Patent Publication Number: US-2023135034-A1

Title: An interlayer, a spar cap and a wind turbine blade

Description:
The present disclosure relates to an interlayer. Particularly, an interlayer for being arranged between a first element and a second element of a conductive fibre reinforced composite material. The present disclosure also relates to a spar cap comprising an interlayer, and a wind turbine blade comprising such spar cap. 
     BACKGROUND 
     Wind power provides a clean and environmentally friendly source of energy. Wind turbines usually comprise a tower, generator, gearbox, nacelle, and one or more rotor blades. The wind turbine blades capture kinetic energy of wind using known airfoil principles. Modern wind turbines may have rotor blades that exceed 90 meters in length. 
     Wind turbine blades are usually manufactured by forming two shell parts or shell halves from layers of woven fabric or fibre and resin. Spar caps or main laminates are placed or integrated in the shell halves and may be combined with shear webs or spar beams to form structural support members. Spar caps or main laminates may be joined to, or integrated within, the inside of the suction and pressure halves of the shell. 
     As the size of wind turbine blades increases, various challenges arise from such blades being subject to increased forces during operation, requiring improved reinforcing structures. The manufacturing of large reinforcing structures, such as spar caps or spar beams, is likewise challenging, in particular when pultruded, carbon fibre-reinforced spar caps are used as the reinforcing members. Carbon fibres are typically lighter than glass fibres by volume and have improved tensile and compressive strength. However, laminate defects, such as voids, wrinkles or misaligned fibres, may have disadvantageous effects on mechanical properties. 
     Use of pultruded elements, for example in the spar cap, in particular when used in combination with a vacuum infused resin transfer moulding (VARTM) process, comprise the difficulty of ensuring complete resin infusion and adherence between the individual pultruded elements. 
     Furthermore, when using pultruded elements, comprising conductive fibres, e.g. pultruded carbon elements, there is a risk that during a thunderstorm a voltage differential may occur between individual pultruded elements if the fibres of one pultruded element and the fibres of other pultruded elements are not electrically coupled. Such voltage differential may cause damage or even fire in the component. 
     SUMMARY 
     It is an object of the present disclosure to provide an interlayer, and associated spar cap and wind turbine, that provides improved performance over the prior art. Particularly, it is an object of the present disclosure to provide an interlayer for being arranged between a first element comprising conductive parts and a second element comprising conductive parts of a fibre reinforced composite material, e.g. a spar cap, such as a spar cap of a wind turbine blade, decreasing the risk of damage caused by lightning and/or facilitating the structural strength of the fibre reinforced composite material. 
     Thus, the present disclosure relates to an interlayer sheet and an interlayer comprising an interlayer sheet, as well as a spar cap comprising an interlayer, and a wind turbine comprising a spar cap with an interlayer. The interlayer may comprise a veil or a sheet, e.g. a thin sheet of fibre material, optionally maintained by a binder. 
     Accordingly, an interlayer is disclosed, such as an interlayer for being arranged between a first element, such as a first pultruded element, and a second element, such as a second pultruded element, of a conductive material, such as a conductive fibre reinforced composite material. For example, the first element and the second element may be carbon elements, e.g. may be of carbon fibre reinforced composite material. The fibre reinforced composite material may be a spar cap of a wind turbine blade. The interlayer may comprise one or more sheets, e.g. comprising an interlayer sheet and optionally a top sheet and/or a bottom sheet. The interlayer sheet comprises one or more fibre layers extending in a fibre layer plane. The one or more fibre layers include a first fibre layer comprising a first plurality of fibres and having a first upper fibre surface and a first lower fibre surface. The interlayer sheet has an upper interlayer surface and a lower interlayer surface and the interlayer comprises a plurality of conductive fibres, such as carbon fibres and/or metal fibres, such as copper fibres and/or steel fibres, wherein each of the plurality of conductive fibres forms part of the upper interlayer surface as well as the lower interlayer surface. 
     Also disclosed is a spar cap for a wind turbine blade. The spar cap comprises a plurality of elements, such as a plurality of pultruded elements, of a conductive material, such as a conductive fibre reinforced composite material, e.g. carbon fibre reinforced composite material. The plurality of elements includes a first element, e.g. a first pultruded element, and a second element, e.g. a second pultruded element. The first element may be a first carbon element, such as a first pultruded carbon element. The second element may be a second carbon element, such as a second pultruded carbon element. The spar cap further comprises an interlayer, such as the interlayer as disclosed above, wherein the interlayer is arranged between the first element and the second element. The interlayer may be arranged between the elements in a width direction (horizontal) and/or between elements in a thickness direction (vertical). The first element and the second element may be adjacent elements in the thickness direction or the first element and the second element may be adjacent elements in the width direction. 
     Also disclosed is a wind turbine blade comprising a spar cap, such as the spar cap as disclosed above. The wind turbine blade may comprise two spar caps according to the spar cap as disclosed herein. For example, the wind turbine blade may comprise a first spar cap in a first blade shell part and a second spar cap in a second blade shell part. The first spar cap may be a pressure side spar cap of a pressure side blade shell part. The second spar cap may be a suction side spar cap of a suction side blade shell part. 
     The plurality of elements of conductive material, such as the first element and/or the second element, may have a length in a longitudinal direction, a width in a width direction, and a height in a height direction. The length may be longer than the width and the width may be longer than the height. The length may be more than 20 meters, such as more than 40 meters, such as more than 70 meters. The width may be between 20-200 mm, such as between 50-150 mm, such as 100 mm. The height may be between 2-10 mm, such as 5 mm. 
     Each of the plurality of elements of conductive material, such as each of the first element and the second element, may have a lower surface and an upper surface extending in the longitudinal direction and the width direction. Each of the plurality of elements may have a first side surface and a second side surface extending in the longitudinal direction and the height direction. Each of the plurality of elements may have a first end surface and a second end surface extending in the width direction and the height direction, 
