HK1114064A - Vehicle bumper beam - Google Patents

Description

Bumper beam for vehicle

Technical Field

The present invention relates to a vehicle bumper beam, and more particularly, to a bumper beam having a continuously shaped front portion and a rear portion connected to the front portion to form a tubular beam of varying cross-sectional dimensions.

Background

Two basic types of bumper beams commonly used on modern vehicles are tubular portions (also referred to as closed portions, such as "B" or "D" shaped) and open portions (such as "C" shaped portions or "hat" shaped portions). The tubular part and the open part each have their own advantages and disadvantages. For example, from an engineering standpoint, bumper beams formed from tubular sections are inherently more rigid and capable of absorbing and/or transmitting more energy upon impact (particularly in terms of strength to weight ratio) as a result of the impact stresses being distributed around and along the tubular shape. In contrast, open portions tend to bend prematurely upon impact because the "legs" of the open portions unfold, kink, and quickly lose shape upon impact. However, the open portion tends to enable more styling and product changes. It is consistently highly desirable to use a high strength material for the bumper because it reduces weight while achieving higher impact strength (as compared to a low strength material). However, as higher and higher strength materials are used, it becomes more difficult to form sheet stock into the desired beam shape because the higher strength materials become stiffer as they pass over the tooling and press. This is particularly true for punches and dies, where the dies are moved vertically relative to the sheet to form the sheet. The roll-forming process has the ability to form higher strength materials relative to stamping, but the roll-forming process is limited to making a constant cross-sectional shape along the length of the roll-formed portion.

Roll forming is a particularly attractive manufacturing method because dimensionally accurate bumper beams can be manufactured in large volumes at relatively high production speeds with minimal manual labor and with high strength materials, but can also be made more durable for tooling than dies when used to form ultra-high strength steels and high strength low alloy steels. Sturrus5,092,512 and Sturrus5,454,504, for example, disclose meaningful roll forming equipment. However, as mentioned above, roll forming suffers from the disadvantage that the roll forming process can only produce a constant cross-section throughout the length of the part. Furthermore, the material thickness and the material strength of the raw material in roll form cannot be changed around a given cross section, since the material starts as a whole material roll. Given the constant cross-section made by roll forming, this often fails to meet the current styling trend that requires either a change in cross-sectional dimensions along the length of the beam due to packaging space on the frame rails (relative to that available on the vehicle centerline), or a longitudinal bend with increased curvature at the corners of the vehicle (e.g., on the fender). These molding conditions require roll-formed tubular parts to be end-formed or taper cut at their ends by secondary operations. These secondary operations, however, are expensive because of the laborious process of shaping the ends and/or taper cutting these parts (especially when such parts are made of high strength materials). In addition, the end forming and/or taper cutting process requires more than one secondary operation. For example, a taper cut requires some sort of cap to cover the sharp edges created by the cutting process, which must be precisely secured and then welded in place. On the other hand, the ends of the tubular portions may be reshaped to better conform to functional and aesthetic styling concerns (see sturrus5,306,058), but it is difficult to deform the ends in precise conformity, thus potentially resulting in unacceptable dimensional deformations and high wear of the tools.

A beam consisting of a C-shaped open section can be shaped into a desired three-dimensional shape including a non-uniform cross-section along its length, but its open section is not inherently as strong as a tube during impact. In particular, the open portion includes rearwardly extending legs that tend to prematurely unfold or collapse upon impact. This greatly reduces the overall cross-sectional impact strength of the beam and reduces its ability to consistently predictably absorb energy. By stabilizing the bracket of the front portion, the front portion may be made more robust and absorb more energy. This is sometimes accomplished in the prior art by incorporating reinforcing members, such as baffles, plates and/or bridges between the brackets to prevent premature deployment of the brackets during an impact. (see figure 1 of the drawings) a closer match to the performance of the tubular portion can be achieved by stabilising the stent in the open portion. However, these additional stiffeners require expensive secondary machining because they use large fastening and welding machines, and often require several additional parts and significant assembly time and resources in the manufacturing process. Furthermore, the process of welding the plurality of stiffeners to the open beam may be difficult to control. Since the multiple parts must be carefully secured individually and each and every part welded in place in good agreement. In addition, the location of each stabilizer strap also greatly affects the impact strength along the beam.

In summary, packaging and performance requirements of bumper beams on vehicles and associated vehicle front (or rear) end components often increase the complexity of bumpers as they result in the addition of other structural members, which may include bridges, bulkheads, radiator supports, instrument panel supports, instrument panels, and the like. Or they may require end treatment of the bumper beam, including end forming or taper cutting, to form an increased angle on the front bumper end of the fender. This increase in complexity results in increased costs due to the numerous secondary operations. In addition, adaptation difficulties arise between functional and styling criteria. It would be desirable to provide a design and process that overcomes the disadvantages of constant cross-section roll formed sections, and also utilizes roll forming as a means of forming the ultra-high strength materials used in bumper beams, as discussed below. It is also desirable to provide design flexibility that allows adjustment of the bumper beam in the bumper development program, which can be very important for time control and for reasons of investment. At the same time, the ultra-high strength steels used for the various components are preferably optional, so that the bumper beam can be designed for an optimally high strength to weight ratio. Also, even with the use of ultra-high strength steels, it is desirable that the apparatus be capable of using less expensive materials and some materials using relatively simple forming and bending tools to minimize capital investment while still being capable of forming ultra-high strength materials without expensive tools and without rapid tool wear. In other words, it is preferable to use stamped or molded stiffeners where possible, and to use a combination with high strength materials where it is practical to do so.

A further problem is that ultra-high-strength materials are difficult to form in a stamping press, or at least do so less optimally. In particular, those skilled in the art do not prefer stamping materials such as Ultra High Strength Steel (UHSS) because UHSS materials are strong enough to be difficult to form without cracking and also damage or wear the dies and punches quickly.

Accordingly, a bumper beam having the above advantages and solving the above problems is desirable.

Disclosure of Invention

In one aspect of the invention, the bumper beam includes a front portion and a rear portion that mate and secure together. The front section is made of metal and has a front wall and top and bottom walls defining a constant cross section and a rearwardly open cavity. The rear section is also made of metal and is mounted against and connected to the rear side of the front section. The rear portion includes a first longitudinal portion defining with the front portion a first cross-sectional shape having a first depth dimension; and includes a second longitudinal portion on an opposite side of the first portion that fits against the front portion to define a second cross-sectional shape. Each second cross-sectional shape has a second depth dimension different from the first depth dimension, wherein at least one of the first and second cross-sectional shapes is tubular and at least one of the longitudinal portions has a protruding portion that fits into the cavity.

