Patent Description:
The blades of modern wind turbines capture kinetic wind energy by using sophisticated blade design created to maximise efficiency. The blades are typically made from a fibre-reinforced polymer material and comprise a pressure side shell half and a suction side shell half. The cross-sectional profile of a typical blade includes an airfoil for creating an air flow leading to a pressure difference between the two sides. The resulting lift force generates torque for producing electricity.

The shell halves of wind turbine blades are usually manufactured using blade moulds. First, a blade gel coat or primer is applied to the mould. Subsequently, fibre reinforcement material is placed into the mould in layers followed by arrangement of other elements within the shell halves, such as core elements, load-carrying spar caps, internal shear webs and the like. The resulting shell halves are resin infused and assembled by being glued or bolted together substantially along a chord plane of the blade.

The spar caps comprise a plurality of carbon pultrusion elements and interlayers arranged between the carbon pultrusion elements. The spar caps may be produced directly in the wind turbine blade moulds or in a separate offline mould where they are resin infused and then subsequently lifted into the main blade shell mould which is then infused with resin.

Different combinations of resins may be used for the spar cap and the main blade shell. It is very important to ensure a sufficiently strong adhesion between the shell and the pre-manufactured spar cap since bonding of resin onto the spar cap is crucial for the structural integrity of the blade. Vinyl ester or epoxy ester resins have good adherence properties and are often used, whereas other resins, such as polyester resin, have an attractive price. However, the adhesion properties of polyester resin are low compared to vinyl ester and epoxy resin.

To decrease the overall price, spar caps may be made offline and infused with vinyl ester resin. The pre-manufactured spar cap may then be placed in a blade shell together with the remaining blade shell parts and infused with polyester resin.

However, because the fracture toughness of the polyester laminate is lower than the fracture toughness of the vinyl ester laminate, any cracks starting at the interface between the spar cap and the remaining blade shell parts will propagate in the interface or kink into the polyester laminate.

Hence, improved methods to ensure a sufficiently strong adhesion between the shell member parts and the pre-manufactured spar cap would be advantageous. Particularly a method of avoiding cracks from propagating into the polyester laminate.

<CIT> and <CIT> are relevant examples of prior art.

It is an object of the present disclosure to provide a wind turbine blade, wherein cracks starting at the interface between the spar cap and the remaining blade shell parts will not propagate in the interface or kink into the blade shell member, particularly when the spar cap is resin infused with vinyl ester or epoxy resin and the blade mould is resin infused with polyester.

The present inventors have found that one or more of said objects may be achieved by arranging a damage tolerant cover sheet according to a first aspect of the present invention at the interface between a pre-manufactured spar cap and the remaining parts of the blade shell member.

By having a damage tolerant cover sheet at the interface between the premanufactured spar cap with a low cohesive strength and low fracture toughness compared to the polyester laminate, any cracks starting at the interface between the spar cap and the remaining blade shell parts will propagate into the damage tolerant cover sheet with low cohesive strength but high fracture toughness.

Thus, in a first aspect the present invention relates to a damage tolerant cover sheet comprising.

wherein the first damage tolerant fibre layer and the second damage tolerant fibre layer are attached to each other in attachment areas by a first plurality of stitching rows and/or by a first binding agent arranged between the first and second damage tolerant fibre layer in the attachment areas, and wherein the attachments areas are separated from each other by a distance between <NUM>-<NUM>, preferably around <NUM>.

The damage tolerant cover sheet comprises a first damage tolerant fibre layer and a second damage tolerant fibre layer. The first damage tolerant fibre layer forms part of a first outer surface of the damage tolerant cover sheet and a second damage tolerant fibre layer forms part of a second outer surface of the damage tolerant cover sheet. The first outer surfaces of the damage tolerant cover sheet is opposite to the second outer surface of the damage tolerant cover sheet and the first and second outer surface of the damage tolerant cover sheet are the two largest surfaces of the damage tolerant cover sheet.

The first damage tolerant fibre layer preferably comprises a first plurality of fibres being unidirectionally arranged along a first fibre direction and the arrangement of the first plurality of fibres are maintained relative to each other by a second plurality of stitching rows and/or by a second binding agent. The distance between the second plurality of stitching rows in the first damage tolerant fibre layer is not important. Furthermore, the second binding agent need not be arranged in specific areas. The importance of the second plurality of stitching rows and second binding agent is just to maintain the fibres in the first damage tolerant fibre layer relative to each other. In preferred embodiments, the first damage tolerant fibre layer is a unidirectional glass fibre layer or a biaxial glass fibre layer.

The second damage tolerant fibre layer preferably comprises a second plurality of fibres being randomly arranged within the second damage tolerant fibre layer. The arrangement of the second plurality of fibres are maintained relative to each other by a third binding agent. Again, the importance of the third binding agent is just to maintain the fibres in the second damage tolerant fibre layer relative to each other. In preferred embodiments, the second fibre layer is a chopped strand mat (CSM) or a continuous filament mat (CFM) comprising or essentially consisting of glass fibres.

The first damage tolerant fibre layer provides strength to the damage tolerant cover sheet and allows it to be handled easily, while the second damage tolerant fibre layer should be a good "filler materials" which can swell up.

In some embodiments, the first and second damage tolerant fibre layers are attached to each other by a first plurality of stitching rows in the attachment areas.

In some embodiments, the first and second damage tolerant fibre layers are attached to each other by a first binding agent arranged between the first and second damage tolerant fibre layer in the attachment areas.