     The first element and the second element may be arranged such that the lower surface of the first element is facing the upper surface of the second element. The interlayer may be arranged between the lower surface of the first element and the upper surface of the second element. 
     The presence of conductive fibres, such as carbon fibres and/or metal fibres, such as steel and/or copper fibres, extending through the interlayer, such as through the interlayer sheet, provides conductivity of the interlayer in a direction perpendicular to the fibre layer plane, i.e. between the lower interlayer surface and the upper interlayer surface of the interlayer sheet. In case the interlayer comprises a top sheet and/or a bottom sheet, these may comprise conductive fibres, or may facilitate electrical conductivity perpendicular to the fibre layer plane by other means. For example, the interlayer sheet, the top sheet and/or the bottom sheet, may be stitched or woven by a plurality of fibres, e.g. including a plurality of conductive fibres. Alternatively or additionally, the top sheet and/or the bottom sheet may be carbon surface veils. Thus, a plurality of conductive paths are established between elements sandwiching the interlayer. When the conductive fibres, such as carbon fibres and/or metal fibres, such as copper fibres and/or steel fibres, are in contact with both elements and electrically couple them through the interlayer, flow of electrons between the two elements are facilitated. Thereby decreasing the risk of having a voltage differential between the two elements. 
     The upper interlayer surface and the lower interlayer surface may be defined as the two largest surfaces of the interlayer sheet. The upper interlayer surface may be opposite the lower interlayer surface. 
     The first upper fibre surface and the first lower fibre surface may be defined as the two largest surfaces of the first fibre layer. The first upper fibre surface may be opposite the first lower fibre surface. 
     The first upper fibre surface may be the upper interlayer surface. The first lower fibre surface may be the lower interlayer surface. 
     The one or more fibre layers may include a second fibre layer having a second upper fibre surface and a second lower fibre surface. Alternatively or additionally, the one or more fibre layers may include a third fibre layer having a third upper fibre surface and a third lower fibre surface. 
     The second upper fibre surface and the second lower fibre surface may be defined as the two largest surfaces of the second fibre layer. The second upper fibre surface may be opposite the second lower fibre surface. The third upper fibre surface and the third lower fibre surface may be defined as the two largest surfaces of the third fibre layer. The third upper fibre surface may be opposite the third lower fibre surface. 
     The first fibre layer may be arranged between the second fibre layer and the third fibre layer. The second lower fibre surface may be the lower interlayer surface. The third upper fibre surface may be the upper interlayer surface. The second upper fibre surface may be facing the first lower fibre surface. The first upper fibre surface may be facing the third lower fibre surface. 
     The first plurality of fibres may comprise a first plurality of glass fibres. Additionally or alternatively, the first plurality of fibres may comprise a first plurality of polymeric filaments. Additionally or alternatively, the first plurality of fibres may comprise conductive fibres, e.g. carbon fibres, such as the plurality of conductive fibres or a first plurality of conductive fibres, such as a first plurality of conductive fibres of the plurality of conductive fibres. The first plurality of conductive fibres may be a first plurality of carbon fibres, The first plurality of fibres may be randomly oriented in the first fibre layer. The plurality of conductive fibres may comprise the first plurality of carbon fibres. 
     The first fibre layer may comprise a binding agent. The binding agent may maintain arrangement of the first plurality of fibres relative to each other. Alternatively or additionally, the first plurality of fibres may be stitched or woven together to maintain arrangement of the first plurality of fibres relative to each other. The first plurality of fibres may be stitched together with one or more threads, e.g. including a first conductive fibre thread, such as a first carbon fibre thread. The plurality of conductive fibres may comprise the first conductive fibre thread. 
     The second fibre layer may comprise a second plurality of fibres. The second plurality of fibres may comprise a second plurality of glass fibres. Additionally or alternatively, the second plurality of fibres may comprise a second plurality of polymeric filaments. Additionally or alternatively, the second plurality of fibres may comprise conductive fibres, e.g. carbon fibres, such as the plurality of conductive fibres or a second plurality of conductive fibres of the plurality of conductive fibres. The second plurality of fibres may be randomly oriented in the second fibre layer. 
     The second fibre layer may comprise a binding agent. The binding agent may maintain arrangement of the second plurality of fibres relative to each other. Alternatively or additionally, the second plurality of fibres may be stitched together to maintain arrangement of the first plurality of fibres relative to each other. The second plurality of fibres may be stitched together with one or more threads, e.g. including a second conductive fibre thread, such as a second carbon fibre thread. The plurality of conductive fibres may comprise the second conductive fibre thread. 
     The third fibre layer may comprise a third plurality of fibres. The third plurality of fibres may comprise a third plurality of glass fibres. Additionally or alternatively, the third plurality of fibres may comprise a third plurality of polymeric filaments. Additionally or alternatively, the third plurality of fibres may comprise conductive fibres, e.g. carbon fibres, such as the plurality of conductive fibres or a third plurality of conductive fibres of the plurality of conductive fibres. The third plurality of fibres may be randomly oriented in the third fibre layer. 
     The third fibre layer may comprise a binding agent. The binding agent may maintain arrangement of the third plurality of fibres relative to each other. Alternatively or additionally, the third plurality of fibres may be stitched together to maintain arrangement of the third plurality of fibres relative to each other. The third plurality of fibres may be stitched together with one or more threads, e.g. including a third conductive fibre thread, such as a third carbon fibre thread. The plurality of conductive fibres may comprise the third conductive fibre thread. 
     The polymeric filaments, e.g. of the first, second and/or third plurality of polymeric filaments, may be polyester filaments, polypropylene filaments and/or polyethylene filaments. The polymeric filaments may be thermoplastic filaments, such as thermoplastic polyester filaments, thermoplastic polypropylene filaments and/or thermoplastic polyethylene filaments. The use of polymeric filaments in the interlayer promotes resin infusion, gives good filling between elements, such as pultruded elements, and reduces the amount of defects. 
