DK177291B1

DK177291B1 - A wind turbine blade

Info

Publication number: DK177291B1
Application number: DKPA201100155A
Authority: DK
Inventors: Lennart Kuehlmeier; Esther Peterslund; Flemming Mortensen
Original assignee: Suzlon Blades Technology B V
Priority date: 2011-03-08
Filing date: 2011-03-08
Publication date: 2012-10-08
Also published as: WO2012119778A1; DK201100155A

Abstract

A composite blade structore for a blade for a wind turbine and a metliod of m.anufac ture thereof is described. The blade core is formed of a first, light-weight, core mate rial and a sec ond core material of a higher grade and/or structural strength. The second material is provided as a relatively thin layer on the outside of the first core material, so that the tip ends of any resin bridges in the core may be contained within the second eore niaterial. As the second core material is of a higher grade than the first, this in creases the resistance of the blade eore to cracks or other faihires originating at the tip of the resin bridge. As only a relatively small amount of second core material is used compared to the first core material, this results in reduced cost and minirnised weight of the total Made.

Description

DK 177291 B1 i A Wind Turbine Blade Field of the Invention

The present invention relates to a blade for a wind turbine, more particularly, a blade 5 comprising a sandwich panel with a core and with external skin layers, and a method of manufacture thereof.

Background of the Invention

In the construction of rotor blades for wind turbines, there is a continual drive to increase reliability and durability of components, while keeping construction costs and 10 component weight down. One outcome of this has been the proliferation of sandwich-core blades.

With reference to Fig. 1, a cross-section of a section of a known sandwich-core blade for a wind turbine is indicated at 10. The blade 10 comprises an inner core 12 of a 15 low-density material, e.g. polyvinyl chloride (PVC), styrene-acrylonitrile (SAN), and an outer laminate skin 14 of a suitable skin material, e.g. glass fibre, aluminium. Such a combination of a strong, durable skin with a low-density, low-cost core material results in a sturdy blade which has a relatively low overall weight shared with minimised cost of materials.

20

The general method of manufacture for such a sandwich-core blade 10 is as follows: Firstly, a planar body of material for the inner core 12 is provided having a given grade or density, e.g. 60 kg/m PVC or SAN. To allow for the core material to be flexed or bent into the desired profile shape for the wind turbine blade design re-25 quired, the material is scored wherein a series of lateral and longitudinal channels or grooves 16 are cut into the planar body of the core material 12, the grooves 16 generally having a triangular cross-section. Any particular arrangement of grooves may be used, with a simple regular grid arrangement normally preferred. In addition, while in some cases all of the grooves are cut onto the same side of the planar body, in other 30 cases the lateral groves are cut into the upper surface of the planar body, with the longitudinal grooves cut into the lower surface of the planar body of core material 12.

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Once the channels 16 have been cut into the body of core material 12, the core material 12 is then positioned in an injection mould, which causes the core to take the shape of a blade profile. Glass fibre layers (or any other suitable skin material) are applied to the surfaces of the core material 12, and appropriate resin is then injected 5 into the mould. Once the resin has cured, the laminate skin 14 has hardened forming a suitable part of a wind turbine blade ready for use.

One of the problems with such blade parts or sandwich panels 10 is that they are susceptible to failure due to cracks forming within the body of the blade/panel 10. It is 10 thought that one reason for such failures can be traced to the manufacture stage. As resin is injected into the mould, the resin accumulates in the grooves 16 of the core material 12, and when cured this results in the formation of resin bridges 18 from the upper to the lower faces of the external skin 14. As most of the cracks in blade failures are found to originate at the narrow end of the resin bridge 18, it is thought that the 15 presence of air pockets and/or higher stresses at the tip of the resin bridge 18 provide a starting point for any stress fractures and subsequent cracks in the blade. An example is illustrated in Fig. 1, wherein a crack 15 is shown originating from a stress concentration area indicated by section A.

20 One possible solution to this problem is to add extra laminate to the outer skin 14 to increase the strength of the blade/panel 10. However, this increases the cost of manufacture of the blade/panel 10, as well as increasing the weight of the blade/panel itself (and thereby the loading on the blades and the rest of the wind turbine generator).