Importantly, the attachments areas are separated from each other by a distance between <NUM>-<NUM>, preferably around <NUM>. In this way, the fibres in the damage tolerant cover sheet can be pulled out over a long distance and the fibres being teared out can create a large bridging zone. When the damage tolerant cover sheet is used at the interface between a premanufactured spar cap and the remaining shell components, the distance between the attachment areas can be designed to twerk the interfacial strength to be just less than the interfacial strength between the pre-manufactured spar cap and the remaining shell components, meaning that cracks will tend to go into the damage tolerant cover sheet instead of into the shell components. The distance between the attachment areas may also be larger than <NUM>. Thus, in some embodiments the attachments areas are separated from each other by a distance between <NUM>-<NUM>, such as between <NUM>-<NUM>, such as between <NUM>-<NUM>, such as between <NUM>-<NUM>.

Preferably, the first plurality of attachment areas are parallel to each other and arranged along a first attachment direction.

In embodiments where the first and second damage tolerant fibre layers are attached to each other by a first plurality of stitching rows, each attachment area is defined by a thread making up a stitching row. In such embodiments, the distance between the attachment areas should be measured from the stitching thread in one stitching row to an adjacent stitching thread in another stitching row. The distance should be measured such that it is taken perpendicular on the first attachment direction of the two adjacent stitching rows.

In embodiments where the first and second damage tolerant fibre layers are attached to each other by a first plurality of stitching rows, each attachment area is defined by the area covered by binding agent. In embodiments where the first and second damage tolerant fibre layers are attached to each other by a first binding agent, the attachment areas are larger than when the first and second damage tolerant fibre layers are attached to each other by a plurality of stitching rows. This is because a line of binding agent with the same width as a stitching thread, would not be sufficient to bind the two layers together. Thus, in such embodiments, the attachment areas have a width larger than a stitching thread, such as at least <NUM> times the width of a stitching thread. In such embodiments, the distance between two adjacent attachment areas should be measured from an outer edge (such as a left edge) of one attachment area to an opposite outer edge (such as a right edge) of an adjacent attachment area. The distance should be measured such that it is taken perpendicular on the first attachment direction of the two adjacent attachment areas.

Preferably, a fibre angle between the first attachment direction of the first damage tolerant fibre sheet <NUM> and the first fibre direction of the plurality of first fibres in the first damage tolerant fibre layer <NUM> is between <NUM> degrees and <NUM> degrees, most preferably <NUM> degrees.

In some embodiments, the area weight of the damage tolerant cover sheet is between <NUM> gsm and <NUM> gsm, preferably <NUM> gsm.

Preferably, the damage tolerant cover sheet is configured to have at least the same length as a pre-manufactured spar cap and substantially the same width.

The damage tolerant cover sheet has a length a width and a thickness. Preferably, the damage tolerant cover sheet is designed to have approximately the same length and width as the two largest surfaces of a premanufactured spar cap. The thickness is determined by the thickness of the first and second damage tolerant cover layers when they are arranged on top of each other. In some embodiments, the damage tolerant cover sheet is longer than a pre-manufactured spar cap. In such embodiments, the damage tolerant cover sheet may be provided in a rolled-up configuration allowing it to be stored without taking up too much place and rolled out over the entire length of the spar cap and cut at a desired length.

The damage tolerant cover sheet according to the present invention is configured to be used in a wind turbine blade. Particularly, the damage tolerant cover sheet of the invention is configured to be arranged at the interface between a pre-manufactured spar cap infused with a first resin, such as vinyl ester resin, and the remaining shell components of a wind turbine blade infused with a second resin, such as polyester resin. The damage tolerant cover sheet should preferably cover at least the two largest surfaces of the pre-manufactured spar cap, including any tapering ends of the pre-manufactured spar cap. By having a damage tolerant cover sheet at the interface between the premanufactured spar cap with a low cohesive strength and fracture toughness compared to the remaining shell components, any cracks starting at the interface between the premanufactured spar cap and the remaining shell components will propagate into the damage tolerant cover sheet with low cohesive strength but high fracture toughness.

In a second aspect, the present invention relates to a pre-manufactured spar cap for a wind turbine blade comprising spar cap structure as well as a first and/or a second damage tolerant cover sheet in accordance with the first aspect of the invention.

The spar cap structure comprises a plurality of fibre-reinforced composite elements arranged in stacked rows and separated by interlayers.

The spar cap structure has a length, a width and a height, wherein the length is longer than the width and the width is longer than the height.

Thus, the spar cap structure is preferably an elongated element having a first surface and a second surface defined by the width and length of the spar cap structure. Furthermore, the spar cap structure has two side surfaces, each defined by the length and the height of the spar cap structure, as well as two end surfaces defined by the width and height of the spar cap structure. The first and second surfaces are the two largest surfaces of the spar cap structure and are arranged opposite each other and may have substantially the same sizes. In the same way, the two side surfaces are arranged opposite each other and have substantially the same sizes, and the two end surfaces are arranged opposite each other and have substantially the same sizes. However, since the shape of the spar cap structure is set according to strength requirements, the thickness may change along the longitudinal direction of the spar cap, resulting in tapering sections at the sides and/or the ends.

In some embodiments, the fibre-reinforced composite elements are elongated planks with a rectangular cross-section and made from carbon fibres or glass fibres and cured resin. Alternatively, the fibre-reinforced composite elements may be hybrid pultrusion elements comprising two types of reinforcement fibres, such as glass fibres and carbon fibres.

