The present invention relates to a use of a composite article as spar component, according to the preamble of the first claim.

WO2008/104174 already describes wind turbines comprising a wind turbine tower and a wind turbine nacelle positioned on top of the wind turbine tower. A wind turbine tower with a number of wind turbine blades is connected to the nacelle through a low speed shaft. From the prior art discussed by WO2008/104174 it appears that a long known measure for increasing the output of a wind turbine is an increasing of the size of the wind turbine blades. WO2008/104174 further describes that wind turbine blades are typically made of glass fibre and resin composite reinforced by metal, wood and/or carbon fibres. The wind turbine blades comprise a straight leading edge, a curved trailing edge, a pointed tip edge and a root which is provided to be connected to the nacelle by means of a hub. The wind turbine blades are usually manufactured by moulding the two blades in halves in two independent moulds. After the blade halves are hardened, a strengthening structure is provided inside one of the blade halves to minimise the risk to bending or breaking of the blade, the connecting surfaces of the blades are provided with an adhesive and placed on top of each other. The strengthening structure described by WO2008/104174 comprises a first and second upright web extending between the two blade halves in a direction perpendicular to the chord of the wind turbine blades along length direction of the wind turbine blades. The first upright web is provided near the leading edge and the second upright web is connected near the trailing edge. The first and second upright webs used to strengthen the wind turbine blade described by WO2008/104174 are also called spar components or spars. It is known to produce spars using composite sandwich panels as for example described in EP1754589. In such composite panels, the upper and bottom face of a core layer of foam material, is positioned between upper and lower layers of fibrous reinforcing material, called the lay up, the whole being impregnated with a resin material. Examples of suitable resins are epoxy and polyester, depending on the rotor blade manufacturing company. Usually such sandwich structures are made using a resin transfer moulding technique, also called infusion technique, which is also described in EP 1754589 A1. In such techniques, the lay up is positioned in a mould to which subsequently a vacuum is applied after which resin is added in the mould. The resin then penetrates into the layer of fibrous material and partially into the foam material and finally the lay up is cured.

Due to the often large forces acting on the spars during operation, the spars are subject to buckling due to which the blades may brake. It has also been found by the inventor that breaking often occurs at impurities, such as dust, cavities and other undesired particles, which are present in the lay up or at the stiffness discontinuities.

In order to nevertheless provide the known spars with the desired properties, the thickness of the core material, the thickness of the lay up and the overall thickness of the spar are often adapted along length direction of the spar. The spar is thus manufactured by fastening a plurality of composite sandwich panels, each having an adapted core thickness, lay up thickness and overall thickness, to each other in length direction of the spar. In this way statistically locations of the spar prone to buckling and/or breaking are provided with a relative strong lay up by adding for example more layers of fibrous material around the foam core. Other locations for example are provided with a weaker lay up since for example less layers of fibrous material are applied around the foam core.

Such spars however present the problem that its production is laborious, time consuming and hardly reproducible, leading to spars with a high weight, variable thickness and/or stiffness discontinuities.

Accordingly, it is an object of the present invention to provide a spar offering a decreased risk for buckling which is easier to produce.

This is achieved according to the present invention by using a composite article showing the technical features of the characterising part of the first claim as spar component. Thereto, the upper and lower layer of fibrous reinforcing material are fastened to each other by means of reinforcing fibrous material which extends in Z-direction of the panel.

The inventor has found that the buckling behaviour occurring with the prior art spars where the upper and lower layers detach from the foamed plastic material, decreases substantially. Without wanting to be bound by any theory the inventor believes that the buckling behaviour is inhibited by the reinforcing fibrous material extending in Z direction of the panel. As the risk for buckling is substantially decreased, the risk that the spar and sometimes even the blade breaks is substantially reduced. The inventor has moreover found that when using such material as spar component, the risk that delamination occurs, decreases substantially due to the presence of the reinforcing fibrous material extending in Z direction.