     The first fibre layer may essentially consist of the first plurality of polymeric filaments. Optionally, the first plurality of polymeric filaments maintained relative to each other by a binder. Thus, the first fibre layer may essentially consist of the first plurality of polymeric filaments and a binder. In some embodiments, the first fibre layer may be a polymeric surface veil, such as a polyester surface veil. 
     Alternatively, the first fibre layer may essentially consist of the first plurality of glass fibre, Optionally, the first plurality of glass fibres are maintained relative to each other by a binder. Thus, the first fibre layer may essentially consist of the first plurality of glass fibre and a binder. 
     The second fibre layer may essentially consist of the second plurality of polymeric filaments. Optionally, the second plurality of polymeric filaments maintained relative to each other by a binder. Thus, the second fibre layer may essentially consist of the second plurality of polymeric filaments and a binder. In some embodiments, the second fibre layer may be a polymeric surface veil, such as a polyester surface veil. 
     Optionally, the second fibre layer is a polyester surface veil. A polyester surface veil is a thin layer of material essentially consisting of polyester fibres and optionally a binder for maintaining the polyester fibres. Typically, the polyester fibres are randomly dispersed throughout the layer. A polyester surface veil gives good adhesion properties and are thus advantageous to have as an outer layer of the interlayer, since it is then configured to be in contact with and adhere to an element, such as a pultruded element. 
     Alternatively, the second fibre layer may be a carbon surface veil. A carbon surface veil is a thin layer of material essentially consisting of carbon fibres and optionally a binder for maintaining the carbon fibres. Typically, the carbon fibres are randomly dispersed throughout the layer. A carbon veil also has high permeability, promotes resin infusion and have good adhesion properties. Furthermore, a carbon surface veil provides conductive properties to the interlayer. 
     The third fibre layer may essentially consist of the third plurality of polymeric filaments. Optionally, the third plurality of polymeric filaments maintained relative to each other by a binder. Thus, the third fibre layer may essentially consist of the third plurality of polymeric filaments and a binder. In some embodiments, the third fibre layer may be a polymeric surface veil, such as a polyester surface veil. 
     Optionally, the third fibre layer is a polyester surface veil. A polyester surface veil is a thin layer of material essentially consisting of polyester fibres and optionally a binder for maintaining the polyester fibres. Typically, the polyester fibres are randomly dispersed throughout the layer. A polyester surface veil gives good adhesion properties and are thus advantageous to have as an outer layer of the interlayer, since it is then configured to be in contact with and adhere to an element, such as a pultruded element. 
     Alternatively, the third fibre layer may be a carbon surface veil. A carbon surface veil is a thin layer of material essentially consisting of carbon fibres and optionally a binder for maintaining the carbon fibres. Typically, the carbon fibres are randomly dispersed throughout the layer. A carbon veil also has high permeability, promotes resin infusion and have good adhesion properties. Furthermore, a carbon surface veil provides conductive properties to the interlayer. 
     One or more of the first fibre layer, the second fibre layer and/or the third fibre layer, may be non-woven fabric layers. Alternatively or additionally, one or more of the first fibre layer, the second fibre layer and/or the third fibre layer may be a woven fabric. For example, the first plurality of fibres may be woven together. 
     Alternatively or additionally, the second plurality of fibres may be woven together. Alternatively or additionally, the third plurality of fibres may be woven together. 
     Each of the plurality of conductive fibres may comprise a first part. Each of the plurality of conductive fibres may comprise a second part and/or a third part. The first part may be between the second part and the third part. The first part of each of the plurality of conductive fibres may extend through the one or more fibre layers. The second part of each of the plurality of conductive fibres may be arranged randomly at the upper interlayer surface. The third part of each of the plurality of conductive fibres may be arranged randomly at the lower interlayer surface. 
     Each of the plurality of conductive fibres may comprise a fourth part. Each of the plurality of conductive fibres may comprise a fifth part. The fourth part may be between the second part and the fifth part. The fourth part of each of the plurality of conductive fibres may extend through the one or more fibre layers. The fifth part of each of the plurality of conductive fibres may be arranged randomly at the lower interlayer surface. 
     Each of the plurality of glass fibres, e.g. the first plurality of glass fibres, the second plurality of glass fibres and/or the third plurality of glass fibres, may comprise a first part. Each of the plurality of glass fibres may comprise a second part and/or a third part. The first part may be between the second part and the third part. The first part of each of the plurality of glass fibres may extend through the one or more fibre layers. The second part of each of the plurality of glass fibres may be arranged randomly at the upper interlayer surface. The third part of each of the plurality of glass fibres may be arranged randomly at the lower interlayer surface. 
     Each of the plurality of glass fibres may comprise a fourth part. Each of the plurality of glass fibres may comprise a fifth part. The fourth part may be between the second part and the fifth part. The fourth part of each of the plurality of glass fibres may extend through the one or more fibre layers. The fifth part of each of the plurality of glass fibres may be arranged randomly at the lower interlayer surface. 
     By a part of a fibre being randomly arranged at the upper interlayer surface and/or the lower interlayer surface is meant that the fibre is arranged such that the part forms part of the upper interlayer surface and/or the lower interlayer surface. 
     A plurality of the one or more fibre layers, such as the first fibre layer and the second fibre layer or the first fibre layer, the second fibre layer and the third fibre layer may be stitched together to maintain arrangement of the plurality of the one or more fibre layers. The plurality of the one or more fibre layers, such as the first fibre layer and the second fibre layer or the first fibre layer, the second fibre layer and the third fibre layer may be stitched together with one or more threads, e.g. including a first conductive fibre thread, such as a first carbon fibre thread. For example, the plurality of the one or more fibre layers, such as the first fibre layer and the second fibre layer or the first fibre layer, the second fibre layer and the third fibre layer may be stitched together by the plurality of conductive fibres. 