In another aspect of the present invention, the bumper beam includes a front portion and a rear portion. The front portion includes front and top and bottom walls defining a constant hat-shaped cross-section with a rearwardly-open cavity and is made of a material selected from the group consisting of HSLA steel and UHSS material. The rear section fits against and is connected to the rear side of the front section. The rear portion has the same length as the front portion and includes a first longitudinal portion extending between the top and bottom walls to define a first shape having a first depth dimension; and includes a second longitudinal portion on an opposite side of the first portion, the second longitudinal portion extending between the top wall and the bottom wall to define a second shape having a second depth dimension. At least one of the first and second shapes is tubular. The rear portion is made of a material selected from the group consisting of UHSS material, HSLA steel, aluminum, and polymeric material.

In yet another aspect of the invention, the bumper beam includes a front portion and a rear portion having the shapes and characteristics defined above, but where the rear portion is made of a lower strength and more formable material than the front portion.

In another aspect of the invention, a method comprises the steps of: roll forming a front section comprising front and top and bottom walls defining a constant cross section and a rearwardly open cavity; and stamping an elongated rear portion from the sheet, the rear portion having a length that is approximately that of the front portion. The method further comprises the following steps: mounting a rear portion against a rear side of the front portion, the rear portion including a first longitudinal portion defining with the front portion a first cross-sectional shape having a first depth; and includes a second longitudinal portion on an opposite side of the first portion that fits against the front portion to define a second cross-sectional shape having a second depth dimension. The method additionally includes connecting the rear portion to the front portion to form a reinforcement beam portion.

In one aspect of the invention, a bumper beam for a vehicle includes a metallic front section of relatively high material strength including a front wall and upper and lower walls defining a rearwardly facing C-shaped cross section and a rearwardly open cavity. The beam also includes a metallic rear section of relatively low material strength, including a rear wall and top and bottom walls defining a forwardly facing C-shaped cross section and a forwardly open cavity. The upper and lower walls of the front section are disposed within a cavity open forwardly of the rear section and telescopically engage the top and bottom walls, respectively, of the rear section and are secured in top and bottom attachment locations which are subjected to shear forces upon impact. Even if one or more of the attachment locations shear fracture loose, the front and rear portions combine to form a tubular portion of varying cross-sectional dimensions to provide significant impact strength upon impact.

In another aspect of the invention, a bumper beam is provided that is adapted to withstand an impact force in a predetermined longitudinal direction of an impacting vehicle. The bumper beam includes a front portion including front and top and bottom walls defining a constant U-shaped cross-section with a rearwardly open cavity, the front portion being elongated in a direction perpendicular to a predetermined longitudinal direction of impact, the front portion being made of a material selected from the group consisting of High Strength Low Alloy (HSLA) steel and Ultra High Strength Steel (UHSS) material. The bumper beam further includes a rear portion mounted against and connected to the rear side of the front portion, the rear portion having a length that is approximately the length of the front portion and including a first longitudinal portion extending between the top and bottom walls to define a first shape having a first depth dimension; and including second longitudinal portions on opposite sides of the first portion, the second longitudinal portions extending between the top and bottom walls to define a second shape having a second depth dimension, at least one of the first and second shapes being tubular, the rear portion being made of a material selected from the group consisting of an ultra-high strength steel (UHSS) material, a High Strength Low Alloy (HSLA) steel, aluminum, and a polymeric material. The front and rear portions have connecting flanges which are telescopically superposed in a direction parallel to the predetermined longitudinal direction of the impact. The connecting flanges are secured together at connecting locations that are subjected to shear stress when the beam receives an impact force in the longitudinal direction, but the connecting flanges of the front portion are disposed within the connecting flanges of the rear portion so that the connecting flanges of the front portion are retained within the connecting flanges of the rear portion even if the connecting locations shear.

In another aspect of the invention, a method comprises the steps of: roll forming a front section comprising front and top and bottom walls defining a constant cross section and a rearwardly open cavity; and stamping an elongated rear portion from the sheet, the rear portion having a length that is approximately that of the front portion. The method further comprises the following steps: mounting a rear portion against a rear side of the front portion, the rear portion comprising: a first longitudinal portion defining with the front portion a first cross-sectional shape having a first depth; and including a second longitudinal portion on an opposite side of the first portion, the second longitudinal portion fitting against the front portion to define a second cross-sectional shape having a second depth dimension; the front and rear portions have attachment flanges which are telescopically engaged in overlying relation in a direction generally perpendicular to the front wall. The method still further includes connecting the attachment flanges together to secure the rear portion to the front portion to form the reinforcing beam portion, the attachment flanges of the front portion being positioned within and retained by the attachment flanges of the rear portion even if some of the attachment locations shear and loosen.

It is an object of the present invention to provide a design that accommodates complexity without the added expense of requiring many secondary operations.

It is another object of the present invention to provide a design and process that overcomes the disadvantages of roll formed sections having constant cross-section, yet allows them to be used to manufacture sections of beams in ultra-high strength materials.

It is another object of the present invention to provide design flexibility that allows adjustment of the bumper beam (early or late) in the bumper development program, which is critical for time control and investment reasons.

It is another object of the present invention to provide a design that enables the use of materials such as ultra-high strength steel for the part so that the bumper beam can be designed with an optimum high strength to weight ratio while still maintaining the ability to achieve optimum beam strength in a particular area of the beam.

It is another object of the present invention to provide an apparatus that facilitates relatively simple forming and bending operations to minimize capital investment while still forming ultra-high strength materials without the use of prohibitively expensive tooling and without rapid wear of tooling and/or punches.

It is another object of the present invention to provide a bumper beam design wherein the dimensions of the tubular cross-section of the beam can be easily and substantially varied throughout the length of the bumper beam, even with very high strength materials. Furthermore, this can be achieved without much secondary machining and/or heat treatment and/or annealing.

Another object is to provide a bumper beam that is optimized for roll forming and stamping to form the component parts of the beam.

The present invention overcomes the disadvantages of roll-formed sections having a constant cross-section by providing an optimal use of geometry and material to produce bumper beams having the strength and rigidity characteristics of tubular bumper sections. The present invention combines manufacturing processes and materials to produce a tubular section having varying cross-sectional geometry along the length of the part and different material properties around the cross-section of the part. The present invention differs from the prior art in that it involves the addition of material to specific areas to achieve local reinforcement.

