The present invention relates to a composite structural element, particularly but not exclu- sively for use in a vehicle suspension, as well as to a method for manufacturing such a structural element.

The materials generally used in mass production of components for vehicle suspensions, such as for instance motorcar suspensions, are steel, cast iron and aluminium, for reasons due to costs of production and performances of the material (weight, stiffness, fatigue life, resistance to limit load conditions). The use of composite materials for the production of motorcar suspension components is traditionally limited to racing cars, such as for instance the Formula 1 cars, or to the so-called supercars, as in this case the weight and performance requirements are of higher importance than the cost.

In the design of components for vehicle suspensions, in particular for motorcar suspensions, a number of requirements conflicting with each other must be complied with. In particular, a motorcar suspension component must be capable of bearing certain kinds of loads (the so called fatigue loads), representative of the normal use of the vehicle, although in heavy conditions. These loads are applied alternately onto the component and this latter must not suffer from crack formation or failures within a given number of fatigue cycles applied. High-strength materials, such as fibre-based composite materials, are suggested in order to fulfil this fatigue life requirement and to limit at the same time the weight. Another structural requirement which must be fulfilled by the motorcar suspension compo- nent is that the component must be able to deform in a predictable and particular way under so-called "misuse" loads, i.e. under "limit load conditions" . With such a kind of stresses the component must be able to deform, reacting with a given reaction load and absorbing a given amount of energy, but connection between wheel and vehicle must always be ensured. In particular, any possible failure must be confined to given zones and must not occur below a given amount of deformation. Ductile materials, such as steel, are suggested in order to fulfil these requirements for controlled deformation and for presence of a deformation witness mark under limit load conditions. US7159880 discloses a structural element, in particular for a vehicle chassis, comprising an elongated body and a pair of connection members mounted at the opposite ends of the elongated body, wherein the elongated body consists of a metal core on which fibre- reinforced plastic material is overmoulded by means of injection moulding so as to provide the elongated body with a cross-section having the desired shape. On the one hand the use of a fibre-reinforced plastic material allows to limit the weight of the component and to ensure at the same time high mechanical properties, while on the other hand the use of a metal core avoids the loss of functionality of the component even in case of damage. Such a known solution is however affected by the drawback that the use of the overmoulding technique to produce the plastic material portion of the structural element makes it possible to obtain only structural elements with a solid cross-section. This inevitably involves limits to the freedom of the designer in designing the cross-section of the element, which limits are excessively penalizing for instance in case of structural elements intended to be used for triangular suspension arms. As is known, in fact, in order to increase the strength of the structural element it is necessary to increase the moment of inertia of the cross-section thereof, in other words to shift the material of the cross-section as far as possible from the middle plane thereof, in the present case from the metal core, which results in an excessive increase in the overall weight of the element. Further examples of structural elements comprising a metal core on which plastic material, if necessary reinforced with fibres, is overmoulded, are known from US6030570, WO2003/039893 and WO2003/039892. Also these structural elements suffer from the same drawback discussed above with reference to document US7159880. US2004/0131418, on which the preamble of independent claim 1 is based, discloses a structural element comprising a first part of metal having a U-shaped cross-section and a second part of plastic material which is attached to the first part to close the U-shaped cross-section thereof, thus forming a hollow structural element having a closed cross- section. The first and second parts are obtained separately from each other and are then joined to each other by bending the edge of the second part onto the edge of the first part, with the use of heat or ultrasounds. If on the one hand this known solution offers the advantage of reducing the overall weight of the structural element due to one of the two parts of which the element consists being made of plastic material, instead of metal, on the other hand the weight reduction allowed by this known solution is minimum, as the cross-section of the structural element is mainly formed by the first part, i.e. by the metal part. It is therefore an object of the present invention to provide a composite structural element which is able to offer similar or better performances with respect to the prior art, with a smaller weight.

This and other objects are fully achieved according to the invention by virtue of a struc- rural element having the features set forth in the characterizing portion of the appended independent claim 1.

Advantageous embodiments of the invention are defined in the dependent claims, the content of which is to be intended as integral and integrating part of the following description.

