Abstract:
Stiffeners are disclosed which can be added to the header section of cast light-metal door panels. The header section casting can be designed for manufacturability, and to meet nominal lateral stiffness specifications while making effective use of material. The stiffeners can be cast in place in the header or attached to the header section after casting via snap-fit features, adhesive or both. The stiffeners can themselves be made of a light-weight metal such as aluminum, and can be produced by roll forming, stamping or extrusion. By effectively yielding a closed-section door header shape, the stiffeners provide maximum incremental bending stiffness in the header while adding a minimum amount of incremental material and mass.

Description:
GOVERNMENT CONTRACT 
       [0001]    This invention was made with U.S. Government support under Agreement No. DE-EE0005753 awarded by the U.S. Department of Energy. The U.S. Government may have certain rights in this invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates generally to enhanced stiffness of an automobile door header and, more particularly, to stiffeners which can be added to cast light-metal door headers, where the headers are part of door inner panels and have open sections by virtue of being cast, and the stiffeners effectively provide a closed section header with greater bending stiffness and minimal additional weight. 
         [0004]    2. Discussion of the Related Art 
         [0005]    Automobile manufacturers are increasingly turning to castings of low density metals such as magnesium, taking the place of parts which were traditionally fabricated from stamped steel. These light-metal castings have found many applications in vehicles, including door inner panels. A cast light-metal door inner panel can replace many stamped steel parts, eliminate the need for multiple welding and other fabrication steps, and reduce door weight. One downside of cast door inner panels is that closed cross-sections cannot be produced by die casting, the preferred method to make thin wall sections. A closed section is particularly desirable in the door header area, where the thickness of the door is lowest. 
         [0006]    Door headers are typically required to meet a prescribed lateral stiffness specification, where an outboard load is applied at the mid-span of the header (above the center of the window) and the deflection cannot exceed a certain value. One approach to meeting the header lateral stiffness specification with cast door panels, while remaining within the cross-sectional area constraints of the header, is to make elements of the header section thicker. However, this approach is sub-optimal, because in an open section most of the added material is fairly close to the neutral axis of bending, and is therefore not very effective at increasing the bending stiffness. 
         [0007]    Furthermore, with automobile manufacturers leveraging vehicle platforms globally, a particular vehicle design may be sold in markets with different specifications for door header lateral stiffness. In such cases, a door header casting which is designed to meet the most stringent lateral stiffness specification will be overdesigned for markets with less stringent specifications, and vice versa. 
       SUMMARY OF THE INVENTION 
       [0008]    In accordance with the teachings of the present invention, stiffeners are disclosed which can be added to the header section of cast light-metal door panels. The header section casting can be designed for manufacturability, and to meet nominal lateral stiffness specifications while making effective use of material. The stiffeners can be cast in place in the header or attached to the header section after casting via snap-fit features, adhesive or both. The stiffeners can themselves be made of a light-weight metal such as aluminum, and can be produced by roll forming, stamping or extrusion. By effectively yielding a closed-section door header shape, the stiffeners provide maximum incremental bending stiffness in the header while adding a minimum amount of incremental material and mass. Furthermore, the stiffeners can be applied to vehicles sold in markets with stringent header stiffness specifications, while other vehicles with the same base header casting can be sold in less stringent markets without the stiffeners. 
         [0009]    Additional features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is an illustration of a vehicle door showing the header area which is subject to stiffness specification; 
           [0011]      FIG. 2  is an illustration of a traditional stamped steel door header cross section and associated trim; 
           [0012]      FIG. 3  is an illustration of a cast door inner panel showing the header region and other regions; 
           [0013]      FIG. 4  is an illustration of a door header cross section and associated trim, where the door header is part of the cast door inner panel of  FIG. 3 ; 
           [0014]      FIG. 5  is an illustration of a cast door header cross section with a first embodiment of a stiffener; 
           [0015]      FIG. 6  is an illustration of a cast door header cross section with a second embodiment of a stiffener; 
           [0016]      FIG. 7  is an illustration of a cast door header cross section with a third embodiment of a stiffener; 
           [0017]      FIG. 8  is an illustration of a cast door header cross section with a fourth embodiment of a stiffener; and 
           [0018]      FIG. 9  is an illustration of a cast door header cross section with a fifth embodiment of a stiffener. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0019]    The following discussion of the embodiments of the invention directed to stiffeners for cast light-metal door headers is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
         [0020]      FIG. 1  is an illustration of a vehicle door  10 , where the door  10  is a left-side door viewed from the left outside of the vehicle. The door  10  includes a header  12 , which is the area above the window of the door  10 . The header  12  has the narrowest cross-section of any part of the door  10 . Furthermore, the header  12  is subject to a lateral stiffness specification, which may vary from one global region to another. For example, in a well-developed region of the world with high-speed road infrastructure, the door header  12  may be required to exhibit a lateral deflection of less than three millimeters (mm) under application of a lateral load of 150 Newtons (N) applied at a mid-span point  14  on the header. In other regions of the world, the header stiffness specification may not be as stringent. 