These and other aspects, objects, and features of the present invention will be understood and appreciated by those skilled in the art upon studying the following specification, claims, and appended drawings.

Drawings

FIG. 1 is a schematic diagram showing a prior art beam structure;

FIG. 2 is a top view of a bumper beam embodying the present invention, the beam including an open front portion (also referred to as a "hat portion") and a rear portion attached to a rear surface thereof;

FIGS. 3-4 are cross-sectional views taken along lines III-III and IV-IV of FIG. 2;

FIG. 4A is a modification of FIG. 1, and FIG. 4B is a cross-sectional view taken along line IVB-IVB;

FIG. 5 is a top view of a bumper beam embodying the present invention, the beam including an open front portion and a rear portion connected to a rear surface thereof;

FIGS. 6-7 are cross-sectional views taken along lines VI-VI and VII-VII of FIG. 5;

FIGS. 8-10 are alternative attachment structures for securing the front and rear portions together; and

FIG. 11 is a flow chart illustrating a method of manufacturing the beam of FIGS. 2 and 5;

FIG. 12 is a perspective view of one half of an improved bumper system embodying aspects of the present invention;

FIG. 13 is an exploded perspective view of FIG. 12;

FIGS. 14-15 are complementary perspective views of FIG. 12, with FIG. 15 being an enlarged view to better illustrate the end of the rear portion; and

fig. 14A-14C are cross-sectional views taken through fig. 14 on lines XIVA, XIVB and XIVC, respectively.

Detailed Description

The present invention is directed to a bumper beam 20 (fig. 2) (and beam 20A, fig. 5; and beam 20B, fig. 4A) that utilizes roll-formed front portions 22, 22A (also referred to as "front channels" or "rolled portions") and stamped or molded rear portions 27, 27A, 27B (also referred to as "rear channels" or "reinforced portions") to mate together to form bumper beams of different tube cross-sectional shapes. More specifically, the present invention provides two solutions that combine to produce a tubular bumper beam having a varying cross-section over the length of the bumper and material properties that vary around the cross-section. The ability to vary the cross-section over the length of the bumper facilitates optimizing the impact performance, weight, and cost of the beam along any selected area of the beam. For example, the use of ultra-high strength steel (UHSS steel) provides desirable characteristics for the impact structure of the beam. The high mechanical properties inherent in UHSS steel support impact design of highly energy absorbing beams with structural members that deform with impact load. The UHSS material also provides desirable spring back characteristics that help restore beam bending and cross-sectional geometry after impact load is released, and also provides excellent strength to weight ratios in each region. Even if the UHSS material presents manufacturing difficulties when stamping is considered, the present invention will still take advantage of the material properties of the UHSS material. For example, UHSS materials are difficult to form due to their ultra-high strength. They also tend to wear the tool quickly. In fact, the mechanical properties inherent in UHSS materials make them a poor choice for stamping. The roll forming process is capable of forming UHSS into complex parts because of the more step-wise method associated with the formation of simple, uncomplicated geometries. In contrast to stamping, the limitations associated with forming UHSS material are not restrictive when roll forming. The present invention takes advantage of the ability to roll form UHSS material and takes advantage of the high mechanical properties associated with UHSS material to produce an impact system of known properties, weight and cost.

In beam 20 (fig. 2), the impact surface of the bumper beam (referred to herein as "front section 22") is a roll-formed section made of UHSS material. The rear face of the impact beam (referred to herein as the "rear portion 27") is a stamped part having a relatively flat portion and is made of High Strength Low Alloy (HSLA) steel. The two halves of the impact beam are joined together at the flanges, for example by welding (fig. 2 and 11), crimping (fig. 9 and 11) or mechanical fixing (fig. 10 and 11). The combination of the two manufacturing methods and the different materials produces an impact beam that can have an infinite number of carefully designed geometries along the length of the impact beam, such as different sized tubular sections and different materials from the front to the rear of the impact beam. This flexibility facilitates the design of impact beams that can be optimized in terms of performance, weight, and cost.

It is apparent from the beam 20 (fig. 2) that the strength of the beam can vary greatly along different portions of the length of the part. However, this advantage is more pronounced by studying beam 20A (FIG. 5), where a "deep" tubular cross-section is formed in the center of beam 20A, and a "shallow" tubular (or sheet) dimension is formed at the ends of beam 20A. For example, the design in FIG. 5 would facilitate more centerline deformation while creating a substantial portion of stiffness and reducing cross-sectional deformation in the frame rails at the ends of beam 20A.

Those skilled in the art of vehicle bumper beams will recognize that increasing the impact beam depth will increase the stiffness of the section and make it more stable during impact, and will also recognize the great advantage of doing so at strategic locations along the beam. Beam 20A (fig. 5) employs a stamped section to increase the cross-sectional depth of the central region of the vehicle while forming a shallower section on the frame rails at the ends of the beam. The shallow depth in the frame rail reduces the packaging space required to package this type of structure and will facilitate a more curved profile at the end of the impact beam. The ability to easily deform shallower depths on vehicle frame rails is overcome by the geometry (i.e., the laminated "zero depth" portion double wall portions 29A and 30A on the rail) and the lack of cross-sectional depth to allow the stamped portion to increase stiffness on the frame rail.

The roll-formed front impact surface (front section 22 or 22A) of the impact beam is of constant cross-section in its central region and may be bent at a constant bend radius or, by means of an in-line tool and roll-forming process, possibly at a combined bend radius. A constrained bend radius will produce more localized loading and possibly more system impact (intrusion into the vehicle) as measured from the inward front surface of the vehicle. A typical compound curved beam will form a flatter surface in the center of the impact beam and a greater curvature at the ends of the impact beam. The combined curve may be more suitable for the current styling trends of vehicles. The combination bending beam facilitates distributing the load over the impact front surface and in turn makes the impact beam less systematic to impact. The ability of a composite bending beam to distribute loads over a larger surface area can also be reproduced with a constant bending beam and an engineered energy absorber. The energy absorber is designed to easily crush over a greater length from the center of the impact beam and in turn load the impact beam over a greater distance extending from the center of the impact beam.

The front and rear portions of the impact beam may be connected using different connection methods. These methods include crimping or flanging (fig. 8), welding (fig. 2-4, 4A-4B, and 5-7), mechanical fastening processes (fig. 9-10), or other joining methods known in the art for securing two structural members together. Each of the illustrated methods may be suitable for joining and each method may result in an impact beam suitable for crashworthiness. The selected connection method for each system constructed in accordance with the present invention is identified and supported through a cost analysis of each method.