In short, the invention is based on the idea of providing a structural element comprising a shell and a core, wherein the shell is made of at least one layer of composite material comprising a fibre-reinforced polymeric matrix, and wherein the core is made of a ductile material, in particular of metal. In the following description and claims, the term "shell" is to be intended as referring to any kind of body defining at least one cavity, wherein such a body may be formed indifferently by a single piece or by several pieces joined to each other and may indifferently have either a closed cross- section or an open cross- section (in which case, naturally, the shell will not be a flat element). The simultaneous presence of a material having high mechanical properties (material of the shell) and of a ductile material (material of the core) allows to meet at the same time the opposite requirements for fatigue life, reaction load, modality of deformation and presence of a deformation witness mark under limit load conditions. The fatigue life and the reaction load under limit load conditions are ensured in particular by the high- strength material of the shell, and in this connection the shell will be suitably shaped to provide the element with the inertia characteristics required each time by the specific application. The ability to absorb impact energy, the ability to provide a deformation witness mark and the ability to avoid failures such as to lead to the separation of parts are ensured, on the other hand, by the ductile material of the core. The use of composite material for the shell of the structural element, which forms the main portion of the structural element in terms of volume of material, clearly allows to reduce the overall weight of the element with respect to the prior art. The polymeric matrix of the composite material of the shell may consist of a thermoplastic polymer or a thermosetting polymer (such as for instance epoxy resin). The fibres of the composite material of the shell may be either oriented fibres or short randomly- oriented fibres. Carbon fibres, Kevlar fibres, glass fibres, metal fibres or fibres of any other material adapted to provide the composite material with the required high mechanical properties may be used. In case of a shell made as a body with a closed cross-section, it advantageously comprises two half-shells joined to each other by heating to a temperature such as to cause the fusion of the polymeric matrix of the composite material. Heat may be supplied by contact with hot surfaces (for instance with a thermo-regulated mould) or by laser welding.

The core is advantageously made of sheet metal, in particular of a sheet of high-strength steel, and is suitably shaped so as to have bosses or changes of plane, if required due to structural reasons. The core may be made as a single piece, obtained for instance by stamping, or alternatively may comprise a plurality of separate pieces, which are each obtained for instance by stamping and are securely connected to each other by various joining techniques, for instance by welding, riveting or gluing.

Since the structural element comprises a part (namely, the part made of composite material) made as a shell, voids are present between the shell and the core, which voids may in- differently be connected to each other to form a single cavity or form separate cavities. The cavities existing between the core and the shell may be filled with filler material having the function of providing the structural element with ductility and/or mechanical strength. Naturally, the filler material may also be provided only in some of the cavities. When it is used as a component for a vehicle suspension, the structural element may be provided with one or more connection members, such as articulation bushes. To this end, the structural element further comprises one or more cylindrical sleeves or pins, intended to form each a seat for the respective bush. When the core of the structural element is made of metal, the cylindrical sleeves may also be made of metal and be directly welded to the core. Alternatively, the cylindrical sleeves may be glued, or joined in any other ways, to the shell of the structural element.

Further features and advantages of the present invention will appear more clearly from the following detailed description of a preferred embodiment thereof, given purely by way of non-limiting example with reference to the appended drawings, in which:

Figures 1 and 2 are a perspective view and a plane view, respectively, of a triangu- lar arm for a motorcar suspension as an example of a structural element according to the present invention;

Figure 3 is a section view of the triangular arm of Figures 1 and 2 through the section plane indicated III-III in Figure 2;

Figure 4 is a section view of the triangular arm of Figures 1 and 2 through the sec- tion plane indicated IV-IV in Figure 2; and

Figures 5 and 6 are section views of two alternative embodiments of a structural element according to the present invention.

