CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to French Application No. 1259912 filed Oct. 17, 2012, and to European Application No. 13166647.1 filed May 6, 2013, which applications are incorporated herein by reference and made a part hereof. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to motor vehicle bodywork parts, whether they be static parts or opening members. 
         [0004]    2. Description of the Related Art 
         [0005]    Most bodywork parts are made of metal sheet, usually steel sheet. Bodywork parts made of plastics material are becoming more and more widespread, such as bumpers, tailgates, fenders, etc., which parts coexist at their interfaces with other parts that are still made of metal, such as fenders, hoods, doors, etc. 
         [0006]    Such parts made of plastics material may present defects that are particularly visible in an outside panel because the panel can be seen from the outside, and defects show up in particular at the periphery of such an outside panel. 
         [0007]    Various ways are known for accommodating differences of expansion under the effect of temperature, in particular when hot because of strong sunlight. 
         [0008]    Thus, it is known to select the component materials of such parts depending on their coefficients of linear thermal expansion (CLTE). It is also known to make such parts, while providing them with reinforcement seeking to avoid excessive deformation. Thus, such parts are dimensioned, in particular in terms of thickness or in the form of ribs, or indeed by fitting them with reinforcement made of stronger material, so as to improve the stiffness of the parts under consideration. 
         [0009]    It is also known to handle defects in local manner, i.e. at the edge of a part where excessive changes in clearance or departures from flush alignments are most visible. 
         [0010]    There is only a limited choice for the plastics materials from which to make such parts. Material choice depends on a technical and economic compromise seeking to comply with various constraining clauses of specifications for the vehicle. It frequently happens under certain conditions laid down in the specification, in particular extreme levels of sunlight, that these parts deform excessively, thereby degrading appearance, and in particular degrading the perceived quality of clearances and alignments that ought to be flush. Panels that are strongly deformed can also give rise to jamming and scratching on painted parts facing such panels, particularly if they are included in a moving system. 
         [0011]    In order to overcome those defects, designers are thus constrained to overdimension components or a set of assembled-together components so as to make them less deformable, which then makes them heavier and more expensive, which therefore goes against the initial objective for choosing to make them out of plastics material. 
         [0012]    Thus, for tailgates, particularly those that are said to be “all plastic”, the components are made up of two main parts, a box situated on the inside of the vehicle and a panel situated on the outside of the vehicle. The box and the panel are assembled together, typically by adhesive. They are also connected to the body and they are subjected to various forces via hinges, actuators, a lock, and a peripheral weather strip. Those various forces vary depending on varying situations, in particular depending on whether the tailgate is closed, open, being subjected to high temperatures, or indeed being slammed shut in particularly rough manner. The panel is often painted and is the part that is visible when the tailgate is closed, and it is therefore with the panel that the above-mentioned defects are observed the most easily, particularly in comparison with surrounding bodywork elements. Under the effect of temperature, the panel suffers both from a loss of performance, i.e. it becomes more deformable, and from expansion of the materials from which it is made. 
         [0013]    The loss of mechanical performance leads to deformation because the tailgate, even when it is in the closed position, is subjected to forces simultaneously by the actuators for assisting opening, by the weather strip compressed against the body, and by the hinge and anchoring points relative to the vehicle such as the hinges, the lock, and the tailgate stops on the vehicle body. Deformation is particularly severe at the hinges, since they are typically made of steel and secured to the body, thereby constituting points where the tailgate is constrained. The tailgate can deform only in zones where it is not constrained, e.g. between the hinges or beyond them, i.e. at the margins of the part. This mechanical deformation is also associated with deformation due to expansion. Expansion occurs within the panel, and also within the box, and indeed in differential manner between those two components, since they may be made of different plastics materials, the box typically having a coefficient of linear thermal expansion that is smaller than that of the panel. The connections between the panel and the box, particularly when they are adhesive connections, are therefore subjected to forces that are particularly intense. 