The inventor has also found that such spar components can be made in a rather automated production process, such as for example pultrusion in combination with tufting for applying the fibrous reinforcing material in Z direction of the spar component. Such a production method allows that the overall-thickness of the upper and lower layer of fibrous material by adding additional layers of fibrous reinforcing material, the thickness of the foamed plastic material and/or the overall thickness of the composite article can be reproducibly and more gradually varied along the length direction of the composite article without negatively affecting the physical properties of the spar. As an effect, construction of the spar component is significantly easier than construction of the known spar components. The spar components can then be made by simply cutting the composite panel in thickness direction into the right shape. Moreover, in such a process the risk that impurities may occur, are dramatically decreased leading to a significant increase in strength of the spar component such that the risk that spar component breaks is further decreased. Moreover, no longer large moulds are needed for creating a vacuum to allow that the resin sufficiently impregnates the lay up and/or to cure the impregnated lay up.

In a preferred embodiment according to the invention the thickness of the foamed plastic material is substantially constant along length and/or width direction of the composite article. The inventor has found that such a composite article is easier to make while substantially maintaining its resistance to buckling.

In preferred embodiments according to the invention the thickness of the upper and lower layer of fibrous reinforcing material is substantially constant along length and/or width direction of the composite article. The inventor has found that such a composite article is easier to make while substantially maintaining its resistance to buckling.

In preferred embodiments according to the invention the overall thickness composite article is substantially constant along length and/or width direction of the composite article. The inventor has found that such a composite article is easier to make while substantially maintaining its resistance to buckling.

The inventor has moreover found that by providing the composite article with such an overall thickness, vertical displacement along length direction of the spar, stress along length direction of the spar and weight along the vertical direction of the spar when subjected to a uniform load along the axis of rotation of the wind turbine blades all do not substantially deviate from vertical displacement along length direction of known spars, stress along length direction of known spars and weight along the vertical direction of known spars. As an effect, known spars can be replaced by spars according to the invention without having to adapt for example the wind turbine blades.

This effect was also observed when a composite article was used having, in addition to the substantially constant overall thickness, a substantially constant thickness of the upper and lower layer and a substantially constant thickness of the foamed plastic material. Such composite article is moreover even easier to manufacture. The invention also relates to use of a composite article as a jig, the composite article comprising a panel comprising a layer of a foamed plastic material, mounted between an upper and a lower layer of a fibrous reinforcing material, the foamed plastic material, the upper and lower layer of fibrous reinforcing material being fastened to each other by means of reinforcing fibrous material which extends in Z-direction of the panel, the panel being impregnated with a resin material.

Although jigs are well-known to the person skilled in the art to support a working object, preferably fittingly receiving the working object, in such jigs no use is made of a composite article as described above.

The inventor has found that when using such a composite article as a jig, the jig can be heated without disadvantageously influencing the structure of the composite material. Without wanting to be bound by any theory the inventor believes that this is caused by the reinforcing fibrous material extending in Z-direction of the panel which substantially increases the stability of the jig when heated since the foamed plastic material, the upper and the lower layer of fibrous reinforcing material are more firmly held together.

Other details and advantages of the device according to the invention will become apparent from the enclosed figures and description of preferred embodiments of the invention.

Figure 1 shows a cross-section of a composite article which can be used for spar component.

Figure 2 shows a cross-section of a wind turbine blade comprising in which a spar component according to the invention is used.

The wind turbine blade 7 is provided with a spar 6. For the spar 6, according to the invention, a composite article is used. The composite article comprises a panel 10 comprising a layer of foamed plastic material 1 mounted between an upper 2 and a lower layer 3 of a fibrous reinforcing material. The foamed plastic material 1 , the upper 3 and lower 4 layer of fibrous reinforcing material are fastened to each other by means of reinforcing fibrous material which extends in Z-direction of the panel 10. The panel 10 is impregnated with a resin material. A cross-section of an example of such a panel 10 is shown in figure 1.

The panel 10 according to this invention comprises a layer of foamed plastic material 1 having an upper face 4 and a lower face 5. To the upper face 4 the upper layer 2 is fastened. To the lower face 5, the lower layer 3 of the panel 10 is fastened.