With reference first to Figures 1 to 4, numeral 10 generally indicates a triangular arm for a motorcar suspension as an example of a structural element according to the present invention. However, as already stated in the introductory part of the description, the invention is applicable to any other structural element for a vehicle, not necessarily an element intended to be used as a suspension component. The arm 10 basically comprises a shell 12 of composite material, a core 14 (Figures 3 and 4) of ductile material and a plurality of seats (in the present case three seats) for receiving each a respective connection member (not shown), such as a bush or a ball joint, for connection of the arm on the one hand to a wheel-carrier (not shown) and on the other hand to the vehicle body (also not shown).

As stated in the introductory part of the description, the shell 12 is a body which is shaped so as to define at least one cavity and, in particular, in the example illustrated in the draw- ings is a hollow body having a closed cross-section. The shell 12 is made of at least one layer of composite material having a fibre-reinforced polymeric matrix. The polymeric matrix may consist of a thermoplastic polymer or of a thermosetting polymer (such as for instance epoxy resin), while the fibres may be oriented fibres, for instance carbon fibres, Kevlar fibres, glass fibres, metal fibres or fibres of any other material suitable to provide the composite material with the required high mechanical properties, or short randomly- oriented fibres (in other words, short non-oriented fibres). In case of oriented fibres the shell will be advantageously made of more layers of composite material overlapped with each other, whereas in case of short non-oriented fibres the shell will consist of a single layer of composite material. Naturally, the orientation of the fibres (in case of oriented fibres), as well as the texture of the fibre and the sequence of overlapping of the various layers, will be chosen so as to provide the shell with the desired mechanical properties. As can be seen in Figures 3 and 4, in case of a shell 12 made as a hollow body having a closed cross-section, it consists of two or more pieces which are produced separately from each other and are then joined to each other so as to obtain a closed cross- section. In the proposed example, the shell 12 consists of two half-shells 12a and 12b, that is to say, an upper half-shell and a lower half-shell, respectively. The two half-shells 12a and 12b are advantageously joined to each other by heating to a temperature such as to cause the fusion of the polymeric matrix of the composite material. In this case, heat may be supplied by con- tact with hot surfaces (for instance, with a thermo-regulated mould) or by laser welding.

Alternatively, the two half- shells 12a and 12b are glued to each other with suitable structural adhesives.

The core 14 is preferably made of metal as ductile material, in particular of high-strength steel. However, the core 14 might also be made of non-metal material, provided it has adequate properties in terms of resiliency and ductility. In the example of Figures 1 to 4, where the shell 12 is a hollow body having a closed cross-section, the core 14 is inserted and attached inside the shell. Preferably, the core 14 extends substantially over the whole plan of the structural element 10, apart from the end zones of the element where the afore- said seats are provided. Alternatively, the core 14 might however extend only in given zones of the structural element, in particular in controlled-deformation zone, where there might be risks of failure of the shell with resulting loss of continuity of the material of the element. The core 14 is advantageously made of sheet metal and is suitably shaped so as to have bosses and/or changes of plane where it is required due to structural reasons. As can be seen in the section view of Figure 3, the core 14 has for instance a cross-sectional profile forming a straight middle section and, at the opposite ends of the middle section, a pair of tabs 16 bent at an angle, for instance at a right angle, to the middle section. On the one hand, the tabs 16 have the function of increasing the bending stiffness of the core 14. On the other hand, in case of the shell 12 being obtained by joining of the two half-shells 12a and 12b, the tabs 16 have the function of supporting the upper half-shell during the step of joining the two half-shells, when this step is carried out by heating. However, the tabs 16 might even not be present. The core 14 may be made as a single piece, obtained for instance by stamping, or may alternatively comprise a plurality of separate pieces, which are each obtained for instance by stamping and are securely connected to each other by various joining techniques, for instance by welding, riveting or gluing. The core 14 is joined to the shell 12 at the flat middle portions of the structural element (straight middle section of the cross-section of the structural element shown in Figure 3). Joining between the core 14 and the shell 12 is obtained by gluing or riveting. In case of a core 14 having lateral tabs 16, the core might be joined to the shell 12 also at these tabs. Moreover, the core 14 may have holes or slots, in which case the half-shells 12a and 12b will be in contact with each other and will be joined by gluing or riveting at these holes or slots.

