Abstract:
A method is provided for connecting hollow profiles ( 1 - 4 ) in a joint ( 10 ) to produce a load-bearing structure ( 5 ). The method includes placing ends of hollow profiles ( 1 - 4 ) in a mold and pressing the ends together with at least one semi-finished product to connect the ends of the hollow profiles to the semi-finished product.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority under 35 USC 119 to German Patent Appl. No. 10 2014 104 638.2 filed on Apr. 2, 2014, the entire disclosure of which is incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a method for connecting hollow profiles in a joint to produce a load-bearing structure. The invention further relates to a load-bearing structure having hollow profiles that are connected firmly together with mutually facing ends in a joint. Moreover, the invention relates to a motor vehicle having a load-bearing structure of this type. 
         [0004]    2. Description of the Related Art 
         [0005]    DE 10 2008 061 463 A1 discloses a load-introduction device and a structural part that comprises at least one fiber composite part. The fiber composite part has at least one insert part that is connected to the structural part and forms a load path. The insert part is formed from a nonmetallic material. DE 10 2011 114 676 A1 discloses a fiber-reinforced plastic composite component with at least one fiber layer that is present in a composite with at least two matrix materials. The DE 10 2012 016 666 A1 discloses a joint element for connecting at least two hollow-profile beams composed of fiber-reinforced plastic in an angled manner. The joint element is formed from light metal or from plastic or from fiber-reinforced plastic. 
         [0006]    It is the object of the invention to simplify the manufacture of stable load-bearing structures with hollow profiles that are connected firmly together with mutually facing ends in a joint. 
       SUMMARY OF THE INVENTION 
       [0007]    The invention relates to a method for connecting hollow profiles in a joint in to produce a load-bearing structure. The hollow profiles are placed with mutually facing ends in a mold and are pressed together with at least one semifinished product to connect the mutually facing ends of the hollow profiles firmly to the semi-finished product. The load-bearing structure preferably is a load-bearing structure of a motor vehicle. Adhesive can be used to connect the hollow profiles to the semi-finished product. However, adhesive is not used in other embodiments, for example if the semi-finished product contains resin that is not cured. 
         [0008]    The semi-finished product may partially surround the mutually facing ends of the hollow profiles. The semifinished product can be molded, for example, so that openings that are open on one side are produced for the hollow profiles. 
         [0009]    The semi-finished product may entirely surround the mutually facing ends of the hollow profiles. As a result, the method enables a form-fitting and cohesive connection between the hollow profiles and the semi-finished product. During manufacture, the semifinished products can be plugged with their mutually facing ends into openings in the semi-finished product to accommodate the half profiles. 
         [0010]    The hollow profiles may placed with their mutually facing ends in the mold and may be pressed together with at least one moldable and curable sheetlike fiber/plastic composite semi-finished product to connect the mutually facing ends of the hollow profiles firmly to the sheetlike fiber/plastic composite semifinished product during the molding and curing of the joint. The moldable and curable sheetlike fiber/plastic composite semifinished product contains for example a thermosetting plastic material that also is referred to as resin or thermoset material. The resin is initially in a liquid form and crosslinks during pressing, preferably under the action of heat, due to chemical reactions. The crosslinking also is referred to as curing. The resin creates a stable connection between the hollow profiles and the semi-finished product in a simple manner during pressing during crosslinking or curing, without the additional use of adhesives. 
         [0011]    The sheetlike fiber/plastic composite semi-finished product may be a sheet molding compound SMC. During the processing of the sheet molding compound, a sheetlike semifinished product is brought into its final form by compression molding, for example extrusion, and cured. The semifinished product contains fibers that are contained in a tangled or sauerkraut-like manner in a carrier material. The carrier material contains curable resin. The fibers are for example carbon fibers. 
         [0012]    The hollow profiles may be formed from a fiber/plastic composite material that may contain carbon fibers for reinforcement. 
         [0013]    The hollow profiles may comprise a foam core, such as a plastic foam, in particular by a rigid foam. The foam core supports the hollow profile from the inside. As a result, the strength and rigidity of the hollow profiles is increased considerably in a simple manner. The increased stability achieved by the foam core enables a fiber content, in particular a carbon fiber content, to be reduced. Furthermore, the foam core prevents undesired collapsing of the hollow profiles under the pressure applied when the semi-finished product is pressed together with the hollow profiles. Thus, it advantageously is unnecessary to insert a core into the hollow profiles when the semifinished product is pressed together with the hollow profiles. 
         [0014]    The semi-finished product may be formed from a similar material to or the same material as the hollow profiles. As a result, undesired thermal stresses in the load-bearing structure can be prevented because the materials have identical or similar coefficients of expansion. 
