Vehicle chassis

Abstract

We disclose a chassis for a vehicle, comprising an interconnected framework comprising a plurality of tubular sections, and at least one sheet bonded to the framework, wherein the tubular sections are of a non-ferrous metallic composition. The non-ferrous tubular sections have a very thin wall; generally, these sections are made by extrusion, which currently allows for wall thicknesses no thinner than about 2.5 mm. We prefer the wall thickness to be about this level, and ideally no greater than 3 mm. Such a thin-walled tube would usually imply a lower resistance to buckling, but as part of the structural element defined above, we have found that the tube does not buckle and in fact has an impact response that is superior to other alternatives. We therefore prefer that the tubular sections have a profile for which the ratio of the minimum area moment of inertia of its cross section to the square of the unsupported length of the section is less than 2 mm.sup.2. Another way of expressing this approach is to consider the aspect ratio of the tubular section, i.e. the ratio of its length to its wall thickness. Sections with a high aspect ratio will be more prone to buckling. Given the low elastic modulus of Aluminium, a low aspect ratio has been preferred, but according to the present invention a higher aspect ratio of more than about 100 or 150 is feasible.

Claims

1. A chassis for a vehicle, comprising an interconnected framework comprising a plurality of tubular sections, and at least one sheet bonded to the framework, wherein the tubular sections are of a non-ferrous metallic composition.

2. The chassis according to claim 1 in which the non-ferrous tubular sections have a wall thickness no greater than 3 mm.

3. The chassis according to claim 1 in which the non-ferrous tubular sections have a profile for which the ratio of the minimum area moment of inertia of its cross section to the square of the unsupported length of the section is less than 2 mm.sup.2.

4. The chassis according to claim 1 in which the non-ferrous tubular sections have an aspect ratio of more than about 100.

5. The chassis according to claim 1 in which the non-ferrous tubular sections have an aspect ratio of more than about 150.

6. The chassis according to claim 1 in which the sheet is of a composite material.

7. The chassis according to claim 6 in which the sheet is a carbon-fibre composite.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] An embodiment of the present invention will now be described by way of example, with reference to the accompanying figures in which;

[0022] FIG. 1 shows the results of an impact test of various test pieces;

[0023] FIG. 2 shows the geometric design of the test pieces used in FIG. 1a, and

[0024] FIG. 3 shows the cross-section of the aluminium test piece used for FIG. 1.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] FIG. 1 shows the results of an impact test applied to a variety of test pieces according to the general geometric layout shown in FIG. 2. This layout comprises a pair of parallel tubular sections 10, 12 which are joined by a flat panel 14. This arrangement is mounted perpendicularly to a baseplate 16, which is attached to a solid surface 18. The tubes 10, 12 have a pattern of notches 20 in their end sections, to act as crush initiators and ensure that deformation is controlled.

[0026] The steel tubes were circular-section tubes 498 mm long and 63.5 mm outside diameter. The Aluminium tubes were an oval profile shown in FIG. 3, 508 mm long, with a minor diameter 22 of 63.5 mm and a major diameter 24 of 83.5 mm. The difference is achieved by a 20 mm wide flat section 26 to define an oval instead of a circular section.

[0027] A sled 28 with a mass of 780 kg is impacted linearly onto the test piece in a direction parallel to the tubular members 10, 12, to crush the test piece against the solid surface. The sled is projected with a speed of 9.5 ms.sup.−1, giving an impact energy of 35.2 kJ. This simulates a 50 kph Full Frontal Barrier (FFB) full vehicle crash test. FIG. 1 shows the results of four scenarios, as follows:

TABLE-US-00001 Wall Mass thickness Line Tube Panel (kg) (mm) 30 Steel Absent 2.7 1.5 32 Steel 1.8 mm 4.4 1.5 Steel 34 Steel Carbon 3.7 1.5 fibre 36 Aluminium Carbon 2.9 2.5 fibre

[0028] The x axis of FIG. 1 shows the displacement of the sled 28 in mm, and the y axis shows the total force exerted in kN. As the sled is provided with the same impact energy in each case, the total enclosed area of the four traces is the same but the profiles differ. Notably, the carbon-fibre reinforced test pieces exhibited a higher crush force than both the unsupported steel tubes 30 and the tubes with a steel panel 32. The addition of the steel panel to the steel tubes appears to make little difference.

