WOUTERSON, Erwin Merijn (460 Corporation Road, #06-01 Parc Vista, Singapore 5, 64981, SG)
NARASIMALU, Srikanth (19 Taman Sireh, Singapore 6, 53520, SG)
WOUTERSON, Erwin Merijn (460 Corporation Road, #06-01 Parc Vista, Singapore 5, 64981, SG)
| CLAIMS 1. A wind turbine blade comprising at least two regions bonded together by a bonding material, wherein the bonding material contains particles having size distribution and stiffness and strength properties which result in the bonding material having higher strength than corresponding bonding material without any particles, wherein at least a part of the wind turbine blade is made from layers of resin- preimpregnated fibres, the resin constituting the bonding material between the layers, and wherein the particles are arranged between at least some of the layers during manufacturing. 2. A wind turbine blade according to claim 1, wherein strength of the bonding material is the shear strength. 3. A wind turbine blade according to claim 1, wherein the strength of the bonding material is the tensile strength. 4. A wind turbine blade according to any of claims 1-3, wherein the particles are made from a magnetisable material. 5. A wind turbine blade according to any of claims 1-3, wherein the particles are made from a material which can be made electro-static. 6. A wind turbine blade according to any of claims 1-3, wherein the particles are selected from: silica, silicate, titanium dioxide, alumina and carbon nanotubes. 7. A method of manufacturing a wind turbine blade, the method comprising bonding at least two regions of the blade together by using a bonding material containing particles having size distribution and stiffness and strength properties which result in the bonding material having higher strength than corresponding bonding material without any particles, the method further comprising the steps of - arranging layers of resin-preimpregnated fibres on a mould surface, providing an airtight and flexible enclosure over the layers of fibres of resin-prei impregnated fibres, evacuating air present in the enclosure, heating the layers of resin-preimpregnated fibres for a predefined period of time to cause a decrease in viscosity of the preimpregnating resin, and curing the resin so that the resin constitutes the bonding material between the layers, wherein the step of arranging layers of resin-preimpregnated fibres on a mould surface comprises arranging particles between at least some of the layers. 8. A method according to claim 7, wherein strength of the bonding material is the shear strength. 9. A method according to claim 7, wherein the strength of the bonding material is the tensile strength. 10. A method according to any of claims 7-9, wherein the particles are selected from: silica, titanium dioxide, alumina and carbon nanotubes. 11. A method according to any of claims 7-9, wherein the particles are made from a magnetisable material. 12. A method according to claim 11, the method comprising the step of arranging one or more magnets adjacent to the mould surface in at least one selected region so that the particles can be attracted by the one or more magnets and thereby be kept in place during the manufacturing. 13. A method according to claim 11, the method comprising use of a magnetisable mould which is made magnetic in at least one selected region so that the particles are kept in place during the manufacturing. 14. A method according to any of claims 7-9, wherein the particles are made from a material which can be made electro-static, and wherein the method further comprises the steps of - applying a positive charge to the mould, and applying a negative charge to the particles. |
FIELD OF THE INVENTION
The present invention relates to wind turbine blades comprising at least two regions which are bonded together by a bonding material and a method for manufacturing such wind turbine blades. In particular the invention relates to wind turbine blades wherein layers of resin pre-impregnated fibres are used in the manufacturing of the blades.
BACKGROUND OF THE INVENTION
A wind turbine blade is typically made from two or more separately manufactured blade parts which are joined by use of a bonding material. Such blade parts may be manufactured by use of layers of resin pre-impregnated fibres, also called pre- pregs, to form a laminated composite structural member. During manufacturing the layers are joined by applying heat and typically also vacuum and/or pressure whereby the resin is made to flow in a direction substantially perpendicular to the plane of the fibres. Hereby a coherent structure is obtained in which the resin in the interface between the layers of fibres can be considered as a bonding material.
The bonding material may have a lower mechanical strength than the remainder of the blade and thereby constitute potential points of weakness when the blade is loaded which may cause debonding between the layers of a laminated composite structure used for a blade part. If debonding occurs, a potential risk of failure arises which means that it may be necessary to inspect the blade at regular intervals or monitor resulting increases in stresses in other regions to be able to prevent hazardous failures of the wind turbine blade.