         [0021]      FIG. 2  is a cross-sectional illustration of a traditional door header and associated trim. In  FIG. 2 , the outboard direction is represented by arrow  20 , while the inboard direction is represented by arrow  30 . The header cross-section includes a stamped steel header  40 , a plastic inner trim piece  42 , and various rubber seals  44 . The header cross-section also includes metal clips  46  for holding the exterior trim and seal parts onto the stamped steel header  40 . 
         [0022]    It can be seen in  FIG. 2  that the stamped steel header  40  provides the majority of the structural rigidity to the door header area, as the other components are either rubber or plastic, or are very thin-section metal. It can also be seen that the cross-section of the stamped steel header  40  is designed for maximum lateral bending stiffness. That is, the stamped steel header  40  has a closed-section shape, and the material has been placed as far as possible to the lateral extremes of the cross-section while remaining within packaging constraints. These design features enable the stamped steel header  40  to meet the lateral bending stiffness specification. 
         [0023]    Vehicle manufacturers are constantly striving to reduce vehicle assembly time, reduce part-to-part variability and reduce vehicle mass. For these and other reasons, some vehicles now use single-piece light-metal castings for door inner panels. Magnesium is a preferred material for such castings; aluminum may also be used.  FIG. 3  is an illustration of a cast door inner panel  60  which may be used in place of numerous stamped parts in a vehicle. The cast inner door panel  60  includes a header region  62 , a latch region  64 , a rocker region  66 , a hinge region  68  and a beltline region  70 . Other components which would be added to the cast door inner panel  60  to produce a complete door assembly—such as an anti-intrusion beam, a window-lift mechanism and a glass guide—are omitted for simplicity. 
         [0024]    The cast door inner panel  60  offers several advantages over a stamped, fabricated design, as discussed previously. However, it is still required to meet header lateral stiffness specifications and this has sometimes been difficult to achieve—due to the lower stiffness of light-metals than steel, and the inherent limitation of die castings to open cross sections. It is generally not feasible to simply add more material to the header region  62  in order to meet bending stiffness requirements—because cross-sectional elements which are excessively thick cannot be cast with good quality. Furthermore, it would be decidedly suboptimal to produce the cast door inner panel  60  without the header region  62 , and then have to fabricate and attach a separate stamped steel header part. Thus, the challenge becomes how to meet the header lateral stiffness specification with the light-metal cast door inner panel  60 . 
         [0025]      FIG. 4  is a cross-sectional illustration of a door header and associated trim, where the door header is part of the cast door inner panel  60  of  FIG. 3 . A cast header section  80 , which is part of the cast door inner panel  60 , replaces the stamped steel header  40  shown previously. It can be seen in  FIG. 4  that the cast header section  80  fits within the same packaging space as the stamped steel header  40 , and can be used with the same trim pieces—namely, the plastic inner trim piece  42 , the rubber seals  44  and the metal clips  46 . The cast header section  80  is one embodiment of a header cross-sectional shape when incorporated in the cast door inner panel  60 . Other shapes are shown in later figures and discussed below. The cast header section  80  is, by necessity, an open section design. Analysis of the cast header section  80  indicates that a neutral axis of bending  90  is located as shown in  FIG. 4 . In order to increase the lateral bending stiffness of the cast header section  80 , it would be desirable to add material as far from the neutral axis  90  as possible, while providing a closed cross-sectional shape. 
         [0026]      FIG. 5  is a cross-sectional illustration of the cast header section  80  with a first embodiment of stiffness enhancement. A stiffener  100  is added to the cast header section  80  in order to increase the area moment of inertia, or resistance to bending, of the cross section. The stiffener  100  has a tube shape, which could either be roll-formed or extruded. The stiffener  100  is preferably made of aluminum, but could be any other metal or even a composite material. The stiffener  100  can be pressed into position in a V-shaped cavity  102  of the cast header section  80 , and permanently bonded with an adhesive. The stiffener  100  could also be formed into a shape matching the V-shaped cavity  102  and cast in place during the casting of the cast door inner panel  60 , where the stiffener  100  would serve as part of the casting die for this portion of the cast header section  80 . The stiffener  100  embodies its own closed section, and also forms a closed section with the cast header section  80  around the V-shaped cavity  102 . The stiffener  100  also provides a considerable amount of material at a near-maximum distance from the neutral axis  90 , while still fitting within the packaging constraints. For these reasons, the stiffener  100  significantly enhances the lateral bending stiffness of the cast header section  80 . 