The foamed plastic material 1 , upper layer 2 and lower layer 3 may be interconnected using conventional fastening techniques known to the person skilled in the art. Examples of known fastening techniques include stitching or needling using a substantially continuous or a non-continuous fibrous reinforcing material. However it is preferred to interconnect upper and lower layer 2, 3 and plastic foamed material 1 by means of a substantially continuous reinforcing fibre at least part of which extends in Z direction of the panel, as with this technique the connecting fibrous reinforcing material gets anchored into the upper 2 and lower 3 layer. More preferably, interconnecting upper and lower layer 2, 3 and plastic foamed material 1 is done by tufting using a substantially continuous tow or thread, preferably a substantially continuous fibrous reinforcing material. Thereby, the fibrous reinforcing material mainly extends in Z-direction, or part of the fibrous reinforcing material may slant with respect to the Z-direction if so desired. If so desired, the plastic foamed material 1 , upper and lower layer

2, 3 may however also be connected by means of stapling or any other equivalent connection technique. The connection preferably extends through the plastic foamed material 1 , the upper layer 2 and lower layer 3. It is however also possible to individually connect the upper and lower layer 2, 3 to the plastic foamed material 1. If so desired, the upper and/or lower face of the composite article 1 may be covered with a layer of a finishing material.

To produce this type of panels use is made of an industrial device, which permits producing panels of a pre-determined length and width adapted to the dimensions desired for the spar. If so desired, at pre-determined positions more or less layers of fibrous reinforcing materials may be applied to locally vary the layer thickness. Similarly, locally the thickness of the foam layer may be varied. This way a reproducible production process is provided in which the thickness varies much more gradually than in the prior art panels.

As the substantially continuous fibrous tufting material, use can be made of fibrous materials in the form of tows, threads, bundles, yarns or rovings, comprising a plurality of fibre bundles or twined or torsioned fibres, which may be built up of a single material or a combination of two or more different materials. The angle under which twined fibres extend with respect to each other will be adapted taking into account the envisaged compressive strength.

The nature of the fibrous material used is not critical to the invention, and may be selected from natural fibres, for example wool, cotton, flax fibres etc; mineral fibres, carbon fibres, metal fibres, glass fibres or synthetic fibres, for example polyester, polypropylene, polyethylene, polyamide, or mixtures of two or more of these fibres. However, because of its high impact strength the use of aramid fibres is preferred. The positioning of the connection is not critical to the invention, although it may be preferred to group the fibrous material connecting the upper and lower layer and core in certain patterns, or in groups or columns of three or four or more fibres or fibre strands or tows. In that case the fibre columns further improve the resistance of the sandwich structure to kinking or bending. The panel 10 of this invention may comprise one single or a plurality of inserts 9 which extend in height direction of the structure and in at least one of the longitudinal and transversal direction of the structure 1. The inserts 9 may however also extend in both the longitudinal and transversal direction of the panel 10 and in any other direction ought suitable by the person skilled in the art taking into account the envisaged application of the panel 10. The at least one insert 9 may be positioned such that it extends mainly in transverse direction of the reinforced sandwich structure, or mainly in longitudinal direction, or in a direction which slants with respect to the longitudinal and/or transverse direction. However, within the scope of the present invention a plurality of inserts 9 may be present which extend in two or more of the afore mentioned directions; thereby the two or more inserts may be positioned adjacent one another or spaced from each other, depending on the desired properties of the composite article. Within the framework of this invention it is also possible to stack one or more inserts 9 on top of each other within the foamed plastic material, or to position consecutive inserts in such a way that they at least partly overlap. In that way locally a higher strengthening may be achieved. Within the framework of this invention it is further possible to position two or more inserts in such a way that their longitudinal side faces lay against each other. The inserts 9 may be positioned at a regular distance from each other, or at varying distances. In case locally a higher stiffness is required, the number of and distance between adjacent inserts 9 may be amended. The insert 9 may be made of a wide variety of materials available to the person skilled in the art and take a wide variety of shapes. The insert 9 may for example be made as a strip or plate, perforated or not; a net or a grid, for example a stamped grid or a grid made of interconnected bars or wires, whereby the grid may extend in only two or in three dimensions. The insert 9 can be made as a mainly flat or pre-shaped strip, plate, net or grid. The insert 9 may for example be made as a zig-zag extending strip, whereby subsequent inserts 9 extend parallel to each other, point towards or from each other. Inserts 9 may however also be positioned in a staggered configuration. The insert 9 may further be shaped in such a way that facing inserts form a honeycomb-like structure in case a structure having a high strength is envisaged. The insert may have the shape of a wave, a rectangular or square wave or any other shape ought suitable by the person skilled in the art. The insert 9 can also be formed as a three-dimensional grid.