In some embodiments, the stacked rows of fibre-reinforced composite elements include a first plurality of fibre-reinforced composite elements arranged adjacent to each other in a first row and a second plurality of fibre-reinforced composite elements arranged adjacent to each other in a second row on top of the first row, wherein the first and second row of fibre-reinforced elements are separated by a first interlayer.

Preferably, the spar cap structure comprises more than two rows of fibre-reinforced elements, such as between <NUM>-<NUM>, such as between <NUM>-<NUM>, such as between <NUM>-<NUM>. Each row may comprise between <NUM>-<NUM> fibre-reinforced composite elements, preferable between <NUM>-<NUM>, such as <NUM>-<NUM>.

The interlayers may be any material promoting resin flow between the fibre-reinforced composite elements and may comprise one or more fibre layers, each comprising glass fibres and/or carbon fibre and/or monofilaments etc. Thus, the interlayers comprise fibre material, such as glass fibres, carbon fibres or polymeric fibres etc. for promoting resin flow between the pultruded carbon planks.

A premanufactured spar cap according to the second aspect of the present invention comprises.

In some embodiments, the pre-manufactured spar cap further comprises a second damage tolerant cover sheet according to the first aspect of the present invention,.

It should be understood that all embodiments described for the damage tolerant cover sheet in accordance with the first aspect of the invention may all be applied to the pre-manufactured spar cap comprising one or more of such damage tolerant cover sheets according to the second aspect of the present invention.

A pre-manufactured spar cap according to the present invention may be provided as follows:
First, a spar cap mould comprising a spar cap moulding surface is provided and a first damage tolerant cover sheet is arranged on the spar cap moulding surface such that the first outer surface of the first damage tolerant cover sheet is in contact with the spar cap moulding surface. Then, a spar cap structure comprising a plurality of fibre-reinforced composite elements in stacked rows, separated by interlayers, is arranged on the first damage tolerant cover sheet, such that the second outer surface of the first damage tolerant cover sheet is in contact with the first surface of the spar cap structure.

More specifically, arranging the spar cap structure may include arranging a first plurality of fibre-reinforced composite elements adjacent to each other in a first row and arranging a second plurality of fibre-reinforced composite elements adjacent to each other in a second row on top of the first row. A first interlayer is arranged between the first and second row of fibre-reinforced elements. Then, a third plurality of fibre-reinforced composite elements are arranged adjacent to each other in a third row on top of the second row. A second interlayer is arranged between the second and third row of fibre-reinforced elements. In the same way, a fourth, fifth and sixth etc. plurality of fibre-reinforced composite elements are arranged adjacent to each other in a fourth, fifth and sixth etc. row and the fourth row is arranged on top of the third row, etc until the desired thickness of the spar cap structure is reached. Again, each row of fibre-reinforced composite elements are separated by an interlayer.

After the spar cap structure has been arranged on the first damage tolerant cover sheet, a second damage tolerant cover sheet is arranged on top of the spar cap structure such that the second outer surface of the second damage tolerant cover sheet is in contact with the second surface of the spar cap structure. Finally, the spar cap structure, the first damage tolerant cover sheet and the second damage tolerant cover sheet are infused with a first resin which is allowed to cure to form the pre-manufactured spar cap.

Thus, in a third aspect, the present invention relates to a method of providing a pre-manufactured spar cap for a wind turbine blade, the method comprising the steps of:.

In some embodiments, the method further comprises arranging a second damage tolerant cover sheet on the spar cap structure such that the second outer surface of the damage tolerant cover sheet is in contact with the second surface of the spar cap structure and wherein the step of infusing the spar cap structure and the first damage tolerant cover sheet with a first resin also includes infusing the second damage tolerant cover sheet with the first resin.

The present invention further relates to a wind turbine blade comprising a spar cap structure and a first and/or second damage tolerant cover layer. Thus, in a fourth aspect, the present invention relates to a wind turbine blade comprising.

In some embodiments, the wind turbine blade further comprises.

Thus, in some embodiments, the wind turbine blade according to the fourth aspect of the present invention may comprise a premanufactured spar cap structure according to the second aspect of the present invention. However, in other embodiments the wind turbine blade comprises a premanufactured spar cap comprising a spar cap structure embedded in a cured resin and individual first and optionally also second damage tolerant cover sheets according to the first aspect of the present invention.

In some embodiments, first cured resin is a vinyl ester resin, a epoxy ester resin or a polyurethane resin, whereas the second cured resin is a polyester resin.

Polyester resin is much cheaper than conventionally used resins, such as epoxy ester and vinyl ester. However, the fracture resistance of the polyester-infused blade shell member parts is significantly lower than the fracture toughness of a vinyl ester or epoxy ester infused blade shell member. The adherence properties and strength of the pre-manufactured spar cap are particularly important. Thus, even though the prices of vinyl ester or epoxy ester are high compared to other resins, these are preferred for the pre-manufactured spar cap. By primarily using polyester resin for the remaining blade shell member and only using vinyl ester or epoxy ester resin for a few parts, such as the pre-manufactured spar cap, the costs of the blade shell member can be greatly reduced. Recent testing shows that the fracture toughness at the interface between a pre-manufactured spar cap infused with vinyl ester resin and the remaining blade shell member part infused with polyester resin is low. Furthermore, the fracture resistance of the polyester infused blade shell member parts is significantly lower than the fracture toughness of the vinyl ester or epoxy ester spar cap. Therefore, any cracks starting at the interface will probably propagate into the interface or kink into the polyester infused blade shell member parts having a lower fracture toughness than the vinyl ester or epoxy ester spar cap. However, with the arrangement of the damage tolerant cover sheets as disclosed herein at the interface between pre-manufactured spar cap and the remaining blade shell member parts, the interfacial strength can be twerked to be just less than the interfacial strength between the pre-manufactured spar cap and the remaining shell components. In this way, cracks will tend to go into the damage tolerant cover sheet where the crack ideally will arrest or at least slow down in propagation speed, instead of into the shell components. Thus, use of the damage tolerant cover sheet around a premanufactured spar cap in a wind turbine blade, decrease the propagation of cracks into the premanufactured spar cap and shell components and the blade shell member can be obtained at a reduced price.