         [0014]    In order to validate a tailgate design, in particular in terms of high temperature performance, the specifications generally require tests to be performed that consist in imposing successive temperature variation cycles on the tailgate, or more particularly on the assembly constituted by the outer panel and the box or inner panel of the tailgate. Those cycles have an amplitude of about 80° C., but that varies between manufacturers, and the number of cycles performed lies in a range of 15 to 20 cycles, approximately. In that type of testing, the tailgate does not always have enough time to return to its initial shape prior to expansion before the next cycle begins. An effect of deformation accumulating from cycle to cycle is therefore observed with the deformation of the tailgate becoming progressively more degraded. 
       SUMMARY OF THE INVENTION 
       [0015]    An object of the invention is to remedy the above-mentioned drawbacks by proposing a motor vehicle bodywork component presenting greater ability to withstand thermal deformation, while still being suitable for making at low cost and using little material. 
         [0016]    According to the invention, this object is achieved by a motor vehicle bodywork component comprising a first panel having a first coefficient of linear thermal expansion and a second panel having a second coefficient of linear thermal expansion greater than the first coefficient of linear thermal expansion, wherein the component further comprises a connection part made of a material having a third coefficient of linear thermal expansion less than the first coefficient of linear thermal expansion, this connection part being fastened to the first panel at at least two fastening points. 
         [0017]    Advantageously, the first panel and the second panel are placed so as to cover each other. 
         [0018]    Advantageously, the first panel is an inside panel, whereas the second panel is an outside, appearance panel. 
         [0019]    Advantageously, the connection part is made of a material selected from the group constituted by steel, aluminum, and composite materials. 
         [0020]    Advantageously, the fastening points fastening the connection part to the first panel are obtained by any of the following means: spot overmolding, screw fastening, riveting, clip fastening, strapping, dogging, adhesive bonding, embedding. 
         [0021]    Advantageously, the connection part is made of a sheet of material having thickness lying in the range 0.5 millimeters (mm) to 1.5 mm. 
         [0022]    Advantageously, the connection part presents a longitudinal direction, and the connection part presents a profile having a slenderness ratio defined in a section perpendicular to the longitudinal direction of the connection part as the ratio of a deployed width of the sheet constituting its profile to a thickness of the sheet, which slenderness ratio lies in the range 20 to 80. 
         [0023]    Advantageously, the slenderness ratio of the connection part is substantially equal to 50. 
         [0024]    Advantageously, the spacing between two consecutive fastening points between the connection part and the first panel lies in the range 30 mm to 80 mm. 
         [0025]    Advantageously, the spacing between two consecutive fastening points between the connection part and the first panel is substantially equal to 50 mm. 
         [0026]    Advantageously, the bodywork component is constituted by movable parts capable of occupying at least two distinct positions. 
         [0027]    Advantageously, the bodywork component constitutes an opening member. 
         [0028]    Advantageously, the bodywork component is selected from the following list: tailgate, a downwardly-opening rear panel (also referred to as a “hatch”), a fender, a hood, a side door, a sill, a capping strip, a trim strip, a roof frame, a bumper, a spoiler, a sun roof. 
     
    
     
       BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
         [0029]    The invention can be better understood on reading the following description given purely by way of example and made with reference to the drawing, in which: 
           [0030]      FIG. 1  is a section view of a prior art motor vehicle bodywork component when subjected to thermal expansion; 
           [0031]      FIG. 2  is a section view on a horizontal plane of a tailgate in an embodiment of the invention, before thermal expansion; and 
           [0032]      FIG. 3  is a section view on a horizontal plane of the same tailgate, after thermal expansion. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0033]      FIG. 1  is a diagrammatic view of a motor vehicle bodywork component. In this example, the component is a structure  10  represented by a continuous line in a situation in which it is not subjected to any expansion, and by a dashed line in a situation where it is exposed to intense solar radiation. 
         [0034]    The structure  10  is fastened to the body of the vehicle by two hinges  21  and  22  represented by triangles. Since the body of the vehicle is made of a material having a smaller coefficient of expansion than the structure  10 , the hinges  21  and  22  are spaced apart by a distance that is constant under both of the presently-considered situations, with or without sunshine. Thus, the structure  10  presents a central portion  11  situated between the hinges  21  and  22 , which central portion  11  is subjected to stress horizontally by the hinges  21  and  22 . An expansion of this central portion  11  thus gives rise to the vehicle being outwardly deformed in bending or “bulging”. By continuity, the margins  12  and  13  of the structure  10  of the bodywork component move towards the inside of the vehicle under a tangential effect around the hinges  21  and  22 . 
