  38 , so that the plies  42  are effectively tailored in their lengths to complementary match the profile of the steps  38 . The plies  42  are layed up to form the inner and outer tube wall portions  32 ,  34  which may be co-bonded along with the cylindrical end section  36  to form a doubled stepped bond joint  39 . The use of the steps  38  may effectively divide the total amount of the residual stress in the resulting bond so that these stresses peak at each step  38 . In some applications, the stepped, double shear bond joint  39  shown in  FIG. 4  may be preferable to the double scarf joint  37  described in connection with  FIG. 2 . 
     Attention is directed to  FIG. 17  along with  FIGS. 10-16  which depict the steps in making the composite tube  22  having co-bonded end fittings  24  described above in connection with  FIGS. 1-9 . As shown in  FIG. 10 , a mandrel rod  44  is provided with an expandable mandrel  46  that may circumscribe rod  44 . In the illustrated example, the expandable mandrel  46  may comprise a flexible, inflatable bladder. Mandrel rod  44  may include a pair of indexing marks  48  on opposite ends thereof, for purposes that will become apparent later. 
     Beginning with step  56 , the mandrel rod  44  may be axially inserted into a female bladder mold  50 , as shown in  FIG. 11 , which has an interior cavity wall  52  corresponding to the desired shape of a mandrel to be formed. The mold  50  may then be evacuated, causing the flexible bladder  46  to expand within the cavity  52 . Next, at step  58 , the expandable mandrel  46  may be filled with a granular material such as, but not limited to sand or ceramic beads. A pressurized source of the granular material may be connected to an axial conduit (not shown) within the mandrel rod  44 , which in turn is connected with the interior of the flexible mandrel  46 . Next, at step  62 , the flexible mandrel  46  may be sealed and evacuated to form a partial vacuum. This partial vacuum may compress the flexible mandrel  46  against the granulated filler material so as to make it somewhat rigid and assume the desired mandrel shape. It should be noted here that other types of constructions could be used to form the flexible mandrel  46 . For example, an expandable metal or break-down mandrel (not shown) could be employed for ply lay-up rather than the flexible bladder  46  illustrated in the drawings. The flexible mandrel  46  or other known, internal bagging material may then be used during lay-up and/or for curing of the inner lay-up  41 . 
     At step  64 , multiple hoop plies of a composite material may be applied to the rigid mandrel  46 , as shown in  FIG. 13 , resulting in the formation of a first, inner lay-up  41  that may define the inner tube wall portion  32 . The plies forming inner lay-up  41  may comprise, for example, successive, uncured layers of carbon reinforced epoxy material in the form of sheets or a tape in which the orientation direction of the reinforcing fiber alternates according to known ply orientation schemes. The inner lay-up  41  may be formed by wrapping each of the hoop plies one revolution (360 degrees) or less around the mandrel  46 . In other words, wrap each hoop ply of the inner lay-up  41  around the mandrel  46  only once or less. By avoiding plies that wrap more than one revolution, the reinforcing fibers are allowed to move radially during subsequent compaction of the inner lay-up  41 . 
     At step  66 , the inner lay-up  41  may be debulked to remove excess air from the lay-up plies and thereby better consolidate the plies. The debulking process may be carried out within a vacuum bag (not shown) using vacuum pressure. 
     Next, at step  68  the end fittings  24  are installed over the inner lay-up  41 . This step is carried out by passing the end fittings  24  over the ends of the mandrel rod  44 , allowing the mandrel rod  44  to pass through the axial openings  28  in the end fittings  24 . The cylindrical end sections  36  of the end fittings  24  are sleeved over the inner lay-up  41 . As previously indicated, the lengths of the plies forming the inner lay-up  41  may be tailored so as to either match the tapered cross section of the cylindrical end section  36  of the end fitting  24  shown in  FIG. 2 , or the steps  38  of the end fitting  24  shown in  FIGS. 4 and 5 . As the end fittings  24  are installed over the outer ends of the inner lay-up  41 , the indexing marks  48  may be used to align the end fittings  24  relative to each other so that the openings  26  in the clevis of the two fittings  24  are in a desired rotational position relative to each other. 
     At step  70 , a female mold  54  may be placed over the inner lay-up  32  and the cylindrical end section  36 , as can be seen in  FIG. 15 . The female mold  54  may be evacuated, creating a partial vacuum that draws bladder  46  shown in  FIG. 12  and the plies in the inner lay-up  41  into contact with the interior walls of the female mold  54  shown in  FIG. 15  thereby compacting the plies. The female mold  54  may be placed in an autoclave and heated to the necessary temperature in order to cure the inner lay-up either during or after the compaction process. 
     Next, the female mold  54  may be removed at step  74 . At this point, the inner lay-up  41  defining the inner tube wall portion  32  may be fully compacted and cured, and may be co-bonded to the inside face of the cylindrical end section  36  of end fitting  24 . Then, at step  76 , the expandable bladder mandrel  46  may be deflated and the mandrel rod  42  is removed from the strut  22   
     At step  78 , multiple, uncured plies of composite material may be applied over the inner tube wall portion  32  as well as over cylindrical end sections  36  to form a second, outer lay-up  43  that defines the outer tube wall portion  34 . The plies in the outer lay-up  43  may be similar or dissimilar to those used in the inner lay-up, comprising, for example, carbon fiber reinforced epoxy resin, in which the plies are arranged in alternating layers of multiple fiber orientations. (e.g. +45/0/90). Other ply orientations may be used. The plies in the outer lay-up  43  may be wrapped one or more times around the inner lay-up  41 . Like the inner lay-up  41 , the plies in the outer lay-up  43  may be tailored in length so as to conform to either the profile of the unstepped tapered cylindrical end section  36  shown in  FIG. 2 , or the stepped cylindrical end section  36  shown in  FIGS. 4 and 5 . It should be noted here that the number of piles used to form the inner and outer lay-ups  41 ,  43  respectively may vary depending on the particular application and performance requirements. In one embodiment for example, a build up of 33 plies was found to be satisfactory for the inner lay-up  41  and 33 plies on the outer lay-up  43  as well. 
     It may also be possible for an inner lay-up  41  or an outer lay-up  43  to not extend the entire length of cylindrical tube  22 . As shown in  FIGS. 4A and 4B , inner lay-up  41  or outer lay-up  43  may taper over a bond to outer lay-up  43  or inner lay-up  41 , respectively. Tapering sections on both tube, ends may form a double butted cylindrical tube  22 . In another embodiment, a single butted tube may be formed. 
     At step  80 , the outer lay-up  43  may be subjected to compaction and curing using conventional techniques. For example, the strut  22  may be vacuum bagged with the vacuum bag being evacuated and placed in an autoclave (not shown) at elevated temperature until the outer lay-up  43  may be fully compacted and cured. As a result of this compaction and curing process, the outer lay-up  43  forming the outer tube wall portion  34  is co-bonded with the inner tube wall portion  32  and with the outer face of the cylindrical end section  36  on the end fittings  24 . 
     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.