25 An alternative solution is to choose a higher grade core material, or an alternative core material with higher overall strength, stiffness and fracture toughness. With increased stiffness the stress concentration at the tip of the resin bridge is reduced and with increased strength and toughness the crack propagation is inhibited. However, again the disadvantage of this method is still an increase in weight and cost of the blade/panel 30 or product.

Accordingly, neither of the known solutions provides a satisfactory low-cost, low-weight improvement to the performance of sandwich-core blades/panels.

DK 177291 B1 3

Object of the Invention

It is an object of the invention to provide a sandwich-core-type blade/panel for a wind turbine and a method of construction thereof, combining reduced susceptibility to stress failure with minimised weight and cost.

5 Summary of the Invention

Accordingly, there is provided a composite blade for a wind turbine, the blade comprising a sandwich panel having: at least one external skin layer; an internal core comprising a first core material; and 10 at least one resin bridge extending from a boundary area of said core into said core, characterised in that the panel further comprises: at least one inter layer comprising a second core material, wherein said inter layer is located between said external skin layer and said core, and wherein said sec-15 ond core material has a greater structural strength relative to said first core material.

The use of a further inter layer of a second core material provides additional strengthening of the wind turbine blade panel, and reduces the risk of failures of the wind turbine blade sandwich panel. It should be noted that the first and second core materials 20 are not any fibre layer type materials, rather are relatively light-weight, high rigidity materials suitable for use in the core of a blade sandwich panel, e.g. balsa, or any suitable monomer- or polymer-based substance, e g. polyvinyl chloride (PVC), urethane, polyethylene terephthalate (PET), styrene-acrylonitrile (SAN), polystyrene, polyme-thacrylimide (PMI), etc.

25

Said second core material has a greater structural strength relative to said first core material. Preferably, said second core material has a greater density relative to said first core material. Preferably, said first core material has a density of between 40-80 kg/m3 and said second core material has a density of between 80-300 kg/m3.

30

Preferably, the panel comprises a second inter layer of the second core material (or of a third core material), wherein said first inter layer and said second inter layer are provided at opposite sides of said internal core.

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Preferably, said at least one inter layer is of a reduced thickness relative to said internal core.

Preferably, the thickness of said at least one inter layer is between 5%-15% of the 5 thickness of the internal core.

Preferably, said second core material has a greater structural strength relative to said first core material.

10 Where said second inter layer is formed of a third core material, preferably said third core material has a greater structural strength relative to said first core material, and wherein said third core material has a greater or lower structural strength relative to said second core material.

15 Preferably, said resin bridge extends through at least a portion of said core and at least one inter layer.

In one embodiment, there is provided a composite blade for a wind turbine, the blade comprising a sandwich panel with: 20 an internal core; an external skin layer, and at least one resin bridge extending from adjacent said external skin layer into said core, wherein said core comprises a layer of first core material and at least one 25 layer of second core material, said layer of second core material provided adjacent said external skin layer, wherein said second core material has a greater structural strength relative to said first core material, and wherein said resin bridge extends through at least a portion of said first core material layer and said second core material layer.

30

The term resin bridge refers to a joint of cured resin which is found in the core section of certain types of composite panels for wind turbine blades, caused due to the features of the manufacturing process used to produce the blades/panels. The resin bridge DK 177291 B1 5 may extend into a portion of the core, or may extend through the entire core, the ends provided in for example a thin mesh provided at either side of the core material. As the second core material is of a higher grade than the first core material, e.g. it is of a stronger type of material, or a material with greater density, this improves the struc-5 tural rigidity of the blade core and reduces the likelihood of cracks forming in the core. As the core is not entirely formed from this second core material, this allows for the lower grade first core material to be used, keeping the overall cost of production of the blade down, as well as reducing the total weight of the blade itself.

10 Preferably, said second core material has a greater density relative to said first core material. Preferably, said first core material has a density of between 40-80 kg/m3 and said second core material has a density of between 80-300 kg/m3.

Preferably, said at least one layer of said second core material is of a reduced thick-15 ness relative to said layer of said first core material.