The present invention shown in beams 20 and 20A (fig. 2 and 5) is an impact beam system comprised of a roll formed UHSS front section and a stamped HSLA rear section. One of ordinary skill in the art will appreciate that various other materials may be used to design a system that may or may not be used interchangeably with design criteria for performance, weight, and/or cost. For example, the front portion 22 or 22A or 22B may be made of UHSS material, HSLA material, tensile grade steel, boron steel that is quenched and heated after forming, high strength aluminum, extruded aluminum, polymeric materials, or other engineered structural materials. The rear portion 27 or 27A or 27B may also be made of HSLA material, tensile grade steel, boron steel that is quenched and heated after forming, high strength aluminum, extruded aluminum, polymeric materials, and other engineered structural materials. In each of these materials, the thickness and hardness can vary within the parameters of commercially available raw materials. It is contemplated that the rear portion could be made of UHSS material if one wishes to produce a greater number of bumpers, but the shape of the rear portion needs to be possibly modified or simplified (such as by modifying the rear portion 27 to include a shallower draw in the central portion 28, or eliminating the flanges and side walls in the portions 28, 31-32) because UHSS material is very tough during tooling and difficult to form due to low elongation. An alternative approach contemplated by the present inventors is to provide a sheet material to produce the rear portion 27 or 27A or 27B from a plurality of strips welded together. For example, for beam 20 (fig. 2), a strip of UHSS material may be welded to opposite edges of a central strip of tensile grade steel. The strips of UHSS material would each have a width sufficient to form portions 29 and 30, while the central strip of stretchable grade steel would have a width sufficient to form portions 28, 31 and 32.

Ultra High Strength Steel (UHSS) materials are well known and are a well defined class of materials in the prior art. UHSS materials typically have a tensile strength of about 120 to 200KSI (or higher). High Strength Low Alloy (HSLA) steel materials are also well known and are a well defined class of materials in the prior art. HSLA steel material with 120KSI, but higher grades of HSLA material are generally considered to be non-stampable. It should be understood, however, that the ability to punch is also related to the material thickness, size and shape of the part being punched, as well as the degree of flow and "stretch" of the material desired. HSLA steel materials that can be stamped typically have a tensile strength of about 80 KSI. Boron steels and heat treatable hardenable steels may also be used. For example boron steel can be formed when at lower KSI strengths and then hardened at the forming process stage or in a secondary operation. Higher strength aluminum materials are also well known in the art. For example, it is contemplated that aluminum series 6000 materials will be used in the present invention. Aluminum series 6000 materials typically have tensile strengths of up to about 40 KSI. Some extrudable grades of aluminum may also be used in forming front section 22, such as extrudable aluminum series 6000 or 7000 materials. The rear portion 22 may also be made of glass reinforced nylon, glass reinforced polyester, or other structural polymers (reinforced or not reinforced).

As mentioned above, the illustrated bumper beam 20 (FIG. 2) includes a front portion 22 and a rear portion 27. The front section 22 includes a front wall 23 and top and bottom walls 24 and 25 that define a constant open cross section (also commonly referred to as a hat section) that defines a rearwardly open cavity 26. The illustrated front portion 22 is longitudinally curved (i.e., curved), such as by the methods disclosed in sturrus5,306,058 and 5,395,036, which are incorporated herein by reference in their entirety for guidance in forming the front portion 22. The bumper beam 20 also includes an elongated rear portion 27, the rear portion 27 being fitted over and attached to the rear side of the front portion 22. The rear section 27 includes a longitudinal central section 28, which longitudinal central section 28 is longitudinally curved to match the relevant central area of the beam 22 and deep drawn to match the overall cross-sectional shape of the front section 22. The rear portion 27 also includes end portions 29 and 30, which end portions 29 and 30 are also longitudinally curved to match the associated end regions of the beam 22, and further includes angled intermediate portions 31 and 32 that interconnect the end portions 29 and 30 to the central portion 28. The central portion 28 is hat-shaped and comprises an intermediate portion which is relatively close to or in contact with the front wall 23 in the central region of the layered structure, thus minimizing the overall depth and strength of the cross-sectional "tubular portion" in the central region. At the same time, the top and bottom of the hat section stiffen and help stabilize the respective wall portions in the center of the front section 22. Note that the center region of the bumper beam 20 must be strong enough that the center of the bumper beam 20 passes the impact test without failing to fail. The central region must also be sufficiently flexible to absorb the energy of a functional impact test or to transmit it so that the vehicle itself does not prematurely fail during a frontal impact.

In the central region shown, the central portion 28 lies relatively closely against or in contact with the front wall 23 of the front portion 22, but it is envisaged that any desired spacing may be provided, and the arrangement shown thus is intended to mean "flat tubes" in the central region as well as "non-flat" or "thin" tubes in the central region. In the end regions, the ends 29 and 30 of the rear portion 27 fit against the rear edges of the top and bottom walls 24 and 25 to form a tubular cross-sectional shape having a "deep" depth dimension D1. It is contemplated that the ends 29 and 30 of the rear portion 27 may be relatively flat (as shown in solid lines in fig. 4) or the ends 29 and 30 may have an inverted hat shape that extends in a direction opposite the hat shape of the central portion 28 of the rear portion 27 (as shown in phantom lines in fig. 4).

The angled intermediate portions 31 and 32 are provided with a tubular shape of varying cross-section that transitions between the central portion and the ends of the beam 20. It is contemplated that intermediate portions 31 and 32 may be deep drawn to form mounting surfaces adapted for attachment to vehicle frame rails, such as the illustrated beam 20B having a rear portion 27B with the rear portion 27B having coplanar and, if desired, spaced apart deep drawn mounting surfaces 29B and 30B (fig. 4A).

It is contemplated that the rear portion 27 will be made by an optimal process. The illustrated rear portion 27 may be stamped using a stamping technique. Simplifying the rear portion 27 (fig. 2) may enable it to be made of High Strength Low Alloy (HSLA) material because it contains a relatively simple bend. It is contemplated that tensile grade steel will always be used whenever the rear section 27 has a "deep" zone requiring material flow. On the other hand, it is contemplated that the rear portion 27 may be molded from a polymeric material.