         [0027]      FIG. 6  is an illustration of a cast door header cross section with a second embodiment of stiffness enhancement. A stiffener  110  has a generally channel-like shape which fits over the open portion of the cast header section  80 . The stiffener  110  includes features  112  and  114  which enable the stiffener  110  to be snap-fit in place on the cast header section  80 . The feature  112  is a web extending from the body of the stiffener  110  inwardly toward the cast header section  80 , where the feature  112  engages with an opposing web on the cast header section  80 . The feature  114  is a curved edge of the stiffener  110  which wraps around a free edge of the cast header section  80 , as shown. 
         [0028]    An adhesive could also be applied to the contact locations on the stiffener  110  and the cast header section  80  just before the stiffener  110  is snap-fit into place, where the snap-fit allows the cast door inner panel  60  to immediately continue on to the next step of the door assembly process while the adhesive cures, and the cured adhesive increases the strength of the attachment between the stiffener  110  and the cast header section  80 . The stiffener  110 , while itself being an open channel section, forms a closed section with the cast header section  80  when assembled. The stiffener  110  also provides a considerable amount of material (in region  116 ) at a maximum distance from the neutral axis  90 , while still fitting within the packaging constraints. Thus, the stiffener  110  significantly enhances the lateral bending stiffness of the cast header section  80 . The stiffener  110  can also be made thinner in region  118 , close to the neutral axis  90  where the material is less effective at increasing bending stiffness, so as to reduce material cost and weight. 
         [0029]      FIG. 7  is an illustration of a cast door header cross section with a third embodiment of stiffness enhancement. A stiffener  120  includes the same design features as the stiffener  110  of  FIG. 6  (and may in fact be identical to the stiffener  110 ), but the stiffener  120  is used with a cast header section  82  which is different than the cast header section  80 . In particular, the cast header section  82  eliminates two shorter transverse webs which do little to increase lateral bending stiffness because of their proximity to the neutral axis  90 . This illustrates how the stiffener  120  can provide an overall more mass-efficient door header design by eliminating material (in the cast header section  82 ) which is close to the neutral axis  90  and replacing it with material (in the stiffener  120 ) which is as far as possible from the neutral axis  90  while still meeting packaging constraints. 
         [0030]      FIG. 8  is an illustration of a cast door header cross section with a fourth embodiment of stiffness enhancement. A stiffener  130  has a generally channel-like shape and includes some of the same design features as the stiffeners  110  and  120 , but the stiffener  130  is used with a cast header section  84  which includes no transverse webs. The stiffener  130  includes features  132  and  134  which enable the stiffener  130  to be snap-fit in place on the cast header section  84  while the adhesive cures. The stiffener  130  also features localized material thinning in region  136 —close to the neutral axis  90 —in order to reduce mass without significantly reducing bending stiffness. The designs of the stiffener  130  and the cast header section  84  illustrate how a very mass-efficient cast door header design can be achieved by emulating the optimal shape of the stamped steel header  40  discussed previously. 
         [0031]    In a preferred embodiment, the stiffeners  110 / 120 / 130  are aluminum extrusions; however, other materials and processes may be used. For example, with some design modifications, the stiffeners  110 / 120 / 130  could be stamped and formed from aluminum or another metal. The stiffeners  110 / 120 / 130  could also be composed of other materials, such as a carbon-fiber composite, where virtually any section shape could be formed. 
         [0032]      FIG. 9  is an illustration of a cast door header cross section with a fifth embodiment of stiffness enhancement. In  FIG. 9 , the cast header section  80  shown in several previous figures is again used. A stiffener  140 , preferably extruded aluminum, has a shape which allows it to be used as a die for the casting of the cast header section  80  of the cast door inner panel  60 . Thus, the stiffener  140  is cast in place and becomes integral to the cast header section  80 . This arrangement not only simplifies the casting of the cast header section  80 , but provides significant incremental bending stiffness. The stiffener  140  can be designed to optimize material usage, with greater thickness in a region  142  (furthest from the neutral axis  90 ) and lesser thickness in a region  144  (near the neutral axis  90 ). 
         [0033]    Using the stiffeners disclosed herein, and other derivations which can be envisioned, cast light-metal door inner panels can be provided with enhanced lateral bending stiffness in the header area. By tailoring the design of the stiffeners, including forming a closed section with the cast header, significant additional lateral bending stiffness of the header can be achieved with minimal added mass. Furthermore, the stiffeners can be designed to meet the bending stiffness specifications of particular global vehicle markets without changing the base casting design, and omitted entirely for markets where incremental stiffness is not required. 
         [0034]    The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.