The shell 12 and the core 14 are shaped in such a manner that voids are defined between these two components, which voids may indifferently be connected to each other to form a single cavity or form separate cavities. In the zones of the structural element where the core is not present, the cavities will be enclosed only by the material of the shell, instead of being enclosed part by the shell and part by the core. The aforesaid cavities may be filled with filler material having the function of providing the structural element with ductility and/or strength. In this connection, Figure 4 shows a cavity (indicated 30 and located around one of the aforesaid seats of the arm) which is filled with filler material.

In the proposed embodiment, in which the structural element is a suspension arm, in par- ticular a triangular arm, the structural element is provided with one or more connection members (in the present case three connection members), such as articulation bushes. To this end, the arm 10 comprises three sleeves or cylindrical tubular elements 18, 20 and 22, of which the first two have a vertical axis and the third one has a horizontal axis, and which are intended to form each a seat for driving the respective articulation bush therein. The cylindrical sleeves 18, 20 and 22 may also be made of metal and be directly welded to the core 14. In addition or as an alternative to being attached to the core 14, the cylindrical sleeves 18, 20 and 22 may be glued or joined in any other ways to the shell 12. Reinforcement layers 24 of composite material may also be attached to the shell 12 and are advantageously made of the same material as that of the shell. The reinforcement layers 24 may be attached to the shell 12 by fusion or gluing. A localized reinforcement of the arm in the areas subject to the highest stresses is thus obtained. With reference finally to Figures 4 and 5, where parts and elements identical or corresponding to those of Figures 1 to 3 have been given the same reference numerals, two example of alternative embodiments of a structural element according to the invention are shown. Differently from the embodiment of Figures 1 to 4, where the two half-shells 12a and 12b are joined to each other with the respective edges (indicated 26a for the upper half-shell and 26b for the lower half-shell in Figures 3 and 4) butt arranged, in the embodiment of Figure 5 the two half- shells 12a and 12b are joined to each other with the respective edges 26a and 26b overlapping with each other. Additionally, as specified above, the two half-shells 12a and 12b may be joined to each other, by gluing or riveting, also at holes or slots provided in the core 14. In the embodiment of Figure 6, the shell 12 of the structural element is made as a single piece of composite material with an open cross- section and the core 14 is attached to the shell 12, in particular by gluing of the tabs 16 of the core to the edges 26 of the shell, so as to form with this latter an element having a closed cross-section. The following steps are basically provided for the manufacturing of a structural element according to the invention:

a) providing a shell 12 made of at least one layer of composite material comprising a fibre-reinforced polymeric matrix;

b) providing a core 14 made of ductile material; and

c) joining the core 14 to the shell 12. In case of the shell 12 being made as a hollow body having a closed cross-section comprising two half-shells 12a and 12b, step a) provides for the two half-shells 12a and 12b being obtained separately from each other, step c) provides for the core 14 being joined to one of the two half-shells (the half-shell 12b in the embodiment of Figure 3 and the half-shell 12a in the embodiment of Figure 5), and a further step d) will also be provided for, in which the second half-shell is joined to the first half-shell and, if necessary, also to the core 14. In this connection, as already mentioned, the two half- shells 12a and 12b are advantageously joined to each other by heating to a temperature such as to cause the fusion of the polymeric matrix of the composite material, the heat required to cause the fusion of the polymeric matrix of the composite material being supplied by contact with hot surfaces (for in- stance with a thermo-regulated mould) or by laser welding. Alternatively, the two half- shells 12a and 12b may be glued to each other with suitable structural adhesives.

As far as step b) is concerned, as already stated above, the core 14 may be produced as a single piece, obtained for instance by stamping, or may alternatively comprise a plurality of separate pieces, which are each obtained for instance by stamping and are securely connected to each other with various joining techniques, for instance by welding, riveting or gluing.

Naturally, the principle of the invention remaining unchanged, the embodiments and the constructional details may vary widely with respect to those described and illustrated purely by way of non-limiting example, without thereby departing from the scope of the invention as defined in the appended claims.