The present invention further relates to two different methods for manufacturing a wind turbine blade shell member comprising a spar cap structure and a first and/or second damage tolerant cover layer. The first method is for manufacturing the wind turbine blade shell member comprising a premanufactured spar cap structure according to the second aspect of the present invention. The second method is for manufacturing a wind turbine blade shell member comprising a premanufactured spar cap comprising a spar cap structure embedded in a cured resin and individual first and optionally also second damage tolerant cover sheets according to the first aspect of the present invention.

In a fifth aspect, the present invention relates to a method of manufacturing a blade shell member for a wind turbine blade, the method comprising the steps of:.

In some embodiments, the premanufactured spar cap is provided by:.

In a sixth aspect, the present invention relates to a method of manufacturing a blade shell member for a wind turbine blade, the method comprising the steps of:.

In some embodiments, the first and second plurality of shell fibre layers comprises glass fibres and/or carbon fibres. In some embodiments, the number of fibre-reinforced layers and further fibre-reinforced layers comprises unidirectional layers and/or biaxial layers and/or triaxial layers.

In some embodiments, the step of arranging the first and/or second plurality of shell fibre layers in the blade mould comprises arranging a plurality of preforms, each comprising a consolidated stack of shell fibre layers in the blade mould. The use of preforms may be advantageous, especially when manufacturing very large blade shell members, since wrinkles in the fibre-reinforced layers may be reduced.

In some embodiments, the step of arranging the first and/or second plurality of shell fibre layers on the blade moulding surface comprises arranging each of the first or second plurality of shell fibre layers on top of each other. The first plurality of shell fibre layers arranged on the blade moulding surface will become the outer shell of the blade shell member. Thus, preferably the first plurality of shell fibre layers should cover the entire moulding surface. The first and/or second plurality of shell fibre layers are between <NUM>-<NUM>, preferably between <NUM>-<NUM>, such as between <NUM>-<NUM>.

In some embodiments, the method further comprises the step of arranging further blade shell member parts, such as sandwich core layers and/or shear webs in the blade mould cavity. The blade shell member parts referred to herein include all parts of the blade shell member.

In some embodiments, the step of infusing the blade mould cavity with resin is based on vacuum-assisted resin transfer moulding (VARMT). When the desired elements have been arranged in the blade mould, a vacuum bag may be arranged on top of the elements arranged on the moulding surface, and the vacuum bag may be sealed against the blade mould. Then, the blade mould cavity within the sealed vacuum bag may be infused with resin. Optionally, the step of resin infusion is followed by curing to obtain the finished blade shell member.

The method for providing a blade shell member may be for providing a suction side shell member or a pressure side shell member. It is to be understood that the same method may be used for providing a suction side shell member as well as a pressure side shell member. The only difference between providing the pressure side shell member and the suction side shell member would be the shape of the blade mould.

In a seventh aspect, the present invention relates to a method of manufacturing a wind turbine blade, the method comprising providing a pressure side shell half and a suction side shell half in accordance with the fifth or sixth aspect of the present invention over substantially the entire length of the wind turbine blade and subsequently closing and joining the shell halves for obtaining a closed shell.

In some embodiments, the method comprises providing a pressure side blade shell member and a suction side blade shell member over substantially the entire length of the wind turbine blade and subsequently closing and joining the pressure side blade shell member and the suction side blade shell member for obtaining a closed shell, wherein providing each of the pressure side blade shell member or the suction side blade shell member comprises the steps of:.

It will be understood that any of the above-described features may be combined in any embodiment of the invention. In particular, embodiments described with regard to the damage tolerant cover sheet may also apply to the premanufactured spar cap or wind turbine blade comprising one or more damage tolerant cover sheets. In the same way, embodiments described with regard to the premanufactured spar cap may also apply to the turbine blade. Furthermore, the embodiments described with regard to the method of manufacturing a premanufactured spar cap may also be applied to the method of manufacturing a blade shell member or a wind turbine blade, and vice versa. Finally, embodiments described with regard to the pre-manufactured spar cap and wind turbine blade shell member may also be applied to the method of manufacturing a premanufactured spar cap, a blade shell member or a wind turbine blade and vice versa.

The invention is explained in detail below with reference to embodiments shown in the drawings, which shall not be construed as limitations.

<FIG> illustrates a conventional modern upwind wind turbine according to the so-called "Danish concept" with a tower <NUM>, a nacelle <NUM> and a rotor with a substantially horizontal rotor shaft. The rotor includes a hub <NUM> and three blades <NUM> extending radially from the hub <NUM>, each having a blade root <NUM> nearest the hub and a blade tip <NUM> furthest from the hub <NUM>.

<FIG> shows a schematic view of a first embodiment of a wind turbine blade <NUM> according to the invention. The wind turbine blade <NUM> has the shape of a conventional wind turbine blade and comprises a root region <NUM> closest to the hub, a profiled or an airfoil region <NUM> furthest away from the hub and a transition region <NUM> between the root region <NUM> and the airfoil region <NUM>.