Hence, an improved wind turbine blade would be advantageous, and in particular a more reliable wind turbine blade would be advantageous. OBJECT OF THE INVENTION
It is an object of the present invention to provide a wind turbine blade with a reduced risk of debonding compared to known art.
It is another object of the present invention to provide a wind turbine blade with improved mechanical properties of the bonding material compared to known art. Mechanical properties of relevance for the bonding are e.g. shear strength, tensile strength, stiffness, fatigue strength, and fracture toughness.
It is another object of the present invention to provide a method of manufacturing such a wind turbine blade.
It is another object of some embodiments of the invention to provide a method of manufacturing a wind turbine blade in which a thinner bonding region than for known art is possible without jeopardizing the mechanical properties. This may result in weight savings and less material consumption depending on the weight and price of the particles and bonding material, respectively.
It is an object of some embodiments of the invention to provide an improved method of manufacturing a wind turbine blade which is also applicable to curved surfaces.
It is a further object of the present invention to provide an alternative to the known art.
SUMMARY OF THE INVENTION
Thus, the above described object and several other objects are intended to be obtained in a first aspect of the invention by providing a wind turbine blade comprising at least two regions bonded together by a bonding material, wherein the bonding material contains particles having a size distribution and stiffness and strength properties which result in the bonding material having higher strength than corresponding bonding material without any particles, wherein at least a part of the wind turbine blade may be made from layers of resin-preimpregnated fibres, the resin constituting the bonding material between the layers, and wherein the particles are arranged between at least some of the layers during manufacturing.
Hereby the mechanical properties, and in particular the inter-laminar shear strength, of the region between fibre layers after curing are improved. The particles may be evenly distributed or the distribution may be optimised to suit an expected loading situation during use.
The strength of the bonding material may be the shear strength or the tensile strength or both. By strength is preferably meant the fracture strength, such as the stress resulting in initiation of a fracture. The strength may be determined by any standardized method known to a person skilled in the art, such as ISO 4587 or ASTM D5045.
The size distribution is relevant at least because it influences the possible maximum volume ratio of particles and thereby the obtainable total stiffness of the particle-containing adhesive. Since a significant cause of crack initiation is stress concentrations due to abrupt changes in local stiffness in a loaded structure, the risk of such crack initiation can be lowered by ensuring at least partial removal of such abrupt changes. Total removal is not always possible in bonding regions as described above, since the stiffness of resin between two layers of fibres typically have a significantly lower stiffness than the surrounding regions. However, even though constant properties in general and thereby also a constant stiffness across an area comprising a bonding region is not possible, an increase in stiffness of the bonding material will decrease the difference in stiffness between the regions and thereby lower the risk of crack initiation related to the change in stiffness properties.
For a load carrying structure, such as a wind turbine blade, both the initiation of a crack and the growth of a crack are critical, and material properties characterising both situations are therefore relevant for the performance. It may therefore be relevant to characterise a bonding material by other properties than the strength, such as the fracture toughness. Since this parameter is dependent on the resistance against crack propagation, it is dependent on the strength of the bonding between the particles and the adhesive or resin. It may therefore be relevant to improve this strength e.g. by treating the particles with a chemical which results in a higher surface roughness or by adding an additive which increases the adhesion between particles and adhesive chemically. This could e.g. be achieved by using a silane coupling agent material.
The particles may be made from a magnetisable material, such as nickel, iron, cobalt and their alloys, or from a material which can be made electro-static. Hereby they can be held in place even on curved surfaces until they are trapped in the cured resin. This will be described in further details below.
Alternatively or in combination therewith the particles may be selected from: silica, silicate, titanium dioxide, alumina and carbon nanotubes.