As the majority of the blade/panel core (or the internal core) is comprised of the first core material, the weight and cost of the total blade is minimised.

20 Preferably, the thickness of said at least one layer of said second core material is between 5%-15% of the thickness of the internal core. Further preferably, between 6%- 10%.

Preferably, said at least one resin bridge comprises a tapered cross-section having a 25 narrow end and a wide end, and wherein the narrow end of said at least one resin bridge is provided proximate said at least one layer of second core material.

As the narrow end of the resin bridge is provided adjacent to or surrounded by the stronger second core material, this improves the resistance of the blade/panel to any 30 cracks that would normally form in the region of the narrow end (or tip) of a resin bridge. By ‘proximate’ it is meant that the narrow end of the resin bridge is provided in, at or adjacent to the at least one layer of second core material. Most preferably, the narrow end of the resin bridge is provided in the at least one layer of second core ma- DK 177291 B1 6 terial, but due to the manufacturing process used, the resin bridge may extend through the at least one layer of second core material, but end close to the layer.

Preferably, said internal core comprises a first layer and a second layer of said second 5 core material, said first and second layers provided at opposite sides of said layer of said first core material, adjacent said external skin layer.

The sandwich arrangement of stronger, higher grade, second core material around the lighter first core material in the centre provides for a stronger overall blade construc-10 tion, while keeping the weight of the core section of the blade/panel to a minimum.

Preferably, said narrow end of said at least one resin bridge is provided proximate a first layer of said second core material and said wide end of said at least one resin bridge is provided proximate a second layer of said second core material.

15

The presence of two layers of second core material at either side of the first core material means that both ends of the resin bridges in the internal panel core will be contained within sections of higher grade core material. This improves the resistance of the blade/panel core to the formation of any cracks or failures that would normally 20 occur in the region of the ends of the resin bridges. Similarly, in the case of blade/panel cores which have tips of resin bridges formed at both the upper and lower surface of the core, the use of this two-layer construction ensures that the tips of all resin bridges in the blade/panel core will be contained within sections of higher-grade core material.

25

Preferably, said first core material and said second core material are formed of the same material, wherein said second core material has a relatively higher grade than said first core material.

30 By higher grade it is implied that the material has a greater strength and/or density.

The higher the strength of the material, the better its ability to prevent the formation of cracks in the core or other core failures.

DK 177291 B1 7

Preferably, said first and said second core material may be of any suitable monomer or polymer substance. Preferably, said first core material and said second core material are selected from the following: polyvinyl chloride (PVC), styrene-acrylonitrile (SAN), polystyrene, polymethacrylimide (PMI).

5

Preferably, said external skin layer may be any suitable fibre or fibre-composite material. Preferably, said external skin layer is formed from one or more of the following: glass fibre, aluminium, carbon, basalt.

There is also provided a wind turbine comprising a wind turbine tower, a nacelle pro-10 vided on said tower, and a rotor provided at said nacelle, said rotor having at least one rotor blade, wherein the wind turbine comprises at least one blade as described.

There is also provided a method of manufacture of a part of a blade for a wind turbine, the method comprising the steps of: 15 providing a planar body of a first core material; laminating at least a portion of a first surface of said planar body with a layer of a second core material to form a sandwich panel core, wherein said second core material has a structural strength greater than said first core material; forming at least one groove in said core to allow flexion of said core, wherein 20 said groove extends through at least a portion of said first and second core materials; adjusting said core into a desired shape in a wind turbine blade mould; performing a resin moulding operation on said core to provide a skin about said core, and curing said resin to form a part of a blade for a wind turbine, such that the 25 portion of resin provided in said at least one groove abuts at least a portion of said first and said second core materials.

Said step of forming at least one groove in said core may be accomplished by any suitable method of making grooves in the core material, e g. cutting. Furthermore, said 30 step of performing a resin moulding operation on said core may include any suitable moulding process, e.g. infusion, pre-preg vacuum bagging, etc.

DK 177291 B1 8

Preferably, said step of laminating comprises laminating at least a portion of said first surface of said planar body and at least a portion of a second surface of said planar body opposite said first surface.