The diameter (or the chord) of the root region <NUM> may be constant along the entire root region <NUM>.

A shoulder <NUM> of the blade <NUM> is defined as the position where the blade <NUM> has its largest chord length.

It should be noted that the chords of different sections of the blade normally do not lie in a common plane since the blade may be twisted and/or curved (i.e. pre-bent), thus providing the chord plane with a correspondingly twisted and/or curved course, this being most often the case in order to compensate for the local velocity of the blade being dependent on the radius from the hub.

<FIG> is a schematic diagram illustrating a cross-sectional view of an exemplary wind turbine blade <NUM>, e.g. a cross-sectional view of the airfoil region of the wind turbine blade <NUM>. The wind turbine blade <NUM> comprises a leading edge <NUM>, a trailing edge <NUM>, a pressure side <NUM>, a suction side <NUM>, a first spar cap <NUM> and a second spar cap <NUM>. The wind turbine blade <NUM> comprises a chord line <NUM> between the leading edge <NUM> and the trailing edge <NUM>. The wind turbine blade <NUM> comprises shear webs <NUM>, such as a leading edge shear web and a trailing edge shear web. The shear webs <NUM> could alternatively be a spar box with spar sides, such as a trailing edge spar side and a leading edge spar side. The spar caps <NUM> may comprise carbon fibres while the rest of the shell parts <NUM>, <NUM> may comprise glass fibres.

<FIG> and <FIG> are schematic diagrams illustrating different views of a damage tolerant cover sheet <NUM> according to two different embodiments of the present invention. <FIG> and <FIG> are a three-dimensional view, <FIG> and <FIG> are a top view and <FIG> and <FIG> are a side view of a damage tolerant cover sheet <NUM> in accordance with the present invention.

The damage tolerant cover sheet <NUM> according to the present invention is configured to be used in a wind turbine blade <NUM>. Particularly, the damage tolerant cover sheet <NUM> of the invention is configured to be arranged at the interface between a pre-manufactured spar cap <NUM> infused with a first resin, such as vinyl ester resin, and the remaining shell components of a wind turbine blade infused with a second resin, such as polyester resin. The damage tolerant cover sheet <NUM> should preferably cover at least the two largest surfaces <NUM>, <NUM> of the pre-manufactured spar cap <NUM>, including any tapering ends of the pre-manufactured spar cap <NUM>. By having a damage tolerant cover sheet <NUM> at the interface between the premanufactured spar cap <NUM> with a low cohesive strength and fracture toughness compared to the remaining shell components, any cracks starting at the interface between the premanufactured spar cap <NUM> and the remaining shell components will propagate into the damage tolerant cover sheet <NUM> with low cohesive strength but high fracture toughness.

As can be seen in <FIG> and <FIG>, the damage tolerant cover sheet <NUM> according to the present invention comprises a first damage tolerant fibre layer <NUM> and a second damage tolerant fibre layer <NUM>. The first damage tolerant fibre layer <NUM> forms part of a first outer surface of the damage tolerant cover sheet <NUM> and a second damage tolerant fibre layer <NUM> forms part of a second outer surface of the damage tolerant cover sheet <NUM>. The first outer surfaces of the damage tolerant cover sheet <NUM> is opposite to the second outer surface of the damage tolerant cover sheet <NUM> and the first and second outer surface of the damage tolerant cover sheet <NUM>, <NUM> are the two largest surfaces of the damage tolerant cover sheet <NUM>.

The first damage tolerant fibre layer <NUM> preferably comprises a first plurality of fibres being unidirectionally arranged along a first fibre direction and the arrangement of the first plurality of fibres are maintained relative to each other by a second plurality of stitching rows and/or by a second binding agent. The distance between the second plurality of stitching rows in the first damage tolerant fibre layer <NUM> is not important. Furthermore, the second binding agent need not be arranged in specific areas. The importance of the second plurality of stitching rows and second binding agent is just to maintain the fibres in the first damage tolerant fibre layer <NUM> relative to each other. In preferred embodiments, the first damage tolerant fibre layer <NUM> is a unidirectional glass fibre layer or a biaxial glass fibre layer.

The second damage tolerant fibre layer <NUM> preferably comprises a second plurality of fibres being randomly arranged within the second damage tolerant fibre layer. The arrangement of the second plurality of fibres is maintained relative to each other by a third binding agent. Again, the importance of the third binding agent is just to maintain the fibres in the second damage tolerant fibre layer <NUM> relative to each other. In preferred embodiments, the second fibre layer <NUM> is a chopped strand mat (CSM) or a continuous filament mat (CFM) comprising or essentially consisting of glass fibres.

The first damage tolerant fibre layer <NUM> and the second damage tolerant fibre layer <NUM> are attached to each other in attachment areas <NUM>.

In the embodiment illustrated in <FIG>, the first and second damage tolerant fibre layers <NUM>, <NUM> are attached to each other by a first plurality of stitching rows in the attachment areas <NUM>. The attachment areas <NUM> are parallelly arranged along a first attachment direction <NUM> of the damage tolerant cover sheet <NUM>.

In the embodiment illustrated in <FIG>, the first and second damage tolerant fibre layers <NUM>, <NUM> are attached to each other by a first binding agent arranged between the first and second damage tolerant fibre layer <NUM>, <NUM> in the attachment areas <NUM>. The attachments areas <NUM> are parallelly arranged along a first attachment direction <NUM> of the damage the damage tolerant cover sheet <NUM>.