It is contemplated that the top and bottom edges of the rear portion 27 may be secured to the front portion 22 by several different methods. For example, where steel is used for the front and rear portions 22, 27, MIG welding or "standard" MIG welding may be used. It is also contemplated that various welds, such as spot welds, may be used to secure the edge flanges of the rear portion 27 and the front portion 22 together. Rivets and other mechanical attachment methods known in the art may also be used. Moreover, the optimum machining will depend on the strength and performance of the rear and front portions 27, 22, and also on the functional requirements of the beam 20. Where a formable material such as stretchable steel is used, it is contemplated that another attachment method may be used, such as a crimped flange 35 (fig. 9) where the edge of the rear portion 27 near the end is folded back on itself to retain the edge 36 of the front portion 22. Where the materials of the front and rear portions are different, a mechanical connection may be preferred, such as a rivet, crimp or toggle locking method.

It is also contemplated that these attachment methods may be used in combination, such as welding in the critical high stress areas, and rivets or other methods in the less stressed attachment areas. The stretchable steel and aluminum may be locked together with the toggle links according to their grades, which is a mechanical connection using the sheet material itself to form a rivet-like connection. A typical toggle lock connection 40 is shown in fig. 9. Note that toggle-locking technology is commercially available. In the toggle-lock connection 40, the edge flanges 41 and 42 abut along the end regions of the rear section 27 and the front section 22. A tool pin (not shown) is forced through the edge flanges 41 and 42 to stretch the flange material to form a double thickness protrusion. The tooling pin is removed (or temporarily put in place during the grit blasting step) and the part is then grit blasted or impacted in a manner that causes the head 44 to mushroom while the neck 45 remains thin. As a result, the material of the rear portion flange 41 in the head 44 is clamped inside by the material of the neck 45 of the front portion after the grit blasting step. The effect is much like a rivet 46, shown in the lower part of fig. 10. It is of course contemplated that the rivets 46 may also be used for fastening. Where the reinforcement material and/or the front portion 22 are substantially different materials (e.g., one is steel and the other is aluminum or plastic), mechanical attachment, such as with rivets 46 or edge crimping, is a possible and desirable option. Crimping of flanges 41-42 (i.e., one flange 41 folded back on itself to retain a mating flange 42) is another attractive attachment method because it uses the material of the portions 22 and 27 themselves without the need for additional parts or components. The flange 41 is shown as continuous, but a slit 48 may be employed.

Another contemplated way is to weld a plurality of strips together to form a long coil from which the rear portion 27 is made. Multiple strips may be selected to have optimal strength and material properties in each of their final positions in the back section 27. For example, the end portions 29 and 30 can be made of one material (e.g., UHSS), while the intermediate portions 31 and 32 and the central portion 28 can be made of a more ductile or lower strength material, such as HSLA steel. In addition, each portion 28-32 may have different material thicknesses and properties. A wide variety of options are possible, as will be readily appreciated by those of ordinary skill in the art of vehicle bumper manufacture and roll forming and stamping.

Bumper beam 20A (fig. 5-7) is similar in many respects to bumper beam 20. To reduce redundant discussion, the same reference numerals are used to designate the same or similar parts, components and features, but with the addition of the letter "a". This is done to reduce redundant discussion and not for other purposes.

Bumper beam 20A (fig. 5-7) is similar to bumper beam 20 in that bumper beam 20A includes a front portion 22A and a rear portion 27A. But in the central region of the bumper beam 20A, the rear portion 27A forms a tubular portion having the front portion 22A. At the same time, the ends 29A and 30A of the rear portion 27A are shown closer to and flat against the end of the front portion 22A. Thus, the bumper beam 20A has a tubular portion on the central area thereof while the end portions thereof are hardened. The end of the rear portion 27A may have a B-shaped cross-section, as opposed to a layered double thickness configuration. The front portion 22A and the rear portion 27A of the bumper beam 20A can be secured together by any of the exemplary attachment means shown in fig. 8-10 and/or other attachment methods discussed herein.

The method of the present invention is illustrated in fig. 11. The method includes selecting a strip material (such as UHSS or UHLA steel material) at step 49 and then roll forming the strip material to form an open front portion 22 (which may be C-shaped, W-shaped or hat-shaped) at step 50, including (optionally) bending the front portion to form a longitudinally bent portion at step 51. The material of the rear part 27 is selected in step 52, prepared in step 53 if necessary, and stamped in step 54. The step 53 of preparing the strip may include welding multiple strips (seam blanks) together and/or heat treating (e.g., annealing) portions of the single strip so that the particular strength characteristics ultimately reach the predetermined locations of the finished rear portion 27. It is contemplated that where heat treatment is employed, such preparation may be performed before, during, or after the stamping step. On the other hand, instead of steps 52-54, the rear portion 27 may be made by molding (or may be made using other forming and bending techniques) in step 54'. The rear portion 27 is then mated 55 with the front portion 22 and then joined 56. As described above, step 55 of mating rear portion 27 to beam 22 may form a variety of different shapes, including various tubular cross-sectional dimensions and depths along the length of beam 20. It is contemplated that the mating step 55 may be performed in-line with the roll-forming machine, or off-line with a secondary operation at the end of the roll-forming operation, thus forming part of a continuous manufacturing process, or off-line with a separate process. Another option is to take the roll formed front section and feed it into an automatic transfer continuous press where it is secured to the rear section after it is stamped. For example, an automatic transfer continuous press may include tooling for stamping the rear section 27. At (or near) the final stage of the stamping process, the roll formed front section 22 is fed onto an automatic transfer press and connected to the front section 22, for example by a crimping process, welding, riveting or toggle locking process to complete the connection. Alternatively, one may use mechanical fasteners or spot welds on the press. It is contemplated that the joining step 56 may include a variety of different joining methods including welding (MIG welding, standard MIG welding, spot welding, mechanical fastening such as crimping, toggle lock joining (see previous discussion of toggle lock and UHSS materials), rivet joining, and other joining means).

Improvements in or relating to

The improved bumper beam 20C (fig. 12-15) includes similar or identical parts, components, and features as the beams 20-20B. In the beam 20C, the same reference numerals are used to designate the same and similar components to reduce redundant discussion. However, it should be understood that the discussion of beams 20-20B also applies to beam 20C.

