Door reinforcement tube

A door reinforcement beam for use in passenger vehicles capable of absorbing lateral impacts includes: a base tube made of a metal having a high degree of strength, toughness and capacity for deformation work and a reinforcing section being disposed and attached to the central portion of the base tube so as to further enhance the door reinforcement's capacity to absorb impact energy.

FIELD OF THE INVENTION 
The present invention relates to a door reinforcement bean being composed 
of metal having a high degree of strength, toughness and capacity for 
deformation work so as to absorb lateral impacts in passenger cars. 
BACKGROUND OF THE INVENTION 
Door reinforcement beams serve as stiffeners for automobile doors by 
absorbing impact energy in the event of a lateral impact and to convert 
impact energy into mechanical work. In this manner, passengers in a car 
can be protected from injury. For such a door reinforcement to perform 
this safety function, the following values, for instance, must be 
satisfied: 
Tensile strength Rm at least 1,100 N/mm.sup.2 
Elastic limit Rt at least 800 N/mm.sup.2 
Elongation at rupture A5 at least 8% or a work capacity W-150 of at least 
1,900 Joules, depending on the dimensions of the section. 
These minimum requirements can be lower or higher depending on the 
specification of the automobile manufacturer. 
The work capacity of a door reinforcement beam can be tested by subjecting 
it to a bending of 150 mm in a 3-point bending test. The force applied is 
recorded with respect to the bending distance and the area below the curve 
is determined. This integral then gives the desired work capacity. 
Since the door reinforcement should be suitable for installation in small 
cars having narrow doors, it is necessary that its dimensions be kept as 
small as possible. Furthermore, it is necessary that the total weight of 
the car be only minimally increased by the installation of the door 
reinforcement. 
A door reinforcement beam of this type is known from Federal Republic of 
Germany Patent 36 06 024. The rectangular, preferably square, hollow 
section with bead portions which extend outward at both ends on the inner 
and outer flanges is an extended section of light metal. One particular 
embodiment as shown in FIG. 9 of the above referred patent is 
characterized by the fact that the central region of the impact beam is 
reinforced over a given length by a length of pipe arranged within it. 
This proposal, however, has the disadvantage that when light metal is 
used, the wall thicknesses of the section must be very large in order to 
obtain the desired work capacity. Furthermore, such a section is poorly 
adaptable to maneuvering around obstacles in an area of installation 
within the door. Furthermore, an expensive extrusion tool must be used 
with the section and once the tool has been selected for use with the 
section, it can no longer be used in the event of a structural change in 
the door or upon a change in the model. 
An alternative solution can be noted from Federal Republic of Germany 
Patent 37 28 476 in which the values for the tensile strength and elastic 
limit of a door reinforcement tube are increased, as compared with the 
above-indicated prior art, by the use of a special steel alloy. This 
alternative however, has the disadvantage that the strength increasing 
effect is achieved only by alloying expensive elements such as nickel and 
molybdenum. 
SUMMARY OF THE INVENTION 
An object of the present invention is to develop a door reinforcement beam 
which, with the use of a low-price steel alloy and while satisfying the 
required mechanical values, is of low weight and can easily be adapted to 
different door designs. 
In the proposed door reinforcement beam, use is made of the discovery that 
work capacity is dependent, inter alia, on the value of the moment of 
resistance of the structural part. Since the test for determining the work 
capacity corresponds approximately to the stressing of a freely lying beam 
with a load acting on its center, the critical cross section for failure 
of the structural part lies at its center. In accordance with known 
equations for static bending tests on a beam shaped as tube, the 
deflection f is inversely proportional to the fourth power of the outside 
diameter with a reduction factor depending on the ratio of the diameter to 
the wall thickness. In order to make optimal use of this known 
relationship with respect to a door reinforcement tube, it is proposed 
that the critical central region of the door reinforcement be reinforced 
specifically against bending and/or kinking. It is, therefore, proposed to 
attach to a base tube a length of tube which is inserted within the base 
tube or pushed over its outside surface and over a given length. A means 
for attachment to the base tube or beam is necessary for the reinforcing 
part to be fixed in position to the base tube. One must distinguish, 
however, between material-locked attachments (for instance, bonding 
soldering, welding), form-locked attachments (for instance, riveting), and 
force-locked attachments (for instance, shrinking). With respect to 
welding techniques, spot welding, projection welding, laser welding or 
capacitor-discharge welding can be used. As an alternative, a slit tube 
can also be used, the size of which is so selected that when it is placed 
over or inserted into the base tube, a clamping action is achieved and 
special fastening in position is generally not necessary. However, in 
order to be certain that the slit tube will not loosen even after 
prolonged use, it is additionally advisable to fix the slit tube in 
position by form-locking or material-locking means. This is also true if a 
variant, such as a shrink-on length of tube is used. 
In accordance with another feature of the invention, the reinforcement beam 
is developed as a separate length of a tube which is attached to the 
sections of the base tube adjoining in on both sides. The equalizing of 
the wall thicknessees in the region of the attachment can be effected 
either by stretching the ends of the thickened section or by bulging the 
ends of the lengths of the adjoining base tube. As a manner of attachment, 
friction welding may be advantageously employed since in this manner 
different materials can be attached to each other in a simple manner and 
within short cycle times. 
Furthermore, it is also proposed to produce the door reinforcement directly 
from the base tube in the form of a section of thickened wall. This has 
the advantage that the transition from the base tube to the thickened 
region is continuous and no sudden changes in the cross section can occur. 