Importantly, the attachments areas <NUM> are separated from each other by a distance <NUM> between <NUM>-<NUM>, preferably around <NUM>. In this way, the fibres in the damage tolerant cover sheet <NUM> can be pulled out over a long distance and the fibres being teared out can create a large bridging zone. When the damage tolerant cover sheet <NUM> is used at the interface between a premanufactured spar cap <NUM> and the remaining shell components, the distance <NUM> between the attachment areas <NUM> can twerk the interfacial strength to be just less than the interfacial strength between the pre-manufactured spar cap <NUM> and the remaining shell components, meaning that cracks will tend to go into the damage tolerant cover sheet <NUM> instead of into the shell components.

In embodiments where the first and second damage tolerant fibre layers <NUM>, <NUM> are attached to each other by a first plurality of stitching rows, each attachment area <NUM> is defined by a thread making up a stitching row. In such embodiments, the distance <NUM> between the attachment areas should be measured from the stitching thread in one stitching row to an adjacent stitching thread in another stitching row. The distance <NUM> should be measured such that it is taken perpendicular on the first attachment direction <NUM> of the two adjacent stitching rows as illustrated by the reference <NUM>. In embodiments where the first and second damage tolerant fibre layers <NUM>, <NUM> are attached to each other by a first plurality of stitching rows, each attachment area <NUM> is defined by the area covered by binding agent. In embodiments where the first and second damage tolerant fibre layers <NUM>, <NUM> are attached to each other by a first binding agent, the attachment areas <NUM> are larger than when the first and second damage tolerant fibre layers <NUM>, <NUM> are attached to each other by a plurality of stitching rows. This is because a line of binding agent with the same width as a stitching thread, would not be sufficient to bind the two layers together. Thus, in such embodiments, the attachment areas <NUM> have a width larger than a stitching thread, such as at least <NUM> times the width of a stitching thread. In such embodiments, the distance <NUM> between two adjacent attachment areas <NUM> should be measured from an outer edge (such as a left edge) of one attachment area to an opposite outer edge (such as a right edge) of an adjacent attachment area. The distance <NUM> should be measured such that it is taken perpendicular on the first attachment direction <NUM> of the two adjacent attachment areas <NUM> as illustrated by the reference <NUM>.

Preferably, a fibre angle between the first attachment direction <NUM> of the first damage tolerant fibre sheet <NUM> and the first fibre direction of the plurality of first fibres in the first damage tolerant fibre layer <NUM> is between <NUM> degrees and <NUM> degrees, preferably <NUM> degrees.

The area weight of the damage tolerant cover sheet <NUM> should be between <NUM> gsm and <NUM> gsm, preferably <NUM> gsm.

<FIG> is a schematic diagram illustrating a spar cap structure <NUM> comprising a plurality of fibre-reinforced composite elements <NUM> arranged in stacked rows <NUM> and separated by interlayers <NUM>.

The stacked rows of fibre-reinforced composite elements <NUM> illustrated in <FIG> includes six rows each comprising three fibre-reinforced composite elements <NUM> arranged adjacent to each other. The six rows are stacked upon each other. Each row of fibre-reinforced elements <NUM> is separated from an adjacent row of fibre-reinforced elements <NUM> by an interlayer <NUM>. Preferably, the fibre-reinforced composite elements <NUM> are longitudinally extending pultruded carbon planks with a substantially square cross-section. The interlayer <NUM> may be any material promoting resin flow between the fibre-reinforced composite elements <NUM> and may comprise one or more fibre layers, each comprising glass fibres and/or carbon fibre and/or monofilaments etc..

The spar cap structure <NUM> has a length, a width and a height, wherein the length is longer than the width and the width is longer than the height. Furthermore, the spar cap structure <NUM> may have a tapering structure at a first and second end (not illustrated). The spar cap structure <NUM> has a first and second surface <NUM>, <NUM>. The first and second surfaces of the spar cap structure <NUM>, <NUM> are the two largest surfaces of the spar cap structure <NUM>, and the second surface of the spar cap structure <NUM> is opposite the first surface of the spar cap structure <NUM>. The second surface <NUM> may include the surface of the tapering structure at the first and second end of the spar cap structure <NUM>.

When the spar cap structure <NUM> is embedded in a first cured resin it constitutes a premanufactured spar cap <NUM>. Such a spar cap in itself does not form part of the invention. However, a wind turbine blade <NUM> comprising such a premanufactured spar cap <NUM> as well as a first damage tolerant cover sheet 10a and a second damage tolerant cover sheet 10b in accordance with <FIG> or <FIG> arranged to cover the larger surfaces of the spar cap structure <NUM>, constitute part of the invention. A cross-sectional close up view of part of a wind turbine blade shell member comprising a first and second plurality of shell fibre layers <NUM>, <NUM> as well as a first and second damage tolerant cover sheet 10a, 10b as described in relation with <FIG> or <FIG>, is illustrated in <FIG>. The close up view shows the part of a wind turbine blade <NUM> illustrated in <FIG> within the square of dotted lines.

