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).