The insert 9 may be made of a wide variety of materials, for example steel, stainless steel, iron or any other metal ought suitable by the person skilled in the art. The insert 9 may also be made of a plastic material, for example a thermosetting resin or a thermoplastic material or a fibrous reinforced thermosetting resin or thermoplastic material. The person skilled in the art will be able to select the most appropriate material for the insert 9 depending on the envisaged application of the sandwich structure.

The insert 9 may however also take the shape of a block, or an enveloped block. A suitable example of such an insert is a block comprising a foamed material, for example a foamed plastic or metal or a mixture of these materials, which is received in an envelope of fibrous reinforcing material. The block may be impregnated with a thermosetting resin or a thermoplastic material. Suitable thermosetting materials for use in the present invention include thermosetting unsaturated polyester resins, vinylester resins, epoxy resins, phenolic resins, polyurethane resins.

The insert 9 may have the same height as the panel 10 as is shown in figure 1. The insert 9 may however also have a smaller height and be completely enveloped by the sandwich structure, as is shown in figure 2. This will for example also be the case when tow or more inserts 9 are stacked on top of each other within the plastic foamed material.

To save material, without this going at the expense of the stiffness of the panel 10, it is however possible to have subsequent inserts alternatingly depending from the upper layer 2, the lower layer 3 and countersunk within the foamed plastic material 1 of the panel 10, taken in cross direction thereof. The insert 9 may however also extend form the upper and lower layer 2, 3. In that case, the end parts protruding from the upper and lower layer 2, 3 will be bent to follow the surface of the upper and lower layer 2, 3.

The insert 9 may extend in longitudinal and/or in transverse direction of the panel 10, depending on the envisaged use of the structure. The insert 9 may extend in longitudinal direction of the panel 10 and have the same, a larger or a smaller length than the panel 10. With inserts 9 having a smaller length and/or positioned in a staggered configuration there is a minimum risk to cutting the sandwich structure by the inserts 9.

The material of which the plastic foamed material 1 is made is not critical to the invention. Suitable foamed materials include metal foams, for example aluminum foam, or plastic foam, for example polyurethane foam, polyethylene foam, polypropylene foam, a foam of an ethylene - propylene copolymer, phenol foam, or any other plastic foam known to the person skilled in the art. The plastic foamed material 1 may however also be made of a mixed metal-plastic foam, glass foam, cementious foam, natural foam, etc.

The plastic foamed material may be made of essentially one piece and of essentially one material. It is however also possible to use a plastic foamed material 1 comprising two or more layers stacked on top of each other and enclosing at least one layer of fibrous reinforcing material between them. The two or more superimposed layers may be made of the same or a different material. The fibrous reinforcing material may be the same material as used in the upper and lower layer 2, 3 or a different material.

The upper and lower layer 2, 3 comprise at least one layer of a fibrous reinforcing material, and may comprise a plurality of superimposed layers of such material. The fibrous reinforcing material will usually take the shape of a fleece, a net, a braiding, a fabric, a mat or a sheet.

The fibrous reinforcing material may be a woven or a non-woven product. The upper and lower layer 2, 3 of the panel 10 may be made of the same or a different fibrous reinforcing material. It is further possible to have one or more of the face layers built up of alternating first and second materials, for example alternating glass fibre mats and mats comprising a mixture of glass fibre and metal fibre. However, such a fibrous reinforcing material may also be present at a more central position of the laminate.

The nature of the material of which the fibrous reinforcement of the upper and lower layer 2, 3 are made is not critical to the invention and may be chosen from natural fibres, for example cotton fibre, flax, wool, carbon fibres; mineral fibres, for example glass fibres or fibres made of plastic material for example polyester, polypropylene, polyethylene, polyamide. The upper and lower layer 2, 3 may however also comprise a combination of two or more of the above mentioned materials. The upper and lower layer 2, 3 may be pre-impregnated with a thermoplastic or thermosetting resin or not, which may be a woven or non-woven product.