In a second aspect the invention relates to a method of manufacturing a wind turbine blade, the method comprising bonding at least two regions of the blade together by using a bonding material containing particles having a size distribution and stiffness and strength properties which result in the bonding material having higher strength than corresponding bonding material without any particles, the method further comprising the steps of arranging layers of resin-preimpregnated fibres on a mould surface, providing an airtight and flexible enclosure over the layers of fibres of resin-preimpregnated fibres, evacuating air present in the enclosure, - heating the layers of resin-preimpregnated fibres for a predefined period of time to cause a decrease in viscosity of the preimpregnating resin, and curing the resin so that the resin constitutes the bonding material between the layers, wherein the step of arranging layers of resin-preimpregnated fibres on a mould surface comprises arranging particles between at least some of the layers.
The method may further comprise the step of assembling at least two blade parts by use of adhesive comprising particles. Other known manufacturing methods may correspondingly be modified by addition of particles where regions are to be bonded together.
In some embodiments of the invention the particles are made from a magnetisable material. In such embodiments, the method may comprise the step of arranging one or more magnets adjacent to the mould surface in at least one selected region so that the particles can be attracted by the one or more magnets and thereby be kept in place during the manufacturing. This is particularly important when the moulding surface is curved, such as double-curved which is typically the case for wind turbine blades. Alternatively or in combination therewith the method may comprise use of a magnetisable mould which is made magnetic in at least one selected region so that the particles are kept in place during the manufacturing. The selected region may be the whole mould.
In some embodiments of the invention, the particles may be made from a material which can be made electro-static, and the method may then further comprise the steps of applying a positive charge to the mould, and applying a negative charge to the particles. This can be done using for example corona electro-static spray guns. The corona gun utilizes a voltage supply to charge the powder particles, thereby negatively charging the particles. If the mould is then grounded positively, the particles will adhere to the mould surface. Suitable methods for utilizing corona spray guns are well-known to a person skilled in the art.
The first and second aspects of the present invention may be combined. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
BRIEF DESCRIPTION OF THE FIGURES
The wind turbine blade and manufacturing method according to the invention will now be described in more detail with regard to the accompanying figures. The figures show one way of implementing the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.
Figure 1 shows schematically a cross sectional view of a wind turbine blade made from two parts which are to be assembled by gluing.
Figure 2 shows schematically a method of manufacturing a composite laminate from resin pre-impregnated fibres.
DETAILED DESCRIPTION OF AN EMBODIMENT
A wind turbine blade is typically made from two or more parts which are manufactured separately and assembled by gluing. A cross sectional view of such parts arranged ready for assembling are shown schematically in figure 1. The illustrated cross section is perpendicular to the length direction of the blade. The blade may comprise further parts, such as spars, not included in the figure. The separate parts may be manufactured by a method as described in relation to figure 2.
The blade section is assembled from a first and a second part 1,2. The resulting blade section may itself constitute only a part of a wind turbine blade. It may e.g. be a tip end which has to be assembled with a root end. The final assembling can be done at the manufacturing site, but it is also possible to transport separate sections to the site where the wind turbine is to be erected before assembly. Hereby the transportation may be easier especially for very large blades.
The blade section shown in figure 1 comprises a first and a second part 1,2 which are glued together by use of an adhesive 3 selected to suit the materials used. The mechanical properties of the adhesive 3 may have been modified by addition of particles to improve the mechanical properties.
Figure 2 shows schematically a method used to manufacture a composite laminate from layers of resin-preimpregnated fibres 4 (also called pre-pregs) which fibres may be arranged in the same direction or in two or more directions. The fibres may also be woven into mats before the impregnation. The method may e.g. be used for manufacturing the parts of the wind turbine blade in figure 1; this will include the use of a curved mould surface.
