The invention relates to a motor vehicle bumper beam system with the features of the introductory part of claim 1.

Motor vehicle bumper beam systems are widely known and are used in practice for more than 100 years now.

Bumper beam systems for motor vehicles have evolved to meet increasingly demanding safety standards imposed by regulations and by vehicle manufacturers' own specifications. Efforts, hitherto, have mainly been directed to improving the safety of occupants of a vehicle's passenger cell in the event of a vehicle impact, although limiting the amount of damage suffered by a vehicle's main structure is also important, hence the common use of 'crush cans' (also known as 'crash boxes') in vehicle chassis design. Systems improving the safety of vehicle passengers have become commonplace.

More recently, efforts have focused on reducing the rate of death and injury to pedestrians in the event of a vehicle/pedestrian collision. In order to meet the relevant required standards, vehicle manufacturers have typically provided energy-absorbing material such as foam in the space between a front fascia ('skin') of a vehicle bumper and the bumper beam itself, so that if the vehicle collides with a pedestrian, at least some of the energy of the impact will be absorbed by the foam, thus reducing the likelihood of causing death or injury to the pedestrian (e.g. US-A-5,803,514 ). Inclusion of such foams and the like can also reduce damage to the bumper beams themselves, in the event of low-speed 'parking collisions', for example. Increasing the amount of foam generally results in an increased (and thus beneficial) amount of energy absorption, but in many vehicles, the amount of space available behind the bumper fascia is limited.

One way of increasing the amount of space available for the energy-absorbing foam, is to extend a front part of the vehicle. In extending the front part of the vehicle in this way, the amount of space available between a forward part of a longitudinal beam of the vehicle chassis and the bumper beam, known as the vehicle's 'package space', which is often occupied by safety devices, remains unchanged, with additional space being created between the front fascia of the bumper and the bumper beam. However, vehicle manufacturers can be reluctant to incur the expense involved in altering the design of vehicles in this way, and technical and aesthetic considerations may preclude such alterations to the design of vehicles, in any event.

From a patent application pending at the European Patent Office, but not yet published ( EP 06 006 250.2 filed March 27, 2006 with priority of April 1, 2005) a way for increasing the effective length of the crush cans in a given "package space" is proposed. Firstly it is proposed to have a bumper beam from high-strength steel with a generally C-shaped cross section so that the front end of each crush can engages directly with an inner face of the front of the bumper beam. Thus a part of the crush can is received within the bumper beam. As a further measure to increase the effective length of the crush can a rearward part of the crush can is engageable with a mounting member which is locatable within a forward part of the longitudinal beam of the vehicle chassis such that the crush can is partially disposed within the longitudinal beam of the chassis itself.

In this way, the amount of space occupied by the bumper beam system forwardly of the longitudinal beams of the vehicle chassis can either be reduced, allowing a greater quantity of energy-absorbing foam to be provided behind a fascia of the vehicle's bumper, or with a given amount of space in the vehicle body the effective length of the crush cans can be considerably increased.

Whereas the crush cans are the essential parts of the bumper beam system for low speed collisions (as extensively discussed above), in order for a bumper beam system to perform well for a high speed impact, a rigid and strong bumper beam is required. This is the reason why in the prior art mentioned above ( US-A-5,803,514 ) the bumper beam is roll-formed as a single piece from high-strength steel sheet material and has a closed, rectangular hollow cross section over its entire length, closed by welding along a lengthwise extending welding line at the centre of the front portion of the bumper beam.

While performing well in high speed tests this kind of bumper beam ruins above explained attempts to increase the effective length of the crush cans which is important for low speed collisions.

Now, the object of the present invention is to provide an improved motor vehicle bumper beam system which performs well in high speed as well as low speed collision tests.

In accordance with the invention this object is achieved by a motor vehicle bumper beam system with the features of the introductory part of claim 1 and, in combination therewith, the features of the characterizing part of claim 1.

Following the teaching of this invention it is possible to generally provide a bumper beam that has a closed hollow cross section over its entire length. Whereas the cross section of the bumper beam in its middle section is rectangular or the like with a full depth helpful for high speed collision, in the end sections of the bumper beam a substantial effective length of the crush cans can be provided by flattening the bumper beam to a reduced depth. So the bumper beam is not opened to a C-cross section at the ends thereof, and no cutting operation is performed there. Cutting is an expensive and complex operation in particular with a high-strength or ultra-high-strength steel that is a preferred material for the bumper beam. Moreover, cutting will result in a permanent strain within the bumper beam leading to an increased risk of fractures and fissures in the steel sheet material.

Instead, in the inventive system the bumper beam is just pressed in the end sections and those end sections are thus flattened to a reduced depth which is much less than the full depth in the middle section. The crush cans are attached to the bumper beam in the flattened end sections thereof. Thus they find more "package space" so that their effective length can be higher than with a bumper beam without flattened end sections.