     The interlayer sheet, such as the one or more fibre layers, e.g. the first fibre layer, may comprise 10-45 wt% carbon fibres, such as 15-45 wt%. The interlayer sheet, such as the one or more fibre layers, e.g. the first fibre layer, may comprise 5-50 wt% polymeric material, e.g. polymeric filaments, such as 10-50 wt%. The interlayer sheet, such as the one or more fibre layers, e.g. the first fibre layer, may comprise 15-50 wt% glass fibres, such as 20-50 wt%. For example, the interlayer sheet, such as the one or more fibre layers, e.g. the first fibre layer, may comprise 10-45 wt% carbon fibres, 5-50 wt% polymeric filaments and 15-50 wt% glass fibres. Alternatively, the interlayer sheet may comprise 15-45 wt% carbon fibres, 10-50 wt% polymeric material and 20-50 wt% glass fibres. 
     The plurality of fibres may be arranged to extend along certain directions. For example, each of the first plurality of conductive fibres may be arranged to extend along a first length direction. Each of the first plurality of glass fibres may be arranged to extend along a second length direction. Each of the first plurality of polymeric filaments may be arranged to extend along a third length direction. For example the plurality of first fibres may be arranged to extend along certain directions in situations where the first fibre layer is a woven fabric or a unidirectional/biaxial/triaxial non-woven fabric. The first length direction and the second length direction may be parallel. The third length direction may be perpendicular to the first length direction and/or second length direction. The first length direction and third length direction may be parallel. The second length direction may be perpendicular to the first length direction and/or the third length direction. 
     The plurality of fibres may be arranged in a plurality of fibre bundles. For example, the first plurality of glass fibres may be arranged in a plurality of glass fibre bundles. The first plurality of conductive fibres may be arranged in a plurality of conductive fibre bundles. The first fibre layer may comprise alternating glass fibre bundles and conductive fibre bundles, e.g. alternating in a direction, such as the first length direction, the second length direction or the third length direction. 
     The plurality of fibres arranged to extend along the first length direction may comprise alternating polymeric filaments and conductive fibres, e.g. alternating perpendicular to the first length direction. The conductive fibres may be bundles of carbon fibres, e.g. since the diameter of carbon fibres are much smaller than that of polymeric filaments. The conductive fibres may also be another type of conductive fibres, e.g. metal fibres, such as copper fibres and/or steel fibres. The plurality of fibres arranged to extend along the first length direction may comprise polymeric filaments and conductive fibres, distributed such that every Xth fibre or fibre bundle arranged to extend along the first length direction comprise conductive fibres, whereas the remaining fibres arranged to extend along the first length direction are polymeric filaments. X may be between 2 and 15, such as between 6-13, such as 10. In this way, the interlayer is resin promoting and conductive at the same time. The plurality of fibres arranged along the first length direction may be stitched or woven together by a plurality of fibres arranged to extend along the second length direction. For example, the fibres used for stitching or weaving together the plurality of fibres arranged to extend along the first length direction, are preferably glass fibres. However, conductive fibres, such as carbon fibres, may also be used, e.g. for every Xth fibre arranged along the second length direction. X may be between 2 and 15, such as between 6-13, such as 10. 
     Some or all of the plurality of fibres, e.g. the first plurality of fibres, such as the first plurality of glass fibres, the first plurality of polymeric filaments or the first plurality of conductive fibres, the second plurality of fibres, such as the second plurality of glass fibres, the second plurality of polymeric filaments or the second plurality of conductive fibres, or the third plurality of fibres, such as the third plurality of glass fibres, the third plurality of polymeric filaments or the third plurality of conductive fibres, may comprise short fibres, e.g. having a length below 100 mm, such as between 5-100 mm. 
     Some or all of the plurality of fibres, e.g. the first plurality of fibres, such as the first plurality of glass fibres, the first plurality of polymeric filaments or the first plurality of conductive fibres, the second plurality of fibres, such as the second plurality of glass fibres, the second plurality of polymeric filaments or the second plurality of conductive fibres, or the third plurality of fibres, such as the third plurality of glass fibres, the third plurality of polymeric filaments or the third plurality of conductive fibres, may comprise continuous fibres, e.g. having a length longer than 100 mm, such as between 100-200 mm. 
     The interlayer may comprise a top sheet. Alternatively or additionally, the interlayer may comprise a bottom sheet. The top sheet may be arranged adjacent the upper interlayer surface. The bottom sheet may be arranged adjacent the lower interlayer surface. The interlayer sheet may be arranged between the bottom sheet and the top sheet. For example, the interlayer sheet may be sandwiched between the top sheet and the bottom sheet. The top sheet and/or the bottom sheet may comprise polymeric or conductive fibres, such as carbon fibres. The top sheet and/or the bottom sheet may be a carbon surface veil. A carbon veil has high permeability, promotes resin infusion and have good adhesion properties. Furthermore, a carbon surface veil provides conductive properties to the interlayer. Alternatively, the top sheet and/or the bottom sheet may be a polyester surface veil. A polyester surface veil has good adhesion properties. In the case the top sheet and/or bottom sheet is a polyester surface veil, the top sheet and/or bottom sheet may comprise conductive fibres, e.g. carbon fibres, extending through the sheet to facilitate electrical conductivity perpendicular to the fibre layer plane, 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Embodiments of the disclosure will be described in more detail in the following with regard to the accompanying figures. The figures show one way of implementing the present disclosure and are not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set. 
         FIG.  1    is a schematic diagram illustrating a wind turbine, 
         FIG.  2    is a schematic diagram illustrating a wind turbine blade and a spar cap structure arranged within the wind turbine blade, 
         FIG.  3    is a schematic diagram illustrating a cross-sectional view of an interlayer arranged between elements, 
         FIG.  4    is a schematic diagram illustrating a three-dimensional view of an interlayer sheet with one and three fibre layers respectively, 
         FIG.  5    is a schematic diagram illustrating a cross-sectional view of an interlayer sheet with one and three fibre layers respectively, 
         FIG.  6    is a schematic diagram illustrating a top view and a cross-sectional view of an interlayer sheet, 
         FIG.  7    is a schematic diagram illustrating a top view, a cross-sectional view and a three-dimensional view of an interlayer sheet, 
         FIG.  8    is a schematic diagram illustrating a top view and a cross-sectional view of an interlayer sheet, 
         FIG.  9    is a schematic diagram illustrating an interlayer sheet, 
         FIG.  10    is a schematic diagram illustrating an exemplary interlayer, and 
         FIG.  11    is a schematic diagram illustrating a cross-sectional view of an exemplary interlayer sheet, 
     