In a known manufacturing method, a predetermined number of pre-pregs 4 which are cut into the desired size and shape are placed on a moulding surface 5 of a mould which surface is typically coated to ease the removal of the composite part after moulding. In relation to the present invention it has been realised that the inter-laminar strength after curing of a composite member, such as a wind turbine blade, can be increased by dispersing particles 6 between the pre-pregs 4 during lay-up. This will be described in more details below. A peel ply 7 is typically placed on top of the pre-pregs 4 to ensure that the surface of the manufactured composite part stays clean until a possible succeeding processing step, such as painting, or until the part is to be used. On top of the peel-ply 7 is a separation foil 8, which is typically a porous plastic foil, and a breather layer 9. All the layers mentioned are covered by an airtight and flexible sheet 10, typically a plastic material, which is sealed to the moulding surface 5 by use of sealing tape 11 to provide an enclosure 12 from which air can be evacuated via a vacuum port (not shown) by use of a vacuum pump (not shown). When substantially all the air present air in the enclosure 12 has been evacuated, heat is applied for a predetermined period of time to cause a decrease in the viscosity of the preimpregnating resin as well as later to cure the resin. Due to the vacuum and the temperature increase, resin will flow in a direction substantially perpendicular to the pre-pregs 4 and towards the breather layer 9. For some applications, a pressure is applied in combination with the heat. This is typically done in an autoclave in which the mould and the entire assembly inside the enclosure are placed. The temperature may be varied during the process, and when the curing is complete, the composite part is removed from the mould.
With this manufacturing method, at least part of the resin between the layers of fibres 4 can be considered as a bonding material between the layers, which layers can be considered as regions to be bonded together. The modification according to the present invention is obtained by addition of particles (not shown in figure 1) which result in an increase in at least one of the following properties of the adhesive bonding after curing: stiffness, shear strength, tensile strength, fatigue strength, and fracture toughness. In the selection of particles for a given adhesive, a high interfacial bonding strength between the particles and the adhesive must be ensured to prevent that a possible crack easily propagates along the interface between the particles and the adhesive. It may therefore be necessary to modify the surface of the particles before they are added to the adhesive. This step may be performed by a supplier of particle-reinforced adhesive, but it may alternatively be performed by the wind turbine manufacturer before adding particles to the adhesive.
The particles may e.g. be selected from: silica, silicate, titanium dioxide, alumina, and carbon nano-tubes, the materials being used alone or in combination. However, any suitable material is considered to be covered by the scope of the present invention. The range of the size distribution may be narrow or wide, such as comprising both nano- and micro-size particles. A wide range may be desired if a large weight or volume ratio of particles is desired. The upper limit for the size range should preferably be well below the thickness of the adhesive bonding to ensure a good bonding to both blade parts. An appropriate weight percentage for nano particles may be in the range of 0-10 %.
The size of the particles 6 is typically chosen so that at least most of them stay at the interface between the layers. However, it may also be advantageous to use a size distribution of particles 6 which result in the smaller of them flowing into the interspaces between the fibres to reduce the possible stress concentrations during loading due to abrupt changes in stiffness. The actual size distribution will in this case depend on the size of the interspaces between the strands of fibres and individual fibres.
The moulding surface 5 in figure in figure 2 is shown as plane but in practice, such as for moulding of wind turbine blades, it will be curved. It must therefore be ensured that the particles 6 stay in the desired place on the pre-pregs 4 until they are fixed by the cured resin. This can e.g. be obtained by using magnetisable particles 6 in combination with one or more magnets 13. Such magnets 13 can e.g. be electromagnets arranged beneath the moulding surface 5 either integrated in the mould or as separate elements. The magnets may cover the whole moulding surface or only selected regions thereof depending on the desired distribution of the particles. Alternatively the particles 6 can be made from a material which can be made static-electric to such an extent that they will stay in place even when heat is applied and the resin starts flowing. This may e.g. be obtained by use of corona electrostatic spray guns to give a negative charge to the particles. If the mould is grounded positively, the particles will adhere to the mould. Suitable materials are any chargeable material, and an actual choice will therefore mainly depend on the desired mechanical properties.
The particles 6 can be evenly dispersed across the surfaces to be bonded together. Alternatively, the particle distribution may be optimised to the expected load distribution during use of the wind turbine blade. Such an optimisation may be with respect to one or more of the following parameters: material, size distribution, volume or weight fraction, aspect ratio and orientation; the two later parameters being relevant for elongated particles, such as carbon nanotubes. Such optimisation may e.g. be performed by computer simulations or by experiment.
The invention has been described in detail with respect to manufacturing by use of pre-pregs. However, other manufacturing methods comprising use of layers of fibres are also considered to be covered by the scope of the present invention. Such methods will be well-known to a person skilled in the art.
Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.
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