Previous studies have shown that a roll-formed bumper beam with a cross section in the form of the letter B performs particularly well. In particular, the combination of a B-section bumper beam with ultra-high-strength steel will result in high performance under high speed impact conditions. And in accordance with the present invention the important function of the crush cans for low speed impacts are improved by the flattened end sections of the bumper beam.

With the middle section of the bumper beam in the form of the letter D or the letter B it is preferable that the crush cans are attached to the bumper beam on the curved (letter D) or the double-curved (letter B) side. This side is the inner face of the bumper beam.

It is generally possible to just leave the bumper beam in its mechanically shaped form but without a welding seam to close this beam where the two halves meet, preferably on the front face of the bumper beam. Nevertheless, closing the bumper beam by welding is a preferred option, in particular closing it on the front face opposite to the crush cans.

For side impact protection beams in motor vehicle bodies it is known not only to use a B-section profile, roll-formed as a single piece from high-strength steel sheet material, but also to have the side impact protection beam in the end sections thereof pressed and thus flattened to a reduced depth much less than the depth of the side impact protection beam in a middle section thereof. Those flattened end sections are used to attach the side impact protection beam to the surrounding framework of the vehicle body, namely the side door by welding, riveting or the like ( US-B-6,591,577 ).

Now, further preferred modifications and improvements of the present invention together with non-limiting embodiments of this invention will be described in greater detail with reference to the accompanying drawings. In the drawings:

Fig. 1

is a top plan view of a motor vehicle bumper beam system according to the invention,

Fig. 2

is an enlarged view of one end section of a preferred embodiment of the inventive bumper beam system,

Fig. 3

is a schematic representation of the cross section of a preferred bumper beam in its middle section and its end section.

Fig. 1 shows the basic structure of an embodiment of a motor vehicle bumper beam system comprising an elongated, preferably somewhat arc-like configured bumper beam 1 with a length that - more or less - conforms to the width of the corresponding motor vehicle body. A crush can 2 is provided at each end of the bumper beam 1, the crush can 2 extending generally perpendicular from the bumper beam 1, and a mounting member 3 is provided associated to each crush can 2 for mounting the crush can 2 to a rigid part of a chassis, in particular to the front face of a longitudinal beam 4 of a chassis of a motor vehicle. The bumper beam 1 is roll-formed as a single piece from high-strength steel sheet material. Two longitudinal beams 4 of the vehicle chassis are indicated schematically. For detailed information reference is made to the prior application ( EP 06 006 250.2 filed March 27, 2006 with priority of April 1, 2005) the content thereof being included herewith by reference.

In the event of a sufficiently forceful impact between a vehicle equipped with the bumper beam system of Fig. 1 and another vehicle or a stationary object, one or both of the crush cans 2 may deform by compressing, thus absorbing at least some of the energy of the impact and reducing the likelihood of death or injury to occupants of the vehicle. In addition, the crush cans 2 offer some protection to the vehicle's chassis, so that it may be possible, after an impact, to repair the vehicle simply by replacing the bumper beam 1 and associated bodywork, without needing to repair the chassis.

Now, in the present invention it is provided that the bumper beam 1 over its entire length has a closed hollow cross section. The cross section of the bumper beam 1 in its middle section 5 may be rectangular, circular, box-like, or any other suitable closed form. It defines a specific full depth D₁ of the bumper beam 1. The bumper beam 1 in the end sections 6 is pressed and thus flattened to a reduced depth D₂ which is much less than the full depth D₁ (Fig. 3). The crush cans 2 are attached to the bumper beam 1 in the flattened end sections 6 of the bumper beam 1.

Without any cutting of the material of the closed hollow bumper beam 1 its effective measure taken in the longitudinal direction of the vehicle chassis, here indicated as the depth D₁ of the bumper beam 1, is substantially reduced in the end sections 6 by mechanically flattening those end sections 6 of the bumper beam 1 to a substantially reduced depth D₂. This is much less than the full depth D₁ in the middle section 5. Fig. 3 shows that the reduced depth D₂ is less than half of the full depth D₁.

Fig. 1 and 2 show that by this means the effective length of the crush cans 2 can be similar to the length of the crush cans 2 in the prior art bumper beam system with a C-section bumper beam 1. However, the bumper beam 1 in the present system is of three-dimensional closed hollow cross section in the middle section 5 with the corresponding positive characteristics for high speed collisions.

The cross section of the bumper beam in its middle section 5 was above described as being rectangular, circular, box-like or the like. The preferred form of the cross section shown in the drawings is that of a letter B. Also the cross section in the form of the letter D is an interesting option. However, the cross section in the form of the letter B has proven to be particularly effective under high speed collision aspects.

As can be seen in the drawings it is preferred that the crush cans 2 are attached to the bumper beam 1 on the curved (letter D) or double-curved (letter B, embodiment of Fig. 3) side thereof.