Beam 20C (FIG. 12) includes a front section 22C and a rear section 27C that are joined together to form a tubular beam of different cross-sectional dimensions along its length. The front section 22C is made of a relatively high strength material, preferably a material such as ultra-high strength steel (UHSS) or an advanced ultra-high strength steel such as a material having a tensile strength of 220 KSI. The front section 22C preferably has a more uniform cross-sectional shape to enable roll forming. The rear portion 27C is made of a material that can be formed by a stamping process. The vertical cross-section defined by the beam 20C has a depth dimension that varies depending on the cross-section taken along the length of the beam, each cross-section being optimally suited for a particular location on the beam 20C for optimal impact strength and energy absorption capability. The illustrated front and rear portions 22C and 27C include top and bottom edge flanges 41C and 42C that can telescopically overlap when the portions 22C and 27C are assembled together. The interface on the edge flange 41C of the front portion 22C and the edge flange 42C of the rear portion 27C defines a horizontal plane that extends along the longitudinal direction. The edge flanges 41C and 42C are secured together, such as by spot welding or by stitch welding or continuous welding, such as MIG welding or by any of the various welding and mechanical joining techniques previously disclosed in this application. Note that the edge flange of the front portion 22C is disposed within the edge flange of the rear portion 27C. This is so that if the beam 20C is impacted sufficiently to shear the connection welds (i.e., to shear a weld bead or other connection means), the front section 22C will slide rearwardly within the top and bottom walls of the rear section 27C until the flange 41C of the front section 22C engages the rear wall of the rear section 27C. By this mechanism, the front portion 22C is contained within the rear portion 27C and the beam 20C retains most of its strength even if some or all of the attachment devices shear prematurely. This is a secondary safety device that may be desirable in some situations and for some vehicles.

Front section 22C (fig. 13), preferably made of UHSS grade with a tensile strength of 220KSI, is longitudinally curved and has a front wall 23C with a channel 52C formed longitudinally therein, front section 22C also having top and bottom walls 24C and 25C extending from front wall 23C. Walls 23C-25C define a rearwardly facing C-shaped cross-section. A hole 53C is formed in the channel 52C at each end.

The rear portion 27C (preferably made of a stamped material such as high strength low alloy or UHSS steel) is longitudinally curved and has an edge flange 42C shaped to mate with the front portion 22C, and includes a central portion 28C and end portions 29C shaped to mate with the walls 24C and 25C of the front portion 27C as needed. The rear wall 55C extends the length of the rear portion 27C. In the central portion 28C, the shape of the rear wall is relatively flat. In the inner portion 56C of the end 29C, the rear wall is pressed forward toward the front wall 23C of the front portion 22C. At an outboard portion 57C of the end 29C, the rear wall is shaped rearwardly to form a flat region that aligns with a similar outboard portion on the other end. The middle portion 58C of the end portion 29C transitions between the two portions 56C and 57C. The outer portion 57C is shown as being flat and adapted to abut and be directly connected to an end cap 59C, which end cap 59C forms the end of a side frame rail on the frame. This configuration eliminates the need for additional parts because brackets do not have to be attached to beam 20C to attach the vehicle frame to bumper 20C. The illustrated end cap 59C is channel shaped and has a central panel 60C connected to the rear wall 55C in the outboard portion 57C and also has a pair of parallel flanges 61C and 62C extending rearwardly to engage the ends of the side frame rails. Reinforcing bosses or channels 63C are formed in the top wall 64C (and bottom wall) of the rear portion 27C, and bosses or channels 65C are also formed on the rear wall 55C of the rear portion 27C as required for strength.

The rear wall 55C is shown terminating short of the end of the front portion 22C (fig. 15). The connecting flange 66C is integrally formed from the end of the rear wall 55C, and the tabs 67C extend from the top and bottom ends of the flange 66C. These tabs 67C are welded and otherwise secured to the top and bottom walls of the rear portion 27C. A post flange 68C extends from the connection flange 66C and a foot flange 69C extends from the post flange 68C. The foot flange 69C abuts the surface of the front wall 23C of the front section 22C. The foot flange 69C is weldable to the front section 22C through the aperture 53C using MIG welding. The foot flange 69C may be attached to the front portion without the aperture 53C using spot welds or mechanical fasteners. Another method of attachment may employ a finger joint 70C that extends from the foot flange 69C through the aperture 53C and curves onto the channel 52C so as not to be an obstruction. This structure, including flanges 66C-70C, supports the ends of front portion 22C to form a bumper 20C with better angular impact strength.

It will be understood that various changes and modifications can be made in the above-described arrangements without departing from the principles of the present invention, and it is also to be understood that the following claims are intended to cover such principles unless these claims by their language expressly state otherwise.

Claims (56)

1. A bumper beam, comprising:

a metallic front member formed to include a front wall and top and bottom walls, the walls defining a constant cross-section and a rearwardly open cavity; and

a rear member formed to fit against the front member and having flanges adjacent to and connected to the front member, the rear member including a first longitudinal center portion defining with the front member a first cross-sectional shape having a first depth dimension; and second longitudinal ends on opposite sides of the central portion, the second longitudinal ends engaging the front piece to define second cross-sectional shapes, each second cross-sectional shape having a second depth dimension different from the first depth dimension, wherein at least one of the first and second cross-sectional shapes is tubular and at least one of the longitudinal portions partially enters the cavity.

2. The bumper beam defined in claim 1, wherein the first and second cross-sectional shapes are tubular.

3. The bumper beam defined in claim 1, wherein only one of the first and second cross-sectional shapes is tubular and the other forms a flat, bi-sheet structure.

4. The bumper beam defined in claim 1, wherein the front member defines a continuously open cross-section adapted to be formed by a roll-forming process.

5. The bumper beam defined in claim 4, wherein the back member is formed from a flat sheet of formable material that is suitable for being formed using a stamping and manufacturing process.

6. The bumper beam defined in claim 1, including a central portion connecting the central portion to the end portions and extending at an angle to each of the central portion and the end portions.

7. The bumper beam defined in claim 1, wherein the front member is formed of an ultra-high strength steel (UHSS) material and the rear member is formed of a material other than an ultra-high strength steel (UHSS) material.

8. The bumper beam defined in claim 7, wherein the other material is selected from one of High Strength Low Alloy (HSLA) steel, tensile steel and aluminum.

9. The bumper beam defined in claim 1, wherein the front member is formed of a High Strength Low Alloy (HSLA) steel material and the rear member is selected from one of an Ultra High Strength Steel (UHSS) material, a stretchable steel, and aluminum.

10. The bumper beam defined in claim 1, wherein the front and rear members are welded together along the flange of the rear member.