         [0015]    The invention further relates to a load-bearing structure having hollow profiles that are connected firmly together with mutually facing ends in a joint by an above-described method. Thus, a very stable load-bearing structure can be produced in a simple manner. 
         [0016]    The invention further relates to a motor vehicle having an above-described load-bearing structure. In this case, the weight of the motor vehicle can be reduced by the load-bearing structure. 
         [0017]    Further advantages, features and details of the invention can be gathered from the following description, in which various exemplary embodiments are described in detail with reference to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a perspective view of a load-bearing structure according to the invention having hollow profiles connected firmly with mutually facing ends in a joint. 
           [0019]      FIG. 2  is a perspective view of an embodiment of the joint of  FIG. 1 . 
           [0020]      FIG. 3  is a perspective view of a variant of the joint from  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    Four hollow profiles  1  to  4  are illustrated in perspective in  FIG. 1 . To produce a load-bearing structure  5 , the hollow profiles  1  to  4  are connected firmly together at their mutually facing ends in a joint  10 . The hollow profiles  1  to  4  have end pieces  11  to  14  at their ends remote from the joint  10 . 
         [0022]    The end pieces  11  to  14  may serve to produce metal joints for attaching the load-bearing structure  5  to further (not illustrated) load-bearing structure elements. The metal joints are formed for example from aluminum or magnesium. In contrast thereto, the joint  10  is formed from a fiber-reinforced plastics material. 
         [0023]    The hollow profiles  1  to  4  are formed from a carbon-fiber-reinforced plastics material. To increase stability, the hollow profiles  1  to  4  have a foam core in their interior. The foam core is preferably a rigid foam core made of a plastic material. 
         [0024]    The hollow profiles  1  to  4  having the foam core are manufactured for example by pultrusion, prepreg compression molding or RTM. Pultrusion is the name given to a method in which fiber-reinforced plastic profiles can be produced quickly and easily. 
         [0025]    Prepreg is the name given to a semifinished product having a plastic matrix in which fibers, in particular continuous fibers, are embedded. The plastic matrix is preferably a resin that is not yet cured. 
         [0026]    The letters RTM stand for resin transfer molding and denotes a method also referred to as transfer molding. Compared with compression molding, a molding material is injected into a mold during transfer molding and cures under heat and pressure. 
         [0027]    The hollow profiles  1  to  4  are supported from the inside by the foam core. As a result, the strength and rigidity of the hollow profiles  1  to  4  can be increased considerably. In addition, the fiber content of the hollow profiles  1  to  4  can be reduced because the foam core contributes to the stability. 
         [0028]    A sheet molding compound SMC may be used to produce the joint  10 . The sheet molding compound SMC advantageously contains carbon fibers. The carbon-fiber-containing SMC also is referred to as a C-SMC. The hollow profiles  1  to  4  can be connected firmly together by way of the SMC. 
         [0029]    The connection between an SMC semifinished product and the hollow profiles  1  to  4  can be carried out by adhesive bonding. Particularly advantageously, however, the connection between the hollow profiles  1  to  4  and the SMC semifinished product is achieved by direct pressing of the SMC semifinished product together with the hollow profiles  1  to  4  placed in the mold. 
         [0030]    The same material preferably is used to produce the hollow profiles  1  to  4  and the SMC semifinished product during the manufacture of the joint  10 . As a result, undesired stresses in the finished load-bearing structure  5  can be prevented. Furthermore, on account of the press fit, the application of the adhesive can be dispensed with. 
         [0031]    A composite body  20  that can produce the joint  10  is illustrated in perspective in  FIG. 2 . The composite body  20  has a total of four openings  21  to  24  that can accommodate the mutually facing ends of the hollow profiles  1  to  4  in  FIG. 1 . 
         [0032]    The openings  21  to  24  are open on their underside. This makes it easier to place the hollow profiles  1  to  4  in the mold with the composite body  20 . During placing in the mold (not illustrated), the composite body preferably is arranged with its underside up. 
         [0033]    A composite body  30  that also can be used to produce the joint  10  in  FIG. 1  is illustrated in perspective in  FIG. 3 . The composite body  30  comprises a total of four openings  31  to  34  to accommodate the mutually facing ends of the hollow profiles  1  to  4 . In contrast to the composite body  20  illustrated in  FIG. 2 , the openings  31  to  34  in the composite body  30  illustrated in  FIG. 3  entirely surround the mutually facing ends of the hollow profiles  1  to  4 . 
         [0034]    During the manufacture of the load-bearing structure  5 , the hollow profiles  1  to  4  easily are plugged with their mutually facing ends into the openings  31  to  34  in the composite body  30 . The hollow profiles  1  to  4  in the plugged-in state then are pressed together with the composite body  30  and preferably heated to produce, or cure or crosslink, the joint  10  in  FIG. 1 .