    
    
     DETAILED DESCRIPTION 
     Various exemplary embodiments and details are described hereinafter, with reference to the figures when relevant. It should be noted that the figures may or may not be drawn to scale and that elements of similar structures or functions are represented by like reference numerals throughout the figures. It should also be noted that the figures are only intended to facilitate the description of the embodiments. They are not intended as an exhaustive description of the invention or as a limitation on the scope of the invention. In addition, an illustrated embodiment needs not have all the aspects or advantages shown. An aspect or an advantage described in conjunction with a particular embodiment is not necessarily limited to that embodiment and can be practiced in any other embodiments even if not so illustrated, or if not so explicitly described. 
       FIG.  1    illustrates a conventional modern upwind wind turbine according to the so-called “Danish concept” with a tower  400 , a nacelle  600  and a rotor with a substantially horizontal rotor shaft. The rotor includes a hub  800  and three blades  1000  extending radially from the hub  800 , each having a blade root  1600  nearest the hub and a blade tip  1400  furthest from the hub  800 . 
       FIG.  2 A  shows a schematic view of a first embodiment of a wind turbine blade  1000 . The wind turbine blade  1000  has the shape of a conventional wind turbine blade and comprises a root region  3000  closest to the hub, a profiled or an airfoil region  3400  furthest away from the hub and a transition region  3200  between the root region  3000  and the airfoil region  3400 . The blade  1000  comprises a leading edge  1800  facing the direction of rotation of the blade  1000 , when the blade is mounted on the hub, and a trailing edge  2000  facing the opposite direction of the leading edge  1800 . 
     The airfoil region  3400  (also called the profiled region) has an ideal or almost ideal blade shape with respect to generating lift, whereas the root region  3000  due to structural considerations has a substantially circular or elliptical cross-section, which for instance makes it easier and safer to mount the blade  1000  to the hub. The diameter (or the chord) of the root region  3000  may be constant along the entire root area  3000 . The transition region  3200  has a transitional profile gradually changing from the circular or elliptical shape of the root region  3000  to the airfoil profile of the airfoil region  3400 . The chord length of the transition region  3200  typically increases with increasing distance r from the hub. The airfoil region  3400  has an airfoil profile with a chord extending between the leading edge  1800  and the trailing edge  2000  of the blade  1000 . The width of the chord decreases with increasing distance r from the hub. 
     A shoulder  4000  of the blade  1000  is defined as the position, where the blade  1000  has its largest chord length. The shoulder  4000  is typically provided at the boundary between the transition region  3200  and the airfoil region  3400 . 
     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.  2 B  is a schematic diagram illustrating a cross sectional view of an exemplary wind turbine blade  1000 , e.g. a cross sectional view of the airfoil region of the wind turbine blade  1000 . The wind turbine blade  1000  comprises a leading edge  1800 , a trailing edge  2000 , a pressure side  2400 , a suction side  2600 , a first spar cap  7400 , and a second spar cap  7600 . The wind turbine blade  1000  comprises a chord line  3800  between the leading edge  1800  and the trailing edge  2000 . The wind turbine blade  1000  comprises shear webs  4200 , such as a leading edge shear web and a trailing edge shear web. The shear webs  4200  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  7400 ,  7600  may comprise carbon fibres while the rest of the shell parts  2400 ,  2600  may comprise glass fibres. 
       FIG.  3 A  is a schematic diagram illustrating a cross sectional view of an interlayer  1  arranged between a first element  50 , such as a first pultruded carbon element and a second element  60 , such as a second pultruded carbon element, e.g. of a conductive fibre reinforced composite material. The elements  50 ,  60  and the interlayer  1  may form part of a spar cap  100  arranged in a wind turbine blade, such as the spar caps  7400 ,  7600  of the wind turbine blade  1000  as illustrated in  FIG.  2   . 
       FIG.  3 B  is a schematic diagram illustrating an exploded view of the interlayer  1  arranged between the first and second elements  50 ,  60 . The interlayer  1 , in the illustrated example, comprises an interlayer sheet  2  having an upper interlayer surface  3  and a lower interlayer surface  4 . In the same way, the first element  50  has a first upper surface  51  and a first lower surface  52  and the second element  60  has a second upper surface  61  and a second lower surface  62 . 
     The first element  50  and the second element  60  are arranged such that the first lower surface  52  of the first element  50  is facing the second upper surface  61  of the second element  60 . The interlayer  1  and the interlayer sheet  2  is arranged between the lower surface of the first element  50  and the upper surface of the second element  60 , e.g. such that the upper interlayer surface  3  is in contact with the first lower surface  52  and the lower interlayer surface  4  is in contact with the second upper surface  61 . 
       FIG.  3 C  is a schematic diagram illustrating a cross-sectional view of a fibre reinforced composite material  100 , e.g. spar cap or part of a spar cap, comprising a plurality of elements, such as pultruded carbon elements, including a first element  50 , such as a first pultruded carbon element, and a second element  60 , such as a second pultruded carbon element. The plurality of elements are arranged in an array with three rows of elements arranged adjacent to each other. Each row of elements are separated by an interlayer  1 . The fibre reinforced composite material  100  may form part of a spar cap arranged in a wind turbine blade, such as the spar caps  7400 ,  7600  of the wind turbine blade  1000  as illustrated in  FIG.  2   . Although not specifically illustrated, interlayers may also be provided between adjacent elements in the width direction, to facilitate conductivity between elements also in this direction. 
       FIGS.  4 A and  4 B  is a schematic diagram illustrating a three-dimensional view of an exemplary interlayer sheet  2  according to two different embodiments, whereas  FIGS.  5 A and  5 B  shows a cross-sectional view through  FIGS.  4 A and  4 B . In  FIGS.  4 C and  5 C  an exploded view of the embodiments shown in  FIGS.  4 B and  5 B  is illustrated. 
     In  FIGS.  4 A and  5 A , the interlayer sheet  2  comprises one fibre layer i.e. the first fibre layer  10 . In this case, the first upper fibre surface  11  is also the upper interlayer surface  3  and the first lower fibre surface  12  is also the lower interlayer surface  4 . 
     In  FIGS.  4 B,  4 C,  5 B and  5 C  the interlayer sheet  2  comprises three fibre layers including a first fibre layer  10 , a second fibre layer  20  and a third fibre layer  30 . 
     The first fibre layer  10  have a first upper fibre surface  11  and a first lower fibre surface  12 . The second fibre layer have a second upper fibre surface  21  and a second lower fibre surface  22  and the third fibre layer  30  have a third upper fibre surface  31  and a third lower fibre surface  32 . 
     The first fibre layer  10  is arranged between the second fibre layer  20  and the third fibre layer  30 . In this case, the third lower fibre surface  32  is also the lower interlayer surface  4  and the second upper fibre surface  21  is also the upper interlayer surface  3 . 
     The first fibre layer  10  comprise a first plurality of fibres, the second fibre layer  20  comprise a second plurality of fibres and the third fibre layer  30  comprise a third plurality of fibres. The first plurality of fibres may comprise a first plurality of glass fibres and/or a first plurality of polymeric filaments, such as polyester filaments, preferably thermoplastic polyester filaments or polypropylene filaments or polyethylene filaments and/or a first plurality of carbon fibres (and/or another conductive fibre, e.g. metal fibre, such as copper fibre and/or steel fibre). In the same way the second plurality of fibres may comprise a second plurality of glass fibres and/or a second plurality of polymeric filaments, such as polyester filaments, preferably thermoplastic polyester filaments or polypropylene filaments or polyethylene filaments and/or a second plurality of carbon fibres (or another conductive fibre, e.g. metal fibre, such as copper fibre or steel fibre) and the third plurality of fibres may comprise a third plurality of glass fibres and/or a third plurality of polymeric filaments, such as polyester filaments, preferably thermoplastic polyester filaments or polypropylene filaments or polyethylene filaments and/or a third plurality of carbon fibres (or another conductive fibre, e.g. metal fibre, such as copper fibre or steel fibre). 