         [0035]      FIG. 2  shows a motor vehicle bodywork component of the invention, specifically a tailgate. This tailgate of the invention presents a structure  30  itself fastened to two hinges  21  and  22  also represented by triangles. The structure  30  is constituted by a box  31  also referred to as the inner panel and an outer panel  32  also referred merely as a panel. In conventional manner, the box  31  and the outer panel  32  are placed so that they overlap each other. In this example they are assembled together by two peripheral zones  33  and  34  of adhesive. 
         [0036]    The tailgate made up of such a structure  30  also presents a connection part  35  in the form of a spar  35  extending so as to cover the box  31 . The spar  35  is placed against a face of the box  31  that faces towards the outer panel  32 , the spar  35  thus being between the box  31  and the outer panel  32 . The spar  35  is thus hidden from view, whether from the outside or from the inside of the vehicle. The spar  35  is fastened to the box  31  via a sequence of fastening points  36  that are distributed along the length of the spar  35 . Thus, during expansion of the box  31  under the effect of heat, the box  31  tends to extend the spar  35  essentially in its length direction. The spar  35  is made of a material having a coefficient of linear expansion (CLTE) that is less than that of the box  31 . The expansion force on the box  31  is thus transferred to the spar  35  which absorbs this force, and very little of this expansion force is transferred to the outer panel  32  or to the hinges  21  and  22 . The presence of this spar  35  also makes it possible to oppose expansions of the box  31 . The presently-described spar  35  is not in direct connection with the hinges  21  and  22 , but in a variant it could be, thereby giving even greater stiffness to the spar  35 . 
         [0037]    As shown in  FIG. 3 , as a result of the various fastening points  36  between the spar  35  and the box  31 , which points are distributed along the length of the spar  35 , overall expansion of the box  31  is prevented in a direction that is preferably horizontal (as has been observed by the inventors on the tailgate of the embodiment described). Such a configuration is found to be sufficient to greatly limit the overall expansion of the tailgate. 
         [0038]    Small local bulging effects  37  do indeed appear between the successive fastening points  36 , nevertheless their amplitude is greatly reduced compared with same-type tailgates, which do not include such a spar  35 , and such local deformations of the box  31  are not transmitted to the outer panel  32 . 
         [0039]    The portion of the bodywork component that is given treatment against expansion deformation is thus the structural inside portion of the tailgate, which inside portion is typically referred to as a box  31  or an inside panel. Such an inside panel or box  31  is structural because its initial role is to carry the visible outer panel  32  and it typically presents a coefficient of linear thermal expansion that is less than that of the outer panel  32 , which is made of plastics material. Although the portion treated by the presence of the spar  35  is the portion that is least subjected to thermal expansion because it is less exposed to solar radiation and because of its small coefficient of linear thermal expansion, it is found that the treatment of this inside portion, specifically the box  31 , greatly reduces the overall deformation of the structure  30 , by reducing indirectly but most effectively the visible deformation of the outer panel  32 , which in this example is an appearance part, i.e. a part that is visible to the user. In addition, the spar  35  is found to play the role of a return spring on a return to the initial temperature. It thus assists the structure  30  in returning more quickly to its initial shape, which effect is particularly advantageous when performing the temperature cycling test. 
         [0040]    Since the spar  35  prevents the box  31  as a whole expanding merely because it opposes longitudinal deformation, it eliminates bulging of the box  31  as a whole. Because it works essentially by opposing longitudinal deformation, there is no need for it to be dimensioned as though it were structural reinforcement. It operates essentially in local traction between two successive fastening points  36 , and its transverse dimensions may thus be particularly small. 
         [0041]    The fastening points between the box  31  and the spar  35  may be implemented by spot overwelding, by screw fastening, by riveting, by clip fastening, by strapping, by dogging, or by other fastening techniques. The fastening points  36  between the connection part  35  and the remainder of the structure  30  may be made for example merely by using discrete fastening points  36  such as the tops of any rib already present in the structure  30  on the box  31 . 