5 Preferably, said step of laminating is configured such that said layer of said second core material is of a reduced thickness relative to said planar body of said first core material.

Description of the Invention

The invention will now be described, by way of example only, with reference to the 10 accompanying drawings, in which:

Fig. 1 shows a cross-section of a portion of a prior art composite blade/panel for a wind turbine;

Fig. 2 shows a cross-section of a portion of a composite blade/panel for a 15 wind turbine according to a first embodiment of the invention;

Fig. 3 shows a cross-section of a portion of a composite blade/panel for a wind turbine according to a second embodiment of the invention; and

Fig. 4 shows a perspective view of a three-bladed wind turbine according to the invention.

20

With reference to the cross-section shown in Fig. 2, a first embodiment of a composite-core blade/panel for a wind turbine is indicated generally at 20, having an upper face 20a and a lower face 20b. The blade/panel 20 comprises an internal core section 22 and an external skin section 24.

25

It will be understood that the external skin section 24 may be formed from any suitable material known for use as an external skin for a wind turbine blade, e.g. a fibre or fibre-composite material. This may include, but is not limited to, composite skins formed from glass fibres, carbon fibres, basalt fibres, aluminium fibre or sheet materi-30 als, etc.

The internal core section 22 comprises a first, central, layer of core material of a first type 26, and an upper and a lower layer of a second core material 28,30. The thin lay- DK 177291 B1 9 ers 28,30 of second core material are provided at either side of the layer of first core material 26, between the first core material 26 and the external skin section 24.

As the internal core 22 is intended as a relatively light-weight section of the 5 blade/panel 20, the first and second core materials are preferably formed from any suitable low-weight, high rigidity material, e.g. balsa, or any suitable monomer- or polymer-based substance, e.g. polyvinyl chloride (PVC), urethane, polyethylene terephthalate (PET), styrene-acrylonitrile (SAN), polystyrene, polymethacrylimide (PMI), etc. It is to be noted that the second core material 28,30 does not include any 10 type of fibre layer material, e.g. a fibre glass layer.

The first core material 26 and the second core material 28,30 differ in that the second core material 28,30 is of a higher strength than the first core material 26. This may be in the form of the first and second materials being the same base core material, but the 15 second core material being of a higher grade (e.g. denser, stronger, etc.), or the second core material being an alternative core material to the first with higher overall strength, stiffness, and fracture toughness relative to the first core material.

It will be understood that said first and second core materials 26,28,30 may be formed 20 as integral parts of a single core body 22, e.g. localised layers of higher density core material formed at the upper and lower sides of the overall sandwich panel core.

Due to the manufacturing process involved (discussed below), a plurality of resin bridges 32 are provided in the internal core section 22. The resin bridges 32 shown in 25 Fig. 2 have a tapered, substantially triangular cross-section, and extend from a first wide end 32a abutting the external skin section 24 at the upper face 20a of the blade/panel 20, to a second tip end 32b proximate to the external skin section 24 at the lower face 20b of the blade/panel 20. The wide ends 32a of the resin bridges 32 are contained within the upper layer of the second core material 28, while the tip ends 32b 30 of the resin bridges 32 are contained within the lower layer of the second core material 30.

DK 177291 B1 10

As the ends 32a,32b of the resin bridges 32 (and particularly the tip ends 32b) are contained within the relatively stronger higher grade second core material 28,30, alleviates the stress concentration within the blade/panel core section 22, and reduces the crack propagation of any initiated damage in the region of the ends 32a,32b of the 5 resin bridges 32. In addition, the use of the relatively light-weight first core material 26 to form the majority of the internal core section 22, while concentrating the relatively heavier second core material 28,30 where it is most effective, the overall weight of the blade/panel 20 is minimised while having improved performance and failure resistance compared to prior art blades/panels.

10

The method of manufacture of such a composite-core blade/panel 20 is as follows.

Firstly, a portion of a first core material is provided in the general dimensions required for a blade for a wind turbine or for a composite sandwich panel. Then, first and sec-15 ond layers of a second core material are adhered to the upper and lower surfaces of the first core material to form a core, wherein the second core material is of a higher grade than the first core material.