[0029] Second, the aluminium tubes reinforced with a carbon-fibre panel showed the same initial impact force of about 185 kN, but maintained that force more consistently and for much longer into the impact than the steel tubes reinforced with a carbon-fibre panel. The latter line 36 drops off quickly to around 140-150 kN whereas the Aluminium-tubed test piece stays in the 170-190 kN range for much longer. This suggests that the Aluminium tubular sections and the reinforcing panel are co-operating under deformation in a manner that the steel tubular sections are not.

[0030] It is also notable that Euler buckling load of the Aluminium tubular sections is considerably lower than that of the steel tubular sections. Taking the well-known Euler equation for the collapse of a column under an axial load, i.e.

[00001]$P_{\mathrm{cr}}=\frac{{\pi }^2\mathrm{EI}}{{\left(\mathrm{KL}\right)}^2}$ [0031] where [0032] P.sub.cr=Euler's critical load (the longitudinal compression load on a column), [0033] E=the modulus of elasticity of the column material, [0034] I=the minimum area moment of inertia of the cross section of the column, [0035] L=the unsupported length of column, and [0036] K=the column effective length factor, reflecting the boundary conditions of the column,

[0037] and approximating the Aluminium tubes as a circular section with an outside diameter of 63.5 mm and a wall thickness of 2.5 mm, the tubular sections have buckling characteristics of:

TABLE-US-00002 Tube E (GPa) I (mm.sup.4) P.sub.cr (kN) Steel 200 281000 559 Aluminium  69 446000 295

[0038] The calculation has been on the basis of K being 2, corresponding to one fixed end and one free end.

[0039] Thus, the Aluminium tube has a buckling strength which is considerably lower than the steel and which is nominally inadequate relative to the failure strength of the test piece, after allowing a suitable safety margin. To increase the buckling strength of the Aluminium tube to match that of the steel tube, the wall thickness would have to be increased to 5.5 mm. Comparing these tube designs:

TABLE-US-00003 Wall Moment Geometric thickness Length of inertia Ratio Aspect Tube (mm) (mm) (mm.sup.4) (mm.sup.2) Ratio Steel 1.5 498 281000 1.1 332 Equivalent 5.5 508 847000 3.3  93 Aluminium Thin 2.5 508 446000 1.7 203 Aluminium

[0040] The geometric ratio noted is intended to reflect the influence of the tube geometry on the buckling performance. It is the ratio of the minimum area moment of inertia of the cross section of the tubes to the square of their unsupported length. As can be seen, the test piece of this-walled Aluminium tube has a ratio less than 2 mm.sup.2, and closer to that of a steel tube than that of an Aluminium tube designed to match the buckling strength of the steel tube. Likewise, the aspect ratio of tube, which is considerably easier to determine in practice, is well above the sub-100 level of the Aluminium tube designed to be equivalent in mechanical strength to the steel tube and is distinctly over 150. Given that the Aluminium has an elastic modulus 2.85 times less than that of steel, the fact that a test piece made up of tubes with an aspect ratio of only 1.6 times less and a geometric ratio of only 1.5 times more achieves the same yield force and a better impact absorption profile indicates that a useful effect is present in the selection of thin-walled Aluminium tubular sections in this context.

[0041] Thus, when combined with a supporting composite panel, Aluminium sections can be provided with a considerably thinner wall than is apparently necessary based on a consideration of their resistance to buckling. This saves material usage, reducing the environmental impact of the vehicle, reduces the weight of the vehicle, and reduces the material cost.

[0042] It will of course be understood that many variations may be made to the above-described embodiment without departing from the scope of the present invention.