         [0042]    The spar  35  needs to present stiffness that is longitudinal only, and its bending stiffness may remain particularly small, so the spar  35  can thus be limited to an element of small section. It therefore presents little extra weight in comparison with the structural reinforcement commonly used for stiffening panels, and it is therefore compatible with the approach that consists in making bodywork components out of plastics material in order to lighten vehicles. 
         [0043]    In this example, the spar  35  is made in the form of a steel sheet that is folded to present a channel section. The steel sheet presents thickness of 0.5 mm and the steel used may present a coefficient of linear thermal expansion that is equal to 1.1×10 −5 . The spar  35  also presents a slenderness ratio, i.e. in its cross-section a ratio of the deployed width of its section to the thickness of the sheet constituting the section, that is of the order of 50. Its slenderness preferably has a value lying in the range 20 to 80 in order to achieve a good compromise between sufficient stiffness in bending and light weight. In addition, the spar  35  may present a cross-section of various shapes, such as a rectangular section, or as in the present example, a channel section. The spar  35  may also present a section of shape that is more complex. 
         [0044]    In this embodiment, the box  31  is made of composite material, specifically polypropylene 40% filled with glass fibers (40% GFPP). In this example the box  31  presents thickness of 3.5 mm and a coefficient of linear thermal expansion of 4×10 −5 . 
         [0045]    The fastening points  36  between the box  31  and the spar  35  in this example are spaced apart by about 50 mm. They are preferably spaced apart by an interval lying in the range 30 mm to 80 mm, with a minimum that is preferably 30 mm. 
         [0046]    In a variant, the spar  35  may be made of aluminum, which presents a coefficient of linear thermal expansion of 2.6×10 −5 . The spar  35  may also be made of composite material, e.g. out of a sheet molding compound (SMC) type composite material. Such a composite material then typically has a coefficient of linear thermal expansion of 1.5×10 −5 . The thicknesses selected for the box  31  and the spar  35  are adapted to the material used, depending on the Young&#39;s modulus of the material and on the feasibility of using it industrially. With aluminum, the thickness of the sheet constituting the spar  35  is advantageously about 0.8 mm and with SMC composite material the thickness of the sheet is advantageously about 1.5 mm. 
         [0047]    In order to enable the spar  35  to operate in traction, a minimum of two fastening points  36  with the box  31  suffices, but it is preferable to provide a larger number. A spacing of about 50 mm between two consecutive fastening points  36  enables the box  31 , and thus the bodywork component as a whole, to be stiffened in particularly effective manner. Advantageously, the spacing between two fastening points  36  lies in the range 30 mm to 80 mm, which spacing is also appropriate for a shape that is rounded or a shape that has changes of level. 
         [0048]    Obtaining such control over deformation makes it possible to select other materials for the box  31  and for the outer panel  32 . It also makes it possible to reduce the structural dimensioning that is sometimes incorporated in such elements for the purpose of opposing their deformation, such as increased thicknesses or ribs. It also makes it possible to reduce and/or eliminate the additional structural reinforcement that is sometimes placed in the outer panel  32 . 
         [0049]    The connection part in this example is in the shape of a spar  35 , however in a variant it could be of some other form, with weight that is lower than in the previously proposed structural configuration so that it can serve to lighten the bodywork component as a whole. 
         [0050]    This type of technique applies not only to a tailgate or to a tailgate zone as described herein, but applies more widely to any tailgate zone and to any other vehicle bodywork component that can be made in part or in full out of plastics materials, such as a fender, a hood, a side door, a sill, a capping strip, a trim strip, a roof frame, or indeed a bumper. 
         [0051]    The invention is not limited to the presently described embodiments and other embodiments will appear clearly to the person skilled in the art. 
         [0052]    While the method and system herein described constitute preferred embodiments of this invention, it is to be understood that the invention is not limited to this precise method and system, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.
The invention relates to a motor vehicle bodywork component comprising a first panel and a second panel having a second coefficient of linear thermal expansion greater than the first coefficient of linear thermal expansion of the first panel, the component further including a connection part having a third coefficient of linear thermal expansion that is less than the first and that is fastened to the first panel.