The tufted fibres preferably extend in height direction of the reinforced sandwich structure, which means that they may extend in Z-direction but also in any other direction which slants more or less with respect to the Z-direction. Thus, the reinforced sandwich structure of the present invention is strengthened in X and Y direction by means of the plies of upper and lower layers, and in Z-direction by the presence of the tufted fibres or piles. The reinforcing fibres used to interconnect the lower layer, upper layer and foamed plastic material, which extend in Z direction of the panel 10 structure are anchored into the panel 10 by impregnating the reinforced sandwich structure 10 with a resin. Thus a structure is obtained showing an improved impact resistance.

The resin used to impregnate the insert may be a thermosetting or thermoplastic resin. Suitable thermosetting materials for use in the present invention include thermosetting unsaturated polyester resins, vinylester resins, epoxy resins, phenolic resins, polyurethane resins. A possible process for producing the reinforced sandwich structure of this invention comprises the steps of

(1 ) forwarding the foamed plastic material 1 ,

(2) forwarding the upper and lower layer 2, 3 of fibrous reinforcing material along opposite sides of the core material 1 to form the panel 10,

(3) if desired inserting a plurality of inserts 9 in longitudinal and/or transversal direction of the sandwich structure 10

(4) interconnecting the upper and lower layer 2, 3 to the core 1 with the reinforcing fibrous material extending in Z direction, preferably by tufting,

(5) impregnating the thus obtained structure with a resin, preferably by pultrusion.

Insertion of the inserts 9 may be done in various ways known to the person skilled in the art. It is possible to insert the inserts 9 after the foamed plastic material 1 has been produced. It is however also possible to add the inserts 9 when producing the foamed plastic material 1.

It is however preferred that after the upper 2 and lower 3 layer and foamed plastic material have been interconnected, the panel 10 is impregnated with a resin material using pulltrusion, to minimise the risk to damaging the needles used in the tufting process.

Furthermore, in particular where use is made of an insert comprising a foamed plastic material enveloped in a sheet of a fibrous reinforcing material, it is advantageous to connect the upper and lower sheet of the foamed plastic material of the panel and the enveloping sheet of reinforcing material to the foamed plastic material of the insert, in one go using the process of tufting.

It is also possible that the panel 10 comprises, stacked on top of each other and fastened to each other, two or more panels as described above, arranged in such a way that a lower layer of a top panel is fastened to an upper layer of a lower panel. If so desired, the stacked panels are over-wrapped with a fibrous reinforcing material and impregnated with at least one resin. The spar component preferably comprises a first and second panel, although a single panel can be sufficient but is not shown in the figure, as described above provided to extend between two blade halves of a wind turbine blade in a direction substantially perpendicular to the chord of the wind turbine blade along length direction of the wind turbine blade. The first panel preferably is provided to be positioned near the leading edge of the wind turbine blade and the second panel preferably is provided near the trailing edge of the wind turbine blade. The first panel preferably is provided to extend along the leading edge and the second panel preferably is provided to extend along the trailing edge. When only a single panel is provided, the position of the panel can be determined in function of the desired properties of the spar.

An example of the first and the second panel positioned in a wind turbine blade are shown in figure 2.

The spar component, although not shown in figure 2, preferably comprises a first flange extending from the first panel towards the second panel. The first flange preferably is provided near a first side of the first and second panel in width direction of the first and second panel. The first flange preferably is provided to be positioned adjacently along a first blade halve of a wind turbine blade and more preferably is provided to extend along a substantial length direction of the first and second panel.

The spar component, although not shown in figure 2, preferably comprises a second flange extending from the first panel towards the second panel. The second flange preferably is provided near a second side of the first and second panel in width direction of the first and second panel, the second side opposing the first side. The second flange preferably is provided to be positioned adjacently along a second blade halve of the wind turbine blade and more preferably is provided to extend along a substantial length direction of the first and second panel.

The first and/or second flange, preferably the first and second flange, preferably comprise substantially the same material as the first and/or the second panel, more preferably the first and the second panel, being a layer of a foamed plastic material, mounted between an upper and a lower layer of a fibrous reinforcing material, the foamed plastic material, the upper and lower layer of fibrous reinforcing material being fastened to each other by means of reinforcing fibrous material which extends in Z-direction of the flange, the flange being impregnated with a resin material, as described above for the panel 10.