As can be seen in <FIG>, the first damage tolerant cover sheet 10a should be arranged at the interface between the first surface of the premanufactured spar cap <NUM> and a first plurality of shell fibre layers <NUM> and the second damage tolerant cover sheet 10b should be arranged at the interface between the second surface of the premanufactured spar cap <NUM> and a second plurality of shell fibre layers <NUM> of the wind turbine blade. The first damage tolerant cover sheet 10a should be arranged such that the second outer surface of the first damage tolerant cover sheet 12a i.e., the second damage tolerant fibre layer, is in contact with the first surface of the spar cap structure <NUM>, whereas the first outer surface of the first damage tolerant cover sheet 11a i.e., the first damage tolerant fibre layer, is in contact with the first plurality of shell fibre layers <NUM>. Furthermore, the second damage tolerant cover sheet 10b should be arranged such that the second outer surface of the second damage tolerant cover sheet 12b i.e., the second damage tolerant fibre layer, is in contact with the second surface of the spar cap structure <NUM>, whereas the first outer surface of the second damage tolerant cover sheet 11b i.e., the first damage tolerant fibre layer, is in contact with the second plurality of shell fibre layers <NUM>.

In embodiments where the wind turbine blade <NUM> comprises a premanufactured spar cap <NUM> comprising a spar cap structure <NUM> as described in relation to <FIG> embedded in the first cured resin, the wind turbine blade components, including the first and/or second plurality of shell fibre layers <NUM>, <NUM>, the first and/or second damage tolerant cover sheets 10a, 10b and the premanufactured spar cap <NUM> are embedded in a second cured resin, such as polyester resin, to adhere all the turbine blade components together.

<FIG> is a schematic diagram illustrating a pre-manufactured spar cap <NUM> for a wind turbine blade <NUM> according to an embodiment of the present invention.

The pre-manufactured spar cap <NUM> according to the present invention comprises a spar cap structure <NUM> as described in relation to <FIG>. Furthermore, the pre-manufactured spar cap <NUM> comprises a first damage tolerant cover layer 10a and/or a second damage tolerant cover layer 10b as described in relation to <FIG> or <FIG>. In <FIG>, the spar cap structure <NUM>, the first damage tolerant cover sheet 10a and the second damage tolerant cover sheet 10b are all embedded in a first cured resin, preferably vinyl ester resin, epoxy ester resin or polyurethane resin (not illustrated). The cured resin holds the spar cap structure <NUM>, the first damage tolerant cover sheet 10a and the second damage tolerant cover sheet 10b together in the desired arrangement. Together the spar cap structure <NUM>, the first damage tolerant cover sheet 10a and the second damage tolerant cover sheet 10b and the cured resin make up a premanufactured spar cap <NUM> according to an embodiment of the present invention.

The first damage tolerant cover sheet 10a is arranged such that the second outer surface of the first damage tolerant cover sheet 12a i.e., the second damage tolerant fibre layer, is in contact with the first surface of the spar cap structure <NUM>. Furthermore, the second damage tolerant cover sheet 10b is arranged such that the second outer surface of the second damage tolerant cover sheet 12b i.e., the second damage tolerant fibre layer, is in contact with the second surface of the spar cap structure <NUM>.

A wind turbine blade comprising a premanufactured spar cap <NUM> as described in relation to <FIG>, as well as an outer shell comprising a first plurality of shell fibre layers <NUM> and/or a second plurality of shell fibre layers <NUM> also forms part of the present invention. Such a wind turbine blade <NUM> will be similar to that described in relation to <FIG>, the only difference being that the first and second damage tolerant cover sheets 10a, 10b are part of the premanufactured spar cap <NUM> i.e. the spar cap structure <NUM>, the first damage tolerant cover sheet 10a and the second damage tolerant cover sheet 10b, are all embedded in the first cured resin. Furthermore, the premanufactured spar cap <NUM>, the first plurality of shell fibre layers <NUM> and second plurality of shell fibre layers <NUM> are all embedded in a second cured resin to adhere these components together and form a blade shell member for a wind turbine blade <NUM>.

In other words, the premanufactured spar cap <NUM> is arranged between the first and second plurality of shell fibre layers <NUM>, <NUM> in the wind turbine blade <NUM> such that the first damage tolerant cover sheet 10a is arranged a first interface between the first plurality of shell fibre layers <NUM> and a first surface of the spar cap structure <NUM> and such that the second damage tolerant cover sheet 10b is arranged at a second interface between the second plurality of shell fibre layers <NUM> and a second surface of the spar cap structure <NUM>.

In some embodiments, the premanufactured spar cap comprises the spar cap structure <NUM>, the first damage tolerant cover sheet 10a and the second damage tolerant cover sheet 10b, which are all embedded in a first cured resin and the wind turbine blade comprises the first plurality of shell fibre layers <NUM>, the second plurality of shell fibres <NUM> and the premanufactured spar cap structure <NUM> all embedded in a second cured resin to adhere these elements together in the desired arrangement within the wind turbine blade <NUM>.

In other embodiments, the first and/or second damage tolerant cover sheets 10a, 10b does not form part of the pre-manufactured spar cap <NUM>. In such embodiments, the premanufactured spar cap <NUM> only comprises the spar cap structure <NUM> embedded in the first cured resin, whereas the first and second damage tolerant cover sheets 10a, 10b are adhered to the premanufactured spar cap <NUM> with a second cured resin together with the first plurality of shell fibre layers <NUM> and the second plurality of shell fibre layers <NUM>.

It should be noted that it is within the scope of the present invention that further surfaces, such as the side surfaces and/or end surfaces of the premanufactured spar cap <NUM>, may also be covered by one or more damage tolerant cover sheets <NUM> as described in relation to <FIG> or <FIG>.