11. The bumper beam defined in claim 1, wherein the flanges are secured together using mechanical fastening means.

12. A bumper beam, comprising:

a front portion including front and top and bottom walls defining a constant hat-shaped cross-section having a rearwardly open cavity, the front portion being made of a material selected from the group consisting of High Strength Low Alloy (HSLA) steel and Ultra High Strength Steel (UHSS) material; and

a rear portion mounted against and connected to the rear side of the front portion, the rear portion having the same length as the front portion and including a first longitudinal portion extending between the top and bottom walls to define a first shape having a first depth dimension; and the rear portion includes a second longitudinal portion on an opposite side of the first portion, the second longitudinal portion extending between the top wall and the bottom wall to define a second shape having a second depth dimension, at least one of the first and second shapes being tubular, the rear portion being made of a material selected from the group consisting of an ultra-high strength steel (UHSS) material, a High Strength Low Alloy (HSLA) steel, aluminum, and a polymeric material.

13. A bumper beam, comprising:

a front portion comprising a front wall and top and bottom walls, the walls defining a constant hat-shaped cross-section with a rearwardly open cavity, the front portion being made of a first material; and

a rear portion mounted against and connected to the rear side of the front portion, the rear portion having the same length as the front portion and including a first longitudinal portion extending between the top and bottom walls to define a first shape having a first depth dimension; and a second longitudinal portion on an opposite side of the first portion, the second longitudinal portion extending between the top wall and the bottom wall to define a second shape having a second depth dimension, at least one of the first and second shapes being tubular, the rear portion being made of a second material having a strength lower than the strength of the first material but being more formable than the first material.

14. A method, the method comprising the steps of:

roll forming a front section comprising front and top and bottom walls defining a constant cross section and a rearwardly open cavity;

stamping an elongated rear portion from the sheet, the rear portion having a length that is approximately that of the front portion;

mounting a rear portion against a rear side of the front portion, the rear portion including a first longitudinal portion defining with the front portion a first cross-sectional shape having a first depth dimension; and including a second longitudinal portion on an opposite side of the first portion, the second longitudinal portion fitting against the front portion to define a second cross-sectional shape having a second depth dimension; and

the rear portion is connected to the front portion to form a reinforcement beam portion.

15. The method of claim 14, comprising the steps of: the strips of different strength are welded together to form a sheet for the fabricated part.

16. The method of claim 14, wherein the step of attaching includes welding the abutment flange of the rear portion to the front portion.

17. The method of claim 14, wherein the step of connecting includes locking together abutting flanged toggle links of the front and rear sections.

18. The bumper beam defined in claim 1, wherein the second longitudinal portion defines a relatively flat coplanar mounting surface at the opposite end of the bumper beam.

19. A bumper beam, comprising:

a front member defining a first top flange and a bottom flange; and

a back member defining second top and bottom flanges that abut and are fixedly connected to the first top and bottom flanges, respectively, along a flange length of the back member from end to end; the back piece has the same length as the front piece but is made of a different material, the front piece and the back piece combining to define a central portion and end portions and a transition portion extending from the end portions to the end portions of the central portion, wherein at least one of the central portion and the end portions is tubular and the other of the central portion and the end portions has a middle portion comprising a longitudinally varying cross-sectional shape extending between a top flange and a bottom flange where the front piece and the back piece are generally disposed proximate to each other.

20. The bumper beam defined in claim 19, wherein the rear member is formed of a material having a different strength than the material of the front member.

21. The bumper beam defined in claim 19, wherein the end portions of the rear member include walls of constant thickness and define relatively flat coplanar mounting surfaces at opposite ends of the bumper beam.

22. The bumper beam defined in claim 19, wherein the rear member is formed from ultra-high strength steel (UHSS).

23. A bumper beam, comprising:

a front member formed to include a front wall and top and bottom walls, the walls defining a rearwardly facing cavity; and

a rear part formed to be mounted against the front part to close the cavity, the rear part having flanges fixedly connected to edges of the front part, the front member being at least partially tubular along a length of the rear member, the front and rear members being joined to define a longitudinal central portion and end portions outside the end portions of the central portion and a central portion, the intermediate portion transitions between and interconnects the end portions and the central portion, the central portion and the end portions forming different tubular shapes, which is adapted to central impacts and corner impacts, respectively, the front and rear parts being of different materials, to adjust these materials and geometries along the length of the bumper beam and around its perimeter for performance, weight, and cost, the rear member at the end has relatively flat wall portions that are coplanar and aligned to form a mounting surface thereon.

24. A bumper beam, comprising:

a front member formed to include a front wall and top and bottom walls and defining a rearwardly facing surface; and

a rear member formed to fit against a rearwardly facing surface of the front member, the rear member having edge flanges fixedly connected to edges of the front member to form a structural beam, the front and rear members combining to define a longitudinal central portion and end portions outboard of the central portion end portions and a central portion transitioning between and interconnecting the end portions and the central portion, the central portion and the end portions forming different shapes that are respectively adapted and designed to enable central impact and angular impact, at least one of the different shapes being tubular, the front and rear members being different materials that can be adjusted along the length and perimeter of the bumper beam for performance, weight and cost purposes, one of the central portion and the end portions having a central portion that includes a longitudinally varying cross-sectional shape, which extends between a top wall and a bottom wall, where the front part and the rear part are arranged close to each other.

25. The bumper beam defined in claim 1, 20 or 24, wherein the rear member is formed of a material having a lower strength than the material of the front member.

26. The bumper beam defined in claim 24, wherein the ends of the rear member define relatively flat coplanar mounting surfaces at opposite ends of the bumper beam.

27. A vehicle bumper beam, comprising:

front and rear structural members extending the same length but made of different materials, the front and rear structural members being fixedly secured together along the edge flanges to form a tubular beam assembly defining different cross-sectional geometries along the length of the assembly, the different cross-sectional geometries and different material properties combining to achieve a specifically designed impact performance at a specific location along the length of the assembly, at least a portion of the cross-sectional geometry being tubular, and at least one vertical intermediate portion of the rear structural member being partially fitted into a cavity formed by the front structural member.

28. The bumper beam defined in claim 27, wherein the rear structural member is formed of a material having a lower strength than the material of the front structural member.

29. The bumper beam defined in claim 27, wherein the ends of the rear structural members define relatively flat coplanar mounting surfaces at opposite ends of the bumper beam.

30. A bumper beam, comprising:

a front member formed to include front and top and bottom walls, the walls defining a cross-section having a rearwardly open cavity; and

a rear member formed to fit against the front member and having flanges connected thereto, the rear member including a first longitudinal center portion defining with the front member a first cross-sectional shape having a first depth dimension; and second longitudinal ends on opposite sides of the central portion, the second longitudinal ends engaging the front piece to define second cross-sectional shapes, each having a second depth dimension different from the first depth dimension, at least one of the first and second cross-sectional shapes being tubular, and at least one of the longitudinal portions partially fitting into the cavity.