A plan outline of a blade/panel is cut from this portion of core, and a series of chan-20 nels or grooves are scored in the core, to allow the moulding or shaping of the core into the desired shape and profile for the wind turbine blade or sandwich panel. The channels or grooves are cut so that the lower end of each channel (e.g. the relatively narrow “tip end” of a triangular-cross-sectional channel) is provided in one of the layers of second core material. In some cases, the channels may be cut such that the 25 channels extend through the core, in which case the opening at the lower end of the channel is provided at one of the layers of second core material.

It will be understood that any suitable method of forming channels or grooves in the core may be used.

A mat of fibres is laid in a mould, to form the lower layer of the outer skin of a part for a blade. The scored composite core is placed into the mould on top of the lower layer of fibres, the channels or grooves of the composite core allowing for the core to 30 DK 177291 B1 11 be shaped into the desired blade profile. A second mat of fibres is then laid on top of the exposed upper surface of the composite blade core, this second mat being used to form the upper layer of the outer skin of a blade.

5 It will be understood that there can be several layers of fibre mats applied on each side of the core, e.g. 10 layers.

The mould is then sealed, and resin infused or injected into the mould. The resin acts to fuse the fibres, and also fills up in the channels formed in the composite core. (In 10 filling up the channels formed in the core, it will be understood that due to the mould-ing/shaping of the core, some of the channels may be relatively more open, while some may be relatively more closed.) The resin is then cured to harden the fibre layers about the composite core, forming the wind turbine blade. The mould can then be opened and the completed blade/panel removed.

15

It will be understood that any suitable alternative methods of moulding may be utilised, e.g. pre-preg vacuum bagging.

As the channels are cut so that the lower ends of the channels are provided in a layer 20 of second core material, this ensures that the lower end of the cured resin contained in the channels (the resin bridges) will also be provided in the layer of second core material. The higher grade second core material provides for increased strength in this region, and reduces the impact of stress fractures occurring in the region of the lower ends of the resin bridges.

25

It will be understood that while the above example describes layers of second core material being adhered to the initial core of first core material, other processes may be used, for example if manufacturing processes allow for a single portion of core material to be provided having areas of different material density.

During the manufacturing procedure, a grid-like arrangement of cut channels in the body of the core section are provided to allow for the flexion of the core about the longitudinal and lateral axes of the blade, so that the core section can be formed into 30 DK 177291 B1 12 an appropriate blade profile prior to the application and curing of the external skin.

Often, the channels are provided such that the longitudinal channels are cut into, say, the upper face of the core, while the lateral channels are cut into the lower face of the core. Such a configuration is particularly suited to the blade/panel 20 described above, 5 as the tip ends of resin bridges formed in such channels will be found at both the upper and lower faces of the core, and as such will be contained within the upper and lower layers of second core material.

However, other configurations of layers of second core material may be utilised. For 10 example, if channels are only going to be cut into one side of the core section, e.g. on the upper face of the core, then the tip ends of the resultant resin bridges will all be provided proximate to the opposite side of the core section, i.e. proximate to the lower face of the core. In this case, the weight and cost of the final blade can be minimised further by having only a single layer of second core material in the core section, pro-15 vided at the lower face of the core section, such that the tip ends of the resin bridges are contained in the relatively stronger second core material.

In some cases, as the channels will be cut so that they extend through all the layers of the core materials, the core material may then be adhered to a thin mesh to form the 20 shape of the internal core.

Similarly, it may be possible to create localised areas, or “pockets”, of second core material at those locations of the core section where it is predicted that the tip ends of the resin bridges will eventually be formed. Such pockets provide the benefit of the 25 stronger, more fault-resistant second core material at the point of likely failure (i.e. the tip ends of the resin bridges), but reduces the amount of second core material required (and consequently the total weight and/or cost of the finished blade/panel) to a minimum.

30 In general, the thickness of the high-grade, second core material layers can be related to the thickness of the low-grade first core material layer. Table 1 provides an example outline of the thickness relationship for an example composite blade (grade di-mensions are provided in kg/m ).