Reference is made to <FIG>, illustrating the process of manufacturing a wind turbine blade shell according to the two embodiments described above i.e. one where the first and second damage tolerant cover sheets 10a, 10b form parts of the premanufactured spar cap <NUM> embedded in the first cured resin (<FIG> and <FIG>) and one where the first and second damage tolerant cover sheets 10a, 10b form part of the remaining blade shell elements and are embedded with the first and second plurality of shell fibre layers <NUM>, <NUM> in the second cured resin (<FIG>and <FIG>).

<FIG> is a schematic diagram illustrating a cross-sectional view of a blade mould <NUM> for a wind turbine blade shell half. The thick black lines illustrate substantially straight areas of the blade mould <NUM> between which a plane Y extends. The area between the plane Y and the moulding surface <NUM> is defined as the moulding cavity <NUM>.

<FIG> is a schematic diagram illustrating an arrangement of a first plurality of shell fibre layers <NUM> on the blade moulding surface <NUM> of the blade mould <NUM>. In <FIG>, the number of the first plurality of shell fibre layers <NUM> is three. The first plurality of shell fibre layers <NUM> are arranged on top of each other, forming a thin outer shell of the blade shell member, as illustrated in <FIG>. In reality, the outer shell is much thinner than illustrated in <FIG>. However, for illustrative purposes, the outer shell proportions are exaggerated. Furthermore, in reality, more than three fibre-reinforced layers <NUM> may be arranged on top of each other, but for an illustrative purpose, only three layers are shown.

<FIG> is a schematic diagram illustrating how a pre-manufactured spar cap <NUM> as described in relation to <FIG> is arranged in the blade mould <NUM>. The premanufactured spar cap <NUM> is arranged on top of the first plurality of shell fibre layers <NUM> such that the first outer surface of the first damage tolerant cover sheet 11a is in contact with the first plurality of shell fibre layers <NUM>.

<FIG> is a schematic diagram illustrating how a pre-manufactured spar cap <NUM> comprising only the spar cap structure <NUM> embedded in a first cured resin as well as the first and second plurality of shell fibre layers <NUM>, <NUM> are arranged in the wind turbine blade mould <NUM>.

As can be seen in <FIG>, a first damage tolerant cover sheet 10a as described in relation to <FIG> or <FIG> is arranged on top of the first plurality of shell fibre layers <NUM>, such that a first outer surface of the first damage tolerant cover sheet 11a is in contact with the first plurality of shell fibre layers <NUM>. Then, as illustrated in <FIG>, the premanufactured spar cap is arranged on top of the first damage tolerant cover sheet 10a, such that a first surface of the spar cap structure <NUM> is in contact with the second outer surface of the first damage tolerant cover sheet 12a. Finally, as illustrated in <FIG>, a second damage tolerant cover sheet 10b as described in relation to <FIG> or <FIG> is arranged on top of the premanufactured spar cap <NUM>, such that a second outer surface of the second damage tolerant cover sheet 12b is in contact with a second surface of the spar cap structure <NUM>.

<FIG> illustrates that after arrangement of the first plurality of shell fibre layers <NUM>, the spar cap structure <NUM> and the first and second damage tolerant fibre sheets 10a, 10b within the blade mould (either as described in relation to <FIG> or as described in relation to <FIG> and 6F-<NUM>), then further blade components, such as core elements and/or shear webs <NUM>, may subsequently be arranged in the blade mould.

<FIG> illustrates that a second plurality of shell fibre layers <NUM> are arranged on the second damage tolerant cover sheet 10b, such that the second plurality of shell fibre layers <NUM> is in contact with a first outer surface of the second damage tolerant cover sheet 11b. After this step, the blade moulding cavity <NUM> is infused with a second resin (not illustrated) to bind all elements within the blade mould together and the second resin is allowed to cure to form a blade shell member.

<FIG> illustrates the manufacture of a pressure side shell part. It is recognised that a suction side shell part may be manufactured in a similar way. The two shell parts can subsequently be assembled to form a closed aerodynamic shell i.e., a wind turbine blade <NUM> according to an embodiment of the invention, e.g. with shear webs between the spar caps as illustrated in <FIG>.

Claim 1:
A pre-manufactured spar cap (<NUM>) for a wind turbine blade comprising
- a spar cap structure (<NUM>) comprising a plurality of fibre-reinforced composite elements (<NUM>) arranged in stacked rows (<NUM>) and separated by interlayers (<NUM>), characterized in that it further comprises
- a first damage tolerant cover sheet (10a) comprising
- a first damage tolerant fibre layer (21a) forming part of a first outer surface of the first damage tolerant cover sheet (11a); and
- a second damage tolerant fibre layer (22a) forming part of a second outer surface of the first damage tolerant cover sheet (12a), the second outer surface of the first damage tolerant cover sheet being opposite to the first outer surface of the damage tolerant cover sheet,
- wherein the first damage tolerant fibre layer (21a) and the second damage tolerant fibre layer (22a) are attached to each other in attachment areas (<NUM>) by a first plurality of stitching rows and/or by a first binding agent arranged between the first and second damage tolerant fibre layer in the attachment areas (<NUM>), and wherein the attachments areas (<NUM>) are separated from each other by a distance (<NUM>) between <NUM>-<NUM>, preferably around <NUM>;
wherein the first damage tolerant cover sheet (10a) is arranged such that the second outer surface of the first damage tolerant cover sheet (12a) is in contact with a first surface of the spar cap structure (<NUM>); and
wherein the spar cap structure (<NUM>) and the first damager tolerant cover sheet (10a) are embedded in a first cured resin.