     The interlayer sheet  2  comprises a plurality of carbon fibres  6  (not illustrated in  FIGS.  4  or  5   ) forming part of the upper interlayer surface  3  as well as the lower interlayer surface  4 . Thus, the plurality of carbon fibres  6  extends through the interlayer sheet  2 , including the one or more layers  10 ,  20 ,  30  in one way or another. Reference is made to  FIGS.  6 - 9    illustrating a plurality of different embodiments where the plurality of carbon fibres  6  form part of the upper interlayer surface  3  as well as the lower interlayer surface  4  of an interlayer sheet  2 . The presence of the plurality of carbon fibres  6  extending through the interlayer sheet  2 , is that the plurality of carbon fibres  6  may facilitate the transfer of electrons between two elements, e.g. two carbon elements, sandwiching the interlayer sheet  2 , by providing indirect contact between such two elements. In this way, the interlayer sheet  2  protects the elements, e.g. containing conductive fibres, against lightning strikes. While embodiments are described with reference to carbon fibres  6 , these may in alternative embodiments, within the scope of the present disclosure, be replaced by or mixed with other conductive fibres, e.g. metal fibres, such as copper fibres or steel fibres. 
     Preferably, the interlayer comprises 10-45 wt% carbon fibres, 5-50 wt% polymeric filaments and 15-50 wt% glass fibres. The polymeric filaments provide good surface properties to the interlayer sheet, such as good adherence properties. The glass fibres add stability and reinforcement to the interlayer sheet and the carbon fibres add conductivity. 
       FIG.  6 A  is a schematic diagram showing a top view of an embodiment of an interlayer sheet  2 .  FIG.  6 B  is a schematic diagram showing a cross-sectional view of the interlayer sheet  2  of  FIG.  5 A  as well as a close-up of part C of the cross-sectional view of the interlayer sheet  2 . 
     The interlayer sheet  2  comprises one fibre layer i.e. a first fibre layer  10 . Thus, the first upper fibre surface  11  is also the upper interlayer surface  3  and the first lower fibre surface  12  is also the lower interlayer surface  4 . 
     The first fibre layer  10  may be a non-woven fabric layer, e.g. essentially consisting of a first plurality of polymeric filaments, such as polyester filaments. Such a layer has good surface properties, including good adherence properties. For example, the first fibre layer  10  may be a polyester surface veil. 
     The interlayer sheet  2  further comprises a plurality of carbon fibres  6  and a plurality of glass fibres  7 , including short and/or continuous fibres with varying sizes. The plurality of carbon fibres  6  and the plurality of glass fibres  7  each comprise several parts, including a first part  6   a ,  7   a  and a second part  6   b ,  7   b . The second part  6   b ,  7   b  of each of the plurality of carbon fibres  6  and the plurality of glass fibres  7  are randomly arranged at the first upper surface  11  of the first fibre layer  10  and forms part of the upper interlayer surface  3 , whereas a first part  6   a ,  7   a  of each of the plurality of carbon fibres  6  extends through the first layer  10  and thereby also forms part of the lower interlayer surface  4 . 
     For example, the plurality of carbon fibres  6  and/or the plurality of glass fibres  7  may be provided by spraying them onto the first fibre layer  10  in a direction substantially perpendicular to the surface of the first fibre layer, e.g. using pressurised air. Thereby at least some of the ends of the carbon and/or glass fibres may upon impact with the layer extend into and through the layer. 
       FIG.  6 B  illustrates how a first part  6   a ,  7   a  of each of the plurality of carbon fibres  6  and glass fibres  7  extends from the upper interlayer surface  3  through the first layer  10  and thereby also forms part of the lower interlayer surface  4 . The glass fibres  7  extending through the first fibre layer  10  adds stability and reinforcement to the interlayer sheet  2 . Due to the plurality of carbon fibres  6  extending through the first layer  10 , the interlayer sheet  2  is conductive when arranged between two elements, such as carbon elements, such as between two pultruded carbon elements, of a spar cap arranged in a wind turbine blade shell. 
       FIG.  7 A  is a schematic diagram showing a top view of an embodiment of an interlayer sheet  2 .  FIG.  7 B  is a schematic diagram showing a cross-sectional view of the interlayer sheet  2  of  FIG.  7 A . 
     The interlayer sheet  2  comprises three fibre layers including a first fibre layer  10 , a second fibre layer  20  and a third fibre layer  30 . The first fibre layer  10  have a first upper fibre surface  11  and a first lower fibre surface  12 . The second fibre layer have a second upper fibre surface  21  and a second lower fibre surface  22  and the third fibre layer  30  have a third upper fibre surface  31  and a third lower fibre surface  32 . 
     The first fibre layer  10  is arranged between the second fibre layer  20  and the third fibre layer  30 . In this case, the third lower fibre surface  32  is also the lower interlayer surface  4  and the second upper fibre surface  21  is also the upper interlayer surface  3 . 
     The first fibre layer  10  may be a non-woven fabric layer essentially consisting of a first plurality of glass fibres. The glass fibres  7  adds stability and reinforcement to the interlayer sheet  2 . 
     The second fibre layer and third fibre layers  20 ,  30  are also non-woven fabrics. Preferably, the second fibre layer  20  essentially consist of a first plurality of polymeric filaments, such as polyester filaments. Such a layer adds good surface properties to the interlayer sheet  2 , including good adherence properties. Furthermore, the third fibre layer  30  essentially consist of a first plurality of polymeric filaments, such as polyester filaments. As a result, both outer surfaces of the interlayer sheet  2  have good adherence properties. 
     The interlayer sheet  2  further comprise a plurality of carbon fibres  6 . The plurality of carbon fibres  6  may be short and/or continuous fibres with varying sizes. The plurality of carbon fibres  6  each comprises several parts, including a first part  6   a  and a second part  6   b . The second part  6   b  of each of the plurality of carbon fibres  6  are randomly arranged at the second upper surface  21  of the second fibre layer-20 and forms part of the upper interlayer surface  3 , whereas a first part  6   a  of each of the plurality of carbon fibres  6  extends through the first, second and third fibre layers  10 ,  20 ,  30 , such that the carbon fibres  6  also forms part of the lower interlayer surface  4 . 
       FIG.  7 B  illustrates how a first part  6   a  of each of the plurality of carbon fibres  6  extends from the upper interlayer surface  3  through the first, second and third fibre layer  10 ,  20 ,  30 , such that the carbon fibres  6  also forms part of the lower interlayer surface  4 . Due to the plurality of carbon fibres  6 , extending through the first layer  10 , the interlayer sheet  2  is conductive in the direction perpendicular to the plane of the interlayer sheet  2 . Thus, when arranged between two elements, e.g. carbon elements, such as between two pultruded carbon elements, of a spar cap arranged in a wind turbine blade shell, the interlayer sheet  2  prevents or reduce build up of a voltage potential between the elements. 
       FIG.  7 C  illustrates that the plurality of carbon fibres  6  may be punched through the interlayer sheet  2 , e.g. using pressurised air, and thus extend through the first, second and third fibre layer  10 ,  20 ,  30  in random directions. Alternatively, as illustrated in  FIG.  7 D , the fibre layers  10 ,  20 ,  30  may be stitched together by the plurality of carbon fibres  6 , forming a controlled pattern of carbon fibres  6  extending through the interlayer sheet  2 . In the later embodiment, the plurality of carbon fibres  6  holds the three fibre layers  10 ,  20 ,  30  together and at the same time adds conductivity to the interlayer sheet  2 . 