DK 177291 B1 13

Core thickness Low grade (40-80) High grade (80-300)

Total [mm] [mm]_[mm]_ 15 13 1 20 16 2 25 21 2 30 24 3 .......................................................................................................................35.....................................................................................................................................29................................................................................................................................................... 3 40 34 3 45 39 3 50 42 4 55 47 4 _60_521_4

Table 1 5 In general, the thickness of the low-grade material layer can be chosen to be between 5%-15% of the thickness of the total composite blade/panel core. In particular, between 6%-10%.

It will be understood that other enhancements of this configuration may be employed, 10 e.g. further layers of different types of core material may be provided at different locations in the core, for example the layers may be configured such that the grade of the material used in the core decreases with increasing distance from the external skin layer.

15 With reference to Fig. 3, a second embodiment of a composite-core blade/panel for a wind turbine is indicated generally at 40, having an upper side 40a and a lower side 40b. The blade/panel 40 comprises a first core section 42. Similar to the embodiment described above, a series of slits 44 are cut or formed into the first core section 42, substantially extending from the upper side 40a of the first core section 42 towards the 20 lower side 40b.

The first core section 42 is placed within a blade mould for forming a composite core sandwich panel blade, the slits 44 allowing for the first core section 42 to be shaped into the desired blade profile in the mould. A second core section 46 is provided on DK 177291 B1 14 the upper side 40a of the first core section 42, with a fibre layer 48 positioned between the first core section 42 and the second core section 46.

The fibre layer 48 may comprise a low-weight pre-impregnated fibre fabric or fibre 5 mat. The fibre layer 48 may be inserted as a separate layer between the first core section 42 and the second core section 46 during the manufacturing process, or the fibre layer 48 may be provided as pre-attached to the lower surface of the second core section 46. The resin carried in the fibre layer 48 will spread into the slits 44 of the first core section 42, such that when the assembly is cured, resin bridges 50 are formed in 10 the slits 44.

It will be understood that after lay-up in a mould, and subsequent curing, the blade/panel 40 may be laminated with at least one external skin layer, as described in relation to the above embodiment.

15

The particular construction of this embodiment provides for greater structural strength of the blade/panel 40, and reduces the impact of any failure at likely resin bridge points. Another advantage provided is that the step of laminating the upper and lower sides 40a,40b of the blade/panel 40 is avoided until after laying up in the blade mould.

20 The use of a pre-impregnated fibre or mat ensures adhesion between the first core section 42 and the second core section 46 during lay-up.

Additionally or alternatively, it will be understood that said fibre layer 48 may be perforated, or may have apertures defined thereon, to facilitate the passage of resin 25 through said fibre layer 48. The fibre or mat 48 may be pre-impregnated using any suitable pre-preg resins, hot-melt resins, hand lay-up resins, etc.

It will be understood that a further layer of second core material 46 may be provided at the lower side 40b of the first core material 42, to further reinforce the blade/panel 30 40 structure. This further layer of second core material 46 may be provided with or without an intervening fibre fabric or fibre mat layer.

DK 177291 B1 15

While the second core section 46 of Fig. 3 is shown as without slits formed in the material, it will be understood that slits may be formed in the body of the material as required, in order to facilitate bending of the second core section 46. However, if the second core section 46 is sufficiently thin, it may be possible to provide the second 5 core section 46 without slits.

Preferably, the second core section 46 is formed from a core material having a higher strength than the material of said first core section. It will be understood that the same basic core material type may be used in the first and second sections, but said second 10 core section should be of a higher grade or density of core material. It will be further understood that if said panel 40 is provided with said further layer of core material, the core material used may be different from said second core material 46, and may comprise a third core material. Preferably said third core material has a higher strength/density than said first core material, to further provide for reinforcement of 15 said panel 40.

It is to be noted that said second and third core materials do not include fibre layer material types, e.g. glass fibre layers.

20 The advantages of the composite-core blade/panel of the invention include: 1) A more robust sandwich for configurations where there is a risk of cracks initiating at resin bridges or other surface stress concentration effects; 2) Possibly a higher peel strength (i.e. the measure of the strength of an adhesive bond - usually the average force required to part bonded materials divided by the 25 width of the sample); 3) Lower sensibility to core damage from ply-drop effects (i.e. the effect of “stepping” the endpoints of layers of a laminate); 4) Low weight impact; and 5) Low cost impact.