31. The bumper beam defined in claim 30, wherein the front and rear members include wall portions that are adjacently disposed in at least one of the central portion and the end portions.

32. The bumper beam defined in claim 30, wherein the end portions of the rear member include flat wall portions that are coplanar and aligned and adapted to form the mounting members.

33. The bumper beam defined in claim 30, wherein the front member comprises a polymeric material.

34. The bumper beam defined in claim 30, wherein the back member comprises a polymeric material.

35. The bumper beam defined in claim 30, including mechanical attachment means connecting the front member to the rear member.

36. The bumper beam defined in claim 35, wherein the front member has a relatively constant cross-section that is roll-formable and the rear member is formed by a non-roll-forming process.

37. The bumper beam defined in claim 36, wherein the rear member is a stamping and has a configuration that can be formed by a stamping process.

38. The bumper beam defined in claim 23, wherein the flat wall portion has a uniform wall thickness.

39. A vehicle bumper beam, comprising:

a front portion of relatively high material strength, the front portion including a front wall and upper and lower walls defining a rearwardly facing C-shaped cross section and a rearwardly open cavity; and

a rear portion of lower material strength, the rear portion including a rear wall and top and bottom walls, the walls defining a forwardly facing C-shaped cross section and a forwardly open cavity;

the upper and lower walls of the front section are disposed within the forwardly open cavity of the rear section and telescopically engage the top and bottom walls, respectively, of the rear section and are secured in top and bottom attachment positions which are subjected to shear forces upon impact; upon impact, the front and rear portions combine to form a tubular portion of varying cross-sectional dimensions to provide significant impact strength, even if one or more of the attachment locations shear away from fracture.

40. The bumper beam defined in claim 39, wherein the rear wall of the rear portion includes ends that are horizontally machined into coplanar alignment with one another, the ends forming an integral mounting surface on the rear portion that is adapted to be attached to the vehicle frame.

41. The bumper beam defined in claim 40, wherein the end of the rear portion rear wall includes an attachment flange integrally formed from the rear wall material and bent forwardly to abut the rear surface of the front portion front wall.

42. The bumper beam defined in claim 41, wherein the attachment flange includes a foot flange that abuts the rear surface of the front wall at a location that is longitudinally inboard of the terminal end of the front wall.

43. The bumper beam defined in claim 42, wherein the attachment flange includes an attachment tab that extends through the aperture in the front wall, the attachment tab being bent to rest on the front surface of the front wall.

44. The bumper beam defined in claim 39, wherein the front portion defines a continuous open cross-section adapted to be formed by a roll-forming process.

45. The bumper beam defined in claim 44, wherein the rear member is formed from a flat sheet of formable material that is suitable for being formed using a stamping process.

46. The bumper beam defined in claim 39, wherein the front portion is formed of an ultra-high strength steel (UHSS) material and the rear portion is formed of a material other than the UHSS material.

47. The bumper beam defined in claim 46, wherein the other material is selected from the group consisting of stampable materials selected from the group consisting of High Strength Low Alloy (HSLA) steel, stretchable steel and aluminum.

48. The bumper beam defined in claim 39, wherein the rear portion is selected from a stampable material selected from the group consisting of Ultra High Strength Steel (UHSS) material, stretchable steel and aluminum.

49. The bumper beam defined in claim 39, wherein the front and rear portions include abutment flanges and are welded together along the edge flanges of the rear portion.

50. The bumper beam defined in claim 39, wherein the front portion defines a continuous cross-sectional shape produced by a roll-forming process and the rear portion defines a discontinuous cross-sectional shape produced by a stamping process.

51. A bumper beam for a vehicle adapted to withstand an impact force in a predetermined longitudinal direction impacting the vehicle, said bumper beam comprising:

a front portion including front and top and bottom walls defining a constant U-shaped cross-section with a rearwardly open cavity, the front portion being elongated in a direction perpendicular to a predetermined longitudinal direction of impact, the front portion being made of a material selected from the group consisting of High Strength Low Alloy (HSLA) steel and Ultra High Strength Steel (UHSS) materials; and

a rear portion mounted against and connected to the rear side of the front portion, the rear portion having a length that is close to the length of the front portion and including a first longitudinal portion extending between the top and bottom walls to define a first shape having a first depth dimension; and including second longitudinal portions on opposite sides of the first portion, the second longitudinal portions extending between the top and bottom walls to define a second shape having a second depth dimension, at least one of the first and second shapes being tubular, the rear portion being made of a material selected from the group consisting of an ultra-high strength steel (UHSS) material, a High Strength Low Alloy (HSLA) steel, aluminum, and a polymeric material;

the front and rear portions have attachment flanges which telescopically overlie one another in a direction parallel to the intended longitudinal direction of impact, the attachment flanges being secured together at attachment locations which are subjected to shear stresses when the beam is subjected to impact forces in the longitudinal direction, but the attachment flanges of the front portion are disposed within the attachment flanges of the rear portion so that, even if the attachment locations shear, the attachment flanges of the front portion are retained within the attachment flanges of the rear portion.

52. The vehicle bumper beam defined in claim 51, wherein the front portion defines a continuous cross-sectional shape produced by a roll-forming process and the rear portion defines a discontinuous cross-sectional shape produced by a stamping process.

53. A method, the method comprising the steps of:

roll forming a front section comprising front and top and bottom walls defining a constant cross section and a rearwardly open cavity;

stamping an elongated rear portion from the sheet, the rear portion having a length that is approximately that of the front portion;

mounting a rear portion against a rear side of the front portion, the rear portion including a first longitudinal portion defining with the front portion a first cross-sectional shape having a first depth dimension; and including a second longitudinal portion on an opposite side of the first portion, the second longitudinal portion fitting against the front portion to define a second cross-sectional shape having a second depth dimension; the front and rear portions having attachment flanges telescopically in overlying engagement in a direction generally perpendicular to the front wall; and

the attachment flanges are joined together to secure the rear portion to the front portion to form a reinforcing beam portion, the attachment flanges of the front portion being located within and retained by the attachment flanges of the rear portion even if some of the attachment locations shear and loosen.

54. A method as claimed in claim 14 or 53, the method comprising the steps of: the rear portion is formed from a material having a lower strength than the front portion, and at least one of the first and second cross-sectional shapes is tubular.

55. The method of claim 14 or 53, wherein the anterior portion is made of a material having a strength of at least about 200 KSI.

56. The method of claim 53, comprising the steps of: the rear portion is formed from a material having a lower strength than the front portion, and at least one of the first and second cross-sectional shapes is tubular.