       FIG.  8 A  is a schematic diagram showing a top view of an embodiment of an interlayer sheet  2 .  FIG.  8 B  is a schematic diagram showing a cross-sectional view of the interlayer sheet  2  of  FIG.  8 A . 
     The interlayer sheet  2  comprises one fibre layer i.e. a first fibre layer  10  comprising a first upper fibre surface  11  and a first lower fibre surface  12 . Thus, the first upper fibre surface  11  is also the upper interlayer surface  3  and the first lower fibre surface  12  is also the lower interlayer surface  4  of the interlayer sheet  2 . 
     The first fibre layer  10  may be a non-woven fabric and comprises a first plurality of fibres, including a first plurality of carbon fibres  6  (illustrated by a black thin line), a first plurality of glass fibres  7  (illustrated by a grey thin line) and a first plurality of polymeric filaments  8  (illustrated by a black thick line). The first fibre layer  10  may further comprise a binding agent, preferably a binding agent being dissolvable by a resin, maintaining arrangement of the first plurality of fibres relative to each other. Alternatively or in addition, the first plurality of fibres may be stitched together, optionally with a carbon fibre thread, to maintain arrangement of the first plurality of fibres relative to each other. 
     The first plurality of fibres  6 ,  7 ,  8  are randomly oriented within the first fibre layer  10 . Due to the random arrangement of fibres in a single layer, at least a plurality  6  of the first plurality of carbon fibres will form part of the upper interlayer surface  3  as well as the lower interlayer surface  4 , making the interlayer sheet  2  conductive when arranged between two elements, e.g. carbon elements, such as between two pultruded carbon elements, of a spar cap arranged in a wind turbine blade shell. 
       FIGS.  9 A and  9 B  are schematic illustrations of two different embodiments of an interlayer sheet  2  comprising a first layer  10  being a woven fabric. 
     The first fibre layer  10  comprises a first upper fibre surface  11  and a first lower fibre surface  12 . Thus, the first upper fibre surface  11  is also the upper interlayer surface  3  and the first lower fibre surface  12  is also the lower interlayer surface  4  of the interlayer sheet  2 . 
     The first fibre layer  10  comprises a first plurality of fibres including a first plurality of carbon fibres  6 , a first plurality of glass fibres  7  and a first plurality of polymeric filaments  8 . The first plurality of fibres  6 ,  7 ,  8  are woven together. 
     The first plurality of glass fibres  7  is arranged in a plurality of glass fibre bundles and the first plurality of carbon fibres  6  are arranged in a plurality of carbon fibre bundles. 
     Each of the first plurality of carbon fibres  6  is arranged along a first length direction, each of the first plurality of glass fibres  7  are arranged along a second length direction and each of the first plurality of polymeric filaments  8  are arranged along a third length direction. 
     In  FIG.  9 A , the first plurality of carbon fibres  6  and the first plurality of polymeric filaments  8  are arranged parallelly and both extend in a first direction X, whereas the first plurality of glass fibres  7  extend in a second direction Y, which is perpendicular to the first direction X. Thus, the first and third length directions are parallel, i.e. parallel with the first direction X, and the second length direction is perpendicular to the first and third length directions. Hence, the second length direction is parallel with the second direction Y. In an alternative embodiment (not illustrated) a number of carbon fibres may be added in the first direction X along the first plurality of glass fibres  7  to further enhance electrical conductivity through the plane. 
     In  FIG.  9 B , the first plurality of carbon fibres  6  and the first plurality of glass fibres  7  are arranged parallelly and extend in the second direction Y, whereas the first plurality of polymeric filaments  8  extend in the first direction X, which is perpendicular to Y. Thus, the first length direction and second length direction are parallel, i.e. parallel with the second direction Y, and the third length direction is perpendicular to the first and second length directions. Hence, the third length direction is parallel with the first direction X. 
     In  FIG.  9 B , the first plurality of carbon fibres  6  and the first plurality of glass fibres  7  are arranged parallelly and extend in the second direction Y, whereas the first plurality of polymeric filaments  8  extend in the first direction X, which is perpendicular to Y. 
       FIG.  10 A  is a schematic diagram illustrating an exemplary interlayer  1 , such as the interlayer  1  as described, e.g. with reference to  FIG.  3   .  FIG.  10    illustrates that the interlayer  1  may comprise a top sheet  70  and/or a bottom sheet  72 , e.g. in addition to the interlayer sheet  2  as described with respect to  FIGS.  4 - 9   . 
     The top sheet  70  is arranged adjacent the upper interlayer surface  3  and the bottom sheet  72  is arranged adjacent the lower interlayer surface  4 . The interlayer sheet  2  may be sandwiched between the top sheet  70  and the bottom sheet  72 . 
       FIG.  10 B  is a cross-sectional view of the interlayer  1  illustrated in  FIG.  10 A . 
     The top sheet  70  and/or bottom sheet  72  may for example be carbon veils, since such veils facilitates electrical conductivity through the plane. Furthermore, a carbon veil has high permeability, promotes resin infusion and have good adhesion properties. Alternatively, the top sheet  70  and/or bottom sheet  72  may be polyester veils, since such veils have good adhesion properties. In such case the top sheet  70  and/or bottom sheet  72  may comprise conductive elements, such as conductive fibres, such as carbon fibres. The presence of conductive fibres, such as carbon fibres, in the top sheet  70  and/or bottom sheet  72  facilitates the electrical connection through the interlayer, such as facilitates electron flow between elements, such as pultruded elements, when sandwiched therebetween. 
       FIG.  11    illustrates a cross-sectional view of an embodiment of an interlayer sheet  2 , wherein the interlayer sheet  2  comprises a first fibre layer  10 , a second fibre layer  20  and a third fibre layer  30 . The first fibre layer  10  comprises a plurality of polymeric fibres  8  and carbon fibre bundles  6  arranged along a first length direction. The carbon fibre bundles  6  are illustrated as black dots, whereas the polymeric filaments  8  are illustrated as white dots. As can be seen in  FIG.  11   , every 4 th  fibre bundle is a carbon fibre bundle  6 , whereas the remaining fibres are polymeric filaments  8 . In other exemplary embodiments, there may be more or less polymeric filaments between each carbon fibre bundle. 
     The second fibre layer  20  and/or the third fibre layer  30  are preferably polyester surface veils or carbon surface veils. The first fibre layer  10 , the second fibre layer  20  and the third fibre layer  30  are stitched or woven together. The thick black line illustrates a thread  64 , e.g. a fibre, such as a glass fibre or a carbon fibre, extending along the second length direction and stitching or weaving the carbon fibres  6  and polymeric filaments  8  arranged along the first length direction together with the second fibre layer  20  and the third fibre layer  30 . The thread  64  may be a conductive fibre and in this way, the interlayer sheet  2  may have conductive properties, even though the second fibre layer  20  and/or the third fibre layer  30  essentially consist of a non-conductive material. In some exemplary embodiments, a plurality of threads  64  may be used along the length of the interlayer sheet  2 , and in such situation the plurality of threads  64  may comprise some conductive threads and some non-conductive threads, e.g. every 10 th  thread may be a conductive thread while the remaining threads may be non-conductive. 
     The disclosure has been described with reference to a preferred embodiment. However, the scope of the invention is not limited to the illustrated embodiment, and alterations and modifications can be carried out without deviating from the scope of the invention. 
     Throughout the description, the use of the terms “first”, “second”, “third”, “fourth”, “primary”, “secondary”, “tertiary” etc. does not imply any particular order or importance but are included to identify individual elements. Furthermore, the labelling of a first element does not imply the presence of a second element and vice versa. 
     REFERENCE SIGNS 
     