With reference to Fig. 4, a wind turbine is indicated generally at 100. The wind turbine 100 comprises a wind turbine tower 102, a nacelle 104 provided at the top of said tower 102, and a rotor hub 106 having a plurality of rotor blades 108 provided at said 30 DK 177291 B1 16 nacelle 104. It will be understood that said rotor blades 108 may be formed from any of the composite-core blades/panels as discussed. Furthermore, it will be understood that the invention is not limited to use in a three-bladed wind turbine rotor blades, and that any number of rotor blades may be employed.

5

The invention is not limited to the embodiments described herein, and may be modified or adapted without departing from the scope of the present invention.

Claims

A composite blade for a wind turbine, comprising a sandwich panel (20, 40. having: 5. at least one outer shell layer (24), - an inner core (26) comprising a first core material, and - at least one resin filling (32), extending from a boundary region into the core and into the core, characterized in that the panel further comprises: 10. at least one intermediate layer (28, 30, 46) comprising another core material, the intermediate layer being located between the outer shell layer (24). and the core (26), and wherein the second core material has a greater structural strength than the first core material.

The blade according to claim 1, wherein the blade comprises a second intermediate layer (28, 30), wherein the first intermediate layer and the second intermediate layer are provided on opposite sides of the inner core (26).

The blade according to claim 2, wherein the second intermediate layer (28, 30) is formed of the second core material. 20

A blade according to any one of claims 1-3, wherein the at least one intermediate layer (28, 30, 46) has reduced thickness relative to the inner core (22).

The blade of claim 4, wherein the thickness of the at least one intermediate layer (28, 30, 46) is between 5% and 15% of the thickness of the inner core (26).

A blade according to any one of the preceding claims, wherein the blade further comprises at least one fiber layer (48) provided between the inner core (42) and the at least one intermediate layer (46). 30

A blade according to claim 6, wherein the fiber layer comprises a pre-impregnated fiber textile or fiber mat layer. DK 177291 B1 18

The blade of claim 7, wherein the second core material has a greater density relative to the first core material.

A blade according to claim 8, wherein the first core material has a density between 40 and 5 80 kg / m3 and the second core material has a density between 80 and 300 kg / m3.

A blade according to any one of the preceding claims, wherein the resin filling (32) extends through at least a portion of the core (26) and at least one intermediate layer (28, 30).

A blade according to any one of the preceding claims, wherein at least one resin filling (32) comprises a decreasing cross-section with a narrow end (32b) and a wide end (32a) and wherein the narrow end (32b) of the at least one resin filling is provided near the at least one layer (30) of the second core material.

A blade according to claim 11, wherein the narrow end (32b) of the at least one resin filling (32) is provided near a first intermediate layer (30) of the second core material and the wide end (32a) of the at least one resin filling (32) is provided near a second intermediate layer (28) of the second core material.

A wind turbine (100) comprising a wind turbine bar (102), a machine cabin (104) located on the tower and a rotor (106) provided on the machine cabin, said rotor having at least one rotor blade (108), wherein the wind turbine comprises at least one blade according to any of the the preceding claims.

A method of manufacturing a sandwich panel for a wind turbine blade, the method comprising the steps of: providing a body of a first core material, laminating at least a portion of a first surface of the body with an intermediate layer of a second core material to form a sandwich panel core wherein the second core material has a structural strength greater than the first core material, forming at least one groove in the core to allow bending of the core, wherein the groove extends through at least a portion of the first core material and second core material, Adapt the core to the desired shape in a wind turbine blade shape, perform a resin casting operation on the core to provide a shell on the core, and cure the resin to form part of a blade to a wind turbine such that the part of the resin which is provided in the at least one groove adjacent to at least one portion of the first and second core materials.

The method of claim 14, wherein the lamination step comprises laminating at least one portion of the first surface of the body and at least one portion of a second surface of the body opposite the first surface.

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