Now, it is possible to have a fully closed hollow bumper beam 1 where the bumper beam 1 is closed by welding along a lengthwise extending welding line. This is used in the prior art bumper beam system ( US-A-5,803,514 ) and may be used here as well. This bumper beam 1 will be welded in its front face where the two halves of this bumper beam 1 meet. This will be done via spot welding or laser welding or any other suitable welding process, in particular high frequency welding, MIG arc welding or MAG arc welding.

Fig. 3 as well shows that in the present and preferred embodiment the flattened end sections 6 are provided via a series of press operation steps that introduce lengthwise extending grooves 7 into side walls of the bumper beam 1. It is important to carefully design the grooves 7 and to carefully define the press operation steps so that the flattening of the end sections 6 will not result in undue strain within the bumper beam 1.

It is common technical knowledge that high-strength steel and, even more important, ultra-high-strength steel is not very formable and has a low elongation, thus rendering it less suitable for mechanical pressing operations. Recent developments in alloyed steel are promising in this respect (for cold-roll-formed profiles from a high-strength deep drawing steel and details of most recent developments in this respect reference is made to the publication " Innovate! 1/05", pages 8 to 13, published by ThyssenKrupp in January 2005 and to the publication in " Max Planck Forschung 3/2004", pages 36 to 41 published by the Max Planck association in Germany ).

In a preferred embodiment of the present system it is provided that in the flattened end sections 6 the high-strength steel of the bumper beam 1 is heat-softened and thus is permanently more malleable and formable than in the middle section 5. With this extra process step the steel material is locally annealed to be more malleable ad formable. This enables the use of a particularly high grade steel and still be able to form the flattened end sections 6 of the bumper beam 1 by a mechanical pressing operation.

So in the end a particularly preferred embodiment of the bumper beam system according to the invention is characterized in that the flattened end sections 6 are provided via a series of press operation steps that include a local annealing process step for heat softening of the high-strength steel of the bumper beam 1 in its end sections 6 only.

Talking about high-strength steel in this application shall refer to steel with a tensile strength above 450 MPa. Talking about ultra-high-strength steel in the present context shall mean steel with a tensile strength between 600 MPa and 800 MPa and more. (Here again reference is made to the publication " Innovate! 1/05", pages 8 to 13, published by ThyssenKrupp in January 2005 and to the publication in " Max Planck Forschung 3/2004", pages 36 to 41 published by the Max Planck association in Germany .)

As far as the crush cans 2 are concerned there are no restrictions and the selection of crush cans 2 is guided by the intended use. In general it is possible that the crush cans 2 are made from mild steel, high-strength steel, or a light metal like aluminum, and/or are generally hollow, and generally square, rectangular, cylindrical, conical or frusto-connical in shape with or without annular grooves, ridges 8 and/or indentations. In the present embodiment the annular ridges 8 and intermediate grooves extend along substantially the entire length of the crush cans 2, as shown in particular in Fig. 2.

The crush cans 2 may be manufactured by pressing, stamping or roll-forming sheet material into two generally semicylindrical or semi-conical halves which are then joined together along their longitudinal edges by welding or any other suitable means. Alternatively, the crush cans 2 may be manufactured by hydroforming a cylinder of material, a method which comprises placing the cylinder in a die formed with a 'negative' of a desired pattern of ridges and grooves and pumping pressurized water or other fluid into the die so that the cylinder is forced to take on the desired shape.

Likewise, the mounting members 3 for the crush cans 2 that are used to attach the crush cans 2 to the longitudinal beams 4 may have any appropriate shape and material. The most basic mounting member 3 will be a simple mounting flange or mounting plate which may even be integrated into the crush can 2 as such. There are ample embodiments of crush cans 2 and mounting members 3 available in the prior art.

However, in the present embodiment and in particular shown in Fig. 2 a specific type of mounting member 3 is provided. This type of mounting member 3 allows for a further increased effective length of the crush can 2. In this regard it is provided that the mounting members 3 for the crush cans 2 are generally C-shaped, U-shaped, cup-shaped, or dish-shaped with a part of the mounting member 3 being locatable within a longitudinal beam 4 of the chassis.

As can be seen in Fig. 2 the cup-shaped mounting member 3 of high-strength deep drawn steel may be attached to an outer edge of the longitudinal beam 4 of the chassis by welding, riveting, bolting, or any other convenient means, Thus the rear end of the crush can 2 is somewhat reset into the longitudinal beam 4 of the chassis. The rear end of the crush can 2 is attached by welding or any other suitable means to an inner face of an end wall of the mounting member 3. So this particular type of mounting member 3 corresponds beneficially to the overall aspect of the invention as far as the effective length of the crush cans 2 is concerned (for reference see again EP 06 006 250.2 filed March 27, 2006 with priority of April 1, 2005).