       
         
           
               
               
            
               
                 
                   1 
                 
                 interlayer 
               
               
                 
                   2 
                 
                 interlayer sheet 
               
               
                 
                   3 
                 
                 upper interlayer surface 
               
               
                 
                   4 
                 
                 lower interlayer surface 
               
               
                 
                   5 
                 
                 fibre layer plane 
               
               
                 
                   6 
                 
                 plurality of carbon fibres 
               
               
                 
                   6 
                   a 
                 
                 First part 
               
               
                 
                   6 
                   b 
                 
                 Second part 
               
               
                 
                   7 
                 
                 plurality of glass fibres 
               
               
                 
                   7 
                   a 
                 
                 first part 
               
               
                 
                   7 
                   b 
                 
                 second part 
               
               
                 
                   8 
                 
                 plurality of polymeric filaments 
               
               
                 
                   10 
                 
                 first fibre layer 
               
               
                 
                   11 
                 
                 first upper fibre surface 
               
               
                 
                   12 
                 
                 first lower fibre surface 
               
               
                 
                   13 
                 
                 first plurality of carbon fibres 
               
               
                 
                   14 
                 
                 first plurality of glass fibres 
               
               
                 
                   15 
                 
                 first plurality of polymeric filaments 
               
               
                 
                   18 
                 
                 Third length direction 
               
               
                 
                   20 
                 
                 second fibre layer 
               
               
                 
                   21 
                 
                 second upper fibre surface 
               
               
                 
                   22 
                 
                 second lower fibre surface 
               
               
                 
                   23 
                 
                 second plurality of carbon fibres 
               
               
                 
                   24 
                 
                 second plurality of glass fibres 
               
               
                 
                   25 
                 
                 second plurality of polymeric filaments 
               
               
                 
                   30 
                 
                 third fibre layer 
               
               
                 
                   31 
                 
                 third upper fibre surface 
               
               
                 
                   32 
                 
                 third lower fibre surface 
               
               
                 
                   33 
                 
                 third plurality of carbon fibres 
               
               
                 
                   34 
                 
                 third plurality of glass fibres 
               
               
                 
                   35 
                 
                 third plurality of polymeric filaments 
               
               
                 
                   40 
                 
                 spar cap 
               
               
                 
                   50 
                 
                 first element 
               
               
                 
                   51 
                 
                 first upper surface 
               
               
                 
                   52 
                 
                 first lower surface 
               
               
                 
                   60 
                 
                 second element 
               
               
                 
                   61 
                 
                 second upper surface 
               
               
                 
                   62 
                 
                 second lower surface 
               
               
                 
                   64 
                 
                 thread 
               
               
                 
                   70 
                 
                 top sheet 
               
               
                 
                   72 
                 
                 bottom sheet 
               
               
                 
                   100 
                 
                 spar cap 
               
               
                 
                   200 
                 
                 wind turbine 
               
               
                 
                   400 
                 
                 tower 
               
               
                 
                   600 
                 
                 nacelle 
               
               
                 
                   800 
                 
                 hub 
               
               
                 
                   1000 
                 
                 blade 
               
               
                 
                   1400 
                 
                 blade tip 
               
               
                 
                   1600 
                 
                 blade root 
               
               
                 
                   1800 
                 
                 leading edge 
               
               
                 
                   2000 
                 
                 trailing edge 
               
               
                 
                   2200 
                 
                 pitch axis 
               
               
                 
                   2400 
                 
                 pressure side 
               
               
                 
                   2600 
                 
                 suction side 
               
               
                 
                   3000 
                 
                 root region 
               
               
                 
                   3200 
                 
                 transition region 
               
               
                 
                   3400 
                 
                 airfoil region 
               
               
                 
                   3800 
                 
                 chord line 
               
               
                 
                   4000 
                 
                 shoulder / position of maximum chord 
               
               
                 
                   4200 
                 
                 shear webs 
               
               
                 
                   7400 
                 
                 first spar cap 
               
               
                 
                   7600 
                 
                 second spar cap 
               
               
                 X 
                 first direction 
               
               
                 Y 
                 second direction