For optimizing the weight, it is particularly advantageous for the base 
tube to be developed, as a whole, as a beam of identical bending stress 
under the assumption that a single load acts centrally and a given 
deflection is permissible. This design has the advantage that within each 
cross-sectional plane the wall is only as thick as is necessary in order 
to take up the load. Such a thickening which extends over a given length, 
regardless of whether the thickening extends radially outward or inward or 
is distributed uniformly towards the outside and the inside, can be 
produced by different methods. One of the possibilities consists, for 
instance, of drawing with a stepped mandrel (Federal Republic of Germany 
30 21 482), or of non-cutting shaping, such as pressing, rolling or 
hammering (Federal Republic of Germany 36 10 481), if necessary, in 
combination with cutting shaping. The advantage of integral thickening 
over placing lengths of tube on the outside or on the inside is that the 
reinforcement can be shifted, in a more focussed manner radially more 
towards the outside or more towards the inside with regard to the 
permissible maximum outside dimension, the weight of the structural part, 
and the size of the load-bearing cross section. In addition, the length of 
the thickened section can also be freely selected. 
Another possibility for reinforcing the central region of the base tube is 
to provide the tube with single layer or multi-layer wrapping. This 
wrapping can consist of wires or strips of metal or high-strength fibers 
of plastics such as, for instance, aramid fibers. This last-mentioned 
variant, has the advantage that considerable weight can be saved and that 
the wrapping technique is very simple. The reinforcing action is 
particularly good if the wires, strips or fibers are wound under tension. 
In the case of plastic fibers, there is also the added possibility of 
embedding the fibers in a hardenable composition which surrounds them, for 
instance epoxy resin. 
All the above embodiments of the invention, have the advantage that the 
door reinforcement is of a larger cross section only in the critical 
central region while the cross sections in the adjoining regions are 
smaller so that the total weight of the structural part is less than 
comparable door reinforcements. Thus, the door reinforcement can be 
obtained by a simple manner of manufacture without the need for using 
expensive alloy material or undergoing expensive manufacturing processes, 
such as, for instance, additional heat treatment. 
The reinforcing action can be further increased, if for instance, the 
inside of the reinforced region is filled with foam. Mixtures of 
substances for such reinforcing foams can be noted, for instance, from 
U.S. Pat. No. 4,861,097. As an alternative, a stiffening element such as a 
tensile spring or a steel strip twisted around the neutral fiber can be 
arranged within the reinforced region. These last-mentioned stiffening 
elements are placed helically against the inner wall of the reinforced 
region and thus increase the resistance of the critical central region 
against kinking. In accordance with yet another feature of the invention, 
the stiffening element is a rectangular hollow section wherein the long 
side of the element lies in the direction of expected impact. 
In the event that tube lengths or slit-tube lengths are used, there is a 
further advantageous aspect. Recognizing that the base tube is much longer 
than the reinforcement region and in order to keep the cost of the 
material for the base tube low, a simple commercial steel can be used, 
such as, for instance, St.52. In order to specifically strengthen the 
critical central region, the section of the tube which is placed on or in 
the base tube or else welded therein has a higher strength and/or 
toughness than that of the base tube. The better mechanical values can be 
achieved by the selection of a specific material, for example, by the 
alloying of specific elements or by a combination of specific shaping and 
proper heat treatment. Yet another possibility is to harden the surface of 
the length of tube, for instance by case hardening or nitrogenizing.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
FIG. 1 shows in a longitudinal section a first embodiment of the door 
reinforcement beam of the present invention having a base tube 1 and a 
tube length 2 pushed thereover. This outer tube section 2 serves to 
reinforce the critical central region of the base tube 1. The manner of 
the attachment of the outer tube section 2 has been described in detail 
above and need not be repeated here. 
A modification of this embodiment is shown in FIG. 2, wherein a reinforcing 
tube section 3 is inserted into the base tube 1. In yet another variation, 
an outer tube 2 can be placed on the outside of the base tube while an 
inner tube 3 can be inserted in the interior of the base tube. These 
possibilities are also present and applicable in the case wherein a 
clamping slit tube is used on the outside and/or inside of the base tube 
1. 
FIG. 3 shows, in a longitudinal section, yet another embodiment, wherein 
the reinforced critical central section 4 is an integral component of the 
base tube 1. This design has the advantage that within each 
cross-sectional plane the wall is only as thick as is necessary in order 
to take up the load. With reference to FIG. 6, it may also be mentioned 
that the wall thickening can also be located entirely or partially on the 
inside of the base tube 1. 
FIG. 4 is comparable to the embodiment shown in FIG. 1, with the mere 
difference that, in addition, a helical steel strip 5 is disposed within 
the reinforced region. This steel strip serves to further stiffen the 
central region of the base tube 1. 
FIG. 5 shows a cross sectional view of an alternative embodiment wherein a 
rectangular hollow section 6 is disposed within base tube 1 and long side 
7 of the rectangular section which lies in the direction of expected 
impact as indicated by arrow 8. The added weight from the stiffening 
element is relatively slifht since the wall thickness of the stiffening 
element 6 can be kept small. In addition, it is also possible to provide a 
web plate 9 (shown here in dashed line), which is even more favorable from 
the standpoint of weight than the hollow section 6. 
The embodiment shown in FIG. 6, is comparable to FIG. 3, wherein there is a 
reinforcing part 12 which is attached to the base-tube sections 1, 1' by 
two weld seams 10, 10'. The connecting seams 10, 10' can, for example, be 
seams produced by friction welding. The dashed line 11 shown along 
reinforced part 12 is intended to indicate that, as an alternative, the 
reinforcing wall thickness can also be disposed entirely or only partially 
on the inside of the base tube 1. The smoothing of the wall in the region 
of attachment can be achieved by deforming the ends of the reinforcing 
part 12 or, as an alternative, by bulging the ends of the two base